Hamstrings

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KEY POINTS

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. As a group, the hamstrings can therefore be trained by exercises that involve either hip extension or knee flexion.

The four hamstrings muscles are: the biceps femoris (long head), the biceps femoris (short head), the semitendinosus, and the semimembranosus. The two biceps femoris muscles are located on the lateral part of the thigh. The semitendinosus and the semimembranosus are located on the medial part of the thigh.

The total volume of the medial hamstrings is greater than that of the lateral hamstrings, the lateral hamstrings are more often injured, but the medial hamstrings are more highly activated during high-speed running. Different exercises may be required to develop the medial and lateral hamstrings, and both groups should be trained for improving sprint running ability.

There are at least two separate regions within the hamstring musculature (upper and lower) that appear to respond differently to the same resistance training exercises. Optimal programming may therefore require multiple exercises to target both regions.

The hamstrings have a large moment arm for hip extension, making them a key hip extensor. They also have a large knee flexion moment arm, making them a key knee flexor. This moment arm increases with increasing knee flexion, making the hamstrings better knee flexors when the knee is bent than when it is extended. This may imply that exercises involving peak contractions when the knee is bent (like leg curls) are more effective at developing 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. Using a range of high and low repetitions, and both slow and fast speeds may be beneficial.

The hamstrings display no clear tendency to greater EMG amplitude at any one joint angle. However, there are differences between individual hamstrings muscles. In contrast to the moment arm findings, this suggests that exercises involving peak contractions at a range of joint angles may be optimal.

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

Some exercises appear to target the medial hamstrings to a greater extent (kettlebell swings and deadlifts) while other exercises target the lateral hamstrings more (leg curls and back extensions). Optimal programs may therefore include a range of exercises that target both medial and lateral hamstrings.

The hamstrings are essential for sprint running performance. Hamstring strains are common in team sports, accounting for around 12 – 16% of injuries. As expected, most strains occur during high-speed running, with the largest proportion affecting the biceps femoris. 

The acute mechanisms producing hamstring strains are unclear. Either fast changes in length in the terminal swing phase or high loading during the early stance phase could be responsible. The mechanisms of recurrent hamstring strain could include alterations in biomechanics, including muscle activation.

Previous hamstring strain increases the risk of an athlete incurring a similar subsequent injury substantially. Strength and conditioning programs should aim to prevent hamstring strains happening in the first place. Eccentric hamstring training, particularly the Nordic hamstring curl exercise, reduces the incidence of both novel and recurrent hamstring strain injury. Compliance is essential in order to prevent recurrent injury.


CONTENTS

Full table of contents

  1. Anatomy
  2. Moment arms
  3. Muscle architecture
  4. Muscle fiber type
  5. Electromyography
  6. Eccentric training
  7. References
  8. Contributors
  9. Provide feedback


ANATOMY

PURPOSE

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

BACKGROUND

Introduction

The hamstrings are important for sporting performance, particularly during high-speed running and sprinting (Higashihara et al. 2010b; Kyröläinen et al. 2005; Morin et al. 2015). 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 & 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. They are usually divided into two groups, the lateral hamstrings (biceps femoris long and short heads) and the medial hamstrings (semitendinosus and the semimembranosus) on the basis of their locations on the rear part of the thigh. 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. The hamstrings as a group can therefore be trained by exercises that involve either hip extension or knee flexion.

Common tendons

Although the hamstrings are generally discussed separately, the semitendinosus and biceps femoris (long head) almost certainly share a proximal origin by way of a conjoined tendon in most people. Additionally, some studies have found that the semimembranosus also shares this same tendon (Neuschwander et al. 2015). When it does have a separate tendon, the proximal tendon of the semimembranosus is located anteriorly and laterally to the shared tendon of the semitendinosus and biceps femoris (long head) (Miller et al. 2007; Philippon et al. 2014; Feucht et al. 2014). In addition, the combined semitendinosus and biceps femoris (long head) footprint on the ischial tuberosity is smaller in length (3.9 ± 0.4 vs. 4.5 ± 0.5cm) and may also be smaller in height (1.4 ± 0.5 vs. 1.2 ± 0.3cm) than the semimembranosus footprint (Feucht et al. 2014).

