Which muscles are the prime movers for shoulder external rotation?

• A. Infraspinatus and Teres Minor
• B. Supraspinatus and Deltoid
• C. Subscapularis and Teres Major
• D. Latissimus Dorsi and Teres Major

Answer: (A)

External rotation of the shoulder is produced mainly by the posterior rotator cuff muscles that pull the humeral head laterally, with the strongest contribution coming from the infraspinatus and teres minor. The infraspinatus sits on the infraspinous fossa and attaches to the greater tubercle of the humerus, making it the primary external rotator across most positions. Teres minor, though smaller, attaches near the same area and adds important external rotation, especially as the arm moves away from the body and helps stabilize the joint. The other muscles listed are more involved in internal rotation (subscapularis, teres major, latissimus dorsi) or in abduction (supraspinatus, deltoid) rather than external rotation, so the combination of infraspinatus and teres minor best fits the role of prime movers for this action.