Supinator is a spiral muscle contained in the posterior compartment of the forearm, along with brachioradialis, and the superficial and deep forearm extensors.
Supinator muscle curls around the proximal part of the radius, connecting it with the ulna. In doing so, the muscle crosses the sagittal axis of the forearm. This interesting anatomy enables the supinator muscle to rotate the radius laterally, producing a movement known as forearm supination. Its famous antagonist is the pronator quadratus muscle which rotates the radius medially, producing the opposite movement of pronation.
In human anatomy, the supinator is a broad muscle in the posterior compartment of the forearm, curved around the upper third of the radius. Its function is to supinate the forearm.[rx]
Structure of Supinator
Supinator consists of two planes of fibers, between which the deep branch of the radial nerve ls. The two planes arise in common — the superficial one by tendinous (the initial portion of the muscle is actually just tendon) and the deeper by muscular fibers —[rx] from the supinator crest of the ulna, the lateral epicondyle of humerus, the radial collateral ligament, and the annular radial ligament.[rx]
The superficial fibers (pars superficialis) surround the upper part of the radius and are inserted into the lateral edge of the radial tuberosity and the oblique line of the radius, as low down as the insertion of the pronator teres. The upper fibers (pars profunda) of the deeper plane form a sling-like fasciculus, which encircles the neck of the radius above the tuberosity and is attached to the back part of its medial surface; the greater part of this portion of the muscle is inserted into the dorsal and lateral surfaces of the body of the radius, midway between the oblique line and the head of the bone.[rx]
The proximal aspect of the superficial head is known as the arcade of Frohse or the supinator arch.
Origin and Insertion of Supinator
The wide supinator muscle consists of superficial and deep layers. These layers differ only in their mode of attachment; the superficial layer arises by tendinous fibers, while the deep layer originates with already formed muscular slips.
Both layers originate from the same landmarks. These involve several osteofibrous structures of the elbow; lateral epicondyle of the humerus, radial collateral ligament of the humeroulnar joint, annular ligament of the superior radioulnar joint, supinator crest of ulna and the adjacent part of the ulnar fossa.
Blood Supply of Supinator
The superficial and deep layers of the supinator muscle are supplied by two different sources;
- The superficial layer receives blood from the radial artery, via its radial recurrent branch
- The deep layer is supplied by the ulnar artery, through its posterior interosseous and posterior interosseous recurrent arteries
Nerve Supply
It is innervated by the deep branch of the radial nerve. The deep branch then becomes the posterior interosseous nerve upon exiting the supinator muscle. Its nerve roots are primarily from C6, with some C5 involvement. There is also possible additional C7 innervation.
The radial nerve divides into deep and sensory superficial branches just proximal to the supinator muscle — an arrangement that can lead to entrapment and compression of the deep part, potentially resulting in selective paralysis of the muscles served by this nerve (the extensor muscles and the abductor pollicis longus.)[rx] Many possible causes are known for this nerve syndrome, known as supinator entrapment syndrome, including compression by various soft-tissue masses surrounding the nerve, and stress caused by repetitive supination and pronation.[rx]
Function
Encircling the radius, the supinator brings the hand into the supinated position. In contrast to the biceps brachii, it is able to do this in all positions of elbow flexion and extension.[rx]
The supinator always acts together with biceps, except when the elbow joint is extended.[rx] It is the most active muscle in forearm supination during unresisted supination, while biceps become increasingly active with heavy loading. Supination strength decreases by 64% if the supinator is disabled by, for example, injury.[rx]
The Supinator muscle rotates the radius laterally at the proximal radioulnar joint. This action puts the radius parallel to the ulna, therefore bringing the hand into the supine position (facing anteriorly, palm up, like holding a bowl of soup).
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