Origins and insertions

Overall, the origins and insertions of each of the hamstrings are as follows:

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.

Muscle weight

From the limited literature, it is 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 when comparing within studies, although there are discrepancies (Wickiewicz et al. 1983; Ito et al. 2003; Horsman et al. 2007; Ward et al. 2009; Kellis et al. 2012). The weight of the biceps femoris (long head) has been recorded at between 55.8 – 245.0g, the weight of the semimembranosus has been recorded at between 109.3 – 146.0g, the weight of the biceps femoris (short head) has been recorded at between 57.1 – 114.0g, and the weight of the semitendinosus has been measured at between 84.7 – 220g (Wickiewicz et al. 1983; Ito et al. 2003; Horsman et al. 2007; Ward et al. 2009; Kellis et al. 2012).

Muscle cross-sectional area

From the limited 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 (Pohtilla et al. 1969; Ito et al. 2003; Woodley and Mercer, 2005). This is in accordance with the data on muscle weight, which gives some confidence that the relative weights and sizes of these muscles is largely correct.

Muscle thickness

Very little research has examined the muscle thickness of the hamstrings (Ikezoe et al. 2011a; 2011b) and no studies have yet compared the muscle thickness of the different hamstrings muscles to one another. The literature is therefore currently too limited to ascertain whether the muscle thickness of any of the hamstrings muscles is substantially different from the others.

Muscle volume

From the limited 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 (Friederich and Brand, 1990; Miokovic et al. 2011; Nakase et al. 2013). This is in accordance with the data on muscle cross-sectional area and muscle weight, which gives some confidence that the relative weights, sizes and volumes of these muscles are largely correct.

Medial and lateral differences

The medial and lateral hamstrings muscles are different from one another in several respects. They are different in weight (Ito et al. 2003; Horsman et al. 2007; Ward et al. 2009; Kellis et al. 2012), cross-sectional area (Pohtilla et al. 1969; Ito et al. 2003), volume (Friederich and Brand, 1990; Miokovic et al. 2011; Nakase et al. 2013), function, and risk of injury, with the lateral hamstrings being more commonly injured (De Smet et al. 2000; Garrett et al. 1989; Slavotinek et al. 2002). Several studies have found that the medial and lateral hamstrings display differences in EMG amplitude in response to common resistance training exercises (Fiebert et al. 2001; Escamilla et al. 2002; Lynn & Costigan, 2009; Simenz et al. 2012; Jakobsen et al. 2012; Zebis et al. 2013; McAllister et al. 2014). In addition, 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). These findings suggest that different exercises may be required to develop the medial and lateral hamstrings, and that both groups should be trained for improving sprint running ability.

Regional differences

The existence of regions within hamstring muscles have been assessed both by anatomical investigation and by using electromyography (EMG). 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 it, which may be responsible for the production of separate regions within this muscle (Garrett et al. 1989; Woodley and Mercer, 2005; Van de Made et al. 2013). In addition, some studies have explored differences in EMG amplitude between individual regions of the same hamstring muscle using EMG. Schoenfeld et al. (2015) explored the EMG amplitude 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 amplitude in the lower region compared with the stiff-legged deadlift. In contrast, there was no difference between exercises in respect of the upper region. This indicates that different exercises do lead to differences in EMG amplitude in different parts of the individual hamstrings muscles. This in turn provides further evidence that there may be separate regions within each hamstring muscle that may require training with different exercises.

SECTION CONCLUSIONS

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. As a group, the hamstrings can therefore be trained by exercises that involve either hip extension or knee flexion.

The four hamstrings muscles are: the biceps femoris (long head), the biceps femoris (short head), the semitendinosus, and the semimembranosus. The two biceps femoris muscles are located on the lateral part of the thigh. The semitendinosus and the semimembranosus are located on the medial part of the thigh.

There are at least two separate regions within the hamstring muscles (upper and lower) that appear to respond differently to the same resistance training exercises. Optimal programming may therefore require multiple exercises to target both regions.

The total volume of the medial hamstrings is greater than that of the lateral hamstrings, the lateral hamstrings are more often injured, but the medial hamstrings are more highly activated during high-speed running. Different exercises may be required to develop the medial and lateral hamstrings, and both groups should be trained for improving sprint running ability.


