Distinct Neural Drives along the Semitendinosus Muscle.

Introduction: Conflicting results have been reported on the functional role of the proximal and distal compartments of the semitendinosus (ST) muscle. This study compared the discharge characteristics of motor units (MUs) in the two compartments at three knee-joint angles (0°: long length, 45°: intermediate length, and 90°: short length).

Methods: Twenty men (21.4 ± 2.3 years) performed steady isometric contractions with the knee flexors at four target forces: 10%, 20%, 40%, and 60% of maximum voluntary contraction (MVC). High-density electromyography (EMG) signals were recorded to examine the MU discharge characteristics in the two compartments. Measurements included recruitment threshold (RT), mean discharge rate (MDR), coefficient of variation for interspike interval (CoV for ISI), and the standard deviation of filtered cumulative spike train (SD of fCST). Additionally, the within- and between-compartment association of the neural drive was calculated.

Results: Analysis of variance indicated that maximal force, absolute EMG amplitude during the MVCs, and force steadiness (CoV for force) were greater at the longest muscle length than the other two lengths (P < 0.05). Linear mixed models showed that both RT and CoV for ISI were similar between compartments (P > 0.05) at each of the three knee-joint angles. However, MDR and the variability in neural drive were greater for the proximal than the distal compartment (P < 0.05). The between-compartment association in neural drive (fCST) was relatively low.

Conclusions: There were distinct differences in motor unit discharge characteristics between the proximal and distal compartments of ST across its operating range of muscle lengths and each compartment received a relatively distinct neural drive. These findings emphasize the importance of recognizing differences in neural control of the ST compartments to guide related interventions and inform rehabilitation strategies.