Small knee flexion angles at initial contact and large ground reaction forces during landing have been identified as risk factors for non-contact anterior cruciate ligament injuries that are common in sports. This study investigated the effects of knee extension constraint on knee flexion angle and ground reaction forces for patients who were 4-6 months post ACL reconstruction surgery, and also compared these patients to healthy subjects' motion patterns. Three-dimensional videographic and force plate data were collected for 12 ACL reconstruction patients and 12 age and gender matched healthy subjects performing level walking, jogging, and stair descending under 3 conditions: knee brace with a constraint to extension, traditional knee brace, and no knee brace. The constrained knee brace significantly increased knee flexion angle at landing, but did not significantly affect the peak ground reaction forces at landing. The constrained knee brace may be useful in the prevention and rehabilitation of non-contact anterior cruciate ligament injuries in sports.