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MUSCLE MOMENT ARMS

[Read more about: moments]

PURPOSE

This section provides a summary of the studies into the muscle moment arms of the hamstrings.

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. Since the hamstrings act as both hip extensors and knee flexors, they 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 much greater. It seems likely that the hamstrings and gluteus maximus therefore have similar muscle moment arms to one another for hip extension and are therefore expected to be involved in this joint action 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 (Spoor et al. 1992; Herzog & Read, 1993; Wretenberg et al. 1996; Lu et al. 1996; Buford et al. 1997; Kellis et al. 1999). In general, there is a trend for hamstrings muscle moment arms to increase with increasing knee flexion angle.

SECTION CONCLUSIONS

The hamstrings have a large moment arm for hip extension, making them a key hip extensor. They also have a large knee flexion moment arm, making them a key knee flexor. This moment arm increases with increasing knee flexion, making the hamstrings better knee flexors when the knee is bent than when it is extended. This may imply that exercises involving peak contractions when the knee is bent (like leg curls) are more effective at developing the hamstrings.


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MUSCLE ARCHITECTURE

[Read more about: muscle architecture]

PURPOSE

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

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). Unlike the quadriceps (Blazevich et al. 2006), the hamstrings are a group of muscles that display very different muscle architecture to one another.

Pennation angle

Only a small number of studies have assessed the pennation angle of the hamstrings (Friederich & Brand, 1990; Horsman et al. 2007; Ward et al. 2009; Kellis et al. 2012). The pennation angle of the hamstrings varies slightly between muscle. In general, 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 were different from one another (Friederich & Brand, 1990; Horsman et al. 2007), later studies found no differences (Ward et al. 2009; Kellis et al. 2012).

Fascicle length

Only a small number of studies have assessed the fascicle lengths of the hamstrings (Friederich & Brand, 1990; Horsman et al. 2007; Ward et al. 2009; Kellis et al. 2012; Kumazaki et al. 2012). 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.

Physiological cross-sectional area

Only a small number of studies have assessed the physiological cross-sectional area of the hamstrings (Friederich & Brand, 1990; Horsman et al. 2007; Ward et al. 2009; Kellis et al. 2012). 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).

Medial and lateral hamstrings

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 (Friederich & Brand, 1990; Horsman et al. 2007; Ward et al. 2009; Kellis et al. 2012). 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.

SECTION CONCLUSIONS

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.


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MUSCLE FIBER TYPE

[Read more about: muscle fiber type]

PURPOSE

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

BACKGROUND

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. However, it is not supported by all of the available studies (Johnson et al. 1973; Garrett et al. 1984; Pierrynowski& Morrison, 1985; Dahmane et al. 2005; 2006). Rather, most studies indicate that the hamstrings 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%.

SECTION CONCLUSIONS

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, and both slow and fast speeds may be beneficial.


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ELECTROMYOGRAPHY

[Read more about: electromyography]

PURPOSE

This section provides a summary of the electromyography (EMG) 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)

Comparing hamstrings exercises

Only a small number of studies have directly compared hamstrings EMG amplitude across a range of common resistance-training exercises (Wright et al. 1999; Andersen et al. 2006; Ebben, 2009; McCurdy et al. 2010; Zebis et al. 2013; McAllister et al. 2014; Schoenfeld et al. 2015). Very few of these (Ebben, 2009; Zebis et al. 2013) included exercises from all joint movement categories and yet did not identify 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, while exercises from the hip extension and knee extension category (i.e. squats) never feature as one of the best exercises. 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.

Back squat

Some strength coaches continue to refer to the back squat as a useful exercise for the hamstrings. However, the literature does not provide 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 & Melrose, 1999; Escamilla et al. 2001; Manabe et al. 2007; Paoli et al. 2009; Li & Chen, 2013; Aspe & Swinton, 2014; Yavuz et al. 2015; Contreras et al. 2015) and it is also apparent that hamstrings EMG amplitude does not always increase with increasing external load (Savelberg et al. 2007; Li & Chen, 2013).

