Reactive signal complexity influences biomechanical variables associated with ACL loading risk

Anterior cruciate ligament (ACL) injuries frequently occur during athletic tasks requiring rapid reactions to changes in the sports environment. This study investigated how reactive signal complexity influenced pre-landing and early landing biomechanical variables associated with ACL loading risk. Thirty-one recreational athletes performed single-leg side and cross-cutting tasks under four conditions: anticipated, simple unanticipated, complex unanticipated without a second signal, and complex unanticipated with a second signal to change the cutting direction. The two complex unanticipated conditions demonstrated increased stance time, slower take-off velocities, decreased minimal knee flexion angles during pre-landing, and increased peak vertical and posterior ground reaction forces during early landing compared to the anticipated and simple unanticipated conditions. Side-cutting and cross-cutting demonstrated different early landing mechanics in the anticipated and simple unanticipated conditions, but became similar in the two complex unanticipated conditions. Participants adopted more reactive movement patterns to maintain landing stability and response accuracy under conditions of uncertainty. However, this delayed response led to decreased cutting performance and landing biomechanical variables associated with ACL loading risk. Screening ACL injury risk under complex situations is recommended. Training athletes to better predict the sports environment to make more tasks from “reactive” to “anticipated” may promote safer movement patterns.