What is the primary role of the supraspinatus in shoulder motion?

• A. Initiates shoulder abduction (first ~15 degrees) and stabilizes the glenohumeral joint.
• B. Extends the shoulder and adducts.
• C. Medially rotates the humerus.
• D. Pronates the forearm.

Answer: (A)

The main idea being tested is the ability of the supraspinatus to start shoulder abduction and to stabilize the glenohumeral joint. The supraspinatus sits in the supraspinous fossa and pulls the humeral head into the glenoid as the arm begins to lift. This creates the initial upward (abducting) moment for the first roughly 15 degrees, before the deltoid takes over most of the lifting. At the same time, it steadies the head of the humerus within the socket, preventing it from migrating upward as the arm elevates, which is crucial for smooth, stable motion.

Other motions listed don’t describe what the supraspinatus primarily does: extending the shoulder and adducting involve other muscles like the latissimus dorsi and pectoralis major; medially rotating the humerus involves muscles such as the subscapularis and teres major; pronating the forearm is a forearm movement, not a shoulder action.