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The elbow joint is a hinge. 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. The joint sureface 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 muscles in the front of the are, when contracted, bends the elbow.
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