The function of the biceps brachii muscle is to help the arm to bend at the elbow, according to WebMD. The muscle does this by extending and contracting as it moves back and forth.
The biceps brachii is a muscle that lies along the inner side of the upper arm. It connects to the radial tuberosity of the radius, and the two heads join together in the middle of the humerus. Although a function of this muscle is to flex the elbow, the primary function is to turn the palm upward, or supinate. The action occurs in conjuction with actions of the supinator muscle. During supination, the elbow must be flexed at least partially. It is not the most powerful muscle of the forearm because the deeper brachialis muscle is a more powerful one.