Curved decelerations result in greater frontal-plane knee loading than straight-line tasks

Background Non-contact anterior cruciate ligament (ACL) injuries often occur during rapid decelerations and change-of-direction tasks. Although most high-intensity movements in team sports follow curved trajectories, the biomechanical consequences of curvilinear deceleration remain unclear. This study examined whether curved decelerations increase frontal-plane knee loading associated with ACL injury risk compared with straight-line tasks. Methods Twelve healthy, recreationally active males performed three maximal deceleration tasks in randomized order: (1) straight-line sprint and deceleration (Straight:Straight), (2) curved sprint with straight-line deceleration (Curved:Straight), and (3) curved sprint with curved deceleration (Curved:Curved). Three-dimensional motion capture and embedded force plates were used to compute center of mass (COM) velocity, peak external knee abduction moment (KAM), resultant ground reaction force (GRF), and frontal-plane moment arm. Repeated-measures ANOVAs with Bonferroni-corrected post-hoc tests assessed differences across tasks. Results COM velocity prior to deceleration was similar across tasks ( P = 0.36). Peak KAM was significantly greater during Curved:Curved compared with both Curved:Straight ( P = 0.007, dz = 1.13) and Straight:Straight ( P = 0.036, dz = 0.87). The resultant GRF did not differ significantly between tasks ( P = 0.056), whereas the frontal-plane moment arm was significantly longer in Curved:Curved ( P = 0.004, dz = 1.04). Conclusion Curved deceleration, independent of approach trajectory, increased knee joint loading through longer frontal-plane moment arms despite comparable entry velocities. These findings identify curvilinear deceleration as an underexplored game situation that may contribute to non-contact ACL injury mechanisms and support the inclusion of curved braking drills in athlete screening and injury mitigation programs.