Strength

Strength is the ability to express force through the production of joint moments. Research comparing the effects of training programs over time can help identify the fastest ways to get stronger.   

If you have some resistance-training experience, train with heavy loads, high volumes, long rest periods, closer to muscular failure, and using a periodized routine.

If you have little resistance-training experience, train with heavy loads, high volumes, long rest periods, fast bar speeds, closer to muscular failure, larger ranges of motion, and using a periodized routine.

CONTENTS

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Summary

Background

Relative load (percentage of 1RM)

Volume

Muscular failure

Frequency (whole body or split)

Rest period duration

Range of motion

Bar speed (isokinetic)

Bar speed (not isokinetic)

Muscle action (isokinetic)

Muscle action (not isokinetic)

Resistance type (variable vs. constant load)

Periodization type

Mechanisms of strength

References

SUMMARY

PURPOSE

This section provides the summary of the evidence provided from long-term trials regarding how best to increase strength using resistance training programs. 

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SUMMARY EVIDENCE FOR STRENGTH GAINS (TRAINED)

Relative load – Heavy loads superior to moderate and light

Volume  – Higher volumes superior to lower

Muscular failure – Closer to failure superior to further from failure

Volume-matched frequency – Higher volume-matched frequency superior to lower

Rest periods – Longer rest periods superior to shorter”

Range of motion (ROM) – Unknown effects

Bar speed – Unknown effects

Muscle action (variable) – Unknown effects

Muscle action (constant load) – Unknown effects

External load type – Unknown effects

Periodization – Periodized routine superior to non-periodized

Periodization – No difference between linear and non-linear periodization

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SUMMARY EVIDENCE FOR STRENGTH GAINS (UNTRAINED)

Relative load – Heavy loads superior to moderate and light

Volume – Higher volumes superior to lower

Muscular failure  Closer to failure superior to further from failure

Volume matched frequency – No difference between different volume-matched frequencies

Frequency to increase volume – Higher frequency leading to higher volume superior to lower frequency with lower volume

Rest periods – Longer rest periods superior to shorter

Range of motion (ROM) – Larger ROM superior to smaller ROM

Bar speed – Fast bar speeds superior to slow

Muscle action (variable) – Eccentric muscle actions superior to concentric

Muscle action (constant load) – Unknown effects

External load type – Variable resistance similar to constant load

Periodization – Periodized routine superior to non-periodized

Periodization – No difference between linear and non-linear periodized routines

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

CONCLUSIONS FOR STRENGTH

If you have some resistance-training experience, train with heavy loads, high volumes, long rest periods, closer to muscular failure, and using a periodized routine.

If you have little resistance-training experience, train with heavy loads, high volumes, long rest periods, fast bar speeds, closer to muscular failure, larger ranges of motion, and using a periodized routine.

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BACKGROUND

PURPOSE

This section provides the background to strength, including the various different measurement methods. 

BACKGROUND

Introduction

Strength is defined as the ability to express force through the production of joint moments. There are many different conditions affecting the way in which strength is displayed. Strength can be displayed maximally or sub-maximally; it can be displayed isometrically (without moving) or dynamically (while moving); during isometric tests strength can be displayed at different joint ranges of motion (leading to long or short muscle lengths); it can be displayed at a single joint (in isolation exercises) or at multiple joints (in compound movements); it can be displayed against an external resistance that has a constant load (isoinertial) or against a changing external resistance (variable, accommodating or isokinetic). In addition, when exerted dynamically, strength can be exerted either while the involved muscle is lengthening or shortening, which are commonly referred to as concentric or eccentric muscle actions.

Overview of strength mechanisms

Strength gains are thought to arise from a combination of factors, including both central and peripheral mechanisms. Central mechanisms refer to adaptations that occur within the central nervous system, while peripheral mechanisms refer to adaptations that occur within the muscle itself. It is unclear to what extent each of the central and peripheral adaptations affect strength gains during resistance training programs. There are indications that the main factor influencing gains in strength is hypertrophy (Erskine et al. 2014). However, other peripheral factors also appear important, as specific tension explains some of the inter-individual variability in strength gains. Specific tension seems to be driven partly by changes in extracellular lateral force transmission and partly by changes in myofibrillar packing density, which are alterations that occur at the level of the individual muscle fiber. Our understanding of the central factors that affect strength is very limited. Traditionally, it has been assumed that large changes in agonist activity or agonist-antagonist co-activation occurred but in reality these are very small. While inter-muscular co-ordination has not been widely studied, it is possible that this may prove to be the largest single central factor affecting strength gains, as many of the other possibilities have been discounted.

Read more about mechanisms

Overview of genetic factors

When performing the same resistance training programs, individuals typically display a wide range of responses. Some enjoy very large increases in strength, while others display almost none. In one of the largest investigations into the inter-individual variability in strength gains yet performed, it was reported that the variance between subjects in 1RM strength gains ranged from 1RM strength gains ranged from 0% to +250%, while changes in maximum isometric strength ranged from -32% to +149% (Hubal et al. 2005). Twin studies have identified that genetic factors are critical for determining both the starting point for muscular strength and the strength gains that result from training (Thomis et al. 1998; Thomis et al. 2000; Tiainen et al. 2004; Mars et al. 2007).

Definitions of strength

Problems

Defining the term “strength” almost seems unnecessary. And yet, since the term is used in so many different circumstances by so many different groups, a comprehensive definition is actually very difficult to provide without resulting in a vague and meaningless concept (Enoka, 1988). The primary complicating factor is the force-velocity relationship (Hill, 1938), although the length-tension relationship is also a key variable. The force-velocity relationship is the observation that muscle force and contraction velocity are inversely related. So where contraction velocity is high, muscle force must be low and vice versa. In practice, this means that testing strength when muscles are changing length must necessarily involve a reduction in maximum force generating capacity. Importantly, the size of this reduction may differ between muscles, individuals and exercises.

Solutions

In order to remove the variability caused by the force-velocity relationship, many researchers have chosen to define strength as the peak level of isometric force production, preferably at a single joint (Enoka, 1988). While this is beneficial as it means that the factors that drive increases in strength can be isolated, it is severely disadvantageous in that there is a discrepancy between the measures of strength that are used in research and those that are directly relevant to strength and conditioning. Therefore, many researchers also make use of multi-joint, dynamic measures of strength, most commonly one-repetition maximum (1RM) lifts in key exercises, such as the squat and bench press. Since 1RM movements are usually performed at very low speeds, they are often referred to as “quasi-isometric” and the relationship between them and maximum isometric strength is thought to be relatively good. Nevertheless, the force-velocity relationship still applies and we should take this into account when considering recommendations for training.

Identifying relevant research

Introduction

In order to review how to achieve optimal strength gains in resistance training programs, it is necessary to identify which sources of research should be used and which should not. It is also necessary to rank sources of research that will be used into types that are most relevant and types that are least relevant. The most relevant types of research are given the greatest weight while the least relevant are given the least weight. This maximises the possibility that the conclusions of the review will be correct.

Long-term trials

Long-term (chronic) trials are those that measure changes in strength over a period of time sufficient for an adaptation to arise (e.g. 6 – 12 weeks). This means that they therefore measure changes in strength directly, by taking a measurement at baseline (before the resistance training program) and afterwards (after the resistance training program). Long-term trials are often used for testing the effectiveness of different elements of resistance training programs. Elements of resistance training programs can include the relative load (percentage of 1RM), volume, or periodization type. Importantly, while new long-term trials are always being published and added to the literature, this does not supersede the findings of the older long-term trials but rather adds to them. Well-constructed long-term trials are therefore extraordinarily valuable to strength and conditioning professionals and researchers alike for ascertaining the ideal means to increase measures of strength. However, as Fisher (2013) has noted, not all long-term trials are well-controlled or reported and often fail to place sufficient control over the details of a training program such that it can provide confidence regarding the factors that have genuinely affected the resulting outcomes.

Short-term trials

Short-term (acute) trials are those that measure immediate changes in a variable that is thought to be related to strength, such as electromyography (EMG). They therefore measure the potential for long-term changes in strength to occur indirectly. Whether this potential is realized depends on how strongly the indirect variable is related to long-term gains in strength and whether our underlying model of how strength gains occur is correct. Importantly, when new short-term trials are published and demonstrate the need to change our underlying model of how strength gains occur, this can completely supersede the findings of the older trials.

Current guidance for strength

Formal guidance

There is no shortage of current guidance for individuals wishing to gain strength. Such guidance ranges from advice from expert strength coaches based on their personal observations (level 4 evidence) through to research-based position stands (levels 1 – 3 evidence) produced by leading institutions, including the American College of Sports Medicine (Kraemer et al. 2002; ACSM, 2009). However, these formal position stands have been criticized on the basis that they draw too much on short-term trials and the currently accepted underlying models of how strength gains occur and cannot substantiate their recommendations using long-term trials. This means that they are at risk of being incorrect (Carpinelli, 2004). Unsurprisingly therefore, more recent attempts have been made to provide better guidance, focusing solely on long-term trials (Fisher et al. 2011).  Such reviews provide a very different perspective on those aspects of a resistance training program that are important for maximizing strength gains.

Expert opinion

Although much of the information that is dispensed is ultimately expert opinion, this source of guidance is far more limited than is generally recognised. Readers familiar with research methodology will readily appreciate the large difference in the level of evidence provided by controlled trials and that provided by expert opinion. However, for those less familiar, there are two very important differences (aside from the inherent fallible nature of human observation described in psychology by the presence of cognitive biases, which are usually conveniently ignored).

Firstly, experts base their opinions on the observation of training programs that they have either used themselves or with their athletes. They therefore make observations about programs (or elements of programs) that worked or that did not work. Rarely (if ever) are they able to compare two programs at the same time in similar groups of athletes. This is the realm in which research stands apart. Therefore, while an expert can be fairly confident establishing whether their program is effective, it is hard (if not impossible) for them to find out whether their program really is better than another program, especially where the differences between programs are marginal.

Secondly, the athletes or trainees that experts work with are training without control groups to compare them with and in an uncontrolled environment. This makes it very difficult for an expert to know whether it is their training program that is producing the rapid gains in strength or something that the athlete or trainee is doing elsewhere. For example, when training youth athletes, it might be easy to assume that very fast strength gains were arising because the athletes were engaged in a superior program, whereas in fact it is simply because they are growing. Similarly, when training athletes for whom there are strong incentives to achieve, it might be easy to assume that very fast strength gains were arising because the athletes were engaged in a superior program, whereas in fact it is simply because they are highly motivated, performing extra training, or even using performance enhancing drugs (PEDs).

Lay-out of this review

In order to reduce the chance of error, this review is structured around the analysis of groups of long-term trials that have explored the effects of specific, individual training program variables (such as relative load, volume, proximity to muscular failure, frequency, rest periods, range of motion (ROM), bar speed (time under tension), muscle action, and periodization type. Each section reviews the literature on the basis of set selection criteria designed to make sure that the included studies present the most fair set of circumstances for investigating that particular training program variable.

CONCLUSIONS FOR STRENGTH

To reduce the risk of error, analysis of the optimal methods for strength gains should be based around a review of well-controlled long-term trials comparing individual resistance training variables.

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RELATIVE LOAD (PERCENTAGE OF 1RM)

PURPOSE

This section explores whether training with heavier loads is more effective for strength gains. This is achieved by looking at long-term studies that compare different programs of training with heavy, moderate and light relative loads for increasing strength.

BACKGROUND

Definitions

Relative load (percentage of 1RM)

Resistance training can be performed with range of loads. The weight lifted may vary from a load that can be lifted only once to loads that can be lifted a great many times. Since the absolute load (in kg or lbs) that an individual can lift varies greatly from one person to the next, it is conventional in resistance training programs to use percentages of one repetition maximum (1RM) when specifying the loads used. Percentage of 1RM during resistance training is sometimes referred to as “intensity”. However, this term is ambiguous as it could be taken to imply a reference to effort that is not intended (see reviews and commentary by Fisher and Smith, 2012; Steele, 2013; Fisher and Steele, 2014; and Schoenfeld, 2014). Effort may not be the same as percentage of 1RM, particularly where inter-individual differences exist in respect of the number of repetitions that can be performed at a given load. For the sake of clarity, alternative suggestions have been made regarding terminology, including “intensity of load” and “relative load”.

Heavy, moderate and light relative loads

Definitions of heavy, moderate and light relative loads vary between researchers. Schoenfeld (2010) proposed defining the ranges as 1 – 5 repetitions being low, 6 – 12 repetitions being moderate, and >15 repetitions being high. These repetition ranges likely correspond to percentage of 1RM as follows: 85 – 100% of 1RM being low, 60 – 85% of 1RM being moderate, and <60% of 1RM being high. The exact relative load that corresponds with a repetition range becomes more difficult to specify with precision as the number of repetitions increases, because of inter-individual differences in fatigue resistance. Some authorities have proposed that the threshold of 15 repetitions corresponds to 60% of 1RM, while others have suggested that 65% is more appropriate (Schoenfeld, 2010; Baechle and Earle, 2008). Thus, when discussing high repetitions, the threshold is sometimes referred to as 60% (ACSM, 2009; Schoenfeld, 2013) and sometimes as 65% (Schoenfeld, 2010).

Popular usage

Powerlifting

Powerlifting is a sport that is heavily dependent upon the ability to produce force with both the upper and lower body. Consequently, the resistance training programs used by powerlifters are specifically intended to produce the greatest possible strength gains. There are many popular powerlifting programs, including Starting Strength, StrongLifts, Madcow’s 5 x 5, The Texas Method, 5/3/1, Sheiko, Smolov, Westside Barbell Method, and more. These programs make use of a range of different relative loads, from 60 – 70% of 1RM in Sheiko, to 70 – 85% of 1RM in Smolov, to around 80 – 85% of 1RM in Starting Strength, and to 100% of 1RM in Westside Barbell. Nevertheless, while there are many supporters of all of these programs, there is is no clear front runner among them, with individuals achieving marked increases in strength as a result of all programs.

Literature usage

Research interest into the effects of relative load on strength gains has been performed mainly subsequent to interest into the effects of relative load on hypertrophy. This is primarily because of the large body of research showing significantly greater increases in strength with heavy loads than with light loads, as well as some research showing significantly greater increases in strength with moderate loads than with light loads.

META-ANALYSES

Meta-analyses indicate that a higher relative load could be superior to a lighter relative load for increasing strength but there is uncertainty on account of a lack of statistical significance.

Schoenfeld et al. (2014b) carried out a meta-analysis in trained and untrained subjects to compare the effects of high (>65% of 1RM) and low (<60% of 1RM) relative loads during resistance training programs on strength gains. It was found that there was a non-significant trend for the pooled effect size for strength gains to be greater with high than with low relative loads loads (effect sizes: 2.30 ± 0.43 vs. 1.23 ± 0.43).

MECHANISMS FOR INCREASING STRENGTH

For a full review of the mechanisms of strength development, see the mechanisms section (read more).

Conceptual basis for strength gains

Peripheral factors

The peripheral factors that might affect strength gains include: hypertrophy, myofibrillar packing density, extracellular lateral force transmission, muscle fiber type shifts and regional hypertrophy. The effect of relative load on hypertrophy is difficult to assess and there is no evidence that increasing relative load above moderate levels is beneficial for increasing muscle size. Equally, the research into the effects of different training variables on specific tension (and its underlying elements) is very limited and it is unclear whether increasing relative load might alter either myofibrillar packing density or extracellular lateral force transmission.

Central factors

The central factors that might affect strength gains include: inter-muscular co-ordination and motor unit firing frequency. Higher relative loads might increase strength gains by altering inter-muscular co-ordination, although this has not been studied in detail. Relative load affects proportional joint moments (Bryanton et al. 2012; Beardsley and Contreras, 2014), movement patterns (Frost et al. 2013; Beardsley and Contreras; 2014a) and muscle activity patterns of different muscles in lower-body multi-joint exercises. This may imply that performing multi-joint exercises with a higher relative load could lead to the development of a better pattern of inter-muscular co-ordination that is optimal for moving very high relative loads (i.e. maximal) than performing the same exercises with lower relative loads.

Higher relative loads might also lead to increased motor unit firing rates, as increases in force production above ~90% of maximum voluntary isometric force are mediated by faster firing rates. Such faster firing rates can be observed indirectly by greater levels of electromyographic (EMG) activity. Indeed, there are many indications that higher relative loads are associated with greater acute levels of agonist muscle activity than lower relative loads (Sundstrup et al. 2012; Akima and Saito, 2013; Cook et al. 2013; Schoenfeld et al. 2014). However, it is unclear whether such higher levels of agonist muscle activity during an exercise have any meaningful effect on the increases in voluntary activation over long-term periods of resistance training, which are generally small (Arnold and Bautmans, 2014).

In summary, a higher relative load might be more effective than a low relative load for gaining strength because of increases in inter-muscular co-ordination that are relevant to maximal strength tests.

Read more about mechanisms

PROBLEMS

Controlling other variables

Where maximal bar speeds are used, the force-velocity relationship is a confounding factor when comparing groups training with different relative loads. This is because the group training with the heavier relative load must use a slower bar speed and consequently perform each repetition with a longer repetition duration and (depending on whether volume is measured as sets x repetitions or sets x repetitions x relative load) potentially also a longer total time under tension for the workout. However, when comparing two groups training with different relative loads in which sub-maximal bar speeds are used, the force-velocity relationship does not cause a problem. This is because the same bar speed can be used in both cases (or a different bar speed in order to maintain total time under tension across the workout by manipulating repetition duration).