Effect of load and speed

Some research has reported that hamstrings EMG amplitude does not increase to the same extent as the EMG amplitude 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 EMG amplitude of most of the lower body muscles increased accordingly, although the increase in the EMG amplitude of the biceps femoris was less marked than that of the other muscles and only significantly increased with the largest load increment. Li & Chen (2013) investigated the differences in EMG amplitude of the lower body muscles during back squats with increasing load and found that although the EMG amplitudes 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 EMG amplitude of the biceps femoris with increasing load. However, Manabe et al. (2007) 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 amplitude, 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 EMG amplitude than narrow stance squats. Similarly, Ninos et al. (1997) reported that there was no difference in hamstrings EMG amplitude 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 amplitude 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 amplitude 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 amplitude with knee flexion angles during the squat, while changes in quadriceps EMG amplitude were noted. Using the same relative loads for each squat depth, Contreras et al. (2015) found that there was no difference in biceps femoris EMG amplitude between parallel and full squats. In respect of lumbar posture, Vakos et al. (1994) compared the hamstrings EMG amplitude 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 & Jensen (2002) compared hamstrings EMG amplitude 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 & 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 EMG amplitude and slightly greater gluteus maximus and hamstrings EMG amplitudes. 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 amplitude, although there were differences in respect of quadriceps EMG amplitude.

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 amplitude during isolated hip extension and isolated knee extension movements performed with 20% of the MVIC moment to hamstrings EMG amplitude with a combined hip and knee extension movement using the same hip and knee extension moments. It was found that hamstrings EMG amplitude 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 amplitude 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 EMG amplitude 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 amplitude (Wright et al. 1999, Escamilla et al. 2002; Ebben, 2009; Zebis et al. 2013; McAllister et al. 2014; Schoenfeld et al. 2015).

Effect of deadlift techniques

Few studies have been performed comparing hamstrings EMG amplitude 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. (2013) compared the hamstrings EMG amplitude 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 amplitude, the researchers did find that there was a time difference in the hamstrings EMG amplitude in that in the straight-leg lift the peak EMG amplitude occurred early in the lift but in the bent-leg lift the peak occurred later on. Ono et al. (2011) assessed hamstrings EMG amplitude during a stiff-legged deadlift and reported that the EMG amplitudes 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 amplitude (Ebben, 2009; McAllister et al. 2014; Vigotsky et al. 2015). In addition,Vigotsky et al. (2015) tested lateral and medial hamstrings EMG amplitude with 50%, 60%, 70%, 80%, and 90% of 1RM and reported steadily increasing levels of EMG amplitude 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 amplitude during less commonly performed exercises. Zebis et al. (2013) measured hamstrings EMG amplitude 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 amplitude 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 amplitude than other strongman movements, including the Atlas stone lift and log lift. Oliver and Dougherty (2009a) investigated hamstrings EMG amplitude in the Razor curl, a variant of the Nordic curl, and found that it produced significant hamstrings EMG amplitude. Oliver and Dougherty (2009b) compared the hamstrings EMG amplitude produced by the Razor curl and the leg curl. They found that the Razor curl produced similar levels of hamstring EMG amplitude 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)

Few studies have directly compared hamstrings EMG amplitude across a range of common rehabilitation exercises (Cook et al. 1992; Graham et al. 1993; Ayotte et al. 2007; Begalle et al. 2012; Orishimo et al. 2015; Youdas et al. 2015; Tsaklis et al. 2015). Few (if any) 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 included any 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. It is noteworthy that in the studies that included exercises involving knee flexion without hip movement (Graham et al. 1993; Orishimo & McHugh, 2015; Tsaklis 2015), these exercises produced the greatest hamstrings EMG amplitude. This is the same finding as for resistance training exercises, where isolated knee flexion exercises (such as leg curls) produced the best results with the greatest regularity.

