Which is a primary action of the subscapularis?

• A. Lateral rotate the shoulder
• B. Protract the scapula
• C. Abduct the arm
• D. Medially rotate the shoulder, stabilize the head of the humerus in the glenoid cavity

Answer: (D)

Subscapularis primarily medially rotates the arm and helps stabilize the head of the humerus in the glenoid cavity during shoulder movement. This muscle lies on the anterior scapula and inserts into the lesser tubercle of the humerus, making its strongest action internal rotation of the shoulder. Holding the humeral head centered in the glenoid is crucial for joint stability, especially when the arm is moving through various directions.

Other options correspond to actions of different muscles: lateral rotation is mainly done by the posterior cuff muscles (like infraspinatus and teres minor), protracting the scapula by serratus anterior (and others), and abducting the arm by supraspinatus early on or by deltoid later. The described combination—medial rotation with stabilization of the humeral head—is the hallmark function of this muscle.