Hamstrings

The hamstrings are a group of four muscles on the back of the thigh. Three of them are two-joint muscles (performing both knee flexion and hip extension) while the fourth performs only knee flexion.

The hamstrings has very different muscle architecture from one another, with a range of fiber lengths, pennation angles and physiological cross-sectional areas. Training the hamstrings with a range of different loads and speeds may therefore be necessary.

Despite the popular belief that the hamstrings are a fast-twitch muscle group, they in fact display a balanced fiber type, with a slight trend towards more slow-twitch fibers. Using a range of high and low repetitions may be beneficial.  

The hamstrings display no clear tendency to greater muscle activity at any one joint angle. However, there are differences between individual hamstrings muscles. This suggests that exercises involving peak contractions at a range of joint angles may be optimal.

Research is limited regarding the best exercises for the hamstrings. Leg curls are a reliable option, while good mornings, Romanian deadlifts, and Nordic curls (glute-ham raises) are good alternatives.

Some exercises appear to target the medial hamstrings to a greater extent (e.g. kettlebell swings, deadlifts, Nordic curls) while other exercises target the lateral hamstrings more (e.g. leg curls and back extensions). Optimal programs may therefore include exercises that target both sub-groups.

CONTENTS

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Summary

Anatomy

Muscle fiber type

Electromyography

Eccentric training

References


SUMMARY

PURPOSE

This section provides the summary of research findings into the hamstrings. 

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SUMMARY EVIDENCE FOR THE HAMSTRINGS

Anatomy – The hamstrings have very different muscle architecture from one another, with a range of fiber lengths, pennation angles and physiological cross-sectional areas. Training the hamstrings with a range of different loads and speeds may therefore be necessary.

Muscle fiber type – Despite the popular belief that the hamstrings are a fast-twitch muscle group, they in fact display a balanced fiber type, with a slight trend towards more slow-twitch fibers. Using a range of high and low repetitions may be beneficial.

Electromyography – The hamstrings display no clear tendency to greater muscle activity at any one joint angle but there are differences between individual hamstrings muscles. Using several exercises involving peak contractions at a range of joint angles may be optimal.

Electromyography – Research is limited regarding the best exercises for the hamstrings. Leg curls are a reliable option, while good mornings, Romanian deadlifts, and Nordic curls (glute-ham raises) are good alternatives.

Electromyography – Some exercises appear to target the medial hamstrings to a greater extent (e.g. kettlebell swings, deadlifts, Nordic curls) while other exercises target the lateral hamstrings more (e.g. leg curls and back extensions). Optimal programs may therefore include exercises that target both sub-groups.

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CONCLUSIONS REGARDING THE HAMSTRINGS

The hamstrings are a very complex group of muscles that require substantial training variety in respect of load, speed, and exercise selection in order to target optimally.

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ANATOMY

PURPOSE

This section provides a summary of the anatomy of the hamstrings. 

BACKGROUND

Introduction

The hamstrings are important for sporting performance, particularly during running (Higashihara et al. 2010b; Kyröläinen et al. 2005). The hamstrings also often require rehabilitation from injury, with hamstring strains accounting for around 12 – 16% of injuries in popular team sports (Woods et al. 2004; Orchard and Seward, 2002). Most such strains seem to occur during high-speed running (Brooks et al. 2006). Therefore, much of the anatomical research into the hamstrings has focused on their role during running and the potential for strain injury.

GROSS ANATOMY

Introduction

There are four hamstrings muscles: the biceps femoris (long head), the biceps femoris (short head), the semitendinosus, and the semimembranosus. The biceps femoris (long head), the semitendinosus, and the semimembranosus are all bi-articular (two-joint) muscles. These bi-articular muscles cross the hip, being attached to the ischiac tuberosity of the pelvis (Batterman et al. 2011), and also cross the knee, being attached to the tibia and fibula, although other insertion points have also been reported (Tubbs et al. 2006). These bi-articular muscles therefore cause both hip extension and knee flexion. The biceps femoris (short head) is a single-joint muscle and causes only knee flexion.

Medial and lateral hamstrings: origins and insertions

The hamstrings can be subdivided into two subgroups (medial and lateral) on the basis of anatomical differences.

Semitendinosus (medial) – originates on the ischiac tuberosity of the pelvis and inserts on the upper anterior medial surface of the tibia.

Semimembranosus (medial) – originates on the ischiac tuberosity of the pelvis and inserts on the postero-medial surface of the medial tibial condyle.

Biceps femoris long head (lateral) – originates on the ischiac tuberosity of the pelvis and inserts on the lateral condyle of the tibia and head of the fibula.

Biceps femoris short head (lateral) – originates on the lower half of the linea aspera and the lateral condyloid ridge of the femur and inserts on the lateral condyle of the tibia and head of the fibula.

