Anterior cruciate ligament injury and hamstrings coactivation.

Two experiments were conducted to determine whether injury to the anterior cruciate ligament influences coactivation patterns of the hamstrings during voluntary isometric knee extension. Normal patterns of hamstrings coactivation were established in a control group during submaximum and maximum effort isometric knee extension torque at two knee flexion angles. Subjects who had undergone anterior cruciate ligament reconstructive surgery and a group of conservatively managed patients were studied using a modified protocol. It was predicted that (a) hamstrings coactivation patterns would vary significantly as a function of knee flexion angle, and (b) control and anterior cruciate ligament reconstructed subjects would differ significantly from the anterior cruciate ligament deficient subjects. Contrary to expectations, (a) hamstrings coactivation patterns were not significantly related to knee joint angle for any of the three groups, and (b) differences between hamstrings coactivation patterns of the anterior cruciate ligament deficient and anterior cruciate ligament reconstructed groups were not significant. The hamstrings coactivation pattern of the combined anterior cruciate ligament group data was significantly different from control patterns and characterized by significantly higher hamstrings coactivation. The coactivation may increase knee joint stability/stiffness and may be a beneficial adaptation to the injury-related knee joint instability. The long-term effect of this adaptation on knee joint function is unknown.