Electromyography profiles of knee joint musculature during pivoting: Changes induced by anterior cruciate ligament deficiency.

Anterior cruciate ligament (ACL) insufficiency and associated rotational instability of the knee joint is a potentially disabling condition. In particular, anterior-posterior laxity increases when the ACL is damaged and decreases with contraction of the hamstring musculature. Because laxity is controllable with muscle activity, comparison of muscular synergy patterns between groups of subjects with ACL-deficient and uninjured knees may identify compensatory mechanisms that could influence therapeutic procedures. All participants selected were between 18 and 40 years of age and had clinical or surgical documentation of ACL deficiency. Each subject was asked to walk and pivot with a stride time of 1 s on a 12-m walkway. The right and left foot-contact patterns and linear envelopes (LE) from the surface electromyographic (EMG) patterns of the gastrocnemius (GS), medial and lateral hamstring, rectus femoris (RF), and vastus lateralis (VL) were measured. Results from 15 subjects with uninjured knees and 12 subjects with ACL-deficient knees showed significant differences between their muscle synergy patterns. The types of differences observed depended on the stride period. All muscles within the ACL-deficient population showed some periods of abnormal activity. The differences in synergies indicate that there is a tendency for a greater net posterior force, flexor moment, and external rotation moment to be produced on the tibia during the time when most external rotation occurs to compensate for the mechanical actions of the lost ligament.