The Human Elbow Joint
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.
The ulna forms the main part of the joint near the elbow and narrows down to the head at the wrist end, which is dominated by the expanded end of the radius. The radial head, at the elbow end, is concerned with rotation of the forearm and the radius is the bone which performs most of the swiveling action that we find so useful in our forearms. This mobility, when combined with the mobility of the wrist and of the fingers and thumb, makes the hand the precise and dominating tool it has become. All elbows exhibit a slight degree of bend when they are relaxed, due to soft tissue tension.
A bony protrusion on the cylindrical end of the humerus articulates with the upper part of the radius which is on the outer side of the elbow when the hand is facing upwards. The radial head is the other part of that articulation, a rounded part somewhat like a cotton reel, placed close to the ulna and held in place during rotatory movements by the radial ligament, a band of ligament surrounding the head. Forearm rotation is predominantly performed by the radius, beginning with it lying in parallel with the ulna and then turning and crossing over that bone to achieve the full pronation movement with the palm down.
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 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.
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.
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.