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Change in Muscle Control after ACL Injury

When the anterior cruciate ligament (ACL) is damaged, the quadriceps muscle may go haywire too. The quadriceps muscle along the front of the thigh helps straighten the knee. It can become weak and out of sorts when the ACL is injured. The change in muscle activity and resulting problems with coordination are called dyskinesia.

Physical therapists at the University of Delaware teamed up with engineers to study this problem. They hope to find better ways to restore total knee function after ACL injury. In this study two groups of 15 subjects each were tested. Group one had ACL problems. Group two had uninjured or "normal" knees.

Two types of knee motion were studied: static (joint doesn't move) and dynamic (joint moves). Nerve tests were used to measure the activity patterns of the muscles around the knee. The authors report major differences between the two groups. The subjects with ACL injuries had less control of the quadriceps muscle in both types of motion.

The biggest finding was that the quadriceps muscle didn't "turn off" in the injured group. During movements when the quadriceps should be relaxed and calm, it was active. This change in the muscle activity pattern was most noticeable for the vastus lateralis (VL). The VL forms the outside (lateral) portion of the quadriceps muscle.

The authors conclude that without a "cease fire" order, the quadriceps can't control knee motion properly. This may explain why the knee is unstable and gives out on some people with weak ACLs. Therapists aren't sure why the muscle fibers don't stop contracting. Further studies are needed to get to the bottom of this mystery.

References: Glenn N. Williams, et al. Altered Quadriceps Control in People with Anterior Cruciate Ligament Deficiency. In Medicine & Science in Sports & Exercise. July 2004. Vol. 36. No. 7. Pp. 1089-1097.

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