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Medial and lateral hamstrings: differences

The medial and lateral hamstrings muscles are different from one another in several respects. They are not equal in size, function or risk of injury. The total volume of the medial hamstrings is greater than that of the lateral hamstrings (Friederich and Brand, 1990; Ito et al. 1996). The medial hamstrings are more strongly activated normalized to maximum voluntary isometric contraction (MVIC) than the lateral hamstrings during running (Jönhagen et al. 1996; Higashihara et al. 2010b). Finally, the lateral hamstrings are more frequently injured when running than the medial hamstrings (De Smet et al. 2000; Garrett et al. 1989; Slavotinek et al. 2002).

Overall size and weight

Unlike the quadriceps (Blazevich et al. 2006), the hamstrings are a group of muscles that display very different muscle architecture to one another. The semimembranosus and biceps femoris (long head) appear to be much larger and heavier than the other two hamstring muscles. However, the difference between these two heavier muscles is unclear. Wickiewicz et al. (1983) reported that the biceps femoris (long head) was the heaviest hamstring muscle, followed by the semimembranosus, while Ito et al. (1996) and Kellis et al. (2012) reported that the semimembranosus was the heaviest hamstring muscle, followed by the biceps femoris (long head). Friederich and Brand reported that the semimembranosus was the largest hamstring muscle by volume, while Ward et al. (2009) and Woodley and Mercer (2005) found that the semimembranosus had the greatest physiological cross-sectional area, followed by the biceps femoris (long head).

Muscle weight

From the limited data available in the literature, it is generally apparent that the biceps femoris (long head) and the semimembranosus are the heaviest muscles, while the biceps femoris (short head) and semitendinosus are usually the lightest.

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Muscle cross-sectional area

From the limited data available in the literature, it is generally apparent that the biceps femoris (long head) and the semimembranosus have the greatest muscle cross-sectional area, while the biceps femoris (short head) and semitendinosus generally have the smallest muscle cross-sectional area.

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Muscle thickness

The literature is currently too limited to ascertain whether the muscle thickness of any of the hamstrings muscles is substantially different from the others.

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Muscle volume

From the limited data available in the literature, it is generally apparent that the biceps femoris (long head) and the semimembranosus have the greatest muscle volume, while the biceps femoris (short head) and semitendinosus generally have the smallest muscle volume.

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MUSCLE MOMENT ARMS

Introduction

Muscle moment arms are often overlooked when determining the precise function of a muscle. However, they are essential for establishing how effective a muscle can be at producing torque at a given joint, at any given joint angle. As a hip extensor and knee flexor, the hamstrings have muscle moment arms at both joints.

Hip extension

Very few studies have reported on the moment arms for the hamstrings in hip extension. Dostal et al. (1986) reported that the moment arms were 5.6cm for the  semitendinosus, 4.6cm for the semimembranosus, and 5.4cm for the biceps femoris (long head). Németh et al. (1985) reported a moment arm for all hamstrings combined of 6.1cm. These figures indicate that the hamstrings are an effective hip extension in the anatomical position. However, exactly how the hip extension muscle moment arms of the hamstrings compare with the gluteus maximus is unclear. Dostal et al. (1986) reported a figure for the gluteus maximus of 4.5cm, which is lower than that seen in the hamstrings. On the other hand, Németh and Ohlsén (1985) reported a figure for the gluteus maximus of 8cm, which is greater. It seems likely that the hamstrings and gluteus maximus therefore have similar muscle moment arms to one another and are therefore expected to be involved in hip extension to a similar extent.

Hip adduction

Very few studies have reported on the moment arms for the hamstrings in hip adduction. Dostal et al. (1986) reported that the moment arms were 0.9cm for the semitendinosus, 0.4cm for the semimembranosus, and 1.9cm for the biceps femoris (long head). These figures indicate that the hamstrings are not particularly active in hip adduction in the anatomical position.

Hip internal rotation

Very few studies have reported on the moment arms for the hamstrings in hip internal rotation. Dostal et al. (1986) reported that the moment arms were 0.5cm for the semitendinosus, 0.3cm for the semimembranosus, and -0.6cm for the biceps femoris (long head). These figures indicate that the hamstrings are not particularly active in hip internal or external rotation (negative numbers) in the anatomical position. However, the presence of small differences between the medial (semitendinosus and semimembranosus) and lateral (biceps femoris) hamstrings in respect of their hip internal and external rotation muscle moment arms may imply a slight difference in function. This slight difference in function might be discerned when the feet are internally or externally rotated during certain hip extension exercises like the back extension in order to place more emphasis upon one set of hamstrings or the other (Fiebert et al. 1992; Fiebert et al. 1997).

Knee flexion: effect of angle

Many studies that have reported muscle moment arms for the various hamstrings muscles for knee flexion with changing knee angle. In general, there is a trend for hamstrings muscle moment arms to increase with increasing knee flexion angle.

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MUSCLE REGIONS

Introduction

Few studies have explored the different regions of the individual hamstrings muscles, most likely because of the substantial differences between the hamstrings themselves. However, certain specific findings have been reported.

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Anatomy

The anatomy of the semitendinosus has been observed to differ substantially from the other hamstrings in several reports. Specifically, it has been noted that the semitendinosus is the only hamstring to display a tendinous inscription that runs proximally to distally through the middle of the muscle (Garrett et al. 1989; Woodley and Mercer, 2005; Van de Made et al. 2013).