Unilateral exercises

Studies investigating hamstrings EMG amplitude during common unilateral exercises have generally found that hamstrings EMG amplitude is low, particularly when compared to quadriceps EMG amplitude. For example, Zeller et al. (2003) investigated leg muscle EMG amplitude during the 1-leg squat and found that hamstrings EMG amplitude was low, particularly in comparison with quadriceps EMG amplitude. They noted that the quadriceps-to-hamstrings ratio of EMG amplitude ranged from 1.2 for females to 3.6 for males. Similarly, Shields et al. (2005) also reported that although hamstrings EMG amplitude increased with increasing load during 1-leg squats, the quadriceps displayed much greater EMG amplitude than the hamstrings at all loads, with the quadriceps-to-hamstrings ratio of EMG amplitude ranging from 2.3 – 3.0. In addition, gender differences may exist in terms of the quadriceps-to-hamstrings ratio of EMG amplitude during 1-leg exercises. For example, Youdas et al. (2007) found that males but not females displayed greater hamstrings EMG amplitude than quadriceps EMG amplitude during the split squat. Similarly, Zeller et al. (2003) found that females displayed a much smaller quadriceps-to-hamstrings ratio of EMG amplitude 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 amplitude. Eom et al. (2013) compared the effects of different support surfaces on hamstrings EMG amplitude during a glute bridge exercise. They found that using a sling to create instability led to twice the hamstrings EMG amplitude as compared with the stable, ground surface. In contrast, Youdas et al. (2007) did not find any significant differences in hamstrings EMG amplitude during 1-leg squats performed on stable and labile surfaces. Similarly, Li and Chen (2013) investigated the differences in hamstrings EMG amplitude 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 amplitude of the hamstrings.

Pelvic restriction

Pelvic restriction seems to have little effect on hamstrings EMG amplitude during back extensions. Da Silva et al. (2009a) investigated the effects of pelvic stabilization and degree of hip flexion on hamstring EMG amplitude during horizontal back extensions. They found a non-significant trend for hamstrings EMG amplitude 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 amplitude 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 amplitude during different exercises with varying results (Fiebert et al. 2001; Escamilla et al. 2002; Lynn & Costigan, 2009; Simenz et al. 2012; Jakobsen et al. 2012; Zebis et al. 2013; McAllister et al. 2014). 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 (Fiebert et al. 2001; Lynn & Costigan, 2009; Jakobsen et al. 2012; Zebis et al. 2013), 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 (Lynn & Costigan, 2009; Zebis et al. 2013; McAllister et al. 2014). 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 amplitude 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 amplitude 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.

Foot position

Directing athletes to use internal tibial rotation during certain movements appears to cause greater medial hamstrings EMG amplitude during a range of different hip extension exercises and movements (Fiebert et al. 1992; 1997; Mohamed et al. 2003; Lynn & Costigan, 2009; Jónasson et al. 2015).

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 amplitude or knee flexion peak torque during knee flexion movements. However, this remains to be demonstrated in the literature (Croce et al. 2000).

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 amplitude, while Lewis & Sahrmann (2009) found that using a hamstrings cue led to increased lateral hamstrings EMG amplitude.

Hamstrings EMG amplitude, ADIM and anterior pelvic tilt

The increasing medial hamstrings EMG amplitude reported by Oh et al. (2007) might not be medial-hamstring-specific, as the EMG amplitude 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 EMG amplitude 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 EMG amplitude 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 AMPLITUDE

Several dynamometry studies have been performed to explore the way in which hamstrings EMG amplitude changes in various hip angles and have generally reported that changing joint angle has little or no effect (Lunnen et al. 1981; Worrell et al. 2001; Mohamed et al. 2002; Guex et al. 2012). 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.

EFFECTS OF KNEE JOINT ANGLE ON HAMSTRINGS EMG AMPLITUDE

Several dynamometry studies have been performed to explore the way in which hamstrings EMG amplitude changes with knee angle and have reported conflicting results (Andriacchi et al. 1983; Fiebert et al. 1996; Worrell et al. 2001; Onishi et al. 2002; Croce et al. 2006; Higashihara et al. 2010a; Kwon & Lee, 2013; Kumazaki et al. 2013). On the one hand, some trials have reported that the hamstrings EMG amplitude is greatest in the middle of the overall knee joint ROM (Worrell et al. 2001; Higashihara et al. 2010a). In contrast, other studies have reported that either medial, lateral or both groups of hamstrings display their greatest EMG amplitude at one end of the overall joint ROM. The effect of knee joint angle on hamstrings EMG amplitude is therefore currently unclear.

