Electromyographic activity of the shoulder muscle at 20 and 90°abduction (20 Abd, 90 Abd) during external rotation was investigated in seven healthy men with no history of injury or instability of the shoulder joint.
Electromyography (EMG) was recorded using intramuscular fine-wire electrodes inserted into the M. Supraspinatus, M. Infraspinatus and M. Teres minor, and with bipolar surface electrodes on the middle and posterior parts of M. Deltoid anti the upper and middle parts of M. Trapezius. To compare activity in different muscles, the integrated EMG (iEMG ) activity of each muscle was normalized.
M. Infraspinatus and M. Teres minor showed significantly higher activity at both the 20 Abd and 90Abd compared with the middle and posterior parts of M. Deltoid and upper parts of M. Trapezius. M. Supraspinatus, the middle and posterior parts of M. Deltoid, and upper and middle parts of M. Trapezius all showed a difference in activity level between the two positions.
These findings suggest that when M. Infraspinatus and M. Teres minor contribute to external rotation as a stabilizer and prime mover, consecutively, M. Supraspinatus, the middle and posterior parts of M. Deltoid, and upper and middle parts of M. Trapezius function according to the positions. Moreover, the activity of the upper and middle parts of M. Trapezius in 90Abd should influence stabilization, adduction and upward rotation of the scapula. Therefore, we conclude that the external rotation position is closely related to shoulder muscle activity and coordination.
