Intra- and Inter-Muscular Variations in Hamstring Architecture and Mechanics and Their Implications for Injury: A Narrative Review.

Understanding the architecture, anatomy, and biomechanics of the hamstrings may assist in explaining the mechanisms that affect and improve their function. The aim of this review is to specifically examine intra- and inter-muscular variations in architecture and mechanical properties of the hamstrings. Of the hamstrings, the long head of the biceps femoris shows the shortest and more pennated fibers. The semimembranosus has a similar muscle architecture with a long head of the biceps femoris but it has a different proximal attachment as well as a different moment arm compared with the long head of the biceps femoris. For the same joint motion, the semitendinosus displays less relative strain than the other hamstrings probably owing to a greater length, longer fascicles and, possibly, a longer tendon. Intra-muscular variations in architecture are documented but their implications are currently unclear. Proximally, the long head of the biceps femoris has shorter and more pennated fibers coupled with a narrower aponeurosis than distally, while the semitendinosus is the only muscle that entails a tendinous inscription. In conclusion, some of the identified intra- and inter-muscular variations in architecture may help explain why some muscles sustain injuries more than others. In the same line, exercises designed for the hamstrings may not provide the same stimulus for all components of this muscle group. Future research could examine whether intervention strategies that target specific muscles or specific areas of the hamstrings may offer additional benefits for injury prevention or rehabilitation of their function.