Volume can be a confounding factor where individuals perform very different numbers of repetitions with the same percentage of 1RM. Thus, where different training groups are being compared who are performing programs using different relative loads, some individuals might perform a greater volume of work than others. While it is noted that several investigations have reported some variation in respect of the number of repetitions that can be performed with a given percentage of 1RM (Hoeger, 1987; Hoeger, 1990; Shimano, 2006; and Moraes, 2014), there does appear to be some degree of reliability in the extent to which prediction equations can be used (Desgorces, 2010). Moreover, the effect of exercise selection seems to be far more important for predicting the number of repetitions that can be performed with a given percentage of 1RM than the exact nature of the population (Hoeger, 1987; Hoeger, 1990; Shimano, 2006; Moraes, 2014; and Desgorces, 2010).

EFFECT OF RELATIVE LOAD ON STRENGTH (UNTRAINED): HEAVY VS. LIGHT LOADS

Study selection

Population – untrained subjects

Intervention – resistance-training, where >2 groups trained with different relative loads and at >1 group used light loads (as defined as <50% of 1RM) and at >1 group used heavy loads (defined as >50% of 1RM)

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 24 studies were identified (click to read): Schmidtbleicher (1981), Anderson (1982), Stone (1994), Pruitt (1995), Aagaard (1996), Hisaeda (1996), Moss (1997), Weiss (1999), Bemben (2000), Campos (2002), Seynnes (2004), Beneka (2005), Tanimoto (2006), Popov (2006), Leger (2006), Fatouros (2006), Holm (2008), Rana (2008), Tanimoto (2008), Scheunke (2012), Mitchell (2012), Ogasawara (2013), Van Roie (2013), Reid (2014). Of these 24 studies, 16 reported significant benefits of heavy loads over light loads while the remainder reported no differences. Heavy loads therefore seem to be beneficial for optimising strength gains in this population.

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EFFECT OF RELATIVE LOAD ON STRENGTH (UNTRAINED): HEAVY VS. MODERATE LOADS

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >2 groups trained with different relative loads and at >1 group used moderate loads (as defined as >60% of 1RM or >15RM) and at >1 group used heavier loads (defined as heavier than the moderate group)

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 11 studies were identified (click to read): Berger (1962), O’Shea (1966), Chestnut (1999), Weiss (1999), Campos (2002), Harris (2004), Kalapotharakos (2004), Beneka (2005), Kalapotharakos (2005), Fatouros (2006), Leger (2006). Of these 11 studies, 6 reported significant benefits of heavy loads over moderate loads while the remainder reported no differences. Heavy loads therefore seem to be beneficial for optimising strength gains in this population.

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

EFFECT OF RELATIVE LOAD ON STRENGTH (TRAINED)

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >2 groups trained with different relative loads and at >1 group used moderate loads (as defined as >60% of 1RM or >15RM) and at >1 group used heavier loads (defined as heavier than the moderate group

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 1 study was identified (click to read): Schoenfeld (2014a). This study reported superior gains for heavy loads compared to moderate loads for one outcome measure and no differences between heavy and moderate loads for another outcome measure. Heavy loads may therefore be beneficial for optimising strength gains in this population.

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

CONCLUSIONS FOR STRENGTH

For untrained individuals, heavier loads seem to be superior to both light and moderate loads.

For trained individuals, evidence is very limited but heavy loads may be superior to moderate loads.

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VOLUME

PURPOSE

This section explores whether training with a higher volume is more effective than training with a lower volume for increasing strength. This is achieved by looking at long-term studies that compare different volumes of training for increasing strength.

BACKGROUND

Definitions

For the purposes of analyzing volume as a training variable in its own right, volume can be very simply defined as the number of sets of an exercise. Thus, in the vast majority of studies investigating the effect of training volume on strength multiple sets of an exercise are compared with single sets. In a small minority, a larger number of sets of a fixed number of repetitions are compared with a smaller number of sets of the same number of repetitions.

For controlling volume when analyzing the effects of other training variables (such as relative load, proximity to muscular failure, range of motion, rest period duration, bar speed, muscle action, or periodization type), at least three methods of equating volume between conditions are possible. Firstly and most easily, volume can be defined as the number of sets x the number of repetitions. However, this is problematic when comparing the effects of training variables that involve different absolute or relative loads, as either the total amount of weight lifted differs or the proximity to muscular failure differs or both. Consequently, other methods of equating volume have been developed. One method involves equating the mechanical work performed by reference to the load lifted (number of sets x the number of repetitions x the total load). However, where different muscle actions, sub-maximal bar speeds, or relative loads are compared this will likely lead to differences in proximity to muscular failure between conditions.

Popular usage

Powerlifting

Powerlifting is a sport that is heavily dependent upon the ability to produce force with both the upper and lower body. Consequently, the resistance training programs used by powerlifters are specifically intended to produce the greatest possible strength gains. There are many popular powerlifting programs, including Starting Strength, StrongLifts, Madcow’s 5 x 5, The Texas Method, 5/3/1, Sheiko, Smolov, Westside Barbell Method, and more. Some of these programs make use of what are generally regarded as high volumes (e.g. Sheiko and Smolov) while others make use of smaller volumes (Starting Strength, StrongLifts, Madcow’s 5 x 5, The Texas Method, 5/3/1). Nevertheless, while there are many supporters of all of these programs, there has is no clear front runner among them, with individuals achieving marked increases in strength as a result of all programs.

Literature usage

Researchers have studied the effect of volume on strength gains more than any other single training variable. This relatively extensive body of literature (in comparison with other training variables) has led to the production of many reviews and one or two meta-analyses.

META-ANALYSES

Meta-analyses by Rhea et al. (2003), Wolf et al. (2004), Peterson et al. (2004), Krieger (2009) and Fröhlich et al. (2010) indicate that a higher volume of resistance training is probably superior to a smaller volume, when comparing multiple sets with single sets. Each of these meta-analyses have been challenged by several researchers (Carpinelli et al. 2004; Winett, 2004; Otto and Carpinelli, 2006; Fisher et al. 2011; Fisher, 2012) on the basis of several points of methodological validity. In turn, the researchers involved in preparing the analyses have provided a defence (Peterson et al. 2005).

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

MECHANISMS FOR INCREASING STRENGTH

For a full review of the mechanisms of strength development, see the mechanisms section (read more).

Conceptual basis for strength gains

Peripheral factors

The peripheral factors that might affect strength gains include: hypertrophy, myofibrillar packing density, extracellular lateral force transmission, muscle fiber type shifts and regional hypertrophy. The peripheral mechanism by which volume affects strength gains is very likely to be hypertrophy, as greater training volume has been found to have a marked, beneficial effect on hypertrophy. In contrast, the research into the effects of different training variables on specific tension (and its underlying elements) is very limited and it is unclear whether increasing volume might alter either myofibrillar packing density or extracellular lateral force transmission. Similarly, the effects of volume on regional hypertrophy and muscle fiber type shifts are unclear.

Central factors

The central factors that might affect strength gains include: inter-muscular co-ordination and motor unit firing frequency. Given that a higher volume implies a higher number of repetitions, this might cause an increase in inter-muscular co-ordination through a repeated practice effect. However, this has not been studied in detail. It is noteworthy that Almåsbakk and Hoff (1996) found that even performing bench press resistance training with a broomstick led to marked strength gains in this exercise over a 6-week period. This suggests that practice in a movement by performing multiple repetitions can lead to strength gains, irrespective of the load used.

In summary, a higher volume might be effective than a lower volume for increasing strength because of its effects on hypertrophy and inter-muscular co-ordination.

[Read more about mechanisms]

PROBLEMS

Controlling other variables

When studying the effect of any individual training variable on strength gains, a major problem is the extent to which other training variables can be fixed between the two groups being compared. The most important training variables to fix are those that have been found to have the biggest effect on strength. In the case of volume, there are few other training variables that have been found to have as large an effect. However, since volume can be increased by simply adding extra sets onto a workout, it is relatively easy to control for other potential confounding factors, such as proximity to muscular failure, frequency, and relative load.

EFFECT OF VOLUME ON STRENGTH (TRAINED)

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >2 groups trained with different volumes (usually by virtue of altering the number of sets)

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 6 studies were identified (click to read): Ostrowski (1997), Hass (2000), Rhea (2002), Kemmler (2004), Marshall (2011), Baker (2013). Of these 6 studies, 3 reported significant benefits of higher volumes over lower volumes while the remainder reported no differences. Higher volumes may therefore be beneficial for optimising strength gains in this population.

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

 

EFFECT OF VOLUME ON STRENGTH (UNTRAINED)

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >2 groups trained with different volumes (usually by virtue of altering the number of sets)

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 22 studies were identified (click to read): Starkey (1996), Borst (2001), Schlumberger (2001), McBride (2003), Paulsen (2003), Galvão (2005), Munn (2005), Esquivel (2007), Rønnestad (2007), Humberg (2007), Marzolini (2008), Cannon (2010), Bottaro (2011), Andersen (2011), Sooneste (2013), Hanssen (2013), Radaelli (2013), Naclerio (2013), Correa (2014), Radaelli (2014), Radaelli (2014a), Radaelli (2014b). Of these 22 studies, 13 reported significant benefits of higher volumes over lower volumes while the remainder reported no differences. Higher volumes may therefore be beneficial for optimising strength gains in this population.

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CONCLUSIONS FOR STRENGTH

For untrained individuals, multiple sets leading to greater total volume likely leads to greater strength gains.

For trained individuals, multiple sets leading to greater total volume likely leads to greater strength gains.

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MUSCULAR FAILURE

PURPOSE

This section explores whether training closer to muscular failure, or closer proximity to fatigue, is more effective for strength gains. This is achieved by looking at long-term studies that compare whether a program where subjects train to muscular failure is better than a program where subjects do not train to muscular failure for increasing strength.

BACKGROUND

Definitions

Muscular failure is a term frequently used in research studies investigating resistance training programs but precise definitions of this term are infrequently discussed. Willardson (2007) defined muscular failure as “the point during a resistance exercise set when the muscles can no longer produce sufficient force to control a given load”. Schoenfeld (2010) tightened this definition by stating that muscular failure involved “the point during a set when muscles can no longer produce necessary force to concentrically lift a given load.” This definition therefore necessitates the use of concentric muscle actions. In their review, Fisher et al. (2011) tightened the definition even further by defining muscular failure as “the inability to perform any more concentric contractions, without significant change to posture or repetition duration, against a given resistance.”

Whether such additions are necessary to the original definition provided by Willardson (2007) is probably a moot point. The important factor of the definition is that muscular failure can only be defined in relation to a given load. This should be immediately apparent when bodybuilders are observed performing repetitions to failure and then immediately dropping the weight and using a lighter weight to continue performing several more repetitions. Thus, muscular failure does not mean that a muscle is incapable of performing further muscle actions and therefore we cannot say that muscular failure is equivalent to being maximally fatigued (Willardson, 2007). Muscular failure means that a muscle is incapable of expressing force at the same level as it was able to previously, such that it is no longer able to move an arbitrary weight that was set for the task in hand.

Popular usage

Muscular failure is often used by individuals in the general population who perform resistance training for reasons relating to health or physical appearance. Additionally, many strength athletes also regularly train to failure, such as bodybuilders. However, some powerlifting groups have also reported training to muscular failure, especially in low repetition ranges. However, not training to muscular failure is also very common.

Literature usage

In the research literature, it is extremely common for all sets of an exercise to be prescribed to muscular failure. In fact, it is quite rare that a study into strength is performed where muscular failure is not reached on all sets of each exercise. This seems to be because it is considered beneficial to ensure that all sets of an exercise are matched between individuals in terms of their fatigue levels at that time. Thus, it is important to note that there is a slight discrepancy between the research literature and general practice. The study of whether muscular failure is important for strength is therefore critical.

MECHANISMS FOR INCREASING STRENGTH

For a full review of the mechanisms of strength development, see the mechanisms section (read more).

Conceptual basis for strength gains

Peripheral factors

The peripheral factors that might affect strength gains include: hypertrophy, myofibrillar packing density, extracellular lateral force transmission, muscle fiber type shifts and regional hypertrophy. Training to muscular failure appears to be beneficial for hypertrophy and therefore likely explains much of the beneficial effect of muscular failure on strength gains. On the other hand, the research into the effects of different training variables on specific tension (and its underlying elements) is very limited and it is unclear whether training closer to muscular failure might alter either myofibrillar packing density or extracellular lateral force transmission. Similarly, the effects of muscular failure on regional hypertrophy and muscle fiber type are also unclear.

Central factors

The central factors that might affect strength gains include: inter-muscular co-ordination and motor unit firing frequency. Training to muscular failure or greater fatigue is known to alter the acute performance of an exercise or movement in many respects, including joint angle movements, muscle activity, and proportional joint moments. However, whether such changes are beneficial and whether they lead to lasting alterations in inter-muscular co-ordination is unknown.

Whether training closer to muscular failure during a set of sub-maximal repetition leads to faster motor unit firing rates in the highest threshold motor units is uncertain. While motor unit firing rates are responsible for increasing voluntary activation after full motor unit recruitment has been reached, other factors may be involved in the process of reaching muscular failure. In addition, even if it were to be found that training closer to muscular failure during a set of sub-maximal repetitions could produce tetanus in the highest threshold motor units, it is unclear whether this would then develop to the ability to produce tetanus in these same motor units during maximal strength tests. Therefore, it is currently unknown whether training closer to muscular failure will lead to increases in motor unit firing rates that are beneficial for strength gains. Moreover, it is noted that increases in voluntary activation following resistance training (which is driven partly by increasing motor unit firing rates) are small (Arnold and Bautmans, 2014).

In summary, it seems likely that training closer to muscular failure is likely to be more effective than training further from muscular failure for strength gains because of superior hypertrophy.

[Read more about mechanisms]

PROBLEMS

Controlling other variables

When studying the effect of any individual training variable on strength gains, a major problem is the extent to which other training variables can be fixed between the two groups being compared. The most important training variables to fix are those that have been found to have the biggest effect on strength (i.e. volume). In the case of muscular failure, it is relatively easy to control for the effect of volume while varying whether individuals train to muscular failure by simply inserting an intra-set rest period.

Ecological validity

In the research literature exploring the effect of muscular failure on strength, it is most common for the effect of muscular failure to be assessed by comparing two groups, one that uses an intra-set rest period and that does not. However, in practice, this is not how individuals who do not train to muscular failure actually perform resistance-training. Such individuals generally stop slightly short of muscular failure, leaving a repetition or two in the tank. Given the observations made above, this could be important. There is therefore a discrepancy between the research literature and general practice, indicating a lack of ecological validity.

EFFECT OF MUSCULAR FAILURE ON STRENGTH (TRAINED)

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >2 groups trained with a different proximity to muscular failure (either by performing an identical number of repetitions but with an intra-set rest period or by stopping short of muscular failure in one group)

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 5 studies were identified (click to read): Lawton (2004), Drinkwater (2006), Izquierdo (2006), Oliver (2013), Giessing (2014). Of these 5 studies, 3 reported significant benefits of closer proximity to muscular failure while the remainder reported no differences. Training closer to muscular failure may therefore be beneficial for optimising strength gains in this population.

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EFFECT OF MUSCULAR FAILURE ON STRENGTH (UNTRAINED)

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >2 groups trained with a different proximity to muscular failure (either by performing an identical number of repetitions but with an intra-set rest period or by stopping short of muscular failure in one group)

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 4 studies were identified (click to read): Rooney (1994), Schott (1995), Folland (2002), Goto (2005). Of these 4 studies, 3 reported significant benefits of closer proximity to muscular failure while the remainder reported no differences. Training closer to muscular failure may therefore be beneficial for optimising strength gains in this population.

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CONCLUSIONS FOR STRENGTH

For untrained individuals, training closer to muscular failure may lead to greater strength gains.

For trained individuals, training closer to muscular failure may lead to greater strength gains.

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FREQUENCY

PURPOSE

This section explores whether training with a higher volume-matched or unmatched frequency (i.e. number of training sessions per week) is more effective for increasing strength than training with a lower frequency. This is achieved by looking at long-term studies that compare whether a program where subjects train with different training frequencies and assessing their relative ability for increasing strength.

BACKGROUND

Definitions

Training frequency is most commonly defined as the number of times per week that resistance training is performed, whether in relation to the whole body or a single muscle.

Popular usage

Training frequency is often a topic of discussion in the bodybuilding community who want to know how many times per week they should train a particular muscle. In general, such questions generally assume that the total training volume over the week is fixed because of the need for recovery. Thus, the question becomes how the training volume is best distributed over the course of the week. More widely, people embarking upon a program of resistance training often wish to know whether they should follow a whole-body or a split routine. Whole-body routines involve training both the upper and lower body in the same workout and workouts are typically performed 3 times per week, with the other days being devoted to rest. Simple split routines for beginners are formed by training the upper and lower body on separate days, most commonly by training 4 times per week, with the other days being devoted to rest. More complex split routines are performed by more advanced trainees and these can involve 5 or 6 days per week of training.

META-ANALYSES

Meta-analyses by Rhea et al. (2003) and Silva et al. (2014) indicate that a higher frequency of resistance training may be beneficial for improving strength gains within certain parameters. However, these meta-analyses are only relevant to the discussion of frequency where volume is not controlled. In each case, the measures of frequency used were not volume-matched but used increased frequency in order to increase volume over the training week.

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

MECHANISMS FOR INCREASING STRENGTH

For a full review of the mechanisms of strength development, see the mechanisms section (read more).

Conceptual basis for strength gains

Peripheral factors

The peripheral factors that might affect strength gains include: hypertrophy, myofibrillar packing density, extracellular lateral force transmission, muscle fiber type shifts and regional hypertrophy. The effect of training frequency on hypertrophy is uncertain. Equally, the research into the effects of different training variables on specific tension (and its underlying elements) is very limited and it is unclear whether altering frequency might alter either myofibrillar packing density or extracellular lateral force transmission. Similarly, the effects of training frequency on regional hypertrophy and muscle fiber type are unclear.

