The elbow joint is a hinge. The joint surface is covered by articular cartilage that protects and cushions the joints. The large muscle in the back of the arm, the triceps, attaches to the point of the ulna (called the olecranon). When this muscle contracts, it straightens out the elbow. The biceps muscle in the front of the arm, when contracted, bends the elbow.

The elbow joint is made up of the humerus (the upper arm bone) and the ulna and radius bones of the forearm. The ulna forms the part of the joint that allows the hinge motion of the elbow. Part of the radius bone of the forearm (the head of the radius) sits against the humerus and turns to allow the forearm to turn.

This motion is called pronation and supination. Pronation is the motion where you turn your palm down and supination is when you turn your palm up.