Hamstrings force-length relationships and their implications for angle-specific joint torques: a narrative review

Abstract

Temporal biomechanical and physiological responses to physical activity vary between individual hamstrings components as well as between exercises, suggesting that hamstring muscles operate differently, and over different lengths, between tasks. Nevertheless, the force-length properties of these muscles have not been thoroughly investigated. The present review examines the factors influencing the hamstrings’ force-length properties and relates them to in vivo function. A search in four databases was performed for studies that examined relations between muscle length and force, torque, activation, or moment arm of hamstring muscles. Evidence was collated in relation to force-length relationships at a sarcomere/fiber level and then moment arm-length, activation-length, and torque-joint angle relations. Five forward simulation models were also used to predict force-length and torque-length relations of hamstring muscles. The results show that, due to architectural differences alone, semitendinosus (ST) produces less peak force and has a flatter active (contractile) fiber force-length relation than both biceps femoris long head (BFlh) and semimembranosus (SM), however BFlh and SM contribute greater forces through much of the hip and knee joint ranges of motion. The hamstrings’ maximum moment arms are greater at the hip than knee, so the muscles tend to act more as force producers at the hip but generate greater joint rotation and angular velocity at the knee for a given muscle shortening length and speed. However, SM moment arm is longer than SM and BFlh, partially alleviating its reduced force capacity but also reducing its otherwise substantial excursion potential. The current evidence, bound by the limitations of electromyography techniques, suggests that joint angle-dependent activation variations have minimal impact on force-length or torque-angle relations. During daily activities such as walking or sitting down, the hamstrings appear to operate on the ascending limbs of their force-length relations while knee flexion exercises performed with hip angles 45–90° promote more optimal force generation. Exercises requiring hip flexion at 45–120° and knee extension 45–0° (e.g. sprint running) may therefore evoke greater muscle forces and, speculatively, provide a more optimum adaptive stimulus. Finally, increases in resistance to stretch during hip flexion beyond 45° result mainly from SM and BFlh muscles.

Biceps Femoris Fascicle Behavior during Submaximal and Maximal Slow Speed Contractions

“…One might speculate that changes in muscle moment arms contributed to a greater muscle length change at more extended angles; that is, an increasing moment arm would have required greater muscle length change to achieve a given joint angle rotation. However, this is not consistent with published data showing a decrease in BFlh (and general hamstrings) moment arm at more extended knee angles (23–25). An alternative explanation is that early joint rotation from flexion toward extension is accompanied by relatively greater stretch of the series elastic component than the muscle during isokinetic eccentric contractions, with later fascicle lengthening as muscle forces no longer increase, resulting in greater muscle length change, similar to the suggestions of Pincheira et al (15).…”

Distal hamstrings tendons mechanical properties at rest and contraction using free‐hand 3‐D ultrasonography

“…Despite the similar morphological characteristics and tendon properties with the SM, the BFlh is more susceptible to injury due to greater changes in MTU length during sprinting 6 . This could be attributed to the larger moment arm in the hip joint and the smaller moment arm in the knee joint observed in BFlh, which collectively increases the risk of injury compared to the SM 4 . The stiffness of tendons plays a crucial role in shaping both the force‐length and force‐velocity profiles of the muscles 1,38 .…”

“…Consequently, the interplay of a shorter lever arm, enhanced force-generating capacity, and tendon stiffness could increase the stress placed on the distal tendon. Even though BFlh and SM display comparable architectural properties 4,9 the fact that SM has greater tendon CSA and greater moment arm compared to the BFlh could explain the reduced stress placed on its tendon.…”

“…Tendon stiffness of the hamstrings may influence the muscle fiber strains during dynamic movements, thus altering the risk for injury 6 . Even though the role of tendons for hamstring muscle function has been highlighted in recent reviews, 4,25 there has been no study that have compared the mechanical properties of the individual hamstring tendons. Such information may help explain why BFlh is more frequently injured during sprinting while SM muscle is more frequently injured during long‐stretching 26 .…”

“…Specifically, our expectation is that the distal tendon of ST, owing to its greater length coupled with a smaller CSA, will exhibit reduced stiffness compared to other tendons. Additionally, we hypothesize that the BFlh, with its shorter lever arm and enhanced force‐generating capacity, 4 coupled with a stiffer tendon, may display greater stress on its distal tendon. Furthermore, our hypothesis extends to the distal hamstrings tendons as a whole, proposing considerable proximo‐distal differences in mechanical properties along their length when subjected to loading.…”