What is a primary action of the infraspinatus muscle?

• A. Medial rotation of the shoulder and stabilization of the humerus in the glenoid cavity
• B. Lateral rotation of the shoulder, adduction of the shoulder, and stabilization of the humerus in the glenoid cavity
• C. Abduction of the shoulder
• D. Flexion of the elbow

Answer: (B)

The main function of the infraspinatus is to externally rotate the arm at the shoulder and to stabilize the humeral head within the glenoid cavity during movement. Because it sits on the posterior scapula and inserts on the greater tubercle of the humerus, contracting this muscle twists the humerus outward (lateral rotation) and helps keep the head of the humerus centered in the socket as the shoulder moves. In addition to external rotation, it can contribute to adduction depending on arm position, and stabilization is a constant, crucial role to prevent the humeral head from sliding out of the glenoid. It does not flex the elbow, which is controlled by other muscles.