The arm is a long lever and the elbow breaks it up into two manageable halves, appearing at first like a straightforward hinge with back and forth movement. However the elbow can do much more than this and performs complex and useful motions. The two bones of the forearm , the radius and the ulna, connect with the humerus to form the elbow. The elbow consists of the junction between the upper expansion of the ulna and the lower expansion of the humerus, and the ulna is easily palpated as the obvious bony prominence behind the elbow. The humeral part of the joint is a cylinder like shape and the ulnar part mirrors it.
The major part of the elbow joint is composed of the the humerus and the enlarged upper end of the ulna, which gradually gets smaller as it travels down towards the wrist where the much larger bone is the radius. Up at the elbow the radial head performs the rotational movements of the forearm which are functionally so useful. This added dimension to the mobile joints of the fingers, thumb and wrist allows the high level of mobility and co-ordination available to the hand. The soft tissues around the elbow are usually dominant in a flexing direction, giving a degree of natural elbow bend at rest.
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.
Rotation of many body joints is a vital movement to possess to aid the manipulation of things with the hands. The thumbs and fingers are very mobile but possess movement mostly in one direction of forwards and back, so the ability to rotate the limb to position the direction of movement correctly for the fingers to be effective is essential. Along with rotation, bending and straightening the elbow joint allows the wrist and hand to be placed precisely in space for complex activities. This function can have a downside as it is so useful we repeatedly perform quite narrow actions and risk overuse problems.
The wrist naturally extends when we reach out for something, bringing the fingers above the object to be grasped and allowing the fingers to exert their power best. Try and flex your wrist downwards, hold it there and grip something powerfully, it just doesn't work. Turning the forearm over so the palm is down is called pronation, and this activity only ever works against the weight of the arm to position the hand for light activities such as grasping small objects or writing.
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.