Electromyographic analysis of the deltoid and rotator cuff muscles in persons with subacromial impingement

The purpose of this study was to compare subjects with subacromial impingem ent and subjects with normal shoulders with respect to muscle activity Fift een subjects in each group were studied by means of Fine-wire electromyogra ph. The middle deltoid and rotator cuff muscles were evaluated during isoto nic scaption from 30 to 120 degrees. Overall, the impingement group demonst rated decreased mean muscle activity in comparison with the group of normal subjects. The magnitude of diminished activity was statistically significa ntly different (P < .05) during the 30- to 60-degrees ore fbr the infraspin atus, subscapularis, and middle deltoid muscles; in addition, the infraspin atus muscle demonstrated significantly depressed activity during the 60- to 90-degrees ore. In the impingement group, the supraspinatus and teres mino r revealed a diminution of muscle function in comparison with shoulders in the normal group; the difference wets not significant. This study demonstra tes that muscle activity in subjects with impingement is most notably decre ased in the first arc of motion. Also of clinical relevance is the fact tha t the inferior force vector (from the infraspinatus and subscapularis) is l ess functional in subjects with impingement than is the superior compressiv e vector (from the supraspinatus). Thus, humeral head depression during the critical first portion of elevation may be insufficient in people with sub acromial impingement.