Quadriceps femoris muscle morphology and function after ACL injury: a differential response in copers versus non-copers.

The morphology (volume and peak cross-sectional area) and voluntary muscle control of 27 athletic people were evaluated with magnetic resonance imaging (MRI) and an established method of testing neuromuscular control in order to explain why some people are able to cope with anterior cruciate ligament (ACL) injury (copers), whereas most cannot (non-copers). Axial spin-echo T1 weighted MRI images were acquired from the level of the ankle mortise to the iliac crest. The subjects' quadriceps, hamstrings, and gastrocnemius muscles were digitally reconstructed from the MRI images. The volume and peak cross-sectional area (CSA) of each muscle were then calculated. Voluntary muscle control was evaluated using an established target-matching protocol that requires subjects to produce and modulate force with control over a range of directions. Electromyographic signals were collected from seven muscles as the subjects performed the experiment. Circular statistics methods were used to calculate a specificity index that describes how focused the activity pattern of each muscle was with respect to its principal direction of action. The results of the non-copers, copers, and uninjured subjects were then compared. The non-copers displayed significantly greater quadriceps atrophy than the copers. The most profound differences were observed in the vastus lateralis muscle. The non-copers also displayed diminished vastus lateralis and lateral gastrocnemius muscle control. Little differences were observed in the results of the copers and uninjured subjects. In general, the copers' results fell between those of the non-copers and uninjured subjects. The results of this study suggest that quadriceps muscle function is a critical factor in the differential response to ACL injury.