Which rotator cuff muscle is the primary external rotator?

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

Answer: (C)

External rotation of the shoulder is produced mainly by the infraspinatus. Located on the posterior scapula, it attaches to the greater tubercle of the humerus and, when it contracts, pulls the humerus outward, rotating the arm away from the body and helping stabilize the humeral head in the glenoid during movement. Teres minor also helps with external rotation, but it’s smaller and acts as a secondary external rotator. Subscapularis sits on the front of the scapula and medially rotates the arm (internal rotation). Supraspinatus isn’t a primary external rotator; it mainly initiates abduction and assists with stabilization. So the infraspinatus is the primary external rotator among the rotator cuff muscles.