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

Several studies have been performed to explore the way in which hamstrings EMG amplitude changes with hip angle during resistance training exercises (Da Silva  et al. 2009a; Zebis et al. 2013). Of note is that Zebis et al. (2012) found that hamstrings EMG amplitude 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 amplitude 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.

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

Several studies have been performed to explore the way in which hamstrings EMG amplitude changes with knee angle during resistance training exercises (Iga et al. 2012; Zebis et al. 2013). It has been found that during Nordic curls, hamstrings EMG amplitude is greater when the torso is closer to the ground than when the torso is more upright.

Implications for hypertrophy

Where exercises display peak hamstrings EMG amplitude 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. (2013b) 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. 2013b) with long-term hypertrophic effects (e.g. Wakahara et al. 2013; Bloomquist et al. 2014).

SECTION CONCLUSIONS

The hamstrings display no clear tendency to greater EMG amplitude at any one joint angle. However, there are differences between individual hamstrings muscles. In contrast to the moment arm findings, 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 hamstring curls (glute-ham raises) are good alternatives.

Some exercises appear to target the medial hamstrings to a greater extent (e.g. kettlebell swings and deadlifts) 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. Hamstring strains account for around 12 – 16% of injuries in popular team sports (Woods et al. 2004; Orchard & Seward, 2002). Most such strains seem to occur during high-speed running (Brooks et al. 2006) in diverse locations throughout the muscle (De Smet et al. 2000; Koulouris & Connell, 2003) and the largest proportion occur in the biceps femoris (De Smet et al. 2000; Garrett et al. 1989; Slavotinek et al. 2002; Ekstrand et al. 2012). This is likely because of the very central role that the hamstrings play in sprinting and the development of the horizontal force that is needed to accelerate and maintain maximal speed (Morin et al. 2015). Hamstring strain injury can be a serious problem for team sports because of the time lost to training and match play for key athletes. Although the mean time to return-to-sport is often reported as only around a week (Cross et al. 2015), the severity of the injury can differ very widely (Cross et al. 2015). Ekstrand et al. (2012) found that the grade of the hamstring strain was a key determinant of the time to return-to-sport, with grades from 1 – 4 requiring an average of 8 ± 3 days, 17 ± 10 days, 22 ± 11 days, and 73 ± 60 days, respectively.

HAMSTRING STRAIN INJURY BIOMECHANICS

Novel hamstring strain injuries

The point at which a hamstring 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 in the terminal swing phase (just prior to ground contact), as this is where the hamstrings muscles are lengthening quickly and reach peak length (Thelen et al. 2005; Chumanov et al. 2007; Chumanov et al. 2011; Schache et al. 2012; Higashihara et al. 2014) and is also where the biceps femoris (long head) displays a peak in EMG amplitude (Higashihara et al. 2014). 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.

Recurrent hamstring strain injuries

The risk factors for recurrent hamstring strain are likely multifactorial, with many factors influencing others (see review by Mendiguchia et al. 2011). Some research has identified that athletes who have previously incurred a hamstring strain injury tend to display altered biomechanics in comparison with athletes who have never experienced such an injury. In particular, injured athletes tend to display reduced biceps femoris EMG amplitude both during eccentric isokinetic testing (Sole et al. 2011; Opar et al. 2013b) and during the Nordic hamstring curl exercise (Bourne et al. 2015) and may also display lower biceps femoris EMG amplitude during running in comparison with other muscles in the trunk and thigh than uninjured athletes (Daly et al. 2015), although there are conflicting findings in this respect (Silder et al. 2011). There are also indications that injured athletes tend to display greater peak anterior pelvic tilt and peak hip flexion on the injured side than on the uninjured side during running, while uninjured athletes do not (Daly et al. 2015). The greater anterior pelvic tilt and peak hip flexion on the injured side may lead to a greater maximum length of this hamstring muscle during running, which may predispose them to greater risk of recurrent hamstring strain injury. However, other investigations have found no differences in the length of the biceps femoris muscle during sprint running between injured and uninjured athletes (Silder et al. 2011).

