The elbow makes up the middle joint of the arm and makes two reasonable length and useful levers of the arm. At first glance the elbow looks like a simple forwards and backwards hinge but on looking more closely it is capable of a lot more. The upper arm bone (humerus) connects in the elbow with the two forearm bones, the ulna and the radius. The upper part of the ulna and the lower part of the humerus form the elbow joint, with the easily felt bony point of the ulna at the back of the joint. The end of the humerus has a rounded cylinder shape which is surrounded partially by the ulnar part of the joint.
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 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.
A similar example of a weak and strong muscle group in a joint is the foot dorsiflexors which pull the feet up so we can take a step and are much weaker than the propulsive calf muscles. Loss of power in the wrist extensor muscles impacts negatively on the ability to hold and grasp objects precisely and safely, inhibiting the strength of the main gripping muscles.
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.