Objectives Compare the lower-limb mechanics and anterior cruciate ligament (ACL) injury risk of athletes using a habitual rear-foot (RF) and fore-foot (FF) fall pattern during unplanned sidestepping (UnSS). Design Experimental cross-sectional. Methods Nineteen elite female field hockey players attended one biomechanical motion capture testing session, which consisted of a random series of pre-planned and unplanned sidestepping sport tasks. Following data collection, participants were classified as possessing a habitual RF or FF fall pattern during UnSS. Hip, knee and ankle joint angles, moments, instantaneous powers and net joint work were calculated during weight acceptance. Between group differences were evaluated using independent sample t-tests (α = 0.05). Results Athletes using a habitual RF fall pattern during UnSS absorbed significantly more work and power through their knee joint (p < 0.001), which was coupled with significantly elevated externally applied peak non-sagittal plane peak ankle moments (p < 0.05) as well as peak flexion and abduction knee moments (p < 0.005). Athletes using a habitual FF fall pattern during UnSS absorbed more power through their ankle joint (p < 0.001). Conclusions A RF fall pattern during UnSS places a large mechanical demand on the knee joint, which is associated with elevated ACL injury risk. Conversely, a FF fall pattern placed a large mechanical demand on the ankle joint. Modifying an athlete's foot fall pattern during UnSS may be viable technique recommendation when returning from knee or ankle injury.