Semitendinosus regeneration influences muscle-specific contributions to knee joint loading after ACL reconstruction

Hamstring tendon autograft is commonly used in anterior cruciate ligament reconstruction (ACLR). Harvesting the semitendinosus (ST) tendon induces morphological adaptations that may alter muscle and joint loading. Although ST tendon regeneration can occur, its post-ACLR function during dynamic tasks remains unclear. Eighteen participants (68 % male) who were 2-4 years post-ACLR underwent bilateral magnetic resonance imaging and three-dimensional motion analysis of sidestep cutting. Personalised musculoskeletal models were simulated under two conditions: an injured-morphology model incorporating MRI-derived ACLR-related hamstring alterations (reduced ST muscle volume, adjusted muscle-tendon parameters, and ST removal where regeneration was absent) and an uninjured-morphology model informed by contralateral hamstring morphology. Both models were driven using the same experimental kinematics and external loads acquired from the ACLR limb. From this motion analysis data, two sets of muscle-specific contributions to knee loading were quantified using sequentially linked static optimisation, followed by induced acceleration and then joint reaction analyses. Between-model differences were assessed using the Peak Model Difference Index (PMDI). Compared with the uninjured-morphology-model, the injured-morphology-model had reduced ST contribution to posterior knee force (PMDI = 47.5 %, d = -1.16), but increased semimembranosus (PMDI = 5.6 %, d = 0.95) and biceps femoris long head (BFLH) contributions (PMDI = 13.6 %, d = 0.72). These between-model differences were amplified in participants without ST tendon regeneration (e.g., semimembranosus (PMDI = 8.9 %, d = 1.55) and BFLH (PMDI = 14.5 %, d = 0.88)) compared with participants with ST regeneration (e.g., semimembranosus (PMDI = 1.8 %, d = 0.38) and BFLH (PMDI = 12.0 %, d = 0.76)). Persistent impairment of ST function post-ACLR results in compensatory redistribution of hamstring contributions to knee-joint loading during a challenging dynamic motor task, particularly when tendon regeneration does not occur. Results underscore the mechanical importance of ST integrity for balanced hamstring load sharing.