Which muscle is primarily responsible for forearm supination?

The muscle primarily responsible for forearm supination is the supinator. This muscle is situated in the forearm and plays a key role in rotating the radius bone around the ulna, allowing the palm to face upwards. The supinator works in conjunction with the biceps brachii during strong supination, but on its own, it is the primary muscle for this movement.

Other muscles mentioned, such as pronator teres, have entirely different functions. The pronator teres is primarily responsible for forearm pronation, which is the opposite movement to supination. The anconeus assists with elbow extension and does not have a significant role in supination. The subscapularis, while an important muscle in shoulder movement, does not influence supination of the forearm.

Therefore, the supinator is the clear choice for the muscle responsible for the action of forearm supination.