Which muscle is the prime mover for Shoulder Internal Rotation?

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

Answer: (A)

Internal rotation of the shoulder is driven most strongly by the subscapularis, the muscle on the anterior surface of the scapula that inserts on the lesser tubercle of the humerus. When it contracts, it pulls the humerus inward, rotating the arm medially toward the body. This makes it the prime mover for internal rotation among the listed muscles.

The other muscles listed contribute in different ways: infraspinatus and teres minor are external rotators, attaching on the posterior side to rotate the humerus outward; supraspinatus mainly initiates and assists with abduction rather than rotation. Because subscapularis has the most effective line of pull to medially rotate the humerus, it sits at the top for internal rotation.