The elbow joint, radioulnar joint and wrist
The elbow is a hinge joint, with the distal end of the humerus articulating with the proximal end of both the radius and ulna. On the proximal end of the ulna is the olecranon process and this fits into the olecranon fossa on the distal end of the humerus. This feature of the joint prevents the elbow joint form hyperextending.
The joint is strengthened by four ligaments, and as the elbow is a hinge joint, movement is possible in one plane only, allowing only flexion and extension to take place.
The muscles on the posterior (back) of the arm are extensors. The prime mover of elbow extension is the triceps brachii muscle, and is assisted by the much smaller anconeus muscle. All anterior (front) arm muscles cause elbow flexion. These muscles are the biceps brachii, brachialis and brachioradialis. The biceps and brachialis contract simultaneously during flexion, and are the chief elbow flexors. The brachialis lies just underneath the biceps brachii and is as important when flexing the elbow.
The biceps muscle also supinates the forearm (turning the palm so it faces up), and is ineffective in flexing the elbow when the forearm is supinated (palm facing down). This is why performing chin ups with the palms facing the athlete is easier than when the palms are facing away. The bulk of the brachioradialis muscle resides mostly in the forearm, and because of this it is quite a weak elbow flexor. It is used when the elbow has been partially flexed by the biceps and brachialis, and the forearm is semi-pronated (palms facing inwards as in the ‘hammer curl’).
The wrist is a condyloid joint, where the distal end of the radius and ulna articulate with 3 of the carpal bones – the navicular, lunate and triquetrum bones. The wrist joint can only flex, extend, abduct and adduct.
The forearm muscles can be divided into two roughly equal groups depending on their function. The first group consists of those forearm muscles that cause wrist movements, and the second consists of those that move the fingers and thumbs. Although many forearm muscles have their origin on the humerus (crossing both the elbow and wrist joints), they have little effect when flexing the elbow.
Like the upper arm, the muscles of the forearm are separated into two compartments, the anterior flexors and posterior extensors. Although most of the anterior forearm muscles cause flexion of the wrist or fingers, there are two muscles in this group which cause pronation. These muscles are the pronator teres and the pronator quadratus.
In contrast, the posterior forearm muscles extend the wrist and fingers. The only exception to this is the supinator muscle, which helps the biceps brachii when supinating the forearm.
Although many different hand movements are possible, the hand contains relatively few of the muscles that control these various movements, these muscles being in the forearm. This helps the hand to be less bulky and more manoeuvrable. The actions of the forearm muscles are assisted and made more precise by small muscles within the hand.