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 or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...

. 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 relativ ...

the forearm.

Structure

The supinator consists of two planes of fibers, between which passes the deep branch of the radial nerve. The two planes arise in common—the superficial one originating as tendons and the deeper by muscular fibers—from the supinator crest of the

ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

, the lateral epicondyle of the

humerus The humerus (; : humeri) 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 (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

, 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 upper fibers (''pars profunda'') of the deeper plane form a sling-like

fasciculus ''Fasciculus vesanus'' is an extinct species of stem-group ctenophores known from the Burgess Shale of British Columbia, Canada. It is dated to and belongs to middle Cambrian strata. The species is remarkable for its two sets of long and sho ...

, 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 or the supinator arch.

Innervation

It is innervated by the deep branch of the

radial nerve The radial nerve is a nerve in the human body that supplies the posterior portion of the upper limb. It innervates the medial and lateral heads of the triceps brachii muscle of the arm, as well as all 12 muscles in the Posterior compartment of the ...

. 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 Paralysis (: paralyses; also known as plegia) is a loss of Motor skill, motor function in one or more Skeletal muscle, muscles. Paralysis can also be accompanied by a loss of feeling (sensory loss) in the affected area if there is sensory d ...

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 arcade of Frohse in remaining cases.

Function

Encircling the radius, supinator brings the hand into the supinated position. In contrast to the

biceps brachii The biceps or biceps brachii (, "two-headed muscle of the arm") is a large muscle that lies on the front of the upper arm between the shoulder and the elbow. Both heads of the muscle arise on the scapula and join to form a single muscle bell ...

, 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 The brachioradialis is a muscle of the forearm that flexes the forearm at the elbow. It is also capable of both pronation and supination, depending on the position of the forearm. It is attached to the distal styloid process of the radius by way ...

, 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