Final answer:
The brachialis muscle is located deep to the biceps brachii on the humerus and assists in flexing the forearm by providing additional power.
Step-by-step explanation:
The muscle that is deep to the biceps brachii and lies on the humerus is the brachialis muscle. This muscle provides power in flexing the forearm and is located deep to the biceps brachii. While the biceps brachii is a well-known muscle that crosses the shoulder and elbow joints to flex the forearm, assist in supinating it, and flex the arm at the shoulder, the brachialis is a synergist muscle that plays a crucial role in forearm flexion by adding additional power to the movement.