Central factors

The central factors that might affect strength gains include: inter-muscular co-ordination and motor unit firing frequency. Training with a higher volume-matched frequency could function to improve inter-muscular co-ordination in a similar way to increased volume. A higher number of repetitions could potentially cause an increase in inter-muscular co-ordination by means of a practice effect though greater repetition. On the other hand, a higher volume-matched frequency does not imply a higher number of repetitions, this would not necessarily bring about an increase in inter-muscular co-ordination by means of a practice effect though greater repetition. However, it is possible that the magnitude of the practice effect might not be linearly related to the number of repetitions performed in each session and there might be a plateau in how much practice can be performed beneficially in a single workout. In this case, a higher volume-matched frequency might allow for more effective practice than a lower volume-matched frequency.

In summary, training with a specific volume-matched frequency might be more effective for increasing strength because of improved inter-muscular co-ordination by virtue of greater practice time.

[Read more about mechanisms]

PROBLEMS

Controlling other variables

Controlling other variables when studying volume-matched training frequency is in theory relatively straightforward. A certain volume of training is identified and then allocated across two workout plans. The workout plan for one group involves a greater workload in a single workout than the other but performs fewer workouts over the course of the week. In practice, when working with trained subjects, it is slightly more complicated, as the only practical way to increase volume-matched training frequency is to train multiple times on the same day, which introduces a time-of-day effect. Training at different times of day has been found by some (Chtourou and Souissi, 2012) but not all (Sedliak et al. 2009) to affect strength gains and may therefore be a confounding factor.

EFFECT OF FREQUENCY ON STRENGTH (TRAINED): VOLUME MATCHED

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >2 groups trained with a different volume-matched frequency from one another

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 3 studies were identified (click to read): Häkkinen and Kallinen (1994), McLester (2000), Hartman (2007). Of these 3 studies, 1 reported significant benefits of a higher volume-matched frequency while the remainder reported no differences. Training with a higher volume-matched frequency could therefore be beneficial for optimising strength gains in this population.

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EFFECT OF FREQUENCY ON STRENGTH (UNTRAINED): VOLUME MATCHED

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >2 groups trained with a different volume-matched frequency from one another

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 6 studies were identified (click to read): Hunter (1985), Calder (1994), Candow (2007), Benton (2011), Arazi and Asadi (2011), Andersen (2012). Of these 6 studies, 1 reported significant benefits of a lower volume-matched frequency while the remainder reported no differences. Altering volume-matched frequency probably has little effect on strength gains in this population.

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EFFECT OF FREQUENCY ON STRENGTH (UNTRAINED): VOLUME UNMATCHED

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >2 groups trained with a different frequency from one another, where frequency was modulated in order to alter total training volume

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 14 studies were identified (click to read): Berger (1965), Rozier (1981), McKenzie Gillam (1981), Graves (1988), Braith (1989), Taaffe (1989), Graves (1990), Carpenter (1991), Pollock (1993), DeMichele (1997), Carroll (1998), DiFrancisco-Donoghue (2007), Kim (2010), Farinatti (2013). Of these 14 studies, 6 reported significant benefits of a higher volume-unmatched frequency, 1 reported a benefit of a lower volume-unmatched frequency while the remainder reported no differences. Increasing volume-matched frequency may therefore have a beneficial effect on strength gains in this population.

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CONCLUSIONS FOR STRENGTH

For untrained individuals, greater training frequency leading to more volume could lead to greater strength gains. However, splitting the same weekly volume out over more sessions is unlikely to be beneficial.

For trained individuals, splitting the same weekly volume out over more sessions might be beneficial but the evidence is very limited.

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REST PERIODS

PURPOSE

This section explores whether training with shorter inter-set rest periods is superior to training with longer inter-set rest periods for increasing strength. This is achieved by looking at long-term studies that compare whether a program where subjects train with short inter-set rest periods (or reducing rest periods) is better than a program where subjects train with longer inter-set rest periods (or with non-reducing rest periods) for increasing strength.

BACKGROUND

Definitions

Resistance training exercises are generally described as being performed in sets of repetitions, where a set is a number of repetitions performed in sequence. Where multiple sets of repetitions are performed, there is a rest between sets, called the inter-set rest period. The length of this inter-set rest period can be referred to as the inter-set rest period duration.

Popular usage

Traditionally, athletes training for strength sports have used relatively long rest periods between sets in order to allow themselves to recover fully before attempting the next set. This is in contrast to the training programs of bodybuilders, which routinely involve short inter-set rest periods. However, not all strength-focused programs involve long inter-set rest period durations. For example, powerlifters following a Westside or Westside-inspired routine involving dynamic lifting days often take relatively short inter-set rests in these workouts.

Literature usage

Research interest into the effects of inter-set rest period duration on strength gains has been performed mainly as a corollary to the effects of inter-set rest period duration on hypertrophy. It was originally hypothesised that the greater metabolic stress that was associated with short rest periods could lead to superior gains in muscle size. However, the literature has failed to support this view to date, although it should be noted that the currently available studies in this area are few (see review by Henselmans and Schoenfeld, 2014).

MECHANISMS FOR INCREASING STRENGTH

For a full review of the mechanisms of strength development, see the mechanisms section (read more).

Conceptual basis for strength gains

Peripheral factors

The peripheral factors that might affect strength gains include: hypertrophy, myofibrillar packing density, extracellular lateral force transmission, muscle fiber type shifts and regional hypertrophy. Rest periods appear to have little effect on hypertrophy, despite many expert recommendations to the contrary. Equally, the research into the effects of different training variables on specific tension (and its underlying elements) is very limited and it is unclear whether altering rest periods might alter either myofibrillar packing density or extracellular lateral force transmission. The effects of inter-set rest period duration on either regional hypertrophy or muscle fiber type are unclear.

Central factors

The central factors that might affect strength gains include: inter-muscular co-ordination and motor unit firing frequency. Given that longer rest periods tend to allow either a higher number of repetitions to be performed or higher relative loads, longer rest periods might allow a greater practice effect that is load-specific and thereby an increase in inter-muscular co-ordination. However, this has not been studied in detail.

In summary, longer rest period durations might bring about greater increases in strength by facilitating increases in inter-muscular co-ordination.

[Read more about mechanisms]

PROBLEMS

The main problem associated with altering rest period duration is controlling volume. As noted above, when reducing rest period duration, this leads to a reduction in the number of repetitions that can be performed in a single set. Thus, in order to control for volume while altering rest period duration while maintaining all sets to muscular failure, an additional set would be required in the condition with the shorter rest period duration.

EFFECTS OF REST PERIOD DURATION ON STRENGTH (TRAINED)

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >2 groups trained with a different inter-set rest period duration from the other

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 4 studies were identified (click to read): Robinson (1995), Ahtiainen (2005), Willardson (2008), De Salles (2010). Of these 4 studies, 2 reported significant benefits of a longer rest period duration while the remainder reported no differences. Increasing rest period duration may therefore have a beneficial effect on strength gains in this population.

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EFFECTS OF REDUCING REST PERIOD DURATION ON STRENGTH (TRAINED)

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >1 group trained with a fixed inter-set rest period duration and >1 group trained with a reducing inter-set rest period duration

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 2 studies were identified (click to read): De Souza-Junior (2010), Souza-Junior (2011). Of these 2 studies, both reported no differences between conditions. Reducing rest period duration may therefore have little effect on strength gains in this population.

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EFFECTS OF REST PERIOD DURATION ON STRENGTH (UNTRAINED)

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >2 groups trained with a different inter-set rest period duration from the other

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 6 studies were identified (click to read): Pincivero (1997), Pincivero (2004), Hill-Haas (2007), Buresh (2009), Gentil (2010), Villanueva (2014). Of these 6 studies, 3 reported significant benefits of a longer rest period duration, 1 reported a benefit of shorter rest period duration, and the remainder reported no differences. Increasing rest period duration may therefore have a beneficial effect on strength gains in this population.

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

CONCLUSIONS FOR STRENGTH

For untrained individuals, longer inter-set rest period durations are probably better for strength gains.

For trained individuals, longer inter-set rest period durations are probably better for strength gains.

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RANGE OF MOTION

PURPOSE

This section explores whether training through larger ranges of motion (ROMs) leads to greater strength gains than training through smaller ROMs. This is achieved by looking at long-term studies that compare whether a program where subjects train through large ROMs is better than a program where subjects train through small ROMs for increasing strength.

BACKGROUND

Definitions

Resistance training exercises are often described as being performed either through full ROMs or partial ROMs. For single-joint exercises, full ROMs can be defined as those in which the joint moves through its entire movement arc, subject to the constraints of passive tissues. Thus, full joint ROM is broadly equivalent to the muscle fully elongating. On the other hand, for multi-joint exercises, full ROMs are more difficult to define, as not all of the joints will necessarily move through their full movement arcs. For example, in a full ROM squat, the ankle joint does not move through its full ROM. Similarly, in a full ROM deadlift, the knee does not move through its full ROM. In the case of the deadlift, greater ROM than full ROM can be achieved by using a snatch grip or by using a platform to perform a deficit deadlift. Thus, for multi-joint exercises, full ROM may need to be defined conventionally as “greater ROM” rather than full ROM.

Popular usage

Within wider strength and conditioning circles, there have been many discussions around ROM in respect of squats, with some favouring deep squats (and concomitantly lighter loads) and others favouring shallow squats (with much greater loads). Other than for squats, however, ROM has never been a particularly common topic of discussion. This is likely because there is no specific sensory feeling associated with ROM. Unlike using shorter rest periods or training to muscular failure, there is no sensation of great effort and burning fatigue within the muscle, which lead the trainee to believe that they are stimulating growth.

Literature usage

ROM has been investigated in two main respects in the study of resistance training. Firstly, researchers have explored the effects of isometric training at different joint angles on ROM-specific gains in isometric strength. Such studies have generally revealed that performing such training leads to ROM-specific strength gains (Kitai and Sale, 1989; Weir et al. 1994; Weir et al. 1995). Secondly, researchers have explored the effects of resistance training protocols using exercises with different ROMs in order to explore their effects on both strength and muscle size.

MECHANISMS FOR INCREASING STRENGTH

For a full review of the mechanisms of strength development, see the mechanisms section (read more).

Conceptual basis for strength gains

Peripheral factors

The peripheral factors that might affect strength gains include: hypertrophy, myofibrillar packing density, extracellular lateral force transmission, muscle fiber type shifts and regional hypertrophy. Training using a full ROM appears to have a beneficial effect on hypertrophy but the effects of ROM on regional hypertrophy and muscle fiber type are less clear. Similarly, the research into the effects of different training variables on specific tension (and its underlying elements) is very limited and it is unclear whether increasing ROM might alter either myofibrillar packing density or extracellular lateral force transmission.

Central factors

The central factors that might affect strength gains include: inter-muscular co-ordination and motor unit firing frequency. Training using different ROMs appears to lead to gains in strength that are specific to the ROM used during the resistance training program, which is an indicator of increased inter-muscular co-ordination. Thus, where strength is tested using full ROM (which is most common), this leads to training using full ROM displaying superior results. Indeed, there is some evidence of task-specificity occurring with ROM (Graves et al. 1989; Graves et al. 1992; Hartman et al. 2012; Bloomquist et al. 2013). Moreover, there is evidence that the task-specificity of ROM is caused by central factors, as indicated by the fact that it displays a cross-over effect (Weir et al. 1994).

In summary, greater ROMs are likely to cause greater increases in strength because of increased hypertrophy. However, task-specificity might also be observed because of changes in inter-muscular co-ordination.

[Read more about mechanisms]

PROBLEMS

Controlling other variables

When studying the effect of any individual training variable on strength gains, a major problem is the extent to which other training variables can be fixed between the two groups being compared. The most important training variables to fix are those that have been found to have the biggest effect on strength (i.e. volume). In the case of ROM, it is relatively easy to equalize volume, particularly where volume is defined as the number of repetitions of the same relative load.

Ecological validity

In the research literature exploring the effect of ROM on strength, there are two types of study. One type compares the effect of training through an arbitrary, partial ROM in a machine exercise with a full ROM of the same exercise. The other explores the effect of training through a partial ROM in a free-weight exercise with a full ROM of the same exercise. In this latter type of study, the partial ROM variation enables the use of a much greater load than the full ROM equivalent because of the torque-angle curve. For example, in the conventional back squat, the torque-angle curve increases steeply with increasing hip or knee angle (i.e. increasing squat depth). This is because the external moment arms at the hip and knee increase steeply with increasing hip and knee angle (i.e. increasing squat depth), while the load stays the same.

Arguably, performing free-weight exercises through a partial ROM is how individuals actually use smaller ROMs in resistance-training. Individuals generally stop slightly short of full squat depth. There is therefore a discrepancy between a portion of the research literature and general practice, indicating a lack of ecological validity.

EFFECT OF ROM ON STRENGTH (UNTRAINED)

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >2 groups trained with different ROM from one another

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 6 studies were identified (click to read): Graves (1989), Graves (1992), Weiss (2000), Massey (2004), Massey (2005), Hartmann (2012), Pinto (2012), Bloomquist (2013), McMahon (2013). Of these 9 studies, 5 reported significant benefits of a larger ROM and the remainder reported no differences. Increasing ROM may therefore have a beneficial effect on strength gains in this population.

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

CONCLUSIONS FOR STRENGTH

For untrained individuals, larger ROM may be superior to smaller ROM for strength gains

For trained individuals, there is currently no evidence available.

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BAR SPEED (ISOKINETIC)

PURPOSE

This section explores whether training with faster bar speeds is superior to training with slower bar speeds for increasing strength, when using isokinetic external resistance. This is achieved by looking at long-term studies that compare the effects of slow and fast bar speeds while training on a dynamometer for increasing strength.

BACKGROUND

Definitions

Resistance training exercises can be performed either maximally or sub-maximally. When performed maximally, the force-velocity relationship is relevant. The force-velocity relationship is the observation that when greater absolute moment is generated at a joint, the angular velocity of that joint must be lower. Force-velocity relationships at joints are largely exponential, with force decreasing very quickly with increasing angular velocity past a certain point. When performed sub-maximally, the force-velocity relationship is not relevant. In fact, as Fisher and Smith (2012) have noted, when performed to muscular failure, a greater number of repetitions are possible with faster bar speeds (i.e. shorter repetition durations) than with slower bar speeds (i.e. longer repetition durations). This in turn suggests that effort and fatigue levels are greater with slower bar speeds (i.e. longer repetition durations).

Popular usage

Most strength athletes who are aiming to improve strength or power make use of fast bar speeds. While many bodybuilders who are training primarily if not solely for hypertrophy use slower bar speeds, this is not directly relevant where strength is concerned as strength gains are a function of many different mechanisms and not solely driven by hypertrophy.

Literature usage

The comparison of different sub-maximal speeds (or maximal with sub-maximal speeds) has been a predominant focus of research. However, it is unclear what underlying mechanism is being investigated.

MECHANISMS FOR INCREASING STRENGTH

For a full review of the mechanisms of strength development, see the mechanisms section (read more).

Conceptual basis for strength gains

Peripheral factors

The peripheral factors that might affect strength gains include: hypertrophy, myofibrillar packing density, extracellular lateral force transmission, muscle fiber type shifts and regional hypertrophy. Bar speed (and repetition duration) appears to have no effect on hypertrophy and seems therefore also unlikely to affect regional hypertrophy. The research into the effects of different training variables on specific tension (and its underlying elements) is very limited and it is unclear whether altering bar speed might alter either myofibrillar packing density or extracellular lateral force transmission. However, it has been suggested that training with higher velocity movements during resistance training could lead to shifts in muscle fiber type from type I to type II muscle fibers (see review by Wilson et al. 2012). This could potentially lead to a favourable change in the strength-size ratio.

Central factors

The central factors that might affect strength gains include: inter-muscular co-ordination and motor unit firing frequency. Bar speed might affect changes in inter-muscular co-ordination, although this has not been studied in detail. Speed of movement affects proportional joint moments (Bryanton et al. 2012; Beardsley and Contreras, 2014), movement patterns (Frost et al. 2013; Beardsley and Contreras; 2014a) and muscle activity patterns of different muscles in lower-body multi-joint exercises. Moreover, the overall pattern for these changes is similar for faster movements and heavier relative loads. This may imply that performing multi-joint exercises with a higher velocity could lead to the development of an better pattern of inter-muscular co-ordination that is optimal for moving very high relative loads (i.e. maximal) than performing the same exercises with lower velocities. Bar speed might also affect motor unit firing rates, although the extent to which changes in motor unit firing rate occurs following resistance training is unknown as the literature is conflicting.

In summary, faster bar speeds might potentially be more effective than slower bar speeds for increasing strength because they cause shifts in muscle fiber type, improvements in inter-muscular co-ordination and/or increases in motor unit firing rate.

[Read more about mechanisms]

PROBLEMS

Controlling other variables

The force-velocity relationship is a serious confounding factor when comparing groups training with different maximal bar speeds. This is because the group training with the faster bar speed must use a lighter relative load. This means that relative load is different between the two conditions. However, when comparing two groups training with different sub-maximal bar speeds, the force-velocity relationship is normally not a problem. This is because the same relative load (for the bar speed) can be used in both cases. It is expected that in order to use slower sub-maximal bar speeds (longer repetition durations) the effort and fatigue are significantly greater than in faster sub-maximal bar speeds (shorter repetition durations). Therefore, it is anticipated that the absolute loads will be smaller for the same relative load in the slower sub-maximal bar speed conditions.

