Which muscle is listed with the action of pronation of the forearm?

• A. Pronator Teres
• B. Biceps Brachii
• C. Deltoid
• D. Masseter

Answer: (A)

Pronation of the forearm is produced by rotating the radius around the ulna at the radioulnar joints. The muscle that does this action most directly is the pronator teres, a superficial forearm muscle that attaches near the medial elbow and inserts on the lateral radius. It actively turns the palm downward and, when the elbow is flexed, assists with elbow flexion. The biceps brachii, while a strong elbow flexor, mainly acts as a supinator of the forearm when the elbow is flexed, turning the palm upward rather than downward. The deltoid is a shoulder muscle involved in lifting and rotating the arm at the shoulder, not forearm rotation. The masseter moves the jaw and has no role in forearm movements.