As. Reddy et al., Electromyographic analysis of the deltoid and rotator cuff muscles in persons with subacromial impingement, J SHOUL ELB, 9(6), 2000, pp. 519-523
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.