Which muscle is primarily responsible for pronation of the forearm?

The pronator teres is primarily responsible for the pronation of the forearm. This muscle runs from the medial epicondyle of the humerus and the coronoid process of the ulna to the midshaft of the radius. When the pronator teres contracts, it effectively rotates the radius over the ulna, resulting in the palm facing downward, which is the action of pronation.

In contrast, the biceps brachii plays a significant role in supination of the forearm rather than pronation. The supinator muscle is involved in supination as well and is not the primary muscle for pronating the forearm. The brachialis primarily functions in flexing the elbow rather than in the pronation-supination movement of the forearm.