The biceps pulls on the radius
The forearm has two bones: the ulna, which lines up with the little-finger side, and the radius, on the thumb side. Near the elbow, the biceps tendon attaches to the radius. When the biceps contracts, it pulls that attachment upward and helps rotate the radius around the ulna, bringing the palm upward. This rotation is called supination.
The same pull can bend the elbow because the biceps crosses the elbow joint. Which action stands out depends on what is held still: with the forearm free to rotate, the muscle can turn it; with the forearm fixed, its pull more directly bends the elbow.
Why a screwdriver recruits the biceps
Turning a screwdriver involves more than gripping the handle. To drive a screw with a palm-upward turning motion, the forearm muscles must rotate the radius, while the hand transmits that twist to the tool. The biceps contributes to the rotation, especially when the elbow is bent and the forearm is working against resistance. Other muscles also help stabilize the arm and produce the turn.
The biceps is therefore not simply an “elbow-bending” muscle. Its attachment gives it a useful pulling angle for both bending and rotating the forearm. That dual role is easy to feel in an ordinary task: hold your elbow bent, then turn your palm upward against gentle resistance. The tension at the front of the upper arm is the same muscle doing a different part of its job.