EFFECT OF ISOKINETIC BAR SPEED ON STRENGTH (TRAINED)

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >2 groups trained with different bar speeds (repetition durations) from each other and where isokinetic external resistance was used in the compared groups

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

Coming soon!

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

EFFECT OF ISOKINETIC BAR SPEED ON STRENGTH (UNTRAINED)

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >2 groups trained with different bar speeds (repetition durations) from each other and where isokinetic external resistance was used in the compared groups

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

Coming soon!

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

CONCLUSIONS FOR STRENGTH

Coming soon!

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BAR SPEED (NOT ISOKINETIC)

PURPOSE

This section explores whether training with faster bar speeds is superior to training with slower bar speeds for increasing strength. This is achieved by looking at long-term studies that compare whether a program where subjects train with fast bar speeds is better than a program where subjects train with slower bar speeds for increasing strength.

BACKGROUND

See previous section: Bar speed (isokinetic)

MECHANISMS FOR INCREASING STRENGTH

See previous section: Bar speed (isokinetic)

PROBLEMS

See previous section: Bar speed (isokinetic)

EFFECT OF BAR SPEED ON STRENGTH (UNTRAINED): MATCHED REPETITION RANGE

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >2 groups trained with different bar speeds (repetition durations) from each other, where constant load (isoinertial) external resistance was used in the compared groups, and where the number of repetitions performed in each set was similar in the compared groups

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 6 studies were identified (click to read): Young and Bilby (1993), Morrissey (1998), Keeler (2001), Munn (2005), Neils (2005), Tanimoto (2006), Pereira (2007), Tanimoto (2008), Rana (2008), Ingebrigtsen (2009), Scheunke (2012). Of these 11 studies, 6 reported significant benefits of a faster bar speed and the remainder reported no differences. Increasing bar speed may therefore have a beneficial effect on strength gains in this population.

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

EFFECT OF BAR SPEED ON STRENGTH (UNTRAINED): MATCHED ABSOLUTE LOAD

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >2 groups trained with different bar speeds (repetition durations) from each other, where constant load (isoinertial) external resistance was used in the compared groups, and where the the absolute load used (or percentage of the same 1RM test) was similar in the compared groups

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 3 studies were identified (click to read): Liow and Hopkins (2003), Watanabe (2013), Watanabe (2013a). Of these 3 studies, 1 reported significant benefits of a faster bar speed and the remainder reported no differences. Increasing bar speed could therefore have a beneficial effect on strength gains in this population.

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

 

CONCLUSIONS FOR STRENGTH

For untrained individuals, a faster bar speed may lead to greater strength gains when using constant-load external resistance, whether matched for repetition ranges to muscular failure or for absolute load.

For trained individuals, there is currently no evidence regarding how bar speed affects strength gains when using constant-load external resistance.

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MUSCLE ACTION (VARIABLE)

PURPOSE

This section explores whether training with eccentric muscle actions is superior to training with concentric muscle actions while using isokinetic (i.e. variable) external resistance for increasing strength. This is achieved by looking at long-term studies that compare isokinetic resistance training programs involving eccentric-only muscle actions are better than isokinetic resistance training programs involving concentric-only muscle actions for increasing strength.

BACKGROUND

Definitions

Eccentric vs. concentric muscle actions

Muscles can be either active or passive, depending upon whether neural signals are sent to them. While being either active or passive, they can either lengthen, shorten, or remain the same length. Shortening active muscles are called concentric muscle actions, lengthening active muscles are called eccentric muscle actions, and when active muscles remain the same length, these are called isometric muscle actions.

Variable load vs. constant load external resistance

External resistance can initially be categorized into two overall categories: (1) external resistance that remains constant throughout a muscle action, (2) external resistance that varies throughout a muscle action. Within the first overall category of external resistance, the two main types are isoinertial and isometric. Isoinertial resistance is simply an object with mass that can be lifted. The mass remains the same at all times and any variation that occurs in how hard it is to lift throughout the joint range of motion depends entirely on the internal or external moment arms. Isometric external resistance is essentially a subcategory of isoinertial resistance but where the mass is too heavy to lift and it therefore becomes an immovable object.

Variable, isokinetic and accommodating external resistance

Within the second overall category of external resistance, the two main types are variable and isokinetic. Variable resistance is simply where the resistance changes with joint range of motion in an unspecified way. Isokinetic resistance is essentially a subcategory of variable resistance but the way in which the resistance changes is so as to maintain a constant velocity throughout the joint range of motion. Isokinetic resistance thereby corrects for the internal and external moment arms at all points. Accommodating resistance is technically identically to isokinetic resistance in biomechanical definitions. However, in popular usage it means an approximation to isokinetic by accounting somewhat for changes in external moment arms and therefore is more correctly referred to as variable resistance.

Popular usage

There is little popular use made of either eccentric-only or concentric-only muscle actions in combination with variable external loading.

Literature usage

Researchers have made extensive use of isokinetic (a form of variable external loading) in order to investigate the effects of either eccentric-only or concentric-only muscle actions on strength gains and increases in muscle size. However, whether such results can be extrapolated to constant load resistance training is hard to establish.

META-ANALYSES

Meta-analyses indicate that training with eccentric-only muscle actions may be superior to concentric-only muscle actions for strength gains, apparently because of greater increases in muscle size. Roig et al. (2009) performed a series of meta-analyses in trained and untrained subjects to compare the effects of eccentric-only vs. concentric-only muscle actions during resistance training programs on strength gains. They found that when relative load was equated but absolute load was not equated, eccentric muscle actions were superior to concentric muscle actions. When absolute load was equated but relative load was not, there was no difference between groups.

MECHANISMS FOR INCREASING STRENGTH

For a full review of the mechanisms of strength development, see the mechanisms section (read more).

Conceptual basis for strength gains

Neural control of eccentric muscle actions

Several researchers have observed that the neural activation strategies may differ between eccentric and concentric muscle actions. This has the potential to affect both central and peripheral factors. Specifically, two features have been observed. Firstly, it has been noted that neural drive is lower during eccentric than during concentric muscle actions. Secondly, some evidence of selective recruitment of larger motor units or a reversal of the size principle has been reported by a small number of studies (see reviews by Enoka, 1996; Enoka and Fuglevand, 2001; Duchateau and Enoka, 2008; Sekiguchi et al. 2013). In general, the prevailing view seems to be that while the possibility exists for a different recruitment strategy during eccentric muscle actions, on balance it seems most likely that the size principle is probably not violated (Sekiguchi et al. 2013).

Peripheral factors

The peripheral factors that might affect strength gains include: hypertrophy, myofibrillar packing density, extracellular lateral force transmission, muscle fiber type shifts and regional hypertrophy. Muscle action may have some beneficial effect on hypertrophy. However, the research into the effects of different training variables on specific tension (and its underlying elements) is very limited and it is unclear whether changing muscle action would alter either myofibrillar packing density or extracellular lateral force transmission. Theoretically, eccentric muscle actions might cause greater disturbance to sarcomeres and potentially increase the connections that enable extracellular lateral force transmission. However this has not been explored. Similarly, the effects of muscle action on regional hypertrophy and muscle fiber type are unclear. If a distinct motor unit recruitment strategy were to be identified during eccentric muscle actions, it might cause increased development of type II muscle fibers. However, this is currently unknown.

Central factors

Overall, it seems likely that the central factors that might affect strength gains include: inter-muscular co-ordination and motor unit firing frequency. Training using different muscle actions most probably leads to gains in strength that are specific to the muscle action used during the resistance training program, which is a feature of increased inter-muscular co-ordination. Indeed, there is some evidence of task-specificity occurring with muscle action (Higbie et al. 1996; Hortobagyi et al. 1996; Hortobagyi et al. 2000; Symons et al. 2005; Nickols-Richardson et al. 2007; Blazevich et al. 2007; Carvalho et al. 2014; and see review and meta-analyses by Roig et al. 2009). Whether training using different muscle actions can alter motor unit firing rates is unclear.

In summary, eccentric muscle actions might cause greater increases in strength because of slightly increased hypertrophy. However, task-specificity might also be observed because of changes in inter-muscular co-ordination.

[Read more about mechanisms]

PROBLEMS

Controlling other variables

Owing to the differences in energy cost and absolute force production between eccentric and concentric muscle actions, it is not an easy matter to control all of the other key variables, particularly volume and relative load. The use of isokinetic external resistance makes this issue even more complex, as force production varies constantly throughout a single repetition, across repetitions of the same set, and between conditions while the velocity does not.

EFFECT OF MUSCLE ACTION ON STRENGTH (TRAINED)

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >1 group trained using predominantly or exclusively eccentric muscle actions, and >1 group trained predominantly or exclusively using concentric muscle actions, and where the external resistance used in all compared groups was isokinetic

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 1 study was identified (click to read): Seger (1998). This study reported no differences between groups. Training with either eccentric or concentric muscle actions may therefore have little effect on strength gains in this population.

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

EFFECT OF MUSCLE ACTION ON STRENGTH (UNTRAINED)

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >1 group trained using predominantly or exclusively eccentric muscle actions, and >1 group trained predominantly or exclusively using concentric muscle actions, and where the external resistance used in all compared groups was isokinetic

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 17 studies were identified (click to read): Komi and Buskirk (1972), Mayhew (1995), Higbie (1996), Hortobagyi (1996), Bast (1998), Hawkins (1999), Hortobagyi (2000), Farthing (2003), Seger (2005), Symons (2005), Miller (2006), Nickols-Richardson (2007), Blazevich (2007), Moore (2012), Cadore (2014), Carvalho (2014), Kim (2014). Of these studies, 3 reported a benefit of eccentric muscle actions in a non-eccentric strength test, 5 reported a benefit of eccentric muscle actions in an eccentric strength test, 1 reported a benefit of concentric muscle actions in a non-concentric strength test, and 2 reported a benefit of concentric muscle actions in a concentric strength test. Training with either eccentric or concentric muscle actions may therefore have little effect on strength gains in this population but there may be some muscle action specificity in strength gains.

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

CONCLUSIONS FOR STRENGTH

For trained subjects, there is no difference between eccentric and concentric muscle actions for increasing isometric, concentric or eccentric strength.

For untrained subjects, eccentric muscle actions may be superior for increasing eccentric strength.

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MUSCLE ACTION (CONSTANT LOAD)

PURPOSE

This section explores whether training with eccentric muscle actions is superior to training with concentric muscle actions while using isoinertial external resistance (i.e. constant load) for increasing strength. This is achieved by looking at whether isoinertial resistance training programs involving eccentric-only muscle actions are better than isoinertial resistance training programs involving concentric-only muscle actions for increasing strength.

BACKGROUND

See previous section: Muscle action (isokinetic)

MECHANISMS FOR INCREASING STRENGTH

See previous section: Muscle action (isokinetic)

PROBLEMS

Controlling other variables

Owing to the differences in energy cost and absolute force production between eccentric and concentric muscle actions, it is not an easy matter to control all of the other key variables, particularly volume and relative load. When comparing eccentric and concentric muscle actions across two groups, there are two common options for equating the load used in each group. Either the same absolute load can be used in both groups or the same relative load can be used in both groups. Another, less-common option is to use an arbitrary, heavier load in the eccentric group. Where the same absolute load is used in both eccentric and concentric groups, this means that the relative load is lower in the eccentric condition (as muscles are stronger during eccentric muscle actions than during concentric muscle actions). Thus, relative load becomes a confounding factor in the investigation. Where the same relative load is used, this eliminates relative load as a confounding factor. However, if the same set and repetition scheme is then employed between the eccentric and concentric groups, then (depending on how you define volume) this leads to an excess of volume being performed in the eccentric condition than in the concentric condition (because the absolute load is greater).

Testing strength

Strength can be measured in a variety of ways. The most common tests of strength are: (1) isometric force or torque production, (2) isokinetic torque production, (3) dynamic 1RM during an exercise. When comparing groups that perform eccentric or concentric muscle actions, researchers sometimes also test eccentric or concentric strength specifically, either using isokinetic or isoinertial external resistance. There are some indications that training using either eccentric or concentric muscle actions may lead to gains in strength that differ when tested using eccentric, concentric, and isometric methods. Consequently, it is important to note the testing method when assessing the results. Additionally, it is important not to compare strength gains in different tests. Consequently, for the following analyses, studies that did not compare strength gains in the same test in both eccentric and concentric groups (e.g. Mannheimer, 1969) were not included.

EFFECT OF MUSCLE ACTION ON STRENGTH (TRAINED)

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >1 group trained using predominantly or exclusively eccentric muscle actions, and >1 group trained predominantly or exclusively using concentric muscle actions, and where the external resistance used in all compared groups was constant load (isoinertial)

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 2 studies were identified (click to read): Seliger (1968), Vikne (2006). Of these studies, neither reported a benefit of either eccentric or concentric muscle actions in an isometric strength test, neither reported a benefit of concentric muscle actions in a concentric strength test, and 1 reported a benefit of eccentric muscle actions in an eccentric strength test. It is therefore difficult to identify whether muscle action has any effect on strength gains in this population.

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

EFFECT OF MUSCLE ACTION ON STRENGTH (UNTRAINED)

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >1 group trained using predominantly or exclusively eccentric muscle actions, and >1 group trained predominantly or exclusively using concentric muscle actions, and where the external resistance used in all compared groups was constant load (isoinertial)

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 10 studies were identified (click to read): Johnson (1976), Pavone (1985), Jones (1987), Ben-Sira (1995), Smith (1995), Raue (2005), Reeves (2009), Farup (2013), Franchi (2014), Farup (2014). Of these studies, 3 reported a benefit of concentric muscle actions in an isometric strength test, 2 reported a benefit of concentric muscle actions in a concentric strength test, 1 reported a benefit of concentric muscle actions in a non-concentric strength test, and 1 reported a benefit of eccentric muscle actions in an eccentric strength test. Training with concentric muscle actions could possibly be superior for strength gains in this population.

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

CONCLUSIONS FOR STRENGTH

For trained subjects, eccentric muscle actions may be superior to concentric muscle actions for increasing eccentric strength, when using constant-load external resistance.

For trained subjects, there is no difference between eccentric and concentric muscle actions for increasing concentric strength, when using constant-load external resistance.

For trained subjects, it is unclear whether eccentric or concentric muscle actions are best for increasing isometric strength, when using constant-load external resistance.

For untrained subjects, concentric muscle actions may be superior to eccentric muscle actions for increasing concentric and isometric strength, when using constant-load external resistance.

For untrained subjects, eccentric muscle actions may be superior to concentric muscle actions for increasing eccentric strength, when using constant-load external resistance.

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RESISTANCE TYPE (VARIABLE VS. CONSTANT LOAD)

PURPOSE

This section explores whether training with variable resistance is superior to training with constant load resistance for increasing strength. This is achieved in two parts. Firstly, we investigate long-term studies that compare commonly-used powerlifting techniques involving variable resistance (i.e. bands and chains) with isoinertial loading (i.e. conventional free weights) for increasing strength. Secondly, we investigate long-term studies that compare machine-based variable resistance with isoinertial loading (i.e. conventional free weights) for increasing strength.

BACKGROUND

Introduction

Strength curves

Many exercises have what appears to be a difficult part of the exercise range of motion (ROM) and an easier part of the exercise ROM. If we draw a graph of the strength of the lifter against the exercise ROM, this produces a “strength curve” (see review by McMaster et al. 2009). Strength curves are caused primarily by changes in the external moment arm lengths of the system load at the key joints and exist in both single-joint and multi-joint exercises.

Ascending strength curves

Where exercises are hard to perform at the bottom of the lift and easy to perform at the top of the lift, we say that these exercises have “an ascending strength curve”. This ascending strength curve is very common in multi-joint, lower-body exercises, such as the deadlift and squat, as the exercise is very difficult when the barbell is closest to the ground. In fact, most multi-joint, lower-body exercises are hardest when the barbell is closest to the ground because this is where the primary joints (hip, knee and ankle) are furthest from the vertical barbell path. This means that this is the point where the external moment arm lengths are greatest. Consequently, the external joint moments as a result of the system load are also greatest. As the barbell gains height, the hip, knee and ankle joints move closer to this vertical barbell path, which shortens the external moment arm length and reduces the external joint moments associated with the same system load. For example, during the deadlift, which is a very hip-dominant exercise, the external moment arm length of the system load at the hip joint starts around 21 cm away from the vertical barbell path at the bottom of the lift but is around 4 cm from the vertical barbell path at the top (Escamilla et al. 2000). This means that during the deadlift, the joint moments at the hip must be very large at the bottom of the lift and much smaller at the top, in order to move the same barbell load.

Accommodating and variable resistance

In powerlifting, the practice of adding bands and chains is usually called “accommodating resistance” because the bands or chains accommodate the variable strength of the lifter by changing the load to match. However, in biomechanics, the term “accommodating resistance” can only be used where the load is varied perfectly with the strength of the lifter, such that the bar speed is identical throughout the movement. In practice, this is extremely difficult to achieve without a dynamometer and there is almost always some variation in joint moments at various points in the exercise ROM. Consequently, in biomechanics, the use of bands and chains is more usually described as “variable resistance” (e.g. see review by Frost et al. 2010).

Bands and chains

Bands and chains are added to a barbell during a multi-joint movement in order to make the easy part of the lift harder but leave the hard part of the lift unchanged. This makes the strength curve less steep. For bands, this generally means looping the band over the barbell from the floor (or under the bench in the case of the bench press) so that the bands begin to stretch as the bar goes upwards and are most stretched at lockout. Bands or chains can be added to any exercise, not just exercises with ascending strength curves. For example, the bench press, chin ups, and hip thrusts don’t have dramatic strength curves, but bands and chains can still be utilized to stress the end range-of-motion of those lifts.

