How can we best train the hamstrings?

How can we design a strength training program that will maximize the growth of the hamstrings? What factors do we need to take into consideration, and how do each of these factors affect the different variables within the training program?

What information do we need?

We can use the research literature to enhance our training programs if we search for information about the gross anatomy, regional anatomy, and internal moment arm lengths of a muscle, in addition to its working sarcomere lengths, and susceptibility to muscle damage. Each of these factors provides information that is useful for different reasons (read more).

• Gross anatomy describes the locations of the attachments of the muscle to the skeleton. Learning the basic anatomy of a muscle helps us figure out suitable exercises, and also helps us see how we might alter them to target different muscles within a group.
• Regional anatomy describes the way in which a muscle divides into several internal regions, and this tells us whether we are going to need multiple exercises to train the muscle.
• The internal moment arm lengths of a muscle determine its leverage on the joint, and therefore its contribution to a joint moment, relative to other agonist muscles. This allows us to see where peak force in an exercise joint range of motion needs to be, to target one muscle within a group (or one region of a muscle). We can alter the point where peak force occurs by our exercise selection and by our choice of external resistance type.
• The working sarcomere lengths describe the lengths of the sarcomeres inside a muscle over its joint angle range of motion. It allows us to see if the muscle can experience (1) active insufficiency (and so will be trained poorly by exercises involving peak forces at very short muscle lengths), and (2) stretch-mediated hypertrophy (and so will be trained more effectively by exercises involving peak forces at very long muscle lengths).
• The susceptibility of a muscle to damage is how easily a muscle is damaged by a workout. It is affected by: (1) muscle fiber type proportion, (2) its level of voluntary activation, and (3) the working sarcomere lengths of its muscle fibers. The amount of muscle damage that a muscle experiences after a workout is the main determinant of the frequency (and volume) we can use for training it, and it also influences our choice of exercises (single-joint vs. multi-joint, single-limb vs. multi-limb, and full vs. partial range of motion).

#1. Anatomy

There are two separate groups of hamstrings muscles, the medial hamstrings (the semimembranosus and the semitendinosus) and the lateral hamstrings (the biceps femoris long and short heads). By volume, the semimembranosus is the largest individual hamstrings muscle, followed by the biceps femoris long head. In line with measurements of volume, the semimembranosus has the largest physiological cross-sectional area, followed by the biceps femoris long head. The biceps femoris short head and the semitendinosus are both similarly small, with small physiological cross-sectional areas.

The two hamstrings muscles with the largest physiological cross-sectional areas are hemi-pennate, and display a greater average pennation angles and shorter normalized muscle fiber lengths than the two smaller muscles. While their muscle fibers do still have a largely parallel arrangement, they run at an angle from the deep surface of the muscle at the origin to its superficial surface at the insertion. In contrast, the semitendinosus is fusiform, which means that its muscle fibers run entirely longitudinally, albeit the muscle is separated into proximal and distal regions by a tendinous inscription. Thus, it has a long muscle fiber length and a smaller average pennation angle. The biceps femoris short head displays an odd pattern of muscle architecture due to its trapezoidal shape, and so has a relatively large average pennation angle as well as a long normalized fiber length.

The medial hamstrings both have their origin on the ischial tuberosity, which is found on the inferior and posterior side of the pelvis. While both medial hamstrings insert on the medial side of the tibia, the semimembranosus inserts on the posteromedial surface of the medial tibial condyle, while the semitendinosus inserts slightly lower down, on the anterior, medial surface of the tibia. Consequently, the medial hamstrings are able to extend the hip, flex the knee, and also internally rotate the hip (which can be observed as rotating the foot inwards).

Unlike the medial hamstrings, the lateral hamstrings have different origins from one another. Like the medial hamstrings, the long head of the biceps femoris originates on the ischial tuberosity, which is found on the inferior and posterior side of the pelvis. However, the short head originates on the lower half of the linea aspera and the lateral condyloid ridge of the femur. The two heads merge together and insert on the lateral condyle of the tibia and also on the fibula. Consequently, only the long head of the biceps femoris can extend and externally rotate the hip (which can be observed as rotating the foot outwards), while both heads can flex the knee.

Indeed, research using electromyography (EMG) to assess muscle activation during various exercises has found that rotating the hip externally to turn the foot outwards during leg curls, single-leg deadlifts, and glute bridges can lead to increased lateral relative to medial hamstrings activation, and conversely that rotating the hip internally to turn the foot inwards during leg curls, single-leg deadlifts, and single-leg glute bridges can lead to increased medial relative to lateral hamstrings activation. Consequently, it seems that foot position is a valid method of preferentially targeting the medial or lateral hamstrings muscle groups during many hamstrings exercises.

Practical implications

In practice, the hamstrings can be divided into two main groups (medial and lateral), with slightly different origins and insertions. Both the medial and lateral hamstrings can extend the hip and flex the knee. Also, the medial hamstrings can internally rotate the hip, while the lateral hamstrings can externally rotate the hip. Rotating the foot outwards during hamstrings exercises will preferentially activate the lateral hamstrings, while rotating the foot inwards can be used to preferentially activate the medial hamstrings.

