The elbow is the central joint of the arm and divides it up into two convenient halves, looking initially merely like a backwards and forwards hinge. On closer examination the elbow shows itself to be a remarkably clever and multi-talented joint. The humerus or upper arm bone connects with the two lower arm bones, the radius and the ulna. The elbow joint itself is formed from the lower end of the humerus and the upper end of the ulna which makes up the bony point of the elbow which can be easily felt. The upper end of the ulna has a ladle like shape which carries in its bowl the end of the humerus.

At the elbow the main component of the forearm joint is made up of the ulna which is a large expanded area, narrowing down to the ulnar head at the wrist where the radius is now much larger. Rotation of the lower arm is the main job of the radial joint with the humerus at the elbow, allowing the very useful twisting action of the lower arm. Along with the highly mobile and coordinated wrist, fingers and thumb, the whole unit makes a highly functional tool of the hand-arm complex. Elbows typically show an angle of bend when they are at rest from tension in the surrounding soft tissues.

The upper end of the radius, which is positioned at the outside of the elbow when the palm is facing upwards, is shaped like a small cotton reel with the top against a bony protrusion on the base of the humerus. This part of the bone is called the head of the radius and is firmly attached to the ulna next to it by the radial ligament, a circular band of strong tissue encompassing the head and allowing it to rotate without slipping from its joint. When the forearm rotates it is the radius which does the vast majority of the work, starting from a position parallel to the ulna and then rotating over and inwards around it until the hand is in a palm down position.

The ability to rotate the forearm is a vital ability in the manipulation of objects. Much human activity and dexterity involves the thumb and fingers which are relatively restricted to forward and back movement. This means other movements are necessary to allow the endlessly varied positions we need to access with our hands. Elbow function combines flexing and extending the joint in combination with rotation, allowing smooth and coordinated movement of the hand. This precision and adaptability can have a negative side because it is so useful it can be repeated many times and to excess.

The action of reaching out for something in front of us demands that we stretch out the elbow, turn the hand palm down and lift the fingers above the object to be gripped, a position which also allows the optimum hand power to be exerted. Trying to grip something strongly when the wrist is bent down is very ineffective and the force available is weak. Turning the forearm over so the palm faces down is known as pronation, and these muscles are weak as they have little more to do than turn the weight of the arm.

The pronators' lack of strength is a bit like the calf muscles which pull our feet up as we step and are much less powerful than the calf muscles which propel us. An inability to extend the wrist or pronate the forearm makes the process of gripping and holding objects difficult and awkward, as the main muscles of gripping are unable to exert full power.

Elbow supination and flexion, the opposite of pronation and extension, occurs when the elbow is actively bent and the palm brought to face up. Typical functions involving this movement are taking food to the mouth with a fork and screwing in screws, so this is both a very common action indeed and one with much more strength than pronation. Supination and flexion of the elbow is primarily performed by the biceps muscle with a contribution from a smaller but strong muscle termed supinator. The common extensor origin is the area on the outer part of the elbow which has the origin of the extensor muscles of the wrist and supinator.