MECHANISMS FOR INCREASING STRENGTH

For a full review of the mechanisms of strength development, see the mechanisms section (read more).

Conceptual basis for strength gains

Peripheral factors

The peripheral factors that might affect strength gains include: hypertrophy, myofibrillar packing density, extracellular lateral force transmission, muscle fiber type shifts and regional hypertrophy. There is currently little research regarding the effects of external resistance type on hypertrophy, regional hypertrophy or muscle fiber type. It is feasible that where certain types of external resistance allow for greater bar speeds that this could lead to shifts in muscle fiber type but this is pure conjecture. Similarly, the research into the effects of different training variables on specific tension (and its underlying elements) is very limited and it is unclear whether altering external resistance type might alter either myofibrillar packing density or extracellular lateral force transmission.

Central factors

The central factors that might affect strength gains include: inter-muscular co-ordination and motor unit firing frequency. It seems likely that training using different types of external resistance will lead to gains in strength that are specific to the type of external resistance used during the resistance training program. Indeed, this has been observed in some studies (Boyer et al. 1990; O’Hagan et al. 1995). However, it is important to note that relative load affects proportional joint moments (Bryanton et al. 2012; Beardsley and Contreras, 2014), movement patterns (Frost et al. 2013; Beardsley and Contreras; 2014a) and muscle activity patterns of different muscles in lower-body multi-joint exercises. Since external resistance might be expected to alter the proportional joint moments during sub-maximal relative load training, it is possible that the proportional joint moments of a movement with one type of external resistance with sub-maximal relative loads could closely resemble the proportional joint moments (and therefore the inter-muscular co-ordination) of the same movement with a different type of external resistance. In this way, training with one type of external resistance with sub-maximal relative loads could transfer well to another type of external resistance with maximal relative loads. It is also possible that where certain types of external resistance allow for greater bar speeds to be performed that this could lead to greater motor unit firing rates. However, this is unclear and remains to be demonstrated.

In summary, it is unclear whether one type of external resistance might lead to greater increases in strength than another type, although some task-specificity might be observed because of changes in inter-muscular co-ordination.

[Read more about mechanisms]

PROBLEMS

Controlling other variables

The primary difficulty when comparing groups using different external resistances in long-term resistance training programs is identifying the work done used in both groups. Work done is quite difficult to calculate when using variable resistance. It is therefore often easier to equate groups on the basis of the volume load (number of repetitions performed multiplied by the relative load).

EFFECT OF POWERLIFTING RESISTANCE TYPE ON STRENGTH (TRAINED)

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >1 group trained using commonly-used powerlifting techniques for creating variable resistance (i.e. bands and chains) and >1 group trained using constant loads (i.e. normal free weights)

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 5 studies were identified (click to read): Anderson (2008), McCurdy (2009), Ghigiarelli (2009), Rhea (2009), Joy (2014). Of these studies, only 1 reported a benefit of variable external resistance compared to constant loads. The use of variable resistance or constant loads may therefore make little difference to strength gains in this population.

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

EFFECT OF POWERLIFTING RESISTANCE TYPE ON STRENGTH (UNTRAINED)

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >1 group trained using commonly-used powerlifting techniques for creating variable resistance (i.e. bands and chains) and >1 group trained using constant loads (i.e. normal free weights)

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 1 study was identified (click to read): Shoepe (2011). This study reported no benefit of variable external resistance compared to constant loads. The use of variable resistance or constant loads may therefore make little difference to strength gains in this population.

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

 

EFFECT OF MACHINE-BASED RESISTANCE TYPE ON STRENGTH (UNTRAINED)

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >1 group trained using commonly-used machine-based exercises that involved variable resistance and >1 group trained using constant loads

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 7 studies were identified (click to read): Pipes (1975), Pipes (1978), Boyer (1990), Manning (1990), O’Hagan (1995), Walker (2013), Matta (2014). Of these studies, none reported a benefit of constant load external resistance compared to variable resistance in respect of isometric strength, 4 reported a benefit of constant load training for improving constant load strength, and 2 reported a benefit of variable load training for improving variable resistance strength. The use of variable resistance or constant loads may therefore make little difference to strength gains in this population, although some specificity of strength gains in respect of external resistance type might exist.

To open a new window and view detailed information in a large table, click HERE (not recommended for small screens)

CONCLUSIONS FOR STRENGTH

For trained subjects, bands and chains leads to similar strength gains as constant loads.

For untrained subjects, bands and chains leads to similar strength gains as constant loads.

For trained subjects, machine-based variable resistance training leads to similar strength gains as constant loads (although some specificity may exist).

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PERIODIZATION

PURPOSE

This section investigates whether periodization is effective for increasing strength. This is achieved by looking at long-term studies that compare whether a periodized program is better than a non-periodized program for increasing strength. The section also investigates which periodized program is best for increasing strength. This is achieved by looking at long-term studies that compare different types of periodized program and their effects on increasing strength.

BACKGROUND

Definitions

The term “periodization” has historically been notoriously difficult to define with any degree of precision or consensus. However, in this analysis, periodization will be defined as “the structure of a training program, where this training program varies over time, either linearly, non-linearly, or in blocks, in order to maximize the results of the athlete.” In contrast, a non-periodization training program will be defined either as a non-varied program or a program that varies randomly. Any training variable can be periodized (i.e. exercise selection, relative-load, volume, frequency, range-of-motion, proximity to failure, rest periods, etc.). However, in practice the two most commonly-varied training variables are relative load and volume. Typically, volume is reduced while relative load is increased and vice versa.

Periodization types

Introduction

Periodization types fall into three main categories: linear, non-linear, and block. In brief, linear (and reverse linear) periodization involves sequential alteration of key training variables over time. Non-linear periodization involves altering training variables from day-to-day or from week-to-week such that all training variables are used similarly within short periods of time. Block periodization involves training for a specific goal in successive, additive cycles.

Linear periodization

Linear periodization is the traditional and earliest form of periodization. This was originally proposed by Matveyev in the 1950s and involves a steady progression from high-volume, low-relative load training at the start of the program through to low-volume, high-relative load training at the end. A variant of linear periodization is reverse linear periodization in which the opposite sequence is followed. It is worth noting that volume and relative load are the most commonly manipulated training variables but essentially there is no reason why other variables cannot also be periodized, such as frequency, range-of-motion, proximity to failure, rest periods and exercise selection. For example, escalating density training (a method of training put forward by Charles Staley that involves steadily reducing rest periods over a period of time) is essentially a form of linear periodization in which a training variable (rest periods) is altered progressively over time.

Non-linear periodization

Non-linear periodization, which encapsulates methods known as undulating periodization and conjugate periodization, involves a less sequential change in training variables than linear periodization over the course of a training cycle. In non-linear periodization, workouts are arranged with training variables being altered across multiple workouts over short periods. This can occur from day-to-day over the course of a single week of workouts (daily undulating periodization) or from week-to-week over the course of several weeks of workouts (weekly undulating periodization). As noted above, while volume and relative load are most commonly investigated and manipulated over the course of periodized programs, there is no reason why exercise selection cannot be changed in the same way. This can be seen in the Westside method, where different exercises are rotated frequently throughout a training cycle.

Block periodization

Block periodization was proposed by Verkoshansky (1998) and involves cycles of sequential training designed to achieve a specific goal. Each block is intended to be the foundation for the next one. Depending on the terminology used, a typical sequence of cycles would be accumulation, transformation and realization, which are elsewhere called hypertrophy, maximal strength and power. The progression from high-volume, low-relative load to lower-volume and higher-relative loads makes it easy to confuse with linear periodization but the premise behind block periodization is different and involves a focus on the goal of the training cycle rather than just the sets and reps. For a discussion of the differences between block and traditional linear periodization, see Issurin (2008).

META-ANALYSES

Meta-analyses indicate that linear and non-linear (daily undulating) periodization are not significantly different in producing strength gains. Harries et al. (2014) recently performed a meta-analysis in both trained and untrained subjects to compare the effect of linear and non-linear periodization during resistance training programs on strength gains. It was reported that there was no significant difference between linear and non-linear periodization types. Linear periodization was non-significantly superior for the squat, while undulating periodization was non-significantly superior for the bench press and leg press.

MECHANISMS FOR INCREASING STRENGTH

For a full review of the mechanisms of strength development, see the mechanisms section (read more).

Conceptual basis for strength gains

Peripheral factors

The peripheral factors that might affect strength gains include: hypertrophy, myofibrillar packing density, extracellular lateral force transmission, muscle fiber type shifts and regional hypertrophy. The effect of periodization on hypertrophy seems to be minimal. Whether periodization could affect myofibrillar packing density, extracellular lateral force transmission, muscle fiber type or regional hypertrophy is unclear. Researchers have suggested that periodization could affect these variables by means of either Selye’s General Adaptation Syndrome or the principle of sequenced potentiation (see further Haff, 2004). Sequenced potentiation suggests that building a foundation of strength with heavy loads allows individuals to maximize the power gains they can achieve from power-training with lighter loads or the size gains that they can achieve from hypertrophy-training with greater volumes. However, how these concepts might drive beneficial changes in the underlying peripheral mechanisms responsible for strength gains is unknown.

Central factors

The central factors that might affect strength gains include: inter-muscular co-ordination and motor unit firing frequency. How periodization might affect these factors through the intermediary mechanisms of Selye’s General Adaptation Syndrome or the principle of sequenced potentiation is unclear. One possible explanation for the beneficial effects of linear, block and non-linear periodization methods (but not reverse linear) on strength gains is that they all involve greater relative loads performed in close proximity to the strength test, which could lead to superior inter-muscular co-ordination, assuming the habitual movement pattern hypothesis. In contrast, the reverse linear periodization model would be expected to perform poorly for strength gains, as the highest relative loads are performed furthest from the post-intervention strength test (which is indeed observed).

The mechanism by which periodization might affect strength gains is unclear. Transient increases in inter-muscular co-ordination might lead to superior strength gains prior to the post-intervention test.

PROBLEMS

Ecological validity

Periodization is very difficult to study in practice, making our ability to draw conclusions from the literature limited. Many researchers and coaches have drawn attention to the limitations of the current literature (e.g. Cissik, 2008), which has not kept pace with research in other areas, such as the effects of certain training variables on strength gains (e.g. relative load, volume, etc.). Primarily, Cissik (2008) observed that it is problematic that the majority of available studies are short-term in nature (approximately the duration of an academic semester), use non-athletic college populations, and primarily involve strength training modalities only. Cissik therefore suggested that this makes it difficult to the current periodization research to athletic populations who structure their training plans over years and performed concurrent training modalities. For the purposes of applying the available research to the achievement of strength gains during recreational resistance-training, however, these are not large concerns.

Methodological validity

More concerning for the application of the available research to recreational resistance-training was raised in a brilliant paper by Kiely (2012), who pointed out that most experimental designs exploring periodization have actually simply compared varied with non-varied interventions. Thus, such studies simply demonstrate that variation is important, and not that periodization is the best way of providing this variation. This relatively simple but radical criticism has unfortunately not been developed since it was raised in 2012. However, until a high quality study compares a randomized program with a periodized program and with a non-varied program, we will continue to lack an understanding of whether variation or structured periodization are of greater importance.

EFFECTS OF PERIODIZED VS. NON-PERIODIZED PROGRAMS ON STRENGTH (TRAINED)

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >1 group trained using a recognised periodization model and the other group did not use a periodization model

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 7 studies were identified (click to read): Willoughby (1992), Willoughby (1993), Baker (1994), Schiotz (1998), Stone (2000), Buford (2007), Monteiro (2009). Of these 7 studies, 4 reported significant benefits of periodization and the remainder reported no differences. Using periodization may therefore have a beneficial effect on strength gains in this population.

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EFFECTS OF LINEAR VS. NON-LINEAR PROGRAMS ON STRENGTH (TRAINED)

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >1 group trained using a linear periodization model and >1 group trained using a non-linear periodization model

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 7 studies were identified (click to read): Baker (1994), Buford (2007), Monteiro (2009), Prestes (2009a), Miranda (2011), Franchini (2014), Harries (2015). Of these 7 studies, only 1 reported significant benefits of non-linear periodization over linear periodization and the remainder reported no differences. Using non-linear periodization may therefore not be superior to linear periodization for strength gains in this population.

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EFFECTS OF LINEAR VS. REVERSE LINEAR PROGRAMS ON STRENGTH (TRAINED)

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >1 group trained using a linear periodization model and >1 group trained using a reverse linear periodization model

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 1 study was identified (click to read): Prestes (2009). It reported significant benefits of linear periodization over reverse linear periodization. Using linear periodization may therefore be superior to reverse linear periodization for strength gains in this population.

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EFFECTS OF LINEAR VS. BLOCK PROGRAMS ON STRENGTH (TRAINED)

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >1 group trained using a linear periodization model and >1 group trained using a block linear periodization model

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 1 study was identified (click to read): Bartolomei (2014). It reported significant benefits of block periodization over linear periodization. Using block periodization may therefore superior to linear periodization for strength gains in this population.

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EFFECTS OF NON-LINEAR VS. BLOCK PROGRAMS ON STRENGTH (TRAINED)

Selection criteria

Population – trained subjects

Intervention – resistance-training, where >1 group trained using a non-linear periodization model and >1 group trained using a block linear periodization model

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 2 studies were identified (click to read): Painter (2012), Hartmann (2009). Both reported no significant benefits of block periodization over non-linear periodization. Using block periodization may be similar to non-linear periodization for strength gains in this population.

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EFFECTS OF PERIODIZED VS. NON-PERIODIZED PROGRAMS ON STRENGTH (UNTRAINED)

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >1 group trained using a recognised periodization model and the other group did not use a periodization model

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 7 studies were identified (click to read): Stone (1982), Stowers (1983), O’Bryant (1988), Herrick (1996), Moraes (2013), Ahmadizad (2014), Souza (2014a). Of these studies, 4 reported significant benefits of periodization over no periodization. Using periodization may therefore be beneficial for strength gains in this population.

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EFFECTS OF LINEAR VS. NON-LINEAR PROGRAMS ON STRENGTH (UNTRAINED)

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >1 group trained using a linear periodization model and >1 group trained using a non-linear periodization model

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 8 studies were identified (click to read): Rhea (2002a), Rhea (2003a), Kok (2009), Apel (2011), Simão (2012), De Lima (2012), Ahmadizad (2014), Souza (2014a). Of these studies, 4 reported significant benefits of non-linear periodization over no periodization while 1 reported significant benefits of linear periodization over non-linear periodization. It is possible that non-linear periodization might be superior to linear periodization for strength gains in this population but there are conflicting reports.

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EFFECTS OF LINEAR VS. REVERSE LINEAR PROGRAMS ON STRENGTH (TRAINED)

Selection criteria

Population – untrained subjects

Intervention – resistance-training, where >1 group trained using a linear periodization model and >1 group trained using a reverse linear periodization model

Comparator – baseline performance or a non-training control group

Outcome – at least one reliable measure of muscular strength, including maximum voluntary isometric contraction strength or 1RM

Results

The following 1 study was identified (click to read): Rhea (2003a). It reported no significant benefits of linear periodization over reverse linear periodization. Using linear periodization may therefore be similar to reverse linear periodization for strength gains in this population.

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CONCLUSIONS FOR STRENGTH

For trained and untrained subjects, periodization is probably superior to no periodization for strength gains.

For trained and untrained subjects, non-linear and linear periodization probably lead to similar strength gains.

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MECHANISMS FOR INCREASING STRENGTH

CONTENTS

Click on the links below to jump down to the relevant section of the page:

Summary

Background

The relationship between size and strength

Specific tension

Peripheral factors

Central factors

PURPOSE

This section describes the mechanisms by which strength increases occur, including both peripheral and central factors. Peripheral factors are those that occur within the muscle. Central factors are those that occur in the central nervous system.

SUMMARY

The following list ranks our confidence for whether a mechanism can explain any of the strength gains following resistance training in order:

Hypertrophy (peripheral)

Inter-muscular co-ordination (central)

Myofibrillar packing density (peripheral)

Extracellular lateral force transmission (peripheral)

Changes in fiber type proportion (peripheral)

Motor unit firing frequency (central)

Regional hypertrophy (peripheral)

Motor unit recruitment (central)

Motor unit synchronisation (central)

Agonist-antagonist co-activation (central)

Fascicle length (peripheral)

Pennation angle (peripheral)

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BACKGROUND

Introduction

Strength is commonly defined as the ability to express force. Force can be displayed in many different ways. It can be displayed either statically (isometrically) or dynamically and during either single-joint (isolation) or multi-joint (compound) movements. The ability to display force under each of these conditions may differ because of differences in the extent to which each of the (peripheral and central) variables that affect force production are involved. As a general rule, force production during less complex movements (e.g. isometric single-joint exercises) involve fewer factors than more complex movements (e.g. dynamic multi-joint exercises). However, most popular measures of strength (e.g. bench press, squat, deadlift) are dynamic, multi-joint exercises.

Force

Forces occur where an object experiences a push or a pull as a result of its interaction with another object (pulling forces are also known as tensile forces). Forces are typically designated with the letter F and are measured in Newtons (N). When muscles perform muscle actions (either eccentric, concentric or isometric), they create a tensile force that pulls on the bones that act at a given joint. Concentric muscle actions pull one end of the muscle closer to the other end, thereby shortening it, while eccentric muscle actions allow one end of the muscle to move further from the other end, thereby lengthening it.

