Biomechanical Compensation Patterns Across Different Phases of Side-Cutting Following Anterior Cruciate Ligament Reconstruction

(1) Background: Anterior cruciate ligament reconstruction (ACLR) alters lower-limb biomechanics. While gait and running are well-studied, the multi-phase side-cutting remains poorly understood, particularly regarding phase-specific adaptations after ACLR. (2) Methods: Thirty-four patients (19 male, 15 female) at nine months post-ACLR participated. Biomechanical data during side-cutting were collected using synchronized motion capture and force platforms. Knee joint kinematics and kinetics were analyzed over three phases: initial contact-deceleration, stance pivot, and push-off. (3) Results: During the initial contact-deceleration, the reconstructed limb exhibited greater knee external rotation at the first posterior ground reaction force (pGRF) peak (8.5° vs. 6.3°, <i>p</i> = 0.021), and reduced knee flexion (43.2° vs. 47.3°, <i>p</i> < 0.001) with a lower extension moment at the second pGRF peak (0.10 vs. 0.14 BW·BH; <i>p</i> < 0.001). The stance pivot phase was marked by significantly lower knee flexion (<i>p</i> = 0.001), extension moment (<i>p</i> < 0.001), and medial/vertical GRFs on the reconstructed side (0.49 vs. 0.52 BW, <i>p</i> = 0.029; 1.98 vs. 2.10 BW, <i>p</i> = 0.012). During the push-off, the involved limb demonstrated a significantly lower extension moment (0.008 vs. 0.014 BW·BH, <i>p</i> = 0.037) and anterior GRF (0.20 vs. 0.23 BW, <i>p</i> = 0.010). (4) Conclusions: This study proposes a three-phase compensation model for side-cutting: "rotational instability" at initial contact, "protective unloading" during the stance pivot phase, and "force-generation deficit" at push-off. This three-phase framework provides a new paradigm for evaluating dynamic knee function after ACLR and guiding phase-specific rehabilitation.