Final answer:
The biceps brachii is a muscle in humans that crosses the shoulder and elbow joints to flex the forearm.
Step-by-step explanation:
The biceps brachii is a two-headed muscle that crosses the shoulder and elbow joints in humans. Its main function is to flex the forearm while also assisting in supinating it and flexing the arm at the shoulder joint. The brachialis muscle is deep to the biceps brachii and provides additional power in flexing the forearm.