Name the primary forearm supinator.

• A. Supinator
• B. Pronator teres
• C. Biceps brachii
• D. Extensor digitorum

Answer: (A)

Turning the palm to face up (forearm supination) is produced primarily by the supinator muscle. It lies deep near the elbow, wraps around the proximal radius, and when it contracts it rotates the radius over the ulna to achieve supination. In a straightened elbow, the supinator does the main work to rotate the forearm, making it the primary supinator. The biceps brachii can also supinate, especially when the elbow is flexed around 90 degrees, where its pull on the radius adds power, but as the primary mover in a neutral or extended elbow, the supinator does the job best. Pronator teres serves the opposite action, turning the forearm inward (pronation), while extensor digitorum mainly extends the fingers and doesn’t contribute to forearm rotation.