The bicep is the muscle that contracts the forearm and is commonly shown when flexing the arm. The tricep is on the opposite side as the bicep and is used to extend the forearm.
to flex the forearm at the elbow, you use the biceps brachii, brachialis, and brachioradial (to a lesser extent).
to supinate the forearm, you use the biceps brachii and the supinator...