#2. Regional anatomy

While each of the hamstrings are innervated separately, each hamstrings muscle also contains anatomically different compartments or regions with differing muscle architecture, which are innervated separately.

Among the medial hamstrings, the semimembranosus has at least two, if not three distinct anatomical regions (proximal, middle and distal), which are innervated by different primary branches of the same nerve. Similarly, the semitendinosus contains two anatomical regions (proximal and distal), and these are innervated by totally separate nerves. Among the lateral hamstrings, the biceps femoris (long head) contains two anatomical regions that are innervated by nerve branches, but these are positioned deep and superficial to one another, instead of proximal and distal to each other. The biceps femoris (short head) contains two (or perhaps three) anatomical regions that are innervated by different nerves. These regions are again positioned deep and superficial to one another.

To a certain extent, the regional innervation of the hamstrings muscles seems to be reflected in their activation in hip extension and knee flexion exercises, although analysis has focused on proximal-to-distal differences for both medial and lateral muscle groups. Indeed, the proximal and lower distal regions of the hamstrings may be activated differently, according to the type of exercise performed. Early research indicated that knee flexion (leg curls) may activate the distal regions of lateral and medial hamstrings to a greater extent, while hip extension exercises may activate the proximal regions more. More recent research has found that which region is most strongly activated during strength training differs more between muscles, and less between exercises, although there do yet seem to be some meaningful differences.

Some research also indicates that there are differences in the activation of the proximal and distal regions of the biceps femoris long head during high-speed running. Specifically, it has been noted that the distal region involves higher activation in the terminal swing phase than the proximal region, possibly due to the high degree of fascicle elongation (whether the relative activation of the regions alters with running speed is less clear). In line with this, sprinters may display greater distal region hamstrings muscle size relative to control subjects. This may be yet another reason to consider implementing leg curls for sprinting performance.

Practical implications

In practice, the hamstrings can be subdivided into separate compartments based on their innervation and anatomy. The medial hamstrings subdivide into proximal and distal regions, while the lateral hamstrings may subdivide both into proximal and distal regions and also into deep and superficial regions. Multiple exercises may be necessary to target all of the areas of each muscle. Knee flexion (leg curls) may be optimal for the distal regions, while hip extension exercise may preferentially target the proximal regions.

#3. Internal moment arm lengths

Hip extension

Overall, the semitendinosus has the longest internal moment arm length of all the hamstrings acting to produce hip extension, but differences between the hamstrings muscles are only really apparent in higher degrees of hip flexion. In contrast, closer to full hip extension, the internal moment arm lengths of the hamstrings are extremely similar to one another. Consequently, hip extension exercises involving peak forces at full hip extension (like supine, straight-leg bridge variations) will likely involve all of the hamstrings to a similar extent. In contrast, hip extension exercises that involve peak forces in greater degrees of hip flexion (like deadlift variations) will likely involve the semitendinosus to a greater extent.

Knee flexion

Overall, the semitendinosus has the longest internal moment arm length of all the hamstrings acting at the knee joint. It therefore has the best leverage for producing knee flexion movements (although this effect is most noticeable in greater degrees of knee flexion). This may be why many studies have found that the semitendinosus is the most active muscle during leg curl variations.

Each of the hamstrings displays an internal moment arm length that peaks at a slightly different knee joint angle. The semimembranosus peaks at full knee extension, the biceps femoris long head peaks between 50–60 degrees of knee flexion, the biceps femoris short head peaks between 70–80 degrees of knee flexion, and the semitendinosus peaks between 90–100 degrees of knee flexion. Nevertheless, because the semitendinosus generally always has a longer internal moment arm than the other muscles, the only meaningful effects of these differences is that the semitendinosus and semimembranosus have similar leverage at full knee extension, whereas the semitendinosus has greater leverage in all other knee angles. Consequently, knee flexion exercises involving peak forces at full knee extension (such as modified Nordic curls with assistance to allow the lifter to perform a concentric phase) will likely involve all of the medial hamstrings to a similar extent. In contrast, knee flexion exercises that involve peak forces in greater degrees of knee flexion (like leg curls against elastic resistance) will likely involve the semitendinosus to a greater extent.

Practical implications

In practice, it is difficult to target each of the hamstrings by manipulating the exercise or the type of external resistance, because the muscles have very similar leverages relative to one another at all joint angles. However, the semitendinosus has better leverage than the other hamstrings when peak forces are exerted in greater degrees of hip flexion in hip extension exercises (as in deadlifts), and when peak forces are exerted in greater degrees of knee flexion in knee flexion exercises (such as in leg curls against elastic resistance).

#4. Working sarcomere lengths

The working sarcomere length ranges of the hamstrings have been estimated by researchers, and it seems that there are meaningful differences between the four muscles.

