Which muscles are responsible for forearm pronation?

• A. Supinator
• B. Pronator teres and pronator quadratus
• C. Pronator teres
• D. Biceps brachii

Answer: (B)

Forearm pronation is the rotation that turns the radius over the ulna, so the palm faces downward. The muscles that accomplish this are the pronators: pronator teres and pronator quadratus. Pronator quadratus sits in the distal forearm as a square muscle, and when it contracts it pulls the distal radius across the ulna, producing pronation with a reliable, direct action. Pronator teres originates higher up, crossing the forearm obliquely from the medial side, and it assists by rotating the radius as well—especially useful in rapid or resisted pronation and when the elbow is flexed. The other muscle named, the supinator, does the opposite action, rotating the radius to turn the palm upward, and the biceps brachii can aid supination when the elbow is bent. So the combination of pronator teres and pronator quadratus best explains forearm pronation.