Altered timing of hamstring muscle action in anterior cruciate ligament deficient patients.

Recent studies indicate that the hamstrings are essential antagonists in ACL deficient patients. In order to evaluate muscle coordination under physiologic conditions, nine patients with arthroscopically verified total ACL ruptures were compared with nine control volunteers. This comparison was made from electromyograms of the thigh muscles and heel contact recordings made while walking on a treadmill. While walking on a horizontal level, the patient's muscle activity did not differ from controls, but when the knee load was increased by walking uphill, the hamstring muscles in patients were activated significantly earlier than among controls. It is proposed that an altered muscle coordination is probably essential in ACL deficient patients to secure knee stability. This study suggests a new approach to investigate the coordination of muscles under physiologic conditions in ACL deficient patients, and may guide rehabilitation programs in the future.