Overall, it seems that the semitendinosus differs substantially from all of the other hamstrings, since it works exclusively on the descending limb of the length-tension relationship. This could help explain why many studies have found that the semitendinosus is more strongly activated than the other hamstrings muscles during eccentric leg curl variations, although the semitendinosus does not actually reach as long a sarcomere length as the semimembranosus. Regardless, the working sarcomere length range of the semitendinosus means that it cannot experience active insufficiency when the muscle is shortened to its maximum extent, such as during bridge or hip thrust exercise variations, which involve hip extension while the knee is flexed. However, it can easily experience stretch-mediated hypertrophy.

In contrast, the semimembranosus and the biceps femoris (long and short heads) work on the ascending limb, the plateau, and the descending limb of the length-tension relationship. Thus, they are able to experience both active insufficiency and also stretch-mediated hypertrophy. Even so, the extent to which each of the four hamstrings travel down the descending limb of the length-tension relationship (and experience stretch-mediated hypertrophy) differs. The semimembranosus travels furthest, then the semitendinosus, and finally the biceps femoris. Consequently, exercises involving stretching the hamstrings using long ranges of motion may lead to the greatest mechanical tension (and therefore hypertrophy) in the semimembranosus, followed by the semitendinosus, and finally the biceps femoris.

Practical implications

In practice, the semitendinosus cannot experience active insufficiency while the other hamstrings can. This may mean that the semitendinosus is trained during exercises like the glute bridge and hip thrust, which involve peak forces being exerted while the hamstrings are at very short lengths. Additionally, while all of the hamstrings can display stretch-mediated hypertrophy, there are differences in the extent to which the sarcomeres of each muscle reach onto the descending limb. The semimembranosus may be most strongly affected by training the hamstrings at long lengths, followed by the semitendinosus, and finally the biceps femoris.

#5. Susceptibility to muscle damage

Most lifters anecdotally report that the hamstrings take a longer time to recover from than the quadriceps, and this is confirmed by the literature. This slower recovery rate could be caused either by a greater proportion of fast twitch fibers or by a tendency to reach a higher level of voluntary activation.

Even though the hamstrings are often believed to be more fast twitch than the other leg muscles, the literature overall does not support this idea. While it is true that one early study suggested that the hamstrings were slightly more fast twitch than other muscles, other investigations have shown that they actually display a fairly balanced slow and fast twitch fiber proportion, much like the quadriceps. Consequently, it is more likely that the hamstrings are more easily damaged than the quadriceps due to a higher level of voluntary activation being reached. Indeed, a recent study reported that the level of voluntary activation for the hamstrings was extraordinarily high.

Practical implications

In practice, the extremely high levels of voluntary activation of the hamstrings make it relatively easy to damage, despite its moderate fiber type. Thus, it recovers more slowly than the other lower body muscles, although not as slowly as the upper body muscles, which have both high levels of voluntary activation and a fast twitch fiber proportion. Therefore, it may be appropriate to train the hamstrings less frequently than the quadriceps.

What is the takeaway?

The hamstrings can be divided into two main groups (medial and lateral), with slightly different origins and insertions. Both the medial and lateral hamstrings can extend the hip and flex the knee. Also, the medial hamstrings can internally rotate the hip, while the lateral hamstrings can externally rotate the hip. Rotating the foot outwards during hamstrings exercises will preferentially activate the lateral hamstrings, while rotating the foot inwards can be used to preferentially activate the medial hamstrings.

The hamstrings can be subdivided into separate compartments based on their innervation and anatomy. The medial hamstrings subdivide into proximal and distal regions, while the lateral hamstrings may subdivide both into proximal and distal regions and also into deep and superficial regions. Multiple exercises may be necessary to target all of the areas of each muscle. Knee flexion (leg curls) may be optimal for the distal regions, while hip extension exercise may preferentially target the proximal regions.

It is difficult to target each of the hamstrings by manipulating the exercise or the type of external resistance, because the muscles have very similar leverages relative to one another at all joint angles. Yet, the semitendinosus has better leverage than the other hamstrings when peak forces are exerted in greater degrees of hip flexion in hip extension exercises (as in deadlifts), and when peak forces are exerted in greater degrees of knee flexion in knee flexion exercises (such as in leg curls against elastic resistance).

The semitendinosus cannot experience active insufficiency while the other hamstrings can. This may mean that the semitendinosus is trained during exercises like the glute bridge and hip thrust, which involve peak forces being exerted while the hamstrings are at very short lengths. Additionally, while all of the hamstrings can display stretch-mediated hypertrophy, there are differences in the extent to which the sarcomeres of each muscle reach onto the descending limb. The semimembranosus may be most strongly affected by training the hamstrings at long lengths, followed by the semitendinosus, and finally the biceps femoris.

The extremely high levels of voluntary activation of the hamstrings make it relatively easy to damage, despite its moderate fiber type. Thus, it recovers more slowly than the other lower body muscles, although not as slowly as the upper body muscles, which have both high levels of voluntary activation and a fast twitch fiber proportion. Therefore, it may be appropriate to train the hamstrings less frequently than the quadriceps.