Which muscle is a powerful supinator of the forearm?

The biceps brachii is indeed recognized as a powerful supinator of the forearm. This muscle has two heads that originate from the scapula and insert on the radius. When the arm is flexed at the elbow, the biceps brachii plays a significant role in rotating the forearm, allowing the palm to face upward. This motion of supination is particularly effective when the forearm is in a flexed position.

While the supinator muscle also contributes to the supination of the forearm, its action is more subtle compared to the biceps brachii, especially when the elbow is flexed. The brachioradialis, primarily involved in the flexion of the elbow, does not play a direct role in supination. The triceps brachii is mainly responsible for elbow extension and does not contribute to supination.

In summary, the biceps brachii is the most powerful muscle for supination, especially when the forearm is flexed, making it the correct answer.