Which muscle is primarily responsible for supinating the forearm?

The biceps brachii is primarily responsible for supinating the forearm due to its anatomical positioning and functional capabilities. When the forearm is in a pronated position, contraction of the biceps brachii not only flexes the elbow but also effectively rotates the radius laterally over the ulna, resulting in supination.

The muscle's dual action is enhanced when the elbow is flexed, making it a powerful supinator during activities that require lifting and turning the palm upward. The biceps brachii has a specific alignment that allows it to exert optimal force for this motion, distinguishing it as the main muscle for supination over others. In contrast, the other muscles listed have different primary functions related to elbow flexion or forearm pronation.