Biceps brachii as a supinator of the forearm

The biceps brachii is described as the main muscle for supination of the forearm. Supination is the movement that turns the forearm so the palm faces upwards when the elbow is bent. For supination, what matters is the lower attachment of the muscle (its moving end), not the origin (where it begins).

The muscle has two heads, both beginning on the scapula (the shoulder blade). Those origins explain what the muscle does at the shoulder, but not what it does at the forearm. For that, look at where the tendon inserts, meaning where it attaches at its moving end. It attaches to the radial tuberosity, a rough bump on the upper radius, the forearm bone on the thumb side.

The tendon pulls on one side of the radius rather than straight along it, so contraction twists the radius around the ulna. That twisting is supination.

Supination by the biceps is described as most powerful with the elbow flexed. With the elbow fully extended (straight), the supinator muscle, a smaller muscle deep in the forearm, does most of the work.

The biceps also flexes the elbow and the shoulder, because it crosses both joints, but these are secondary actions that do not turn the forearm. Pronation is the opposite movement, turning the palm downwards. In the forearm, the muscles pronator teres and pronator quadratus produce it.

For a memory aid, think of a screwdriver: in the right hand, supination turns it clockwise, which tightens an ordinary screw.