Supinator muscle curls around the proximal part of radius, connecting it with the ulna. In doing so, the muscle crosses the sagittal axis of 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.
|Origin||Lateral epicondyle of humerus, radial collateral ligament, annular ligament, supinator crest of ulna|
|Insertion||Lateral, posterior, and anterior surfaces of proximal third of radius|
|Action||Proximal radioulnar joint: Forearm supination|
|Innervation||Posterior interosseous nerve (C7, C8)|
|Blood supply||Radial recurrent artery, posterior interosseous artery, posterior interosseous recurrent artery|
This article will discuss the anatomy and function of supinator muscle.
Origin and insertion
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 humerus, radial collateral ligament of humeroulnar joint, annular ligament of the superior radioulnar joint, supinator crest of ulna and the adjacent part of ulnar fossa.
From here the muscle wraps (spirals) around the proximal third of the radius, inserting to the upper third of its lateral, posterior and anterior surfaces.
Supinator is found deep in the forearm, superficial only to the parts of the radius and ulna over which the muscle lies. Along with the brachialis muscle, it forms the floor of the cubital fossa. Extensor carpi radialis brevis and longus cover the muslce’s radial surface, while the inferior part of the anconeus muscle overlies it from the ulnar side.
The deep branch of the radial nerve gives off the posterior interosseous nerve just proximal to the supinator muscle. The nerve then passes between the muscle’s superficial and deep heads to enter the extensor compartment of the forearm. Sometimes an inconsistent fascial band called the oblique cord passes over the deep head of supinator. The cord connects the radius and ulna by extending between the ulnar and radial tuberosities.
Supinator is innervated by the posterior interosseous nerve (C7, C8), a branch of the radial nerve.
The superficial and deep layers of the supinator muscle are supplied by two different sources;
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).
When producing a slow and unopposed supination movement, the supinator muscle suffices on its own, and is the prime mover. An example of such a movement is the simple rotation of the hand when grabbing popcorn from a bowl.However, for a quick, strong, forceful supination movement, or when the movement happens against resistance, the supinator muscle is assisted by the biceps brachii muscle. Note that biceps brachii cannot act as a supinator when the forearm is fully extended, so the most powerful supination occurs when the elbow joint is flexed to 90 degrees. This action is seen, for example, when turning a screwdriver or pulling out the cork from a bottle of wine.