Compensatory Movement Strategies during the Rearfoot Elevated Split Squat after ACL Reconstruction

Purpose: To compare inter- and intra-limb compensation strategies between anterior cruciate ligament (ACL)-involved and non-involved limbs during the rearfoot elevated split squat (RFESS) and examine the influence of trunk flexion angle on compensation strategy. Methods: Twenty-five individuals (3.9 ± 0.9 months post-ACL reconstruction) performed RFESS using three-dimensional motion capture and force plates. Joint kinematics, kinetics, and vertical ground reaction forces (vGRF) were analyzed for the lead limb at the bottom position of the RFESS. Internal joint moments were normalized to body mass and vGRF were normalized to body weight. Relative joint contributions were calculated as [(joint moment/total support moment) x 100)]. Paired samples t-tests assessed between-limb differences (adjusted p =.0038). Linear regression examined the relationship between trunk flexion angle and compensation strategy. Results: The ACL-involved limb demonstrated both inter- and intra-limb compensation, reflected by lower vGRF (–7.3%, p <.001, d = –1.26) and total support moment (–0.29 N·m/kg, p =.002, d = –0.72), alongside a redistribution of demand away from the knee (–12.8%, p <.001, d = –2.81) and toward the hip (+9.8% p <.001, d = 2.02) and ankle (+3.2%, p <.001, d = 1.10). Greater trunk flexion was associated with higher vGRF (R² = 0.52, p <.001) but not knee contribution (p =.84). Conclusions: Inter- and intra-limb compensation strategies were observed in the ACL-involved limb during the RFESS. Performing the RFESS with a more upright trunk may exacerbate inter-limb compensation, promoting offloading of the lead limb. Collectively, these findings suggest that without careful monitoring, the RFESS may be ineffective as a knee extensor strengthening exercise during mid-stage ACL rehabilitation.