Muscle · Upper arm

Brachialis

Musculus brachialis

The muscle lying underneath the biceps, and the real workhorse of bending the elbow. Because it runs from the humerus to the ulna it crosses only the elbow and cannot rotate the forearm at all, so it flexes the elbow in every position of the hand — palm up, palm down or thumb up — while the biceps fades whenever the palm is turned down. It has a larger cross-section than the biceps, so most of the force in an everyday lift comes from a muscle you cannot see.