Electromyography

Historically, the majority of electromyographic (EMG) investigations exploring the hamstrings muscle group have tended to either use a single set of surface electrodes for the whole complex or study the lateral (biceps femoris) and medial (semitendinosus and semimembranosus) hamstrings separately. However, few have explored potential differences in EMG activity between individual regions of the same hamstring muscle. Schoenfeld et al. (2015) explored the EMG activity of the proximal (upper) and distal (lower) regions of the medial and lateral hamstrings during the stiff-legged deadlift and the lying leg curl exercises in resistance-trained males. They found that the lying leg curl produced greater medial and lateral EMG activity in the lower region compared with the stiff-legged deadlift. However, there was no difference between exercises in respect of the upper region. This indicates that different exercises do lead to differences in EMG activity in different parts of the individual hamstrings muscles, which suggests that there may be separate regions within each muscle.

MUSCLE ARCHITECTURE

Introduction

Muscle architecture describes the arrangement of muscle fibers within the overall framework of the muscle itself, which is surrounded by fascia. It has been described as “the macroscopic arrangement of muscle fibers” (see review by Lieber and Fridén, 2000). Since muscles are roughly cylindrical structures comprising fascicle bundles that run at an angle to the axis of force generation, there are three main measurements of the structure of a muscle: normalized fiber length, physiological cross-sectional area, and pennation angle. Muscle architecture of the hamstrings is of particular interest for the prevention and rehabilitation of hamstring strain injury, as studies have reported differences in muscle architecture between previously strained and healthy muscles in the same individual (e.g. Timmins et al. 2014).

Pennation angle

Only a small number of studies have assessed the pennation angle of the hamstrings. The pennation angle of the hamstrings varies slightly between muscle. The bigger, heavier semimembranosus seems to be more pennated than the semitendinosus. Exactly how the biceps femoris (long head) and biceps femoris (short head) compare is less clear. While early studies indicated that they followed a similar pattern to the medial hamstrings, later studies found no differences.

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Fascicle length

Only a small number of studies have assessed the fascicle lengths of the hamstrings. The fascicle lengths of the hamstrings varies slightly between muscles. The bigger, heavier semimembranosus seems to be shorter than the semitendinosus. Similarly, the biceps femoris (long head) is longer than the biceps femoris (short head), although this is likely a function of differences in the placements of the origins.

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Physiological cross-sectional area

Only a small number of studies have assessed the physiological cross-sectional area of the hamstrings. The physiological cross-sectional area of the hamstrings varies slightly between muscle. The bigger, heavier semimembranosus seems to be greater in size than the semitendinosus. Similarly, the biceps femoris (long head) is usually found to be greater in size than the biceps femoris (short head).

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Practical implications

From the above analysis, it is interesting to note that across the medial and lateral hamstrings, there is one muscle that has a high normalized fiber length and a low physiological cross-sectional area and another muscle that has a low normalized fiber length and a high physiological cross-sectional area. Since the moment arm lengths for hip extension appear to be similar between the semitendinosus, semimembranosus and biceps femoris (long head) (Dostal et al. 1986), this may imply that one muscle in each subgroup is better suited for producing large excursions with high joint angular velocities while the other may be better suited for performing very forceful muscular contractions over short excursions (see review by Lieber and Fridén, 2000). Additionally, the difference in normalized fiber lengths between the two muscles in each group implies that each of the hamstrings will produce their individual maximum forces at different joint angles and muscle lengths. Training the hamstrings with a range of different loads and speeds may therefore be necessary for maximum development.

CONCLUSIONS REGARDING THE HAMSTRINGS

The hamstrings has very different muscle architecture from one another, with a range of fiber lengths, pennation angles and physiological cross-sectional areas. Training the hamstrings with a range of different loads and speeds may therefore be necessary.

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MUSCLE FIBER TYPE

PURPOSE

This section provides a summary of the studies into the muscle fiber type of the hamstrings.

BACKGROUND

Introduction

Many strength and conditioning coaches believe that the prevailing hamstrings tend to display a prevailing type II muscle fiber type (fast twitch). This assumption has been rarely challenged by other coaches and yet is not supported by the literature. On balance, studies indicate that the hamstrings in fact display a fairly balanced muscle fiber type. If anything, there is a slight trend for hamstrings to display a predominance of type I muscle fibers (slow twitch), with type I fiber proportions ranging from around 49% – 67%.

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CONCLUSIONS REGARDING THE HAMSTRINGS

Despite the popular belief that the hamstrings are a fast-twitch muscle group, they in fact display a balanced fiber type, with a slight trend towards more slow-twitch fibers.

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ELECTROMYOGRAPHY

PURPOSE

This section provides a summary of the electromyography (EMG) activity studies into the hamstrings.

BACKGROUND

Introduction

Both strength and conditioning coaches and rehabilitation specialists often have need to find suitable exercises to develop the hamstrings in their athletes and clients. The hamstrings are considered to be important players in sprint running, which is a key attribute of many team sports athletes, and are often injured, meaning that they need to be rehabilitated and trained in order to return to sport. Therefore, it is important to identify the best hamstrings exercises, which can be used both in standard training and during rehabilitation and in the post-injury period prior to return-to-sport.

