What is the primary supinator of the forearm?

The biceps brachii is primarily responsible for supinating the forearm. During the action of supination, the biceps brachii contracts, helping to rotate the radius over the ulna. This muscle is particularly effective in supination when the elbow is flexed, which allows for greater leverage and force.

The anatomy of the biceps brachii, with its two heads originating from the scapula and its insertion on the radial tuberosity, contributes to its function. When the biceps brachii contracts, it pulls the radius, resulting in the supination movement that turns the palm upward.

In contrast, other muscles listed do not primarily perform this action. The pronator teres is mainly involved in pronation, while the triceps brachii and brachialis are focused on elbow extension and flexion, respectively, without a role in supination.