HAMSTRING STRAIN INJURY EPIDEMIOLOGY

Incidence

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 (Brooks et al. 2006; Elliott et al. 2011).

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 (Seward et al. 1993; Bennell et al. 1996; Orchard & Seward, 2002; Woods  et al. 2004).

RISK FACTORS FOR HAMSTRING STRAINS

Introduction

Overall, the factors that drive hamstring strains and the optimal strategies for rehabilitation remain largely unclear (Mendiguchia et al. 2011; Brukner, 2015). Indeed, 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 (Mendiguchia et al. 2011). This links into the common strategies for rehabilitation which frequently take multiple factors into account (Valle et al. 2015). Mendiguchia et al. (2011) proposed that hamstring strains are not only multifactorial, but also that each of the individual factors can have an influence on the others.

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 (Orchard et al. 1997; Bennell et al. 1998; Arnason et al. 2004; Hägglund et al. 2006; Gabbe  et al. 2006a; Engebretsen et al. 2010), while the relative risk ranges between 2.1 – 2.4 times (Orchard, 2001; Opar et al. 2014).

Hamstring weakness

Studies exploring the retrospective relationships between hamstring strength and the risk of hamstring strain injury have historically reported conflicting results (Worrell et al. 1991; Brockett et al. 2004; Opar et al. 2013a; Opar et al. 2013b; Timmins et al. 2014). Strength measures were traditionally recorded using isokinetic methods (Worrell et al. 1991; Brockett et al. 2004; Opar et al. 2013b) but some more recent assessments have used isoinertial (eccentric) and isometric tests instead (Opar et al. 2013a; Timmins et al. 2014). Similarly, studies exploring the prospective relationships between hamstring strength and the risk of hamstring strain injury have also reported conflicting results (Orchard et a l. 1997; Bennell et al. 1998; Sugiura  et al. 2008; Croisier et al. 2008; Yeung et al. 2009; Opar et al. 2014; Goossens et al. 2014). Again, strength measures were traditionally recorded using isokinetic methods (Orchard et a l. 1997; Bennell et al. 1998; Sugiura et al. 2008; Croisier et al. 2008; Yeung et al. 2009) but more recent assessments have used isoinertial (eccentric) and isometric tests instead (Opar et al. 2014; Goossens et al. 2014). Overall, the literature indicates that hamstrings weakness, when measured both retrospectively and prospectively, can indicate a greater risk of strain injury.

ECCENTRIC TRAINING FOR HAMSTRING STRAIN PREVENTION AND REHABILITATION

Introduction

Eccentric training has been proposed as a method of training for the hamstrings that may be useful both for preventing hamstring strains from occurring and for rehabilitation of hamstring strain injury after is has occurred. 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).

Nordic hamstring curl

INTRODUCTION

The Nordic hamstring curl is the primary exercise used for performing eccentric training of the hamstring musculature during long-term trials investigating hamstring strain injury prevention (Gabbe et al. 2006b; Engebretsen et al. 2008; Arnason et al. 2010; Petersen et al. 2011; Van der Horst et al. 2015) although a range of others have also been developed that may also be suitable (Askling et al. 2013; Orishimo & McHugh, 2015). Consequently, a number of investigations have explored this exercise (Small et al. 2009; Iga et al. 2012; Zebis et al. 2013; Mendiguchia et al. 2013a; 2013b; Ditroilo et al. 2013; Bourne et al. 2015; Marshall et al. 2015). Additionally, it is commonly recommended as the primary exercise to perform in order to prevent and rehabilitate hamstring strain injury (Schmitt & McHugh, 2012; Bahr et al. 2015). This is important, as few other conservative treatments have any support (Reurink et al. 2011). Despite this common advice, the majority of elite soccer teams fail to use the Nordic hamstring curl in either prevention or rehabilitation programs, which may explain the continued high incidence of both novel and recurrent hamstring strain injury (Bahr et al. 2015).

