We experienced cases that can elevate the humerus but can't maintain the position in shoulder flexion. It is generally thought that the inner muscles stabilize the humeral head in the glenoid, and the deltoid, which is one of the outer muscles, elevates the humerus during shoulder flexion. Kido et al. recently reported that all portions including the anterior, middle and posterior deltoid muscles have functions in stabilizing the shoulder. In this study, in order to evaluate the activity of the deltoid muscle, not only the anterior portion but the middle and posterior portions were analyzed at several positions of flexion by surface electromyography (EMG). Our results show that the deltoid muscle activity of the anterior portion increased with flexion, and especially the activity at over 90°C flexion was significantly increased over that at 30°C flexion. The activity of the middle and posterior portions over 120°C flexion were significantly increased over those at 30°C and 60°C flexion. In addition, at 150°C flexion, the activities of the middle and posterior deltoid muscles were significantly increased over that at 90°C flexion. This study demonstrates that the anterior portion of the deltoid muscle, mainly has the function of active flexion, and the middle and posterior portions as well as the inner muscles have functions which stabilize the glenohumeral joint during shoulder flexion.