Moments

When forces act on a system containing a pivot, they are called moments (i.e. torque, or turning forces). Moments are slightly different from most other forces, as they cause angular motion rather than linear motion. Moments are typically designated with the Greek letter τ (tau) and are measured in Newton meters (Nm). They can be calculated as the linear force exerted multiplied by the perpendicular distance between the force and the pivot. This perpendicular distance is known as the moment arm length. Although muscles act in order to produce force on the bones that they are attached to, the net result is a joint moment, as the bones are attached to a joint, which acts as a pivot. Thus, while the muscle produces a certain amount of force that is purely dependent upon its contractile ability, the end result is a joint moment. It is important to appreciate that this joint moment is not only a function of the force expressed by the muscle but is also dependent upon the moment arm length (Baxter and Piazza, 2014), which is not primarily a function of contractile ability. The analysis of joint moments is particularly useful when discussing single-joint movements (isolation exercises).

Single- and multi-joint movements

Introduction

The human body can perform both single-joint (isolation) and multi-joint (compound) movements. During single-joint movements (e.g. a knee extension or a biceps curl), the movement takes an angular path, with the joint at the center as a pivot. On the other hand, during multi-joint movements (e.g. a leg press or a bench press), the joints reposition themselves in relation to one another so that although each joint rotates, the end result is that the end segment takes a linear path throughout the motion. This requires an extremely complex and co-ordinated pattern of interactions between the individual joint moments that occurs as a result of detailed and changing muscle activation patterns, even while many other factors (including the moment arm lengths) are changing as well.

Single-joint measurements

The joint moment produced in single-joint movements is generally measured either isokinetically or isometrically using a dynamometer, although strength can also be measured by reference to a 1RM in a suitable single-joint exercise. In the case of the isokinetic joint moment, this is usually recorded as the peak torque exerted during the movement. Isokinetic external resistance is where the velocity is controlled but the force exerted is voluntary, while isometric external resistance is where there is no movement. This joint moment is not only a function of the force expressed by the muscle but is also dependent upon the moment arm length, which is not primarily a function of contractile ability.

Multi-joint measurements

The force produced in multi-joint movements is generally measured during isometric muscle actions with a linear force plate, although strength can also be measured by reference to the maximum amount of external load (i.e. 1RM) in a suitable dynamic multi-joint exercise. As explained above, the force in multi-joint movements is produced by a combination of joint moments, and during dynamic multi-joint movements the joints reposition themselves in relation to one another so that although each joint rotates, the end result is that the end segment takes a linear path throughout the motion. This means that when looking at the factors that make up multi-joint strength, there are the same factors that comprise single-joint moments (but at several joints), as well as factors relating to the interactions between these joints. Consequently, the expression of muscular strength as measured by joint torque or by force during a multi-joint movement can be quite different from the tensile force produced by a muscle. The force exerted by the muscle is transmitted through several layers of rotational and linear systems before it is measured.

THE RELATIONSHIP BETWEEN SIZE AND STRENGTH

Introduction

Although muscle size is a good predictor of strength, several key observations have been made that indicate the presence of other factors. Firstly, while it is well correlated with force producing ability, it does not entirely predict inter-individual differences in strength (Ikai and Fukunaga, 1968; Maughan et al. 1983; Maughan et al. 1983a; Maughan and Nimmo, 1984; Brechue et al. 2002; O’Brien et al. 2010; Verdijk et al. 2010; Baxter and Piazza, 2014). Secondly, muscle size does not fully predict changes in strength following a long-term resistance training program (Hubal et al. 2005; Erskine et al. 2014). Thirdly, increases in strength are greater than increases in muscle size as a result of resistance training programs (Erskine et al. 2008; Erskine et al. 2010; Erskine et al. 2011). Fourthly, the loss in strength that occurs with age is much greater than the loss in muscle size (see reviews by Clark and Manini, 2008; Manini and Clark, 2011; Mitchell et al. 2012; Clark and Manini, 2012). These observations mean that changes in strength must occur as a result of a combination of factors (most likely including both central and peripheral changes), of which an increase in muscle size is just one (see review by Gabriel et al. 2006).

THE RATIO OF STRENGTH-TO-SIZE AND SPECIFIC TENSION

Strength-size ratio

A simple way of tracking whether strength increases at a faster rate to muscle size is to monitor the ratio of strength to muscle size. The strength-size ratio can be calculated by taking any measure of muscle strength that is relevant to a single muscle or muscle group (e.g. isometric knee extension torque or 1RM knee extension) and then expressing it relative to muscle size (e.g. quadriceps volume or cross-sectional area). Studies often report that the strength-to-size ratio increases during resistance training programs, because strength gains occur at a faster rate than increases in muscle size. The reasons for this difference in strength and size adaptations are unclear but are thought to involve both central and peripheral factors.

Factors affecting the strength-size ratio

Adaptations resulting from resistance training fall into either peripheral or central types. Peripheral factors are those that occur at the muscular level and involve structural alterations to the muscle. The most important of these is hypertrophy, which is most commonly measured as a change in muscle cross-sectional area but can also be measured as a change in muscle mass or volume. The other key peripheral changes include alterations in the other aspects of muscle architecture (muscle fascicle length and pennation angle), shifts in muscle fiber type, and changes to the individual muscle fiber and its surrounding extracellular environment. On the other hand, central factors are those that occur within the central nervous system and are thought to be mainly related to alterations in neural drive (which is a function of both motor unit recruitment and motor unit firing frequency) and inter-muscular co-ordination.

Specific tension

Specific tension is sometimes considered to be the same as the ratio of strength to muscle size. However, it is not the same. Specific tension is defined as the maximal (involuntary) isometric muscle force per unit physiological cross-sectional area for an individual muscle. On the other hand, the ratio of strength to muscle size is simply the maximal muscle force (isometric or dynamic) per unit anatomical cross-sectional area for an individual muscle. Therefore, it is possible for the ratio of strength to size ratio to be affected by factors that do not affect specific tension. Such factors include the level of voluntary activation, muscle architecture, and the force-velocity relationship.

Measuring specific tension

Specific tension can be measured in two ways. Firstly, it can be measured with an in vitro model of skinned, isolated muscle fibers at standardized fiber lengths. This is sometimes referred to as “single fiber specific tension” (Erskine et al. 2011). Secondly, it can be measured using an in vivo model, starting from a single-joint exercise and then accounting for central factors, the muscle moment arm, and muscle architecture separately. In this latter case, researchers control for confounding variables by estimating their impact and removing them individually. However, different approaches to the in vivo model involve different assumptions and the way in which each confounding variable is controlled for could affect the results obtained. In particular, the extent to which the effects of muscle architecture are genuinely accounted for has been criticized on the basis that both pennation angle and fascicle length change substantially with joint angle (Narici et al. 1996). Nevertheless, several studies (Magnaris et al. 2001; Erskine et al. 2009) have demonstrated that in vivo measurements are feasible and have produced results that are both reliable and in line with previous in vitro models.

Changes in specific tension

Changes in specific tension have been found to explain a substantial element of the inter-individual variability in strength gains (Erskine et el. 2010a). This has two important implications. Firstly, it means that if we can identify why specific tension changes following resistance training, we can explain why strength increases can occur in the absence of size increases. Secondly, it means that many of the other factors that have been proposed to affect the strength-size ratio may not be quite so important (e.g. central factors, muscle pennation angle, muscle fascicle length, and muscle moment arms). Unfortunately, researchers are still unclear why specific tension increases as a result of resistance training. The most recent studies indicate that either myofibrillar packing density or increases in extracellular lateral force transmission could be responsible for increases in specific tension.

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PERIPHERAL FACTORS AFFECTING STRENGTH

Introduction

There are a number of peripheral factors that have been proposed to contribute to strength gains following from resistance training. The main factor is an increase in muscle size, which is also known as hypertrophy. Importantly, while this change in muscle size primarily affects strength by altering contractile ability, muscle size can also affect moment arm length. A larger muscle can bring about a greater joint moment by increasing the moment arm length even without changing the tensile force exerted by the muscle itself (Sugisaki et al. 2010; Akagi et al. 2012; Akagi et al. 2014; Sugisaki et al. 2014). Additionally, the change in the shape of the muscle during a muscle contraction appears to affect the moment arm length (Akagi et al. 2012; Akagi et al. 2014).

An increase in overall muscle size is simply one peripheral adaptation to resistance training. Many other peripheral changes occur that have an affect on strength, including alterations in the shape of the muscle (regional hypertrophy), increases in pennation angle, alterations in fascicle length, and shifts in muscle fiber type. Researchers often group muscle size (when expressed as physiological cross-sectional area) together with muscle fascicle length and pennation angle into a composite concept known as muscle architecture. It has been argued that this measure is the single best predictor of muscle function (see reviews by Ward et al. 2009; Lieber and Ward, 2011).

Hypertrophy

[Read more about hypertrophy]

Regional hypertrophy

Introduction

Hypertrophy is the increase in cross-sectional area or size of a muscle. Regional hypertrophy was first reviewed by Antonio (2000), where it was defined as a change in the shape of a muscle for the purposes of bodybuilding. More recently, regional hypertrophy has been used to refer to differences in hypertrophy along the length of a muscle, where proximal or distal sections may display greater or less increases in size from one another.

Effects of regional hypertrophy

Potentially, regional hypertrophy over sustained periods of time might lead to changes in the ability of the muscle to display strength at different joint angles. Since many multi-joint exercises have a “sticking point” or a portion of the ROM that is harder than other parts, this could then lead to regional variation in muscle size having an effect on the ability to exert force at different points in the ROM during such exercises.

Mechanisms of regional hypertrophy

There are two main mechanisms by which regional hypertrophy might occur (see review by Antonio, 2000). Firstly, the compartmentalization of muscles could mean that certain areas were activated to perform certain ranges of motion of a joint action or certain movements at a joint where multiple movements are possible (such as at the hip or shoulder). Indeed, most individual muscles are not comprised of single compartments but are in fact made up of several segments, which display different features. For example, the gluteus medius has 4 compartments (anterior, anterior-middle, posterior-middle, and posterior). These compartments have different nerve branches and varying pennation angles (ranging from +33.1 through to -29.5 degrees), which means that they are probably best suited to slightly different movement tasks (Flack et al. 2014).

Secondly, it has been reported that there are differences in muscle fiber type between one region of a muscle and another. Lexell et al. (1983) found that type I fibers were predominant in the deep vastus lateralis, while type II fibers were predominant in the superficial vastus lateralis. Similarly, Sola et al. (1992) found a greater proportion of type I fibers in the deep latissimus dorsi and more type II fibers superficially. Other research has also found differences between regions from proximal-to-distal as well as between regions from deep-to-superficial. This means that where different repetition ranges are used and thereby target different muscle fiber types, this could lead to preferential growth in certain parts of a muscle. More recently, Wakahara et al. (2013) found that differences in EMG activity in certain parts of a muscle correlated with the increases in muscular size. Miyamoto et al. (2013) found differences in muscle tissue oxygenation saturation was between distal and middle regions of the vastus lateralis during knee extension exercise.

Challenges

There has been little research performed investigating the effects of regional hypertrophy on strength gains. It is therefore unclear whether any changes in the shape of a muscle by regional hypertrophy would be substantial enough to have a meaningful effect on the improvement in the overall strength-to-size ratio that occurs following resistance training.

Muscle pennation angle

Introduction

The pennation angle of a muscle fiber is the angle between the prevailing direction of the muscle fiber and the line of action between the origin of a muscle and its insertion. Different parts of a muscle can have different pennation angles (Infantolino and Challis, 2014). This is because the commonly-held view that muscle fibers run all linearly the way from the origin to the insertion is not strictly correct. Rather, in most muscles, there are groups of muscle fibers running in different directions in various parts of the overall muscle. These groups of fibers run variously from the origin to the outer fascia of the muscle, from the outer fascia of the muscle to the insertion, or even from the outer fascia of the muscle on one side to the outer fascia of the muscle on the other side (see review by Lieber and Ward, 2011).

Effects of changing pennation angle

Increasing pennation angle is actually detrimental for force production of the muscle for two reasons. Firstly, the angle of pull of the muscle fibers in relation to the line of action between the origin and insertion changes with changing pennation angle. Greater pennation angles mean that less of the force exerted by the muscle fibers is transferred in the direction of the overall tensile force produced by the muscle. Secondly, since muscle fibers span shorter distances in heavily pennated muscles (i.e. from one side of the muscle to the other), this means that there are fewer sarcomeres in parallel than lightly pennated muscles (i.e. from one end of the muscle to the other in completely unpennated muscles. This means that heavily pennated muscles have a lower maximal shortening velocity than completely unpennated muscles. This makes them less able to produce force at higher velocities during dynamic movements.

Explanation for increasing pennation angle

Increasing pennation angle seems detrimental for force production but in fact it serves a very important purpose. Increasing the pennation angle of a muscle or of a region within a muscle means that more muscle fibers can be fitted into the same space. More muscle fibers means that the force being produced in total is greater. Thus, there is an apparent trade-off between the detrimental effects of increasing pennation angle and the beneficial effects of increasing muscular cross-sectional area for improving force production.

Challenges

Changing pennation angle is likely to have a small adverse effect on the strength gains that occur following resistance training. Therefore, changes in pennation angle are unlikely to be relevant for explaining the improvements in the strength-to-size ratio that occurs.

Muscle fiber length

Introduction

Muscle fibers are commonly-believed to run the full length of a muscle, from origin to insertion. In reality, muscle fibers run from one side to the other in a wide variety of different patterns, forming various compartments. The angle at which muscle fibers run to an imaginary line drawn between the origin and the insertion is known as the pennation angle. More pennated muscle fibers tend to have shorter fibers (because muscles are usually longer than they are wide) and therefore muscle fiber length is broadly reduced when pennation angle is increased.

Measuring muscle fiber lengths

There are two different types of measurement regarding muscle fiber lengths. The first is the raw muscle fiber length, which is fixed by the distance between its start and end points, which could be the origin and insertion of the muscle or other points within the muscle, such as an aponeurosis. The second is the normalized fiber length, which is simply a count of the number of sarcomeres that exist in series in the muscle fiber. Normalized fiber lengths are calculated by measuring the raw fiber length and then standardising the sarcomere lengths within this raw fiber length to 2.5μm. The length of a muscle fiber at the point when it exerts force affects the magnitude of that force. This is known as the “length-tension” relationship. Consequently, if the strength of a muscle is tested before and after a resistance training program that causes a change in normalized fiber length, the change in normalized fiber length will affect the change in strength. Whether the change in normalized fiber length will cause an increase in strength or a decrease will depend on how long each sarcomere is within the muscle at the point where the strength measurement is being taken.

The length-tension relationship

The length-tension relationship is a very important concept that aims to explain the ability of individual muscle fibers (which are chains of sarcomeres) to exert force by reference to the behavior of the individual sarcomeres in the chain. It is thought that length-tension relationship can be explained by reference to two underlying mechanisms: the active and passive length-tension relationships. The active length-tension relationship is thought to occur as a result of the degree of overlap between the actin and myosin filaments within an individual sarcomere. Too much or too little overlap leads to sub-optimal tension being developed but where the overlap is “just right” the maximal tension is developed. The passive length-tension relationship is thought to occur much more simply as a result of the elastic elements within a sarcomere, within a muscle fiber and within the muscle itself. Thus, the passive length-tension relationship is largely unnoticed at small muscle fiber lengths but becomes very important very quickly once the muscle is stretched beyond a certain length. The combination of the active and passive length-tension relationships explains the overall length-tension relationship. Overall, where the active length-tension relation predominates (i.e. at short lengths), the curve rises, plateaus and then falls back down. The rising part of this section is known as the “ascending limb” and the falling part is known as the “descending limb”.

Challenges

The idea that changing muscle fiber length can have a marked effect on the strength-size ratio is challenged firstly by the fact that alterations in specific tension are known to occur following training, which account for the length-tension relationship by taking measuring measurements at a fixed sarcomere length. Secondly, the role of muscle fascicle length in the strength-size ratio has been directly investigated. Buchanan (1995) modeled the specific tension of the elbow flexors and extensors by experimentally recording maximum joint moments at a range of different joint angles and incorporating this data along with anatomical parameters reported in the literature into a musculoskeletal model. They found that the specific tension for elbow flexors was larger than for the elbow extensors and that the length-tension relationship was not a good predictor of the differences.

Muscle fiber type

Introduction

Muscle fibers can be classified in various ways, although all current methods are dependent upon the assumption that limiting factor for the speed at which cross-bridge cycling can occur is the speed at which the ATPase of the myosin head can hydrolyze ATP to power the process.

Relationship between muscle fiber type and strength

Individuals who possess a greater proportion of type II muscle fibers are believed to be stronger than those who possess a smaller proportion of type II muscle fibers. Indeed, some studies have reported strong relationships between type II muscle fiber proportion and strength (Tesch and Karlsson, 1978; Clarkson et al. 1980; Schilling et al. 2005). Also, the specific tension of type II muscle fibers appears to be greater than that of type I muscle fibers (Young, 1984; Grindrod et al. 1987; Bottinelli et al. 1996; D’Antona et al. 2006; Pansarasa et al. 2009).

