In vivo assessment of the passive stretching response of the bi-compartmental human semitendinosus muscle using shear wave elastography

The semitendinosus muscle contains distinct proximal and distal compartments arranged anatomically in-series but separated by a tendinous inscription, with each compartment innervated by separate nerve branches. Although extensively investigated in other mammals, compartment-specific mechanical properties within the human semitendinosus have scarcely been assessed in vivo. Experimental data obtained during muscle-tendon unit stretching (e.g., slack angle) can also be used to validate and/or improve musculoskeletal model estimates of semitendinosus muscle force. The purpose of this study was to investigate the passive stretching response of proximal and distal humans semitendinosus compartments to distal joint extension. Using two-dimensional shear wave elastography, we bilaterally obtained shear moduli of both semitendinosus compartments from 14 prone-positioned individuals at ten knee flexion angles (from 90° to 0° [full extension] at 10° intervals). Passive muscle mechanical characteristics (slack angle, slack shear modulus, and the slope of the increase in shear modulus) were determined for each semitendinosus compartment by fitting a piecewise exponential model to the shear modulus-joint angle curves. We found no differences between compartments or legs for slack angle, slack shear modulus, or the slope of the increase in shear modulus. We also found the experimentally determined slack angle occurred at ~15-80° higher knee flexion angles compared to estimates from two commonly used musculoskeletal models, depending on participant and model used. Overall, these findings demonstrate that passive shear modulus-joint angle curves do not differ between proximal and distal human semitendinosus compartments, and provide experimental data to improve semitendinosus force estimates derived from musculoskeletal models.