Final answer:
The biceps brachii, brachialis, and brachioradialis function as synergists to flex the forearm. The biceps brachii also assists with supination and shoulder flexion, while the brachialis adds power to forearm flexion.
Step-by-step explanation:
The biceps brachii, brachialis, and brachioradialis are three muscles that work together to flex the forearm. The biceps brachii is a well-known two-headed muscle that crosses the shoulder and elbow joints, allowing it not only to flex the forearm but also to assist in supinating the forearm at the radioulnar joints and flexing the arm at the shoulder joint. The brachialis muscle, positioned deep to the biceps brachii, provides considerable power during forearm flexion. Lastly, the brachioradialis, found in the forearm, can also flex the forearm either quickly or assist in lifting a load slowly. These muscles are synergists when it comes to flexing the forearm, meaning they work together to accomplish this movement.