RESISTANCE TRAINING EXERCISES

Introduction

Since 3 of the 4 hamstrings muscles are biarticular, hamstrings exercises can involve either hip or knee movement, or both. When both hip and knee movements are involved, exercises involving the hamstrings can cause a wide range of muscle length changes, from very small to very large, through a range of different combinations of joint movements. Hamstrings exercises can usually be placed into one of the following categories:

Hip extension and knee extension (e.g. squat)

Hip extension with partial knee extension (e.g. deadlift)

Hip extension without knee movement (e.g. back extension)

Knee flexion without hip movement (e.g. leg curl)

Hip extension and knee flexion (e.g. glute-ham raise)

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Comparing hamstrings exercises

Few studies have directly compared hamstrings EMG activity across a range of common resistance-training exercises. Fewer still have included exercises from all joint movement categories and those that have been performed have not produced consistent results. In general, it seems that exercises from the knee flexion category (i.e. leg curls) almost always feature as one of the best exercises, exercises from the hip extension and knee extension category (i.e. squats) never feature, and it is unclear how the other categories should be viewed, with exercises from the hip extension with partial knee extension, hip extension without knee movement, and hip extension and knee flexion categories all appearing in the best exercises category in some but not all studies.

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Back squat

Many strength coaches continue to refer to the back squat as a useful exercise for the hamstrings. However, the literature does not provide substantial support for this view. Indeed, studies have reported that the hamstrings are not activated to the same extent as the quadriceps during squats (Isear et al. 1997; McCaw and Melrose, 1999; Escamilla et al. 2001; Paoli et al. 2009) and do not increase with increasing external load (Savelberg et al. 2007; Li and Chen, 2013).

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Effect of load and speed

Some research has reported that hamstrings EMG activity does not increase to the same extent as the activity of other lower body muscles during back squats with increasing load. Savelberg et al. (2007) found that as the load increased in a sit-to-stand movement, the activity of most of the lower body muscles increased accordingly, although the increase in the activity of the biceps femoris was less marked than that of the other muscles and only significantly increased with the largest load increment. Li and Chen (2013) investigated the differences in lower body muscle activity during back squats with increasing load and found that although the activities of the soleus, vastus medialis, gluteus maximus, and upper lumbar erector spinae all increased as the load was increased, there was no significant increase in the activity of the biceps femoris with increasing load. However, Manabe et al. reported that the hamstrings were significantly more active during squats performed with a fast repetition velocity than during normal and slow squats.

Effect of back squat techniques

Different squat techniques, including foot position, depth, lumbar posture, and type of load (i.e. conventional loading or accommodating resistance) appear to have little effect on hamstrings EMG activity, although load position appears to have a significant effect. In respect of foot position, Escamilla et al. (2001), Paoli et al. 2009) and McCaw and Melrose (1999) all reported that wide stance squats do not lead to greater hamstrings activity than narrow stance squats. Similarly, Ninos et al. (1997) reported that there was no difference in hamstrings EMG activity when using either a self-selected stance or a stance that was 30 degrees of external rotation from the self-selected position. In respect of depth, Gorsuch et al. (2013) reported that the biceps femoris did not display different EMG activity between partial and parallel squats with the same relative load. Caterisano et al. (2002) also reported that the biceps femoris did not display different EMG activity between partial and parallel squats with the same absolute load (the relative loads used were therefore different). Similarly, Ninos et al. (1997) found no changes in EMG activity with knee flexion angles during the squat, while changes in quadriceps EMG activity were noted. In respect of lumbar posture, Vakos et al. (1994) compared the hamstrings EMG activity during squats with kyphotic and lordotic postures and found no differences between the two variations.

Effect of back squat load type

In respect of load type, Ebben and Jensen (2002) compared hamstrings EMG activity in squats with conventional barbell loading and using barbells in combination with either bands or chains. No differences were observed between the conditions. In respect of load position, Lynn and Noffal (2012) used dumbbells in two different positions (on the shoulders and with arms outstretched) to compare the effect of squatting by “sitting back” with squatting normally. They found that “sitting back” led to much reduced rectus femoris activity and slightly greater gluteus maximus and hamstrings activity. However, whether the same effect would be achieved by simply performing a squat using a different technique with the load in the same position is unclear and further research is needed in this area. In a related study performed not in free-weight squats but with a leg press, Da Silva et al. (2008) explored the differences between high and low foot positions in a horizontal leg press and found that there were no significant differences in hamstrings EMG activity, although there were differences in respect of quadriceps EMG activity.

Why is the squat a poor hamstrings exercise?

