Quadriceps atrophy in the anterior cruciate insufficient knee.

Atrophy of the quadriceps musculature is frequently associated with chronic knee instability. This atrophy may persist despite the patient's ability to return to athletic competition. When open biopsies of the quadriceps muscle from unstable knees were compared with those from normal acutely injured knees, it was found that clinical evidence of atrophy correlated with a relative decrease in the size of Type II or fast twitch muscle fibers. This supports the subjective, clinical impression that the unstable knee is less effective in explosive push-off. The findings also indicate the necessity of including exercise specific for the training of Type II fast twitch fibers in any program designed to rehabilitate the injured knee.