Anticipation-angle interaction identifies 45° unanticipated change of direction as a high-demand movement with altered ACL injury-related biomechanical characteristics.

Unanticipated change-of-direction (COD) maneuvers represent a key scenario for non-contact lower-limb injury exposure in sport. This study investigated how anticipation modulates lower-limb mechanics across varying cutting angles (45°, 90°, and 180°) to identify phase-specific dynamic joint vulnerabilities. Twenty-three high-level male soccer players performed anticipated (ANT) and unanticipated (UNA) COD tasks. Kinematics, kinetics, and muscle activation were analyzed using two-way repeated-measures ANOVA and Statistical Parametric Mapping (SPM1D). Significant angle effects occurred for temporal and force variables (p < 0.05). At 45°, ANT showed higher impact forces and stiffness (p < 0.01). Condition × Angle interaction effects emerged for medial gastrocnemius preactivation (p = 0.004) and vastus medialis cushion (p = 0.007). SPM1D revealed interactions for CoM velocity (p < 0.005), early-cushion knee angles (p = 0.034), and mid-cushion moments (p = 0.024). UNA-45 exhibited greater negative knee power during mid-cushion (p < 0.001), while trunk lateral bending interaction occurred during early preactivation (p = 0.039). Anticipation effects were maximal at 45°, whereas 90° and 180° mechanics were primarily angle-driven (p < 0.001). While 90° and 180° maneuvers provide a temporal buffer for movement reorientation, the 45° task forces athletes to process extreme loads within a compressed timeframe. Under unanticipated conditions, altered feedforward neuromuscular preparation combined with an upright landing posture was accompanied by greater eccentric power absorption, indicating increased multi-planar mechanical demands. These findings suggest that 45° UNA cutting may serve as a challenging biomechanical paradigm for examining biomechanical characteristics relevant to ACL injury mechanisms, revealing potential neuromuscular control adaptations under constrained preparation conditions that may remain less evident during larger-angle or pre-planned maneuvers.