Anatomical knowledge of the hamstrings is necessary to better understand the function of each muscle. The hamstrings are located in the posterior compartment of the thigh with the BF-L and BF-S (lateral hamstrings) in the lateral part and with the ST and SM (medial hamstrings) in the medial part of the posterior thigh. The ischial tuberosity is the common origin site of the BF-L, ST, and SM, while the BF-S origin is from the most proximal part of the lateral lip of the femur. The BF-L attaches on the fibular head and the BF-S shares a common distal tendon with the BF-L. The ST and SM attach on the medial surface of the upper part of tibia and on the protruded medial surface of the medial condyle of the tibia, respectively. The BF-L, ST, and SM are biarticular muscles that cross from the hip to the knee joints, whereas the BF-S crosses only the knee joint. The hamstrings have similar anatomical features with each other, but previous studies have reported intra- and inter-muscular architectural variations among these muscles. Considering the close relationship between muscle architecture and function (Fukunaga et al. 1996), the differing anatomical features of individual muscles likely indicate a functional difference among the hamstrings.

The FL, pennation angle (PA), and PCSA of a muscle have been considered important parameters that affect muscle function. The FL is thought to be proportional to maximum muscle excursion (or velocity), and the PCSA proportional to maximum muscle force (Lieber and Friden 2000). A muscle with relatively long fibers (a large number of sarcomeres arranged in series) with a smaller PA is suited for joint excursion, whereas those composed of relatively short fibers (a large number of fibers arranged in parallel) with a larger PA and PCSA are suitable for force generation (Woodley and Mercer 2005). The BF-L and SM are typically classified as pennate muscle due to their structure, while the ST is regarded as a fusiform muscle with a parallel fiber configuration (Chleboun et al. 2001; Kellis et al. 2010, 2012; Woodley and Mercer 2005). Each muscle differs with respect to architectural parameters such as the muscle length, FL, PA, PCSA, and volume (Kellis et al. 2010, 2012; Kumazaki et al. 2012; Woodley and Mercer 2005). Woodley and Mercer (2005) suggested that the BF-L is designed to allow for long excursion since its FL is relatively long and its PCSA is intermediate in size. On the contrary, Kellis et al. (2012) inferred that the BF-L have a higher force generation capacity with shorter pennate fibers and larger PCSA. The SM is designed for force production rather than excursion because this muscle has a relatively short FL but a relatively large PCSA (Kellis et al. 2012; Woodley and Mercer 2005). The ST has a parallel fiber configuration and longer FL coupled with a very long distal tendon (Kellis et al. 2010, 2012; Kumazaki et al. 2012; Woodley and Mercer 2005), indicating that this muscle has a higher excursion capacity. The BF-S possesses a relatively long FL among the hamstrings, but the PCSA is smaller compared with the other hamstrings (Kellis et al. 2012; Woodley and Mercer 2005). Therefore, the magnitude of force exerted by the BF-S is likely to be small.

The BF-L and BF-S form lateral hamstrings running along the lateral side of the thigh, whereas the ST and SM run medially within the thigh as medial hamstrings. The medial hamstrings have more distal attachments than the lateral ones. Therefore, functional differences between medial and lateral groups in knee flexion have been suggested. For instance, the ST has an almost double moment arm (MA) as compared to the BF-L, suggesting that the ST is superior in the production of knee flexion torque (Herzog and Read 1993). On the other hand, Kellis et al. (2012) reported that the pairs of lateral (BF-L and BF-S) and medial (ST and SM) muscles show a low similarity in their muscle architectures. According to their results, the BF-L and SM have shorter pennate fibers and larger PCSA among the hamstrings, whereas the ST has the longest FL coupled with a very long distal tendon. The BF-S has a longer FL, but possesses the smallest PCSA among the hamstrings. Therefore, they suggest that there are noticeable differences between medial or lateral muscles such that each pair has one muscle which is designed for excursion (BF-S or ST) and another muscle which is suitable for force production (BF-L or SM).