Exactly why the squat is a poor exercise for the hamstrings is not entirely clear. It may relate to the bi-articular nature of the hamstrings musculature. While the squat exercise involves hip extension, for which the hamstrings are a prime mover, it also involves knee extension, for which the hamstrings are an antagonist. Yamashita (1988) compared hamstrings EMG activity during isolated hip extension and isolated knee extension movements performed with 20% of the MVIC moment to hamstrings EMG activity with a combined hip and knee extension movement using the same hip and knee extension moments. It was found that hamstrings EMG activity in combined hip and knee extension was only 42% of the level in the isolated hip extension movement, despite the hip extension moment being identical in both cases. It was concluded that hamstrings EMG activity is depressed when combined hip and knee extension are performed compared to during isolated hip extension. This may occur because the hamstrings change length to a greater extent when performing isolated hip extension compared to when performing combined hip and knee extension, where they remain largely the same length. Alternatively, it is plausible (but as yet unexplored in the literature) that the motor strategy during combined hip and knee extension takes into account the need for the quadriceps to counteract the knee flexion moment that would be generated when the hamstrings are activated and consequently hamstrings activity is actively suppressed.

The deadlift

Introduction

The conventional deadlift and its variations (sumo deadlift, RDL, stiff-legged deadlift, and unilateral stiff-legged deadlift) all appear to lead to relatively high levels of hamstrings EMG activity (Wright et al. 1999, Escamilla et al. 2002; Ebben, 2009; Zebis et al. 2013; McAllister et al. 2014).

Effect of deadlift techniques

Few studies have been performed comparing hamstrings EMG activity during the deadlift and its variations while varying load, speed, depth, stance width or variation. Escamilla et al. (2002) compared conventional and sumo deadlifts and found no differences between the two variations. In addition, Bezerra et al. compared the hamstrings EMG activity during the deadlift and stiff-legged deadlift and also reported no differences between the two variations. Nemeth et al. (1984) compared four types of deadlift with a 12.8 kg load, including lifts with straight knees and lifts with flexed knees. While the load was low and therefore only led to small-to-moderate levels of hamstrings EMG activity, the researchers did find that there was a time difference in the hamstrings EMG activity in that in the straight-leg lift the peak activity occurred early in the lift but in the bent-leg lift the peak occurred later on. Ono et al. (2011) assessed hamstrings EMG during a stiff-legged deadlift and reported that the activity levels of the biceps femoris and of the semimembranosus were significantly higher than that of the semitendinosus.

The good morning

The good morning appears to lead to relatively high hamstrings EMG activity (Ebben, 2009; McAllister et al. 2014; Vigotsky et al. 2015). In addition,Vigotsky et al. (2015) tested lateral and medial hamstrings EMG activity with 50%, 60%, 70%, 80%, and 90% of 1RM and reported steadily increasing levels of muscle activity with increasing load. This confirms previous assumptions that the hamstrings are a prime mover in this exercise.

Unconventional exercises

Some researchers have investigated hamstrings EMG activity during less commonly performed exercises. Zebis et al. (2013) measured hamstrings EMG activity separately between the medial and lateral hamstrings during 1-leg glute bridges, two-hand kettlebell swings, Nordic curls, supine slide-board curls, horizontal back extensions, weighted horizontal back extensions, RDLs, seated leg curls, and lying leg curls. It was found that all of the exercises displayed >60% and >50% of peak EMG in the medial and lateral hamstring, respectively. McGill and Marshall (2012) compared different kettlebell exercises and found that the snatch and swing activated the biceps femoris to a similar extent. McGill et al. (2009) compared a variety of strongman exercises and found that the tire flip led to greater biceps femoris EMG activity than other strongman movements, including the Atlas stone lift and log lift. Oliver and Dougherty (2009a) investigated hamstrings EMG activity in the Razor curl, a variant of the Nordic curl, and found that it produced significant hamstrings EMG activity. Oliver and Dougherty (2009b) compared the hamstrings EMG activity produced by the Razor curl and the leg curl. They found that the Razor curl produced similar levels of hamstring EMG activity to the leg curl.

REHABILITATION EXERCISES

Introduction

As with resistance training exercises, hamstrings rehabilitation exercises can involve either hip or knee movement, or both. When both hip and knee movements are involved, exercises involving the hamstrings can cause a wide range of muscle length changes, from very small to very large, through a range of different combinations of joint movements. As with resistance training exercises hamstrings rehabilitation exercises can be placed into one of the following categories:

Hip extension and knee extension (e.g. single-leg squat)

Hip extension with partial knee extension (e.g. single-legdeadlift)

Hip extension without knee movement (e.g. back extension)

Knee flexion without hip movement (e.g. sliding leg curl)

Hip extension and knee flexion (e.g. glute-ham raise)

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Few studies have directly compared hamstrings EMG activity across a range of common rehabilitation exercises. Fewer still have compared exercises from more than one joint movement category. Those studies that have only compared exercises within a single joint movement category (e.g. Beutler et al. 2002) or in exercises that involve joint movements not covered by the above system (e.g. Andersen et al. 2006) have been excluded. From a review of the literature, it is immediately apparent that very few studies have compared rehabilitation exercises in the hip extension and knee flexion, and hip extension without knee movement categories. This may reflect a lack of variation in exercises for the hamstrings being commonly programmed among rehabilitation professionals.

