The Elbow Joint
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.
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.
For manipulating objects it is very important to have the abilities of joint rotation. The thumb and fingers, the most important part of human coordination and precision, are relatively restricted in their freedom of movement. In order to permit the vast number of varying positions which are required to manage hand use, rotation is a vital function. Extension and flexion of the elbow joint, when combined with rotation, allows the hand to be placed in position with precision. At times this ability can be unhelpful as its usefulness may mean we repeat the exact same action a large number of times.
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.