Dominic Stone
The supinator is a broad muscle in the posterior compartment of the forearm. It consists of two layers of fibres, superficial and deep, which originate from the ulna and humerus. The supinator muscle rotates the radius laterally, producing a movement known as forearm supination. The deep branch of the radial nerve passes through the belly of the supinator in 70% of cases. This arrangement can lead to entrapment and compression of the deep part, potentially resulting in selective paralysis of the muscles served by this nerve.
| Characteristics | Values |
|---|---|
| Location | Posterior compartment of the forearm |
| Shape | Broad, spiral |
| Function | Supinates the forearm |
| Layers | Superficial, deep |
| Innervation | Deep branch of the radial nerve |
| Nerve Roots | C5, C6, C7, C8 |
| Blood Supply | Superficial layer: Radial artery |
| Deep layer: Ulnar artery | |
| Antagonist | Pronator quadratus muscle |
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What You'll Learn
The supinator muscle's function is to supinate the forearm
The supinator muscle is a broad, spiral muscle located in the posterior compartment of the forearm. It is innervated by the deep branch of the radial nerve, which passes through the belly of the supinator in 70% of cases. The deep branch then becomes the posterior interosseous nerve upon exiting the supinator muscle. This nerve innervates the supinator muscle and is a branch of the radial nerve.
The supinator muscle consists of two layers of fibres, or planes, that originate from the same landmarks, including several osteofibrous structures of the elbow. The superficial layer of fibres has a tendinous origin, arising from tendons, and surrounds the upper part of the radius. The deeper layer of fibres consists of muscular fibres and encircles the neck of the radius above the radial tuberosity.
The supinator muscle rotates the radius laterally at the proximal radioulnar joint, bringing the hand into the supine position, or supinating the forearm. This movement involves the palm facing upwards, as opposed to pronation, where the palm faces downwards. Supination can be tested by pronating the forearm at the radioulnar joints while the elbow joint is held in the extended position.
The supinator muscle always acts together with the biceps muscle, except when the elbow joint is extended. The biceps become increasingly active during heavy loading, while the supinator is the most active muscle during unresisted supination. If the supinator is disabled, supination strength decreases by 64%.
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The supinator muscle anatomy
The supinator is a broad muscle in the posterior compartment of the forearm. It is curved around the upper third of the radius and consists of two layers of fibres, or planes. These layers are the superficial and deep layers, which differ in their mode of attachment. The superficial layer originates from tendinous fibres, while the deep layer originates from already formed muscular slips. Both layers originate from the same landmarks, which include several osteofibrous structures of the elbow. These structures are the lateral epicondyle of the humerus, the radial collateral ligament of the humeroulnar joint, the annular ligament of the superior radioulnar joint, the supinator crest of the ulna, and the adjacent part of the ulnar fossa.
From these origins, the supinator wraps around the proximal third of the radius, inserting into the upper third of its lateral, posterior, and anterior surfaces. This spiral anatomy enables the supinator to rotate the radius laterally, producing a movement known as forearm supination. This action puts the radius parallel to the ulna, bringing the hand into the supine position. The supinator is the prime mover in slow and unopposed supination movements, such as rotating the hand to grab popcorn from a bowl. However, for quick and forceful supination, or when there is resistance, the supinator is assisted by the biceps brachii muscle.
The supinator is innervated by the deep branch of the radial nerve, which becomes the posterior interosseous nerve upon exiting the muscle. Its nerve roots are primarily from C6, with some C5 involvement, and possible additional C7 innervation. The radial nerve divides into deep and sensory superficial branches just proximal to the supinator, which can lead to entrapment and compression of the deep branch. This nerve syndrome, known as supinator entrapment syndrome, can potentially result in selective paralysis of the muscles served by the nerve.
The supinator is found deep in the forearm, with only the radius and ulna lying superficially to it. Along with the brachialis muscle, it forms the floor of the cubital fossa. Extensor carpi radialis brevis and longus cover the muscle's radial surface, while the inferior part of the anconeus muscle overlies it from the ulnar side.