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Unilateral exercises

Studies investigating hamstrings EMG activity during common unilateral exercises have generally found that hamstrings EMG activity is low, particularly when compared to quadriceps EMG activity. For example, Zeller et al. (2003) investigated leg muscle EMG activity during the 1-leg squat and found that hamstrings EMG activity was low, particularly in comparison with quadriceps EMG activity. They noted that the quadriceps-to-hamstrings ratio of EMG activity ranged from 1.2 for females to 3.6 for males. Similarly, Shields et al. (2005) also reported that although hamstrings EMG activity increased with increasing load during 1-leg squats, the quadriceps displayed much greater EMG activity than the hamstrings at all loads, with the quadriceps-to-hamstrings ratio of EMG activity ranging from 2.3 – 3.0. In addition, gender differences may exist in terms of the quadriceps-to-hamstrings ratio of EMG activity during 1-leg exercises. For example, Youdas et al. (2007) found that males but not females displayed greater hamstrings EMG activity than quadriceps EMG activity during the split squat. Similarly, Zeller et al. (2003) found that females displayed a much smaller quadriceps-to-hamstrings ratio of EMG activity in the 1-leg squat (1.2) compared to males (3.6).

Stability and instability

Different support surfaces appear to have some effect on hamstrings EMG activity. Eom et al. (2013) compared the effects of different support surfaces on hamstrings EMG activity during a glute bridge exercise. They found that using a sling to create instability led to twice the hamstrings EMG activity as compared with the stable, ground surface. In contrast, Youdas et al. (2007) did not find any significant differences in hamstrings EMG activity during 1-leg squats performed on stable and labile surfaces. Similarly, Li and Chen (2013) investigated the differences in hamstrings EMG activity when squatting either on the ground or on the Reebok core board with three different loads. They found that the unstable surface had no effect on the EMG activity of the hamstrings.

Pelvic restriction

Pelvic restriction seems to have little effect on hamstrings EMG activity during back extensions. Da Silva et al. (2009a) investigated the effects of pelvic stabilization and degree of hip flexion on hamstring EMG activity during horizontal back extensions. They found a non-significant trend for hamstrings EMG activity to be increased during horizontal back extensions with pelvic restriction. Udermann et al. reported a similar non-significant trend. On the other hand, Da Silva et al. (2009b) found a non-significant trend for decreasing hamstrings EMG activity in the order of: unrestrained, partially restrained, and fully restrained pelvis.

EXERCISES FOR THE MEDIAL AND LATERAL HAMSTRINGS

Introduction

Many studies have compared the medial and lateral hamstrings EMG activity during different exercises with varying results. In general, it appears that leg curls of varying kinds (prone leg curl and supine slide-board curl), back extensions and lunges may be useful for targeting the lateral hamstrings, while kettlebell swings, deadlifts of varying kinds (Romanian and 1-leg), good mornings and glute-ham raises may be superior for targeting the medial hamstrings. Programs aimed at improving the strength and size of the hamstrings muscle group may therefore benefit from including exercises from both of these groups in each training session.

Care should be taken in the interpretation of these findings, as differences may also exist between individual medial and lateral hamstrings muscles. Ono et al. (2010) found that EMG activity of the semitendinosus was significantly higher than that of the semimembranosus during eccentric leg curls and Kubota et al. (2007) found that muscular soreness and signal intensity was greatest in the order semitendinosus > biceps femoris (long head) > semimembranosus following eccentric leg curls. The exact ratio of medial-to-lateral hamstrings EMG activity may therefore depend upon the precise muscles measured. For example, it may be the case that preferential stimulation of the semitendinosus in certain exercises occurs because the muscle is fusiform and is therefore more easily damaged during lengthening exercises than the other more pennate hamstring muscles.

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Foot position

Directing athletes to use internal tibial rotation during certain movements appears to cause greater medial hamstrings EMG activity during a range of different hip extension exercises and movements.

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Ankle position

Since the gastrocnemius is both a knee flexor and a plantar flexor, it is possible that ankle position may affect either the hamstrings EMG activity or knee flexion peak torque during knee flexion movements. However, this remains to be demonstrated in the literature.

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Internal and external cues

Although external cues are widely used, as they appear to enhance performance (see review by Wulf, 2007), the use of internal cues may be useful in order to alter the degree to which the medial and lateral hamstrings are activated during certain movements. Oh et al. (2007) reported that using the Abdominal drawing-in maneuver (ADIM) led to increased medial hamstring EMG activity, while Lewis and Sahrmann (2009) found that using a hamstrings cue led to increased lateral hamstrings EMG activity.

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Hamstrings activity, ADIM and anterior pelvic tilt

The increasing medial hamstrings EMG activity reported by Oh et al. (2007) might not be medial-hamstring-specific, as the EMG activity of the lateral hamstrings was not reported. It is interesting that Oh et al. (2007) noted that the use of the abdominal drawing-in maneuver also led to reduced anterior pelvic tilt during the prone hip extension exercise. Tateuchi et al. (2012) found that during prone hip extension, increased activity of the hip flexor (tensor fasciae latae) relative to that of hip extensors (gluteus maximus and semitendinosus) was significantly associated with increased anterior pelvic tilt. Thus, increased activity of the hip extensors and abdominals both seem to lead to reduced anterior pelvic tilt during hip extension movements.

