The possibility that functional impingement results from muscular imbalance has been discussed. This study investigates whether disturbance of muscular coordination is reflected in a shift in the balance of fast-twitch and slow-twitch fibers, as fast-twitch fibers enable a rapid reaction or contraction and slow-twitch fibers enable slow contractions and sustained performance. The supraspinatus and deltoid muscles in 37 patients with a supraspinatus syndrome and partial or complete rupture of the rotator cuff underwent biopsy. All three groups of patients showed a reduction in size, increased variability, and change in the frequency distribution of fiber cross sections. The fast-twitch fibers were especially affected. These findings were confirmed objectively by morphometric measurements. It might be concluded from the changes mentioned above that disturbance in coordination of the musculature can cause a functional impingement. We term this neuromuscular insufficiency of the rotator cuff. The mechanical factors relating to subacromial impingement may only have a role in promoting its manifestation by restricting the compensatory range of the joint.