Which muscle stabilizes the head of the humerus in the glenoid cavity?

• A. Infraspinatus
• B. Subscapularis
• C. Levator scapula
• D. Trapezius

Answer: (B)

Dynamic stabilization of the humeral head within the glenoid relies on the rotator cuff muscles pulling the head of the humerus into the socket as the shoulder moves. The muscle that sits on the anterior surface of the scapula, attaches to the lesser tubercle of the humerus, and acts to press the head of the humerus into the glenoid is the subscapularis. This anterior stabilizer helps prevent forward (anterior) translation of the humeral head, especially during movements that challenge the joint like external rotation and abduction. The other options either act mainly on the posterior aspect (infraspinatus and teres minor), assist with different motions (levator scapulae elevates the scapula), or stabilize the scapula without directly centering the humeral head in the socket (trapezius).