In human anatomy, the supinator is a broad muscle in the posterior compartment of the forearm, curved around the upper third of the

radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...

. Its function is to

supinate Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...

the forearm.

Structure

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 —'' Gray's Anatomy'' (1918), see infobox from the

supinator crest The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...

of the

ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...

, the lateral epicondyle of

humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a roun ...

, the radial collateral ligament, and the annular radial ligament. 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 pronator teres is a muscle (located mainly in the forearm) that, along with the pronator quadratus, serves to pronate the forearm (turning it so that the palm faces posteriorly when from the anatomical position). It has two attachments, to ...

. 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. The proximal aspect of the superficial head is known as the

arcade of Frohse Arcade of Frohse, sometimes called the supinator arch, is the most superior part of the superficial layer of the supinator muscle, and is a fibrous arch over the posterior interosseous nerve. The arcade of Frohse is a site of interosseous poster ...

or the supinator arch.

Innervation

It is innervated by the deep branch of the radial nerve. The deep branch then becomes the

posterior interosseous nerve The posterior interosseous nerve (or dorsal interosseous nerve) is a nerve in the forearm. It is the continuation of the deep branch of the radial nerve, after this has crossed the supinator muscle. It is considerably diminished in size compared ...

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 Paralysis (also known as plegia) is a loss of motor function in one or more muscles. Paralysis can also be accompanied by a loss of feeling (sensory loss) in the affected area if there is sensory damage. In the United States, roughly 1 in 5 ...

of the muscles served by this nerve (the extensor muscles and the abductor pollicis longus.) Many possible causes are known for this nerve syndrome, known as supinator entrapment syndrome, including compression by various soft-tissued masses surrounding the nerve, and stress caused by repetitive supination and pronation.

Variation

The deep radial nerve passes through the belly of supinator in 70% of cases and via the

in remaining cases.

Function

Encircling the radius, 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. Supinator always acts together with biceps, except when the elbow joint is extended. It is the most active muscle in forearm supination during unresisted supination, while biceps becomes increasingly active with heavy loading. Supination strength decreases by 64% if supinator is disabled by, for example, injury.

History

Etymology

The term "supinator" can also refer more generally to a muscle that causes supination of a part of the body. In older texts, the term "supinator longus" was used to refer to the brachioradialis, and "supinator brevis" was used to describe the muscle now known as the supinator.

Additional images

Notes

References

* * * * * {{Authority control Muscles of the upper limb