Effect of preferential muscle fiber type hypertrophy on strength gains

Since type II muscle fiber type proportion seems to be associated with superior strength, it is possible that increases in strength irrespective of alterations in muscle size could occur by preferential hypertrophy of type II muscle fibers. Indeed, it is generally accepted that type II muscle fiber area increases to a greater extent than type I muscle fiber area following resistance training, although both muscle fiber areas do increase substantially (see review by Ogborn and Schoenfeld, 2014). It has been reported that type II muscle fiber area increases by ~50% more than type I muscle fiber area, although substantial inter-individual variability has been observed (see review by Ogborn and Schoenfeld, 2014). Thus, by this mechanism, an increase in total fiber area by means of hypertrophy would naturally be expected to be accompanied by a shift in proportional fiber area in favour of type II muscle fibers, leading to increases in the strength-size ratio. However, the amount of this increase in the strength-size ratio that can be explained by this alteration in proportional fiber area is unclear. Additionally, the type of resistance training that is performed may affect the extent to which preferential hypertrophy of type II muscle fibers occurs. Ogborn and Schoenfeld (2014) suggested that training with lower relative loads to muscular failure would lead to greater stimulus of type I muscle fibers while training with higher relative loads might lead to greater stimulus of type II muscle fibers.

Effect of muscle fiber type shifting on strength gains

Since type II muscle fiber type proportion seems to be associated with superior strength, it is possible that increases in strength irrespective of alterations in muscle size could occur by actual shifts between type I and type II muscle fibers. Regarding actual fiber type shifting, it is clear that there are shifts between hybrid fiber types and between type IIX and type IIA muscle fibers and there are some indications that shifts between type I and type II muscle fibers could occur, particularly subsequent to high-velocity resistance training (see review by Wilson et al. 2012).

Challenges

Although there seems to be a relationship between type II muscle fiber proportion and strength, there are many strong challenges to the idea that muscle fiber type is a key driver for the substantial increases in strength that occur with resistance training. Firstly, several studies have identified that specific tension can increase in type I and type II muscle fibers individually (Harber et al. 2004; Trappe et al. 2006; Pansarasa et al. 2009; Parente et al. 2011) or even change differently in response to the same training program in each type of muscle fiber (Trappe et al. 2001). Secondly, many studies have reported that the differences in the strength-size ratio between individuals cannot be explained by differences in muscle fiber type (Maughan and Nimmo, 1984) and that muscle fiber type is not well-correlated with strength (Clarkson et al. 1982; Clarkson et al. 1982a; Schantz et al. 1983; Schilling et al. 2005a; Beck et al. 2009). Thirdly, some studies have not found good correlations between changes in muscle fiber type and the changes in specific tension following resistance training (Erskine et al. 2011). Fourthly, evidence regarding the extent to which meaningful shifts in muscle fiber type proportion or muscle fiber type area occur subsequent to long-term resistance training is conflicting and while some studies report shifts, many others do not.

Myofilament packing density

Introduction

Some researchers have raised the possibility that an increase in myofilament packing density could occur following resistance training, which would lead to a change in specific tension because the muscle would essentially become denser. This phenomenon has been investigated directly only to a small extent and therefore it is helpful to consider studies of both hypertrophy and atrophy, as well as studies of single fibers (in vitro) specific tension.

Hypertrophy

An early study investigating the hypertrophic effects of resistance training found changes in myosin filament concentration, in the distance between myosin filaments, and in the number of actin filaments in orbit around a myosin filament (Penman, 1970). However, a later study (Claassen et al. 1989) found that a 6-week period of resistance training led to marked increases in both strength and muscle size without any change in the distance between myosin filaments or in the ratio of actin to myosin filaments.

Atrophy

In a trial investigating the atrophic effects of a 17-day period of bed rest on the ultrastructure of soleus muscle cells in humans, Riley et al. (1998) found that the concentration of thick filaments was unchanged, while the concentration of thin filaments decreased substantially. In a rodent study using single, skinned muscle fibers, Riley et al. (2005) found that disuse atrophy led to a reduction in specific tension and a loss in the concentration of thin filaments, which they interpreted as being consistent with reduced cross bridge formation. It was also found that by osmotically compacting the myofilaments with a 5% dextran solution, it was possible to return the concentration of thin filaments to previous levels and simultaneously revert levels of specific tension to pre-intervention values.

Single fiber specific tension

Positive findings for increases in single fiber (in vitro) specific tension are generally taken to be supportive of the idea that greater myofilament packing has occurred. This is largely because there is practically no other factor that we can identify that could possibly be relevant, given that central factors, muscle moment arms, and muscle architecture are completely removed (Pansarasa et al. 2009; Parente et al. 2011).

Challenges

The concept of myofibrillar packing density has been challenged as some researchers have found changes in measures of in vivo specific tension or the strength-size ratio without similar-sized changes in measures of single fiber specific tension (Trappe et al. 2000; Trappe et al. 2001; Widrick et al. 2002; Erskine et al. 2011). However, the fact that some studies have found differences suggests that myofibrillar packing density is likely a factor but is very likely not the sole factor that can explain changes in specific tension.

Extracellular lateral force transmission

Introduction

Until the first in vitro experiments on single muscle fibers from frogs by Street (1983), it was thought that force transmission in muscle fibers occurred solely in a longitudinal direction, from one end to the other. However, Street (1983) found that much of the force developed by a single muscle fiber was transmitted laterally, into the extracellular matrix. The occurrence of the phenomenon of extracellular lateral force transmission was quickly adopted and assumed to occur in other animals. However, it was only very recently confirmed to happen in mice and rats (Ramaswamy et al. 2011).

Lateral force transmission and costameres

Like the concept of extracellular lateral force transmission itself, the explanation for this phenomenon has quickly been focused on a single idea: costameres. It is thought that the connection between costameres and the concept of extracellular lateral force transmission was first put forward by Pardo et al. (1983), shortly after the original frog experiments by Street (1983). However, the relationship between costameres and mechanical force transmission by the muscle fiber has now become firmly established (see review by Ervasti, 2003).

Lateral force transmission and specific tension

Increases in specific tension could arise as a result of an increase in the number of lateral attachments between sarcomeres and the extracellular matrix, which would increase lateral force transmission between neighbouring myofibrils and effectively raise the number of sarcomeres in parallel (Jones et al. 1989). Since lateral force transmission is thought to be mediated by attachments between individual sarcomeres and the extracellular matrix by means of the costameres (Danowski et al. 1992; Bloch and Gonzalez-Serratos, 2003; Ramaswamy et al. 2011), it is interesting that increases in collagen synthesis do occur following acute bouts of resistance training (Miller et al. 2005; Moore et al. 2005) and that proteins associated with costameres (e.g. desmin and dystrophin) are similarly elevated (Woolstenhulme et al. 2006; Kosek & Bamman, 2008).

Challenges

The concept of lateral force transmission can be challenged on the basis that it has not been directly investigated and confirmed but rather inferred by the discovery of strong challenges to all other known plausible mechanisms.

CENTRAL FACTORS AFFECTING STRENGTH

Introduction

There are a number of central factors that have been proposed to contribute to strength gains following from resistance training (see reviews by Behm, 1995; Aagard, 2003; Aagaard, 2003; Gabriel et al. 2006; Folland and Williams, 2007). Historically, these have included changes in: inter-muscular co-ordination, agonist muscle activity (driven by increases in motor unit recruitment, motor unit discharge rates, or motor unit synchronisation), and agonist-antagonist co-activation. Despite being frequently referred to, almost all of the evidence for central involvement in strength gains is indirect (see reviews by Carroll et al. 2001; Carroll et al. 2011) and the individual central factors that are most important remain unclear. Overall, it seems likely that the largest single centrally-mediated factor is inter-muscular co-ordination, although even the evidence for this factor is weak.

Justification for central factors

Introduction

Many arguments have been put forward to justify the role of central factors in explaining the inter-individual variability in strength and muscle size, both cross-sectionally and longitudinally during resistance training programs. Unfortunately, most of the evidence for central factors is indirect (see reviews by Carroll et al. 2001; Carroll et al. 2011) and quite weak when considered in the light of studies into specific tension. In addition, there are many inconsistencies in the direct research into central factors at present, which precludes any strong conclusions (see review by Carroll et al. 2011).

Difference between strength and size gains

One of the most common arguments in favour of the role of central factors in strength gains is the observation that gains in strength are typically much greater than gains in muscle size. We can measure this by a change in the strength-size ratio and also by a more refined calculation, called specific tension. Specific tension is the maximal muscle force per unit physiological cross-sectional area for an individual muscle. Specific tension differs substantially from the strength-size ratio as it excludes many variables, including all central factors. Nevertheless, despite the removal of these central factors, peripherally-mediated changes in specific tension have been found to explain a substantial element of the inter-individual variability in strength gains (Erskine et el. 2010a). Since such peripherally-mediated changes in specific tension can be used to explain a large proportion of the difference in magnitude of the gains in strength and size, it would appear presumptuous to assume that centrally-mediated factors must have occurred following a resistance training program purely on the basis that the gains in muscle size cannot explain the gains in strength.

Early phase adaptations

Early trials studying the long-term gains in muscle size and strength often reported no changes in muscle size over an initial 6 – 7 weeks of training. This gave rise to the now commonly-held idea that gains in strength during this period must be attributable largely to central factors. Initial challenges to this idea were limited to the observation that muscle protein synthesis was elevated post resistance training even within this early phase (Phillips, 2000) but there is now substantial evidence that gains in muscle size do in fact occur during periods of resistance training as short as 3 weeks (see review by Wernbom et al. 2007). Moreover, as noted above, peripherally-mediated changes in specific tension seem to explain a large proportion of the difference in magnitude of the gains in strength and size. Therefore, it is incorrect to assume that centrally-mediated factors must have occurred following a resistance training program purely on the basis that the gains in muscle size cannot explain the gains in strength.

Speed of strength gains

A more plausible justification for the presence of central factors in strength gains following resistance training can be found in studies that report extremely fast strength gains. For example, even testing a maximum voluntary isometric contraction has been found to lead to measurable gains in strength over subsequent days (see review by Gabriel et al. 2006). Such rapid gains are not thought to be possible via peripheral mechanisms, although it is noted that clearly some peripheral adaptations do begin to occur even at the earliest stage.

Cross-education effect

Possibly the most robust justification for the existence of centrally-mediated factors driving changes in strength following a resistance training program is the presence of the cross-education effect. The cross-education (or cross-transfer or cross-over) effect is the observation that long-term resistance training of a limb can lead to gains in voluntary strength in the contralateral limb. This effect has well-established, with a meta-analysis of 13 randomized controlled studies of voluntary unilateral resistance training (using >50% of 1RM and lasting >2 weeks) showing contralateral effects of resistance training of around an ~8% (range -3% – +22%) increase on initial strength levels, or ~35% of the change in the ipsilateral limb (Munn et al. 2004).

In order to explain the cross-education effect, it has been suggested that there exist central factors that are altered with resistance training that affect both ipsilateral and contralateral sides similarly, while the ipsilateral limb also benefits from peripheral (muscular) adaptations (see reviews by Carroll et al. 2006; Hendy et al. 2012). Currently, the underlying mechanisms for the cross-education effect are unknown but may include changes at cortical, subcortical or spinal levels (see reviews by Carroll et al. 2006; Hendy et al. 2012). The possibility of changes in many (but not all) peripheral factors has been explored and rejected (Houston et al. 1983).

Imagined contractions

Related to the cross-education effect is the observation that imagined contractions can lead to meaningful gains in strength. In a remarkable study, Yue and Cole (1992) found that left hand fifth digit metacarpophalangeal abduction torque increased after a 4-week period of resistance training in both the conventionally-trained and imagined contractions groups (by 30% and 22%, respectively) and that both of these groups also displayed cross-education effects to the right hand (by 14% and 10%, respectively. A control group displayed minimal changes in measurements for both left and right hands. Other researchers have also reported marked gains in strength following imagined contractions (Herbert et al. 1998; Zijdewind et al. 2003; Smith et al. 2003; Ranganathan et al. 2004; Sidaway and Trzaska, 2005; Reiser et al. 2011; see review by Folland and Williams, 2007). In addition, there appears to be an additive effect of imagined contractions and conventional resistance training, particularly in respect of the lower body (Lebon et al. 2010).

INTER-MUSCULAR CO-ORDINATION

Introduction

Inter-muscular co-ordination has been defined as “a distribution of activation or force among individual muscles to produce given combination of joint moment” (see review by Prilutsky, 2000). Inter-muscular co-ordination is often confused with other neural adaptations, such as motor unit synchronisation (for example, see review by Carroll et al. 2001) but it is distinct. It represents the ability of the central nervous system to co-ordinate the distribution of neural drive to all muscles in an order and proportion that optimises the production of force for a given set of parameters (range of motion, velocity, muscle action and type of resistance). Unfortunately, few studies have explored the extent to which changes in inter-muscular co-ordination or motor programs occur as a result of long-term resistance training programs. Therefore, we do not yet know for certain to what extent they can explain the inter-individual differences in strength gains.

Improvements in inter-muscular co-ordination

Although many reviewers have identified inter-muscular co-ordination as a potential key central factor mediating strength gains (see reviews by Rutherford, 1988; Carroll et al. 2001; Carroll et al. 2011), few studies have actually explored its direct contribution to strength gains over a long-term resistance training program. Rutherford and Jones (1986) demonstrated specificity of posture by comparing resistance training programs involving similar exercises but with different degrees of support. The exercises with most support caused the greatest increases in maximum voluntary isometric contraction strength. Almåsbakk and Hoff (1996) compared the long-term increases in strength between groups performing bench press resistance training 3 times per week for 6 weeks using either a broomstick or heavy loads. Both groups focused on maintaining good technique throughout. The broomstick group gained a significant amount of strength in the bench press movement and there was little difference in the strength gains between groups.

Task specificity of resistance training

The presence of task-specificity is indirect evidence that inter-muscular co-ordination is a key mechanism mediating strength gains over a long-term resistance training program. Task-specific gains in strength have been observed in resistance training very frequently (see reviews by Morrissey et al. 1995; Carroll et al. 2001). In addition to velocity, task specificity in strength gains has been observed in relation to many relevant resistance training variables included in this review, including range of motion (Graves et al. 1989; Graves et al. 1992; Hartman et al. 2012; Bloomquist et al. 2013), muscle action (Higbie et al. 1996; Hortobagyi et al. 1996; Hortobagyi et al. 2000; Symons et al. 2005; Nickols-Richardson et al. 2007; Blazevich et al. 2007; Carvalho et al. 2014) and external load type (Boyer et al. 1990; O’Hagan et al. 1995). Additionally, where task specificity has been investigated directly following a long-term period of resistance training, it has been shown to occur. For example, it has been reported that leg press resistance training caused quadriceps hypertrophy without increasing MVIC knee extension strength (Sale et al. 1992), that isoinertial training led to superior gains in isoinertial strength compared to isokinetic strength (Abernethy and Jürimäe, 1996; Pearson and Costill, 1998), and that bilateral training leads to greater bilateral strength gains than unilateral training and vice versa (Taniguchi, 1997).

Template motor patterns

Introduction

In order to understand how changes in inter-muscular co-ordination might enable strength gains following resistance training, it is necessary to have a working model for how the central nervous system controls movement. This enables hypotheses to be formed regarding how changes in this system of control can occur in response to training. The theory of template motor patterns is the current, most widely-accepted model for how the central nervous system controls movement. The theory of template motor patterns was originally proposed to explain how humans and other animals are able to perform similar but not identical movements with very little preparation or calculating time. These template motor programs are thought to involve a set pattern of inter-muscular co-ordination for all sub-maximal and maximal variations of the same basic movement (Van Zandwijk et al. 2000).

Template motor patterns and neural drive

Some researchers have proposed a very strict interpretation of the theory of template motor patterns. In their model, it is proposed that the central nervous system uses the same pattern of muscle activity for all versions of the movement and that it is solely the magnitude of neural drive to each muscle that is increased. However, this has not been found in direct investigations. For example, Van Zandwijk et al. (2000) found that the proportional muscle activity and onset times of the various leg muscles differed substantially between maximal and sub-maximal vertical jumps, indicating that not only was there a change in the magnitude of the neural drive to each muscle but also that there was a change in the relative drive to the various muscle groups. Moreover, the exact joint angle movements were significantly different. Thus, template motor programs do not simply involve scaling up muscle activity across all involved muscles with increasing force production but rather that another function is involved that alters the relationship between muscles with changing overall force production. This implies that inter-muscular co-ordination may differ between movements performed with different loads and speeds. Consequently, a practice effect might exist in order to develop the ability to produce force with different loads and speeds.

Effect of load and speed

Some researchers have noted that there is no apparent effect of load on muscle activity patterns during basic movements (i.e. magnitude and timing of EMG signals). Indeed, a lack of effect of load has generally been found to in the jump squat, which routinely displays similar muscle activation patterns with different loads (Eloranta et al. 1995; Nuzzo and McBride, 2013; Giroux et al. 2014; Giroux et al. 2015). This has been taken by those researchers who have proposed a very strict interpretation of the theory of template motor patterns as evidence that template motor programs do not involve changes in inter-muscular co-ordination as loads become heavier or speeds faster. However, there have been several contrasting observations in studies of a range of dynamic, multi-joint movements (squat, deadlift, lunge, vertical jump, and running) to the effect that different loads and speeds do in fact involve different contributions from different joints and muscle groups (see Frost et al. 2013; reviews by Beardsley and Contreras, 2014; Beardsley and Contreras, 2014a).

Concomitant changes in joint angle movements

In addition, it is interesting to note that the changes in joint contribution (Beardsley and Contreras, 2014) and muscle activation patterns Beardsley and Contreras, 2014a) in dynamic, multi-joint movements appear to occur in tandem with slightly different movement patterns, as measured by joint angles (Frost et al. 2013). Indeed, in their direct study of the concept of template motor patterns, Van Zandwijk et al. (2000) found significant differences in movement patterns between sub-maximal and maximal vertical jumps in addition to alterations in the pattern of inter-muscular co-ordination. Nevertheless, changes in inter-muscular co-ordination have also been observed to occur when the movement pattern is fixed, as when performing slower vs. faster cadences or lower vs. higher power outputs during maximal and sub-maximal cycle ergometer sprints (Bieuzen, et al. 2007; Dorel et al. 2012).