EFFECTS OF HIP JOINT ANGLE ON HAMSTRINGS EMG ACTIVITY

Several dynamometry studies have been performed to explore the way in which hamstrings EMG activity changes in various hip angles and have reported that changing joint angle has little or no effect. However, there are key differences between the study protocols used in the literature. For example, Lunnen et al. (1981) studied a much greater hip flexion angle (135 degrees) than many of the other researchers (e.g. Mohamed et al. 2002; Guex et al. 2012) and it is possible that the large stretch in this position moved the muscle up the passive arm of the length-tension curve, thereby reducing neural drive. Additionally, Lunnen et al. (1981) made use of surface electrodes while Guex et al. (2012) used fine wire electrodes, which may have also led to differences in the results observed.

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EFFECTS OF KNEE JOINT ANGLE ON HAMSTRINGS EMG ACTIVITY

Several dynamometry studies have been performed to explore the way in which hamstrings EMG activity changes with knee angle and have reported conflicting results. On the one hand, some trials have reported that the hamstrings EMG activity is greatest in the middle of the overall knee joint ROM (Worrell et al. 2001; Higashihara et al. 2010a). Other studies have reported that either medial, lateral or both groups of hamstrings display their greatest EMG activity at one end of the overall joint ROM.

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EFFECTS OF HIP JOINT ANGLE ON HAMSTRINGS EMG ACTIVITY DURING RESISTANCE TRAINING EXERCISES

Several studies have been performed to explore the way in which hamstrings EMG activity changes with hip angle during resistance training exercises. Of note is that Zebis et al. (2012) found that hamstrings EMG activity was greater with increasing hip angle in the Romanian deadlift, 2-hand kettlebell swing and seated leg curl. In contrast, they also found that EMG activity was greater at with reduced hip angle in the supine slide-board curl, prone leg curl, Nordic curl, 1-leg glute bridge, horizontal back extension, and back extension.

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EFFECTS OF KNEE JOINT ANGLE ON HAMSTRINGS EMG ACTIVITY DURING RESISTANCE TRAINING EXERCISES

Several studies have been performed to explore the way in which hamstrings EMG activity changes with knee angle during resistance training exercises. It has been found that during Nordic curls, hamstrings EMG activity is greater when the torso is closer to the ground than when the torso is more upright.

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Implications for hypertrophy

Where exercises display peak hamstrings EMG activity at different degrees of knee flexion, this may imply that they could lead to increases in strength and hypertrophy in different parts of the hamstring muscles. Using magnetic resonance imaging (MRI) scans, Mendiguchia et al. (2013) reported that the signal intensity in various regions of three different hamstring muscles differed depending on the exercise selected. Similar results have been observed in other muscle groups, which have confirmed the association between acute observations of signal intensity (Mendiguchia et al. 2013) with long-term hypertrophic effects (e.g. Wakahara et al. 2013; Bloomquist et al. 2014).

CONCLUSIONS REGARDING THE HAMSTRINGS

The hamstrings display no clear tendency to greater muscle activity at any one joint angle. However, there are differences between individual hamstrings muscles. This suggests that exercises involving peak contractions at a range of joint angles may be optimal

Research is limited regarding the best exercises for the hamstrings. Leg curls are a reliable option, while good mornings, Romanian deadlifts, and Nordic curls (glute-ham raises) are good alternatives

Some exercises appear to target the medial hamstrings to a greater extent (e.g. kettlebell swings, deadlifts, Nordic curls) while other exercises target the lateral hamstrings more (e.g. leg curls and back extensions). Optimal programs may therefore include exercises that target both sub-groups.

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ECCENTRIC TRAINING

PURPOSE

This section provides a summary of the long-term studies performed using eccentric training exercises for the hamstrings, either for injury prevention or for injury rehabilitation.

BACKGROUND

Introduction

Hamstring strain injury is a particularly prevalent form of non-contact injury in many sports involving high-speed running or sprinting. However, the precise point at which the strain injury occurs in the gait cycle remains unclear. It was originally suggested that hamstring strain injury occurred most commonly during the early stance phase, as this is where both knee flexion and hip extension moments are highest (Mann and Sprague, 1980). However, later researchers proposed that hamstring strains most likely occur during the terminal swing phase (just prior to ground contact), as this is where the hamstrings muscles are lengthening quickly (Thelen et al. 2005; Chumanov et al. 2007; Chumanov et al. 2011; Schache et al. 2012). The hamstrings lengthen quickly while the hip is flexing and while the knee is extending because the hamstrings are both hip extensors and knee flexors. Since Lieber and Fridén (1993) have explained that muscle damage is not a function of force but rather of mechanical deformation (i.e. relative change in length), this may suggest that this is the point in the gait cycle that is most dangerous for the hamstrings. While some researchers still argue in favour of either one of these explanations, recent research by Sun et al. (2015) indicates that both may in fact be similarly likely. Sun et al. (2015) noted that their analysis of intersegmental dynamics suggests that the hamstrings experience very high loads in both early stance and late swing phases.

