Which muscle is the primary external rotator of the glenohumeral joint?

• A. Infraspinatus
• B. Supraspinatus
• C. Teres minor
• D. Subscapularis

Answer: (B)

External rotation of the shoulder is driven mainly by the posterior rotator cuff muscles, especially the infraspinatus. This muscle sits on the back of the scapula and attaches to the greater tubercle of the humerus, pulling the humeral head outward to rotate the arm laterally. Teres minor also contributes to external rotation, reinforcing the movement as a secondary actor.

The supraspinatus, by contrast, is best known for initiating and assisting early abduction of the arm rather than rotating it externally. Subscapularis is an internal rotator, pulling the humerus inward. So the strongest external rotator is the infraspinatus, with teres minor providing additional support.