An electromyographic analysis of the shoulder during cones and planes of arm motion.

Shoulder motion has traditionally been described in reference to arbitrarily defined planes of motion (e.g., sagittal/flexion, coronal/abduction). This study examined shoulder muscle recruitment during conical arm movements, which include all planes of motion. Electromyographic (EMG) data was collected with intramuscular wire electrodes from ten muscles in five normal volunteers. Each muscle showed peak EMG activity in association with a direction of action consistent with its anatomic alignment. These findings were similar for movements in both the sagittal and coronal planes, calling into question conventionally held designations of shoulder muscles as flexors and abductors. Muscle recruitment was as follows: Clavicular pectoralis major, to move the arm medially along the horizontal; anterior deltoid, to move the arm obliquely upward inclined toward the midline; middle deltoid, to move the arm obliquely upward inclined away from the midline; posterior deltoid, to move the arm laterally along the horizontal; and teres major and latissimus dorsi, to move the arm obliquely downward away from the midline. Rotator cuff muscles were maximally active during elevation, consistent with the concept of a force couple.