ROTATIONAL ACTION OF THE SUPRASPINATUS MUSCLE ON THE SHOULDER JOINT

The shoulder joint allows three-dimensional movement. In order to anal yze the function of the muscles which act on the shoulder joint, three -dimensional movements, including rotation, must be considered. Among muscles participating in the shoulder joint movement, the supraspinatu s muscle is known to have abduction and stabilization effects on the s houlder joint. However, the rotational function of the supraspinatus m uscle has not been identified, because few studies have been reported on it. This study investigates the rotating function of the supraspina tus muscle using electrical stimulation, magnetic resonance imaging (M RI) and anatomical examination. Electrical stimulation was applied sel ectively to the supraspinatus muscle of healthy subjects using percuta neous wire electrodes. The electrical stimulation was given at differe nt positions of the shoulder joint. It was found that the electrically induced rotational movements changed their direction depending on the position of the shoulder joint. When the humerus was relatively in in ternal rotation, internal rotation resulted. When it was in external r otation, external rotation occurred. Regarding the abduction angle of the shoulder joint, external rotation was induced with an increase in the abduction angle, whereas internal rotation occurred when the abduc tion angle was decreased. By the dissection of cadavers and MRI examin ation, it was indicated that the relation between the running directio n of the supraspinatus muscle and the center of rotation of :he humera l head was dependent on the position of the shoulder joint. Those find ings supported the results of electrical stimulation of the supraspina tus muscle at various shoulder positions. These results indicate that the bi-directional rotating function of the supraspinatus muscle is ch aracterized by an anatomical relationship between the running directio n of the supraspinatus muscle and the center of rotation of the humera l head. (C) 1998 Elsevier Science Ltd. All rights reserved.