Muscle-Specific Neural Modulation in Biarticular Semitendinosus and Biceps Femoris during Isometric Torque Generation

In this study, we aimed to determine whether the semitendinosus (ST) and biceps femoris long head (BF) exhibit systematically distinct preferred directions (PDs) and to evaluate the biomechanical mechanisms underlying the PDs. A total of 21 healthy young adults performed isometric torque tasks in 16 sagittal-plane directions, while surface electromyography (EMG) signals were recorded from six lower limb muscles. Joint torque was estimated using inverse dynamics, and muscle PDs were determined through linear regression of EMG against torque data. Optimal PDs were predicted using static optimization modeling that incorporated both moment arm- and physiological cross-sectional areas. Mechanical-only PDs were also estimated based solely on moment arm geometry. Experimentally derived PDs differed significantly between ST and BF (p < 0.001). The ST exhibited significantly greater alignment with optimization-based predictions (mean cosine similarity = 0.96 ± 0.08) than with mechanical-only predictions (0.82 ± 0.15, p = 0.0007), whereas the BF showed comparable levels of agreement with both models (optimization = 0.70 ± 0.35, mechanical = 0.76 ± 0.20, p = 0.3096). These findings indicate that muscle-specific neural modulation contributes to directional tuning, particularly in the ST, and highlight the necessity of incorporating both biomechanical and neural factors to understand spatial organization of muscle activity during complex multi-joint motor tasks.