The rotator cuff muscles are very important for the motions and the dynamic stabilization of the shoulder girdle. The purpose of this study was to analyze the inner muscle activity during shoulder flexion and abduction by using surface and fine wire electromyography (EMG). EMG activity was examined in 5 static positions from 0 degrees to 120 degrees for 5 seconds in each position during exercises of shoulder flexion and abduction. We evaluated supraspinatus muscle activity on Needle EMG and infraspinatus muscle on surface EMG. 6 healthy volunteers participated in the study. EMG data were collected for analysis during maximal voluntary isometric contraction and 3 repetitions of each exercise. We used Telemyo System 2400(Noraxon USA Inc) and analyzed by MyoVideo and MyoResearch. The relative activity of both supraspinatus and infraspinatus muscles gradually increased during abduction.
The relative activity of infraspinatus muscle gradually increased during flexion. The relative activity of supraspinatus muscle at 90 degrees in flexion was less than at 60 degrees. This study shows that infraspinatus muscle has an important role in the dynamic stabilization, at 90 degree flexed position.
