What is the primary muscle involved in forearm pronation?

The primary muscle involved in forearm pronation is the pronator teres. This muscle is located in the anterior compartment of the forearm and is specifically designed to facilitate the action of pronating the forearm, which is the process of rotating the forearm so that the palm of the hand faces downward.

The pronator teres acts to pull the radius over the ulna during this motion, effectively allowing for the rotational movement that's characteristic of pronation. This muscle works in conjunction with another muscle, the pronator quadratus, which is also involved in the pronation process but the pronator teres is the more prominent one taking on this function during most forearm activities.

Other muscles mentioned, such as the supinator, are involved in opposing actions, specifically supination, which is the opposite movement. The biceps brachii primarily assists in forearm supination and elbow flexion rather than pronation. The triceps brachii mainly functions as an extensor at the elbow and does not contribute to forearm rotation. Understanding these specific functions of the muscles helps clarify why the pronator teres is the correct answer for the action of forearm pronation.