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A tear of the anterior cruciate ligament (ACL) disrupts the delicate balance of static stabilizers of the knee, leading to significant alterations in joint kinematics. Little is known about the dynamic compensatory responses of the patient to these kinematic alterations. This lack of quantitative information on the muscle synergy patterns has limited the surgeon's ability to evaluate various operative and rehabilitative techniques. Twelve subjects with documented ACL deficiency for at least 1 year and 15 normal participants were studied. Each subject was asked to walk at free and fast speeds on a 12 m walkway. The right and left foot contact patterns and the linear envelopes from the surface electromyogram (EMG) patterns of the gastrocnemius, medial and lateral hamstrings, rectus femoris, and vastus lateralis were measured. Significant differences were found in the muscle synergy patterns during walking. During the swing-to-stance transition, the ACL-deficient subjects showed significantly less activity in the quadriceps and gastrocnemius muscles and more activity in the biceps femoris than in the normal group. During early swing, the vastus lateralis is more active than normal, and during midstance and terminal stance, the hamstrings appear to be less active than normal subjects. These dynamic compensatory mechanisms suggest that use of the hamstring tendons in reconstructive procedures may alter important compensatory mechanisms about the knee joint. Application of dynamic EMG techniques to the study of reconstructive procedures should provide additional information that will assist the clinician in the rational choice of a surgical procedure.

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