Biceps brachii

What it is

The two-headed muscle on the front of the upper arm, and the most recognised muscle in the body.

Both heads arise from the scapula, not the humerus — the long head from just above the shoulder socket, its tendon running through the joint itself and down the groove between the tubercles of the humerus; the short head from the coracoid process. They join into one belly and insert onto the radius, at the radial tuberosity.

So this muscle spans from shoulder blade to forearm and has no attachment to the upper arm bone it lies on. That is unusual and it is why it acts on both the shoulder and the elbow.

A flat sheet of connective tissue, the bicipital aponeurosis, spreads from its tendon across the forearm flexors.

What it does

Three things.

Supinates the forearm — turns the palm upward — and it is the strongest supinator you have. Because it attaches to the radius, contracting it spins that bone. This is its most powerful action, and it is strongest with the elbow bent to a right angle.

Flexes the elbow, though it is not the main muscle for that — brachialis underneath it does more, particularly when the forearm is palm-down.

Assists shoulder flexion, via both heads crossing the joint.

The supination-plus-flexion combination is the motion of turning a screwdriver, or of raising a glass. Right-hand screw threads are conventionally tightened clockwise, which suits the stronger supination of a right arm — a frequently repeated and pleasing claim, though as an explanation of the convention it is folk history rather than documented fact.

What loads it

Curling with the palm up, pulling movements, and any resisted supination.

The Repertoire has no arm movement yet.

What is commonly got wrong about it

That it is the main elbow flexor. Brachialis is, by cross-section and by leverage. Biceps contributes, and contributes most when the palm faces up.

"Peak" as a training outcome. The height of the bump when you flex is determined by the length of the muscle belly and where its tendon starts — anatomy you were born with. Training enlarges the muscle; it does not lengthen the belly or move the tendon.

That the long head tendon problem is a muscle problem. Pain at the front of the shoulder frequently involves this tendon where it runs in its groove, and it commonly accompanies other shoulder findings. That is a clinical matter.

That it has two heads because it does two jobs. The heads share an insertion and act together.