Muscle · Shoulders

Biceps brachii

Musculus biceps brachii

The famous two-headed muscle on the front of the upper arm. It bends the elbow, but its more distinctive job is supination — turning the palm upward — and it is by far the strongest supinator when the elbow is bent, which is why you turn a stiff screw or a doorknob with the elbow at a right angle rather than straight. Both heads come from the shoulder blade, so the biceps also crosses the shoulder; the long head's tendon runs inside the shoulder joint and down a groove at the top of the humerus, and that awkward course is why front-of-shoulder pain and biceps problems so often travel together.