The passive stretching response of the human biceps femoris long head muscle varies regionally

Abstract

The in vivo passive behavior of biceps femoris long head (BFlh) across different regions remains poorly understood despite its relevance for improving localized muscle force estimation in musculoskeletal models and understanding the mechanisms of hamstring strain injury. We investigated the region-specific passive stretch response of BFlh during passive knee extension in 20 healthy participants (17 males, 3 females). Using shear wave elastography, the shear modulus of BFlh was assessed at proximal, middle, and distal regions at 5° increments from 90° to 0° of knee flexion. A piecewise exponential model was fitted to the shear modulus-knee joint angle relationship to determine the slack angle (i.e., the knee joint angle at which the shear modulus began to increase), slack shear modulus (i.e., shear modulus before slack angle), and the exponential increase beyond slack angle (α). Slack angle differed significantly across regions (p = 0.040), occurring at a higher knee flexion angle in the distal (63.8 ± 14.1°) compared to proximal (52.8 ± 10.6°, p = 0.031) region. The distal region (0.0123 ± 0.0069) had a larger α than the proximal region (0.0080 ± 0.0048, p = 0.039), but this effect was not observed when assessing only males (p = 0.135). No significant regional differences were observed for slack shear modulus. Overall, the passive stretch response of BFlh to knee extension varies across regions, with slack angle at more flexed knee angles and steeper increase in shear modulus in the distal than the proximal region, though the latter was evident only when including female participants. However, given the low number of female participants, this finding should be interpreted with caution, and future studies including larger female cohorts are warranted. These results have important implications for BFlh muscle function, injury risk, and validity of musculoskeletal modeling estimates