Hip–angle dependent changes in the shear modulus of biceps femoris short head with knee extension

PURPOSE: Previous cadaveric studies using Thiel-embalmed specimens have suggested that the tension of the biceps femoris short head (BFsh) may depend on hip joint angle, despite the BFsh being a monoarticular muscle. However, whether this hip-angle dependence exists in vivo remains unclear. This study examined the effect of hip joint angle on the in vivo shear modulus of the BFsh using shear wave elastography (SWE). METHODS: Fifteen healthy young men participated. Shear modulus was assessed in both the BFsh and the biceps femoris long head (BFlh) at 30% along the ischial tuberosity-to-fibular head line under four configurations: (1) hip 0°-knee 90°, (2) hip 0°-knee 0°, (3) hip 90°-knee 90°, and (4) hip 70°-knee 0°. SWE was used to evaluate shear modulus, and values were compared using a two-way repeated-measures ANOVA. RESULTS: A significant interaction was found between muscle and joint position (P < 0.01). In the BFsh, knee extension at 0˚ hip angle did not significantly increase shear modulus (17.9 ± 16.6 to 26.0 ± 15.9 kPa, P = 0.184), whereas knee extension when the hip was flexed significantly increased shear modulus (51.9 ± 27.7 to 128.3 ± 65.0 kPa, P < 0.01). CONCLUSION: These findings indicate that the passive mechanical behavior of the BFsh is influenced by hip joint position despite being a monoarticular muscle. This observation suggests the presence of mechanical interactions within the biceps femoris complex that may contribute to the regulation of muscle stiffness under elongated conditions, potentially reflecting intermuscular force transmission between the long and short heads.