The role of shoulder muscles is task specific

The aim of this study was to compare activity in shoulder muscles during unsupported internal and external rotation to examine their functional roles, and determine whether they retain their ‘stabiliser’ or ‘mover’ role regardless of the shoulder task. Electromyographic recordings in the dominant shoulder of 15 normal subjects were taken from 13 shoulder muscle sites using a combination of surface and intramuscular electrodes during isometric shoulder internal and external rotation in an unsupported abducted position under conditions of increasing load. During internal rotation significantly higher activity levels were found in subscapularis (p < 0.001). During external rotation significantly higher activity levels were demonstrated in supraspinatus, infraspinatus, trapezius and serratus anterior (p < 0.05). There were no significant differences in activity levels in deltoid, pectoralis major and latissimus dorsi during internal and external rotation. As rotational forces increased there was a significant increase in activity in all muscle sites activated above low levels (r2 = 0.93 ± 0.07; mean ± s.d.). This study has shown that shoulder muscle function is task specific and that shoulder muscle motor strategy for a particular task does not change with increasing torque. As the only shoulder rotators that demonstrated direction specificity the rotator cuff muscles are likely to be functioning to provide rotation torque while the deltoid is likely to be providing dynamic shoulder stability during the task examined. Higher scapulothoracic muscle activity during external rotation indicated the need for greater dynamic scapular stability as a result of higher rotator cuff activity during external than internal rotation.