The human semitendinosus muscle is characterized by a tendinous inscription separating proximal (STprox) and distal (STdist) neuromuscular compartments. As each compartment is innervated by separate nerve branches, potential exists for the compartments to operate and be controlled independently. However, the morphology and function of each compartment have not been thoroughly examined in a human adult population. Further, the distal semitendinosus tendon is typically harvested for use in anterior cruciate ligament reconstruction (ACLR) surgery, which induces long-term morphological changes to the semitendinosus muscle-tendon unit. It remains unknown if muscle morphological alterations following ACLR are uniform between STprox and STdist. Here, we performed magnetic resonance imaging on ten individuals who had undergone ACLR involving an ipsilateral distal semitendinosus tendon graft 14 ± 6 months prior, extracting morphological parameters of the whole ST muscle and each individual muscle compartment from both the (non-injured) contralateral and surgical legs. In the contralateral non-surgical leg, volume and length of STprox were lower than STdist. No between-compartment differences in volume or length were found for ACLR legs, likely due to greater shortening of STdist compared to STprox after ACLR. The maximal anatomical cross-sectional area of both compartments was substantially smaller on the ACLR leg, but did not differ between STprox and STdist on either leg. The absolute and relative differences in STprox morphology on the ACLR leg were strongly correlated with the corresponding between-leg differences in STdist morphological parameters. Specifically, greater morphological differences in one compartment were highly correlated with large differences in the other compartment, and vice versa for smaller differences. These relationships indicate that despite the heterogeneity in compartment length and volume, compartment atrophy is not independent or random. Further, the tendinous inscription endpoints were generally positioned at the same proximodistal level as the compartment maximal anatomical cross-sectional areas, providing a wide area over which the tendinous inscription could mechanically interact with compartments. Overall, results suggest the two human semitendinosus compartments are not mechanically independent.

The authors have declared no competing interest.

This work was supported by a Griffith University Postgraduate Research Scholarship (to AK), the Erasmus+ progamme of the European Union (to AK and LS), and the Academy of Finland (#324529 to RKK, #332915 to LS).

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