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The brachioradialis is a muscle of the forearm that flexes the forearm at the elbow. [1] [2] It is also capable of both pronation and supination, depending on the position of the forearm. [2] It is attached to the distal styloid process of the radius by way of the brachioradialis tendon, and to the lateral supracondylar ridge of the humerus.
Pronation at the forearm is a rotational movement where the hand and upper arm are turned so the thumbs point towards the body. When the forearm and hand are supinated, the thumbs point away from the body. Pronation of the foot is turning of the sole outwards, so that weight is borne on the medial part of the foot. [33]
The radius can thus glide on the capitulum during elbow flexion-extension while simultaneously rotate about its own main axis during supination-pronation. [2] Between the capitulum and the trochlea of the humerus is the capitulotrochlear groove. A semi-lunar surface around the circumference of head is shaped to articulate continuously with this ...
Though the elbow is similarly adapted for stability through a wide range of pronation-supination and flexion-extension in all apes, there are some minor differences. In arboreal apes such as orangutans , the large forearm muscles originating on the epicondyles of the humerus generate significant transverse forces on the elbow joint.
Supinator always acts together with biceps, except when the elbow joint is extended. [7] It is the most active muscle in forearm supination during unresisted supination, while biceps becomes increasingly active with heavy loading. [8] Supination strength decreases by 64% if supinator is disabled by, for example, injury. [9]
Additionally, as the forearm moves from pronation to supination, the interosseous membrane fibers change from a relaxed state, to a tense state in the neutral position. They once again become relaxed as the forearm enters pronation. The interosseous membrane is composed of five ligaments:
This causes significant pain, partial limitation of flexion/extension of the elbow and total loss of pronation/supination in the affected arm. The situation is rare in adults, or in older children, because the changing shape of the radius associated with growth prevents it.
These three muscles act as flexors at the elbow joint. [5] The extensor carpi radialis brevis and longus are both weak flexors at the elbow joint. Brevis moves the arm from ulnar abduction to its mid-position and flexes dorsally. Longus is a weak pronator in the flexed arm and a supinator in the outstretched arm.
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