There are also intramuscular differences in the architecture of each hamstring muscle. Kellis et al. (2010) revealed that the BF-L has greater PA and longer FL at the proximal side than at the distal one, whereas the ST exhibits a significantly smaller PA and shorter FL proximally than distally. The SM consists of three distinct regions (proximal and intermediate unipennate, and distal bipennate regions) on the basis of fascicular orientation, and the distal region has the largest PCSA (Woodley and Mercer 2005). These data suggest that the architecture of each muscle is not uniform. Therefore, modeling each muscle by assuming a uniform architecture along muscle length can yield an inaccurate representation of hamstring muscle function (Kellis et al. 2010). Moreover, it is possible that intramuscular architectural variation produces higher tension in some regions of the muscle, while it allows for greater efficiency of force transfer in line with the tendon in other regions (Kellis et al. 2010; Scott et al. 1993).

Measured torque value at any joint is the sum of torques produced by several muscles. The amount of torque produced by a muscle depends on the number of motor units activated, the sarcomere length (based on a length–tension relationship), and the MA of the muscle during force generation in addition to muscle size. Moreover, the non-contractile component of the muscle (the endomysium, perimysium, epimysium, and tendon) can be an important factor in joint torque production because stretched connective tissues produce passive tension that is added to the active tension generated by muscle contraction (Magnusson 1998). Therefore, the connective tissues of the hamstrings are likely more stretched with a flexed hip and/or an extended knee, which may contribute to total joint torque production (Lunnen et al. 1981; Mohamed et al. 2002).

Each hamstring muscle possesses relatively long tendons at both the proximal and distal sides (except for the proximal side of the BF-S). The long proximal and distal tendons of each hamstring muscle extend into the muscle bellies, thereby forming elongated musculotendinous junctions. The BF-L and SM have proximal and distal tendons which overlap to some extent within the belly of the muscle, and the distal tendon of the BF-L receives fascicles from the BF-S (Woodley and Mercer 2005). The ST possesses a much longer tendon on the distal side than on the proximal side (Kellis et al. 2012; Woodley and Mercer 2005), and unlike other muscles, has a tendinous inscription within the muscle belly that divides it into two partitions, with each region receiving innervation from one muscle nerve, or from a primary branch of the nerve (Woodley and Mercer 2005). Considering that hamstring strain injuries often occur at the musculotendinous junction (Koulouris et al. 2007; Slavotinek et al. 2002), the structures of tendons and musculotendinous junctions may be key factors. In addition, Kumazaki et al. (2012) revealed that muscle fibers of the BF-L and SM with unipennate architecture are elongated more than those of the ST and BF-S during knee extension. These authors suggest that BF-L and SM have a higher risk of muscle strain during extension of the knee joint. Moreover, there is a gender difference. Blackburn et al. (2009) showed that musculotendinous stiffness of the BF-L is greater in males than in females, but that the elastic modulus does not differ significantly across the sexes. They also indicated that the male BF-L has a greater capacity for resisting changes in length imposed via joint motion from a structural viewpoint. However, this property is at least partially attributable to a greater muscle size in males, and is probably similar between genders from a material viewpoint.

Many previous studies used cadaveric lower limbs to study the architecture of the hamstrings (Chleboun et al. 2001; Kellis et al. 2010, 2012; Kumazaki et al. 2012; Woodley and Mercer 2005). However, the shrinkage of muscle fibers, which can occur during or after the embalming process, could potentially affect the fascicular dimensions. Also, elderly specimens may have smaller FL and volume than younger ones. Although these limitations might have led to some inconsistent findings among earlier studies, understanding the detailed anatomy of the hamstrings is important to develop biomechanical models of relationship between morphology and function for each hamstring muscle. Moreover, these morphological data should be useful for discussing injury mechanisms as well as for proposing injury prevention and treatment procedures for individual hamstring muscles.