Regional mechanical differences in hamstring muscles after removal of surrounding connective tissue

Regional differences in the passive mechanical properties of the hamstring muscles remain unclear. Previous studies using isolated muscle-tendon units have reported inconsistent findings regarding whether proximal or distal regions exhibit greater stiffness. These discrepancies may arise from differences between isolated specimens and in situ anatomical conditions. Therefore, this study investigated regional variations in the mechanical properties of individual hamstring muscles under different joint positions and tissue conditions. Six lower limbs from Thiel-embalmed cadavers were examined. The biceps femoris long head (BFlh), semitendinosus (ST), and semimembranosus (SM) were evaluated. Mechanical properties were assessed under two joint conditions representing slack and extended muscle states defined by hip and knee positions. Two tissue conditions were tested: intact and removal of surrounding connective tissues. Measurements were obtained at two locations along each muscle defined as 33% (proximal) and 66% (distal) of the muscle length. A three-way repeated-measures ANOVA revealed significant effects of tissue condition and joint position in all muscles (P ≤ 0.007). Significant three-way interactions (condition × position × region) were identified in BFlh (P = 0.017) and SM (P = 0.027), whereas no significant interaction was observed in ST (P = 0.104). Under intact conditions, shear modulus was greater in the distal region during the extended position; however, this regional difference diminished after removal of surrounding connective tissues. These findings indicate that passive mechanical behavior of the hamstring muscles exhibits muscle-specific regional characteristics. In particular, regional differences in BFlh and SM disappeared after removal of surrounding connective tissues, suggesting that surrounding connective tissues may contribute to region-dependent force transmission.