28 athletes with knee flexion angles of less than 30 degrees were randomized to investigate the efficacy of a knee extension constraint device in improving knee flexion angles during peak impact posterior ground-reaction force, and in decreasing peak impact ground-reaction forces during landing of a stop-jump, and side-cutting task. The evidence presented in this study demonstrated that the use of the knee extension constraint device while playing sports for 4 weeks was associated with significant increases in knee flexion angle at the time of peak impact posterior ground-reaction force, and decreases in the peak impact posterior ground-reaction force during landings of the stop-jump and side-cutting tasks. These benefits were retained 4 weeks after training but were diminished in magnitude.