Effect of strength gains

Under the strict interpretation of the theory of template motor programs described above, researchers have suggested that there are no changes in inter-muscular co-ordination as loads become heavier or speeds faster. In this model, it is assumed that increasing jump height is a simple matter of enhancing the force production of the involved muscles. The difference between sub-maximal jumps and maximal jumps should be explained by an increase in the muscle activation of the involved muscles while maintaining the same pattern of inter-muscular co-ordination. Consequently, increasing the strength of the relevant muscles should be immediately translated into increases in vertical jump height. However, this has not been observed in practice, and studies have reported increases in leg strength without increases in vertical jumping ability (Clutch et al. 1983). In order to investigate this problem, Bobbert and Van Soest (1996) constructed a musculoskeletal model. They found that if the control pattern remained unchanged while muscle strength was increased, vertical jump height actually decreased. On the other hand, when control pattern was tuned to take advantage of greater muscle strength, vertical jump height increased. This implies that inter-muscular co-ordination does differ between movements performed with different loads and speeds and that a practice effect probably exists that can be used develop the ability to produce force with different loads and speeds.

Effects of fatigue

Many researchers have found that fatigue causes marked changes in inter-muscular co-ordination, as measured by EMG activity. For example, O’Bryan et al. (2014) studied the changes in inter-muscular co-ordination in tandem with a 60% reduction in power output during a 30-second stationary cycle ergometer sprint. They found a decrease in EMG amplitude for all muscles except the hamstrings. They also noted that two bi-articular muscles decreased in EMG amplitude to the greatest extent (rectus femoris and gastrocnemius). They concluded that this alteration in inter-muscular co-ordination as a result of fatigue might be expected to result in a redistribution extent to which each joint contributed to the overall force exerted on the pedal during the sprint. How these changes can be described within the context of the general framework of template motor patterns is unclear.

Explanations for changes in inter-muscular co-ordination

Introduction

If changes in inter-muscular co-ordination occur during resistance training and can be used to explain the strength gains that occur, how do such improvements happen? Several theories have been put forward to explain this phenomenon. However, in order to understand this, it is necessary first to address the wider problem of how the central nervous system co-ordinates movement. This problem is called motor redundancy or the degrees of freedom problem (see review by Mohan and Morasso, 2011), or sometimes the force distribution problem (see review by Prilutsky and Zatsiorsky, 2002). The essence of this problem is that the musculoskeletal system has many times more degrees of freedom (and muscles to help produce movement) than there are directions to travel in. Consequently, there is nearly an infinite number of ways in which joints can be repositioned with respect to one another in order to produce a single movement, which makes many of the options redundant. Indeed, there is a tendency for broadly the same movement pattern to be observed by individuals performing the same task on multiple occasions and between individuals performing the same task (see reviews by Prilutsky and Zatsiorsky, 2002; Mohan and Morasso, 2011).

Optimal muscle activation pattern hypothesis

In order to solve the motor redundancy problem, the optimal muscle activation pattern hypothesis has been proposed. This hypothesis states that the central nervous system continually seeks out the single movement pattern solution that produces the best outcome. Exactly what outcome is desired is currently unclear, however, and various possibilities have been explored, including options that are the most metabolically efficient, those that involve the least fatigue, or those that require the least perceived effort (see review by Prilutsky and Zatsiorsky, 2002). Within this model, it would be assumed that during deliberate practice, better and better solutions for a movement would be observed and identified and then used in the future. In this way, resistance training could lead to superior performance in a given movement by way of repeated practice.

Criticisms

The hypothesis of optimal muscle activation patterns during movement has been criticised by many researchers on a variety of grounds (e.g. see review by Loeb, 2012; discussion by Bobbert et al. 2013). Loeb (2012) argued that in order for there to be a requirement for an hypothesis of optimal muscle activation patterns, we would first have to demonstrate an evolutionary need for a single, known cost function to be optimised, among other things. This remains to be demonstrated, particularly as the general rule for most biological systems is to become “good enough” to meet requirements rather than to become optimal. Loeb proposed that a contrary hypothesis that is more consistent with other biological observations is that humans are constantly in the process of using trial-and-error learning to develop a repertoire of sensorimotor behaviors that have previously been found to work. Constant trial-and-error thereby leads to the development of better-and-better solutions for a range of movement problems. Thus, exposure to a wider range of movement problems might be expected to lead to superior learning experiences and the development of a better set of possible solutions.

Habitual movement pattern hypothesis

As an alternative to the hypothesis of optimal movement patterns, some researchers have reported findings indicating that individuals generally perform habitual movement patterns (Ganesh et al. 2010; De Rugy et al. 2011). De Rugy et al. (2011) carried out a series of musculoskeletal simulations and found that in contrast to what might be expected from optimal control theory, inter-muscular coordination is not continuously optimized at the level of individual muscles. Rather, they found that habitual inter-muscular coordination is surprisingly resilient to a wide variety of modeled, challenging circumstances including the loss or damage of one of the muscles. Ganesh et al. (2010) performed an experimental trial in which subjects performed a task several times freely in what was an optimal way, after which they performed the same task several times in a much more inefficient way, before finally performing the task several times freely. Most of the subjects performed the final repetitions of the task in the inefficient way, which is a strong challenge to the optimal muscle activation hypothesis and an indication that individuals tend to adopt movement patterns habitually rather than out of a clear perception that they are optimal. Nevertheless, in this model, resistance training could still lead to superior performance in a given movement by way of repeated practice, although it would also be possible to develop adverse habits that could cause losses in strength.

Challenges

The primary challenge to the proposal that inter-muscular co-ordination is a driver for strength gains following resistance training is the lack of direct research. The second challenge is our current lack of understanding of precisely how the central nervous system controls movement and how resistance training or other training modalities might modify this control process.

AGONIST MUSCLE ACTIVITY (VOLUNTARY ACTIVATION)

Background

Introduction

Agonist muscle activity (also called voluntary activation or neural drive) is a measure of the strength of the signal from the central nervous system to the muscle. Individual muscle fibers within the muscle are innervated in groups by motor units. The number of motor units that are innervated to perform a given action is thought to be determined by the size of the signal from the central nervous system. The strength of this signal has historically been believed to be determined by at three main factors: the magnitude of the electrical stimulus, its firing frequency, and the synchronisation of these impulses (see review by Behm, 1995).

Common drive

Although it is sometimes supposed that individual motor units are controlled separately, there is actually a common drive from the central nervous system to all of the motor units in a muscle (De Luca and Erim, 1994). This phenomenon was identified when researchers observed close correlations in the variations in motor unit firing rates between simultaneously activated motor units during muscle contractions. The concept was termed “common drive” because all motor units in a muscle appear receive the same common signal from the central nervous system. Consequently, whether they are activated (and their absolute firing rates) is simply a function of their excitatory threshold (De Luca and Erim, 1994).

Measuring agonist muscle activity

The extent to which agonist muscle activity is maximal during any given muscle action can be assessed by the difference in force production exerted during maximum voluntary contractions and maximum involuntary contractions, most commonly by means of the interpolated twitch technique (see review by Shield and Zhou, 2004). In this technique, an electrical stimulus is administered to a muscle during a voluntary contraction as well as while the muscle is at rest. This electrical stimulus is thought to cause those motor units that have not already been recruited to respond by contracting and generating force, thereby allowing a percentage of maximum possible voluntary activation to be calculated (Herbert and Gandevia, 1999; see reviews by Shield and Zhou, 2004; Gabriel et al. 2006). Taking these measurements before and after a resistance training program can provide an indication of how important alterations in neural drive are for increasing strength.

Agonist muscle activity changes in adults

Surprisingly, despite the commonly-held belief that changes in neural drive occur with resistance training, voluntary activation is not thought to be a major contributor to inter-individual differences in strength or in the magnitude of strength gains following conventional, heavy resistance training. Indeed, it is generally believed that voluntary activation is generally within ~5% of maximum involuntary activation during maximal voluntary isometric contractions (see reviews by Shield and Zhou, 2004; Gabriel et al. 2006). Additionally, voluntary activation appears to increase with resistance training in normal, healthy adult populations by only ~2% (Knight and Kamen, 2001; Gabriel et al. 2006) if they increase at all (Lee et al. 2009). However, such measurements are typically taken in the context of isometric contractions and different results might be observed if dynamic contractions are observed (see Buckthorpe et al. 2014). Nevertheless, in contrast to the above general findings for conventional resistance training, it is possible that high-velocity, ballistic resistance training or plyometrics might cause increases in neural drive that explain a substantial proportion of the associated strength gains (e.g. Häkkinen et al. 1998; Kyröläinen et al. 2005; Behrens et al. 2014; Behrens et al. 2015; see also review by Markovic and Mikulic, 2010). This may be because of the greater level of motor unit recruitment that has been found to occur during high-velocity muscle actions for the same level of force output (see review by Duchateau et al. 2006).

Agonist muscle activity changes in the elderly

In elderly populations, there is some evidence for voluntary activation playing a role in explaining the loss of strength with age, as deficits in voluntary activation are observed between young and old people (see reviews by Clark and Manini, 2008; Manini and Clark, 2011; Mitchell et al. 2012; Clark and Manini, 2012). This deficit may be connected with the loss of voluntary activation that was reported in early disuse studies (Wolf et al. 1971; Duchateau and Hainaut, 1987). Alternatively, it may relate to an increase in intramuscular fat content. Intramuscular fat has been observed to increase with age (Delmonico et al. 2009), and appears to affect voluntary activation (see review by Gabriel et al. 2006; Yoshida et al. 2012).

Similarly, there is some evidence that the gains in strength that occur following resistance training may also be affected by changes in voluntary activation in the elderly (see reviews by Clark and Manini, 2008; Manini and Clark, 2011; Mitchell et al. 2012; Clark and Manini, 2012). Such changes might again be the result of centrally-mediated factors but might also be affected by changes in intramuscular fat content, which can be altered through resistance training (see review by Marcus et al. 2010). Nevertheless, the magnitude of the increase in voluntary activation following resistance training is still relatively small. Arnold and Bautmans (2014) performed two meta-analysis of the changes in voluntary activation following long-term resistance training programs in elderly people: one for the plantar flexors and one for the knee extensors. They found that voluntary activation increased significantly in both muscle groups but the size of the pooled increase was ~9% for the plantar flexors and ~2% for the knee extensors.

Motor unit recruitment

Introduction

Voluntary activation is affected by two main factors: motor unit recruitment and motor unit firing frequency. The way in which motor units are recruited is described by the “size principle” or often “Henneman’s size principle” after the researcher who discovered it (Henneman et al. 1965; see reviews by Enoka and Stuart, 1984; Duchateau et al. 2006; Heckman and Enoka, 2012: Bawa et al. 2014). The size principle describes how the order of motor unit recruitment in response to the signal from the central nervous system is determined by spinal and peripheral factors determining the excitability of the motor units, with large motor units being less excitable than smaller ones because of their lower input resistance (Duchateau et al. 2006; Dideriksen and Farina, 2013).

Factors affecting motor unit recruitment

It is important to note that motor unit recruitment is not the same as voluntary activation (which can be assessed indirectly using electromyography [EMG]). Nevertheless, they are frequently confused in the literature as well as in the lay press. Motor unit recruitment describes whether a motor unit is active or not. Voluntary activation involves both motor unit recruitment and motor unit firing frequency and therefore describes both the activation status and the proximity to tetanus of the motor unit. Additionally, there are several key factors that affect the level of force at which full recruitment occurs. Firstly, it has been shown that motor unit recruitment is likely muscle specific (Kukulka and Clamann, 1981; De Luca et al. 1983). Masakado (1994) concluded that differences between muscles could be explained by their size: small muscles reach full recruitment at 50% of MVIC and thereafter rely upon increases in motor unit firing rate for further increases in force, while large muscles only reach full recruitment at around 80 – 90% of MVIC. Secondly, it seems that the level of force at which full recruitment is reached is velocity-dependent, with faster speeds leading to greater recruitment at lower levels of force (Duchateau et al. 2006). Finally, it is important to note that there are various limitations to take into account when drawing inferences from EMG studies regarding motor unit recruitment (see review by Farina et al. 2004).

Afterhyperpolarization

As described above, voluntary activation is affected by two main factors: motor unit recruitment and motor unit firing frequency. The relationship between motor unit recruitment and motor unit firing rate is somewhat unclear. Some researchers have found that motor unit recruitment is largely responsible for increases in force production at lower levels of force, while increases in motor unit firing rates are largely responsible for increases in force production at higher levels of force (Kanosue et al. 1979) although this relationship may be muscle specific (De Luca et al. 1983). At least two frameworks for the relationship between motor unit recruitment and motor unit firing rates have been proposed. The first framework to reach wide acceptance has become known as the afterhyperpolarization model. This model was treated as the de facto explanation for the relationship between motor unit recruitment and motor unit firing rates for several decades. It rests upon the observation that larger motor units display shorter afterhyperpolarization and faster motor unit firing rates than smaller motor units and assumes a relationship between the duration of the afterhyperpolarization period and motor unit firing rate (De Luca and Contessa, 2012). However, later experiments have challenged this concept.

Onion skin model

The second framework for modeling the behavior of motor unit recruitment and motor unit firing rates is called the onion skin model. In this model, the firing rate of motor units follows a clear hierarchical progression: motor units recruited earlier display faster firing rates than motor units that are recruited later. In building this model, researchers noted that the largest motor units rarely reach the point of maximal stimulation (tetanus), presumably because they are extremely fatiguable and cannot sustain this level of activity for very long (De Luca and Erim, 1994; De Luca and Contessa, 2012). Whether this implies that there is the possibility of developing the ability to display extremely high levels of force for very short durations of time by learning how to stimulate these high threshold motor units all the way to the point of tetanus is currently unknown.

Motor unit firing rates

Introduction

Motor unit firing rates (also called rate coding and motor unit discharge rates) are key to the immediate production of force. As described above, the central nervous system uses both motor unit firing rates and motor unit recruitment to control the acute application of muscle force. Motor unit firing rates appear to be key for the development of force once motor unit recruitment is complete. Although motor unit firing rates have been identified as a key factor that may be responsible for changes in voluntary activation after resistance training, recent reviewers have been pessimistic about their ability to contribute substantially to the gains in strength that are observed after resistance training (see review by Carroll et al. 2011).

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Motor unit synchronisation

Historically, it was believed that motor units did not always fire in synchronisation with one another and that training the central nervous system to co-ordinate motor unit firing with resistance training could thereby lead to enhanced force production (see reviews by Behm, 1995; Aagaard, 2003; Gabriel et al. 2006). However, this concept has been challenged strongly over the last 20 years and many researchers now hold that motor units are always synchronised by virtue of common drive or synaptic input (see reviews by De Luca and Erim, 1994; Duchateau et al. 2006; De Luca and Contessa, 2012; Farina and Negro, 2015). Consequently, the idea that motor unit synchronisation is a factor that can be manipulated with resistance training in order to enhance strength gains seems unlikely.

ANTAGONIST CO-ACTIVATION

Background

Introduction

It has been suggested that resistance training could increase strength by modulating a spinal inhibitory pathway and bringing about a reduction in antagonist co-activation (Arnold and Bautmans, 2014). By reducing antagonist co-activation, this may permit greater net joint torque to be produced. In addition, it has been reported that elderly people display different antagonist co-activation strategies (see review by Gabriel et al. 2006). Indeed, there is some evidence that elder people display greater antagonist co-activation (Macaluso et al. 2002), which may be in order to increase joint stability.

Antagonist co-activation in adults

In healthy adult populations, the literature provides conflicting views on the effects of resistance training on antagonist co-activation (see review by Gabriel et al. 2006). On the one hand, some studies have found that antagonist co-activation alters substantially with resistance training (e.g. Carolan and Cafarelli, 1992; Tillin et al. 2011; Stock and Thompson, 2014a). On the other hand, other studies have reported either increases in antagonist co-activation (e.g. Gabriel et al. 1997) or no alteration (e.g. Häkkinen et al. 1998; Colson et al. 1999). Recently, a series of interesting studies intended to explore the effects of resistance training in combination with cues to perform maximal antagonist co-contraction unanimously reported no effects on antagonist co-activation when tested before and after training (Maeo et al. 2013; Driss et al. 2014; Maeo et al. 2014). These studies are informed by another in which bodybuilders were compared with recreational control subjects. The bodybuilders were able to perform voluntary co-contractions with greater muscle activity but this did not affect their level of antagonist co-activation during a normal strength task (Maeo et al. 2013a). On balance, it seems likely that changes in antagonist co-activation do not contribute to increases in strength as a result of resistance training in most adults.

Antagonist co-activation in the elderly

In elderly populations, the balance of evidence suggests that antagonist co-activation does not play a major role in explaining the changes in strength that occur subsequent to a long-term resistance training program. Arnold and Bautmans (2014) performed two meta-analyses of the changes in antagonist co-activation following long-term resistance training programs in elderly people: one for the plantar flexors and one for the knee extensors. They found that antagonist co-activation did not increase significantly in either muscle groups. The size of the pooled change was +0.6% for the plantar flexors and -1.8% for the knee extensors. Therefore, it seems that changes in antagonist co-activation do not contribute to increases in strength as a result of resistance training in elderly people.

CONCLUSIONS FOR STRENGTH

The peripheral factors that might affect strength gains include: hypertrophy, myofibrillar packing density, extracellular lateral force transmission, fiber type shifts and regional hypertrophy.

The central factors that might affect strength gains include: inter-muscular co-ordination and motor unit firing frequency.

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