What is the role of the supinator?

• A. Pronates the forearm
• B. Flexes the elbow
• C. Extends the wrist
• D. Supinates the forearm, especially when the elbow is flexed

Answer: (D)

The key idea is forearm rotation: turning the palm up is supination, and the muscle that does this rotates the radius around the ulna. The supinator wraps around the upper part of the radius and, when it shortens, twists the radius to rotate the forearm so the palm faces upward. It’s particularly effective when the elbow is flexed, because in that position it provides strong, controlled rotation without relying on other muscles that are less advantaged in a flexed elbow. So the role of the supinator is to supinate the forearm, especially when the elbow is flexed. (Pronation is done by pronator muscles, elbow flexion by flexors like the biceps, and wrist extension by extensors—these aren’t the actions of the supinator.)