EMG AMPLITUDE

Exploring the EMG amplitude of the hamstrings during the Nordic hamstring curl, Iga et al. (2012) found that EMG amplitude of the hamstrings was higher when the knee was extended than when the knee was flexed, indicating that the exercise trains the hamstrings at long muscle lengths. However, Zebis et al. (2013) did not find any effect of joint angle on EMG amplitude during the Nordic hamstring curl. Bourne et al. (2015) found that the Nordic hamstring curl produced preferentially higher semitendinosus EMG amplitude; but again, Zebis et al. (2013) did not report any preferential activation; Mendiguchia et al. (2013a) reported preferential biceps (short head) activation; and Ditroilo et al. (2013) reported that biceps femoris EMG amplitude exceeded maximum voluntary eccentric contraction levels by some margin. Therefore, whether there is any difference between medial and lateral hamstrings EMG amplitudes in the Nordic hamstring curl (and whether it in fact matters) remains unclear.

EFFECTS OF FATIGUE

Exploring multiple sets of the Nordic hamstring curl exercise, Marshall et al. (2015) noted that a single set of 5 repetitions led to substantial reductions in peak eccentric knee flexion moments during the exercise, with even further reductions in subsequent sets, implying that performing the Nordic hamstring curl prior to practice or other exercise might not be advisable. Nevertheless, training under fatigued conditions may have benefits if carefully managed. Small et al. (2009) found that long-term training using the Nordic hamstring curl either before or after practice had different effects. Training before practice led to greater strength gains being displayed when measured before a simulated game but training after practice led to greater strength gains being displayed when measured after the simulated game. This indicates that performing hamstrings training under conditions of fatigue may benefit the demonstration of hamstrings strength under fatigued conditions.

Meta-analysis

The effects of eccentric hamstring training on 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. This finding is supported by more recent investigations (Tyler et al. 2015), where athletes who were compliant with an eccentric training rehabilitation program did not incur any recurrent hamstring strain injury after a mean follow-up period of 24 ± 12 months, whereas 4 of 8 non-compliant athletes (50%) incurred a recurrent hamstring strain (Tyler et al. 2015).

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 (Askling et al. 2003; Gabbe et al. 2006b; Engebretsen et al. 2008; Arnason et al. 2008; Petersen et al. 2011; Van Van der Horst et al. 2015). In these studies, the most commonly-used eccentric hamstring exercise is the Nordic hamstring curl (Gabbe et al. 2006b; Engebretsen et al. 2008; Arnason et al. 2008; Petersen et al. 2011; Van Van der Horst et al. 2015). However, there are several other similar exercises, which have been reviewed in detail by Brughelli and Cronin (2008), and which may also be valuable. The odds ratio for hamstring strain injury between players taking part in the injury prevention program and those not taking parts ranged between 0.13 – 0.28 times as likely, while the relative risks ranged between 0.30 – 1.55). Overall, there is a strong indication that eccentric training for the hamstrings is beneficial for reducing the risk of novel hamstring strain injury.

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 (Petersen et al. 2011; Askling et al. 2013; Tyler et al. 2015). There is an extremely strong indication that eccentric training for the hamstrings is beneficial for reducing the risk of recurrent hamstring strain injury. The relative risk is 0.14 (Petersen et al. 2011), the period of time to return to sport was shorter (28 ± 15 days vs. 51 ± 21 days), and there is no recurrence in athletes who are compliant to the program, while there is a 50% re-injury rate in non-compliant athletes (Tyler et al. 2015). Therefore, eccentric training for the hamstrings is strongly recommended for the  rehabilitation of injured athletes.

SECTION CONCLUSIONS

The hamstrings are essential for sprint running performance. Hamstring strains are common in team sports, accounting for around 12 – 16% of injuries. As expected, most strains occur during high-speed running, with the largest proportion affecting the biceps femoris. 

The acute mechanisms producing hamstring strains are unclear. Either fast changes in length in the terminal swing phase or high loading during the early stance phase could be responsible. The mechanisms of recurrent hamstring strain could include alterations in biomechanics, including muscle activation.

Previous hamstring strain increases the risk of an athlete incurring a similar subsequent injury substantially. Strength and conditioning programs should aim to prevent hamstring strains happening in the first place. 

Eccentric hamstring training, particularly the Nordic hamstring curl exercise, reduces the incidence of both novel and recurrent hamstring strain injury. Compliance with eccentric hamstring training is essential to prevent hamstring strain injury.


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