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.
If the palm is facing upwards the radius is along the outside of the forearm and the upper part of the bone is formed into a rounded prominence resembling a cotton reel, articulating with a bony area on the lower humerus. This radial head is closely positioned next to the ulna just inside it and there is a circular structure of ligamentous material, the radial ligament, which surrounds the head and holds it in place as it rotates. The radius performs the greatest amount of movement in forearm rotation, starting in line with the ulna and ending up rotated over it so the radius is on the inner side and the hand is palm downwards.
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.
When we decide to reach forward for something we extend the elbow, turn the hand over, extend the wrist and position the fingers above the object, permitting at the same time the maximum power from the fingers. If you try and grip something firmly whilst flexing the wrist downwards you will find yourself unable to exert much strength. Pronation is the name given to the action of twisting the forearm so the palm faces down, and as these muscles only do this action they are not very strong.
This means that the pronators are not powerful muscles as the only job they have to do is position the arm, a bit like the muscles which pull our feet up when walking. A lack of pronation or the ability to extend the wrist back when reaching makes gripping objects a clumsy procedure, with the primary power muscles of gripping inhibited in use.
The opposite movement, that of bending the elbow and turning the palm up, is known as flexion and supination. It is a much stronger movement than pronation and performed many times during a typical day from bringing food up to the mouth to turning a screwdriver. The biceps is the main muscle which flexes the elbow and it also supinates the forearm, with a further powerful supination action coming from a small muscle near the elbow joint called the supinator. The supinator originates in the same areas as the extensor muscles of wrist, an area called the common extensor origin.
Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapy, back pain, orthopaedic conditions, neck pain, injury management and Physiotherapists in Coventry. Jonathan is a superintendant physiotherapist at an NHS hospital in the South-West of the UK.
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