Incidence of hamstring strains

The incidence of hamstring strain injury has been explored in rugby union and American football and ranges between 0.27 – 5.6 injuries per 1,000 exposure hours, depending upon the sport and on the exact definition of exposure.

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Proportion of injuries comprising hamstring strains

The proportion of total injuries comprised of hamstring strains in common team sports varies between 12 – 15% in Australian Rules Football, track and field, and soccer.

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RISK FACTORS FOR HAMSTRING STRAINS

Introduction

Previous reviews have identified that there are many different individual risk factors for hamstring strain injury, which include previous hamstring strain injury, hamstrings weakness and various other factors (see review by Mendiguchia et al. 2011). Mendiguchia et al. (2011) proposed that hamstring strains are not only multifactorial but that each of the individual factors can have an influence on the others, as shown in the diagram below:

Hamstrings strains

 

Previous hamstring strain injury

In reviewing the literature relating to previous hamstring strain injury, Mendiguchia et al. (2011) concluded that previous hamstring strain injury increases the risk of re-injury substantially and suggested that previous hamstring strain injury is likely the greatest individual risk factor for future injury. However, whether this increased risk arises because of some feature of the initial injury or because of a failure to perform sufficient rehabilitation is currently unclear. The odds ratio associated with previous hamstring strain injury ranges between 1.4 – 16.5 times.

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Hamstring weakness: retrospective

Studies exploring the retrospective relationships between hamstring strength and the risk of hamstring strain injury have historically reported conflicting results. Strength measures were traditionally recorded using isokinetic methods but more recent assessments have used isoinertial (eccentric) and isometric tests instead. However, the literature currently appears to indicate that hamstrings weakness, when measured retrospectively, can indicate a greater risk of strain injury.

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Hamstring weakness: prospective

Studies exploring the prospective relationships between hamstring strength and the risk of hamstring strain injury have historically reported conflicting results. Strength measures were traditionally recorded using isokinetic methods but more recent assessments have used isoinertial (eccentric) and isometric tests instead. However, the literature currently appears to indicate that hamstrings weakness, when measured prospectively, can indicate a greater risk of strain injury.

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ECCENTRIC TRAINING FOR HAMSTRING STRAIN INJURY PREVENTION

Introduction

Eccentric training has been proposed as a method of training for the hamstrings that may be useful for preventing hamstring strains from occurring. There are at least two possible reasons why this type of training may be effective for this purpose. Firstly, eccentric training of any muscle has been found to shift the optimum length at which torque is developed in the hamstrings (Brockett et al. 2001). This change in the optimal length at which torque is developed appears to occur because of an increase in length of the individual muscle fibers (sarcomerogenesis). Increasing muscle length may help reduce the risk of strain injury because it allows the muscle fibers to change length more quickly and with less resistance. Secondly, since several studies have found that eccentric strength of the hamstrings is a risk factor for hamstring strain injury, eccentric hamstring training may be useful for addressing this problem. Indeed, eccentric hamstring training has been found to be more effective than concentric hamstring training for improving eccentric hamstring strength (Mjølsnes et al. 2004) as well as hamstring strength overall (Kaminski et al. 1998).

Meta-analysis

The ability of eccentric hamstring training to reduce the incidence of hamstring strain injury was recently subjected to a review and meta-analysis by Goode et al. (2014). The review included 4 of the following trials in order to determine the effect of eccentric hamstring strengthening on the risk of hamstring injury and specifically investigated the effect of intervention non-compliance on outcomes. It was found that while the trials involving eccentric hamstring training did not significantly reduce the risk of hamstring injury (risk ratio of 0.59 times), this was because of significant heterogeneity. Importantly, most of this heterogeneity came from compliance. When considering only those subjects compliant with the eccentric strengthening, the reviewers found an overall significant reduction in hamstring injury risk (risk ratio of 0.35 times) and this effect had little heterogeneity.

Effect of eccentric hamstring training on hamstring strain injury incidence

A small number of studies have explored the effects of eccentric training on novel hamstring strain injury. In these studies, the most commonly-used eccentric hamstring exercise is the Nordic hamstring curl. However, there are several other types of similar exercise, which have been reviewed in detail by Brughelli and Cronin (2008). In general, there is a strong indication that eccentric training for the hamstrings is beneficial for reducing the risk of novel hamstring strain injury.

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Effect of eccentric hamstring training on recurrent hamstring strain injury incidence

A very small number of studies have explored the effects of eccentric training on recurrent hamstring strain injury.There is an extremely strong indication that eccentric training for the hamstrings is beneficial for reducing the risk of recurrent hamstring strain injury and therefore such exercises are strongly recommended for rehabilitation of injured athletes.

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CONCLUSIONS REGARDING THE HAMSTRINGS

Previous hamstring strain increases the risk of an athlete incurring a similar subsequent injury substantially. Therefore, strength and conditioning programs should be designed to prevent hamstring strains happening in the first place.

Eccentric hamstring training, particularly using the Nordic hamstring curl exercise, reduces the incidence of both novel and recurrent hamstring strain injury.

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