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The supinator's relationship with the radial nerve
The supinator muscle is a broad muscle located in the posterior compartment of the forearm. It is responsible for supinating the forearm, or rotating the radius laterally, to bring the hand into a supine position. This muscle works in conjunction with the biceps brachii to achieve forearm supination, except when the elbow joint is extended. The supinator muscle is innervated by the deep branch of the radial nerve, which becomes the posterior interosseous nerve upon exiting the supinator muscle.
The radial nerve divides into deep and sensory superficial branches just proximal to the supinator muscle. This arrangement can lead to entrapment and compression of the deep branch, resulting in selective paralysis of the muscles served by this nerve, including the extensor muscles and the abductor pollicis longus. This nerve syndrome is known as supinator entrapment syndrome and can be caused by compression by soft tissue masses or stress from repetitive supination and pronation.
The deep branch of the radial nerve passes through the belly of the supinator in 70% of cases, with the remaining cases passing through the arcade of Frohse. The arcade of Frohse refers to the proximal aspect of the superficial head of the supinator muscle. The radial nerve innervates the supinator muscle, with nerve roots primarily from C6 and some C5 involvement, and possible additional C7 innervation.
The supinator muscle consists of two layers or planes of fibres: a superficial layer and a deep layer. These layers differ in their mode of attachment, with the superficial layer arising from tendinous fibres and the deep layer originating from muscular slips. The deep layer is supplied by the ulnar artery, while the superficial layer receives blood from the radial artery. Between these two layers passes the deep branch of the radial nerve, which then exits the supinator muscle as the posterior interosseous nerve.
The supinator muscle plays a crucial role in forearm supination and its relationship with the radial nerve is essential to its function. The radial nerve provides innervation to the supinator muscle, allowing it to rotate the radius and bring the hand into a supine position. However, the anatomical arrangement of the radial nerve in relation to the supinator muscle also makes it susceptible to nerve entrapment and compression, which can lead to muscular paralysis in severe cases.
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The supinator's origin and insertion
The supinator is a broad muscle in the posterior compartment of the forearm. It is a long, flat fusiform type of skeletal muscle that consists of superficial and deep layers. The supinator muscle has a broad origin from the ulna and humerus. The two layers of fibres originate from the supinator crest of the ulna, the lateral epicondyle of the humerus, the radial collateral ligament, and the annular radial ligament. The superficial layer of fibres has a tendinous origin, while the deep layer originates with already formed muscular slips. The supinator muscle belly travels inferolaterally over the elbow joint, spiralling over the superior third of the radius, and travels inferomedially to its insertion site. The muscle wraps around the proximal third of the radius, inserting to the upper third of its lateral, posterior and anterior surfaces. The insertion site is superior to the insertion site of the pronator teres muscle.
The supinator muscle rotates the radius laterally at the proximal radioulnar joint. This action puts the radius parallel to the ulna, bringing the hand into the supine position (facing anteriorly, palm up). When producing a slow and unopposed supination movement, the supinator muscle is the prime mover. However, for quick, strong, or forceful supination movements, or when there is resistance, the supinator muscle is assisted by the biceps brachii muscle. The 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.
The supinator muscle is innervated by the deep branch of the radial nerve. The nerve passes between the muscle's superficial and deep heads to enter the extensor compartment of the forearm. The deep branch then becomes the posterior interosseous nerve upon exiting the supinator muscle. The supinator muscle is supplied by two different sources: the superficial layer receives blood from the radial artery, while the deep layer is supplied by the ulnar artery.
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The supinator's antagonist, the pronator quadratus muscle
The supinator muscle is a broad muscle in the posterior compartment of the forearm. It is responsible for supinating the forearm, or in other words, rotating the radius laterally. This movement brings the hand into the supine position (facing anteriorly, palm up). The supinator muscle works in tandem with the biceps muscle, except when the elbow joint is extended.
The supinator's famous antagonist is the pronator quadratus muscle. This muscle is responsible for producing the opposite movement of pronation, or the rotation of the radius medially. The pronator quadratus is a flat, short, quadrilateral muscle that originates from the anterior surface of the distal shaft of the ulna. It is part of the deep group of forearm flexors, along with flexor digitorum profundus and flexor pollicis longus. These muscles are overlaid by the superficial group of forearm flexors.
The pronator quadratus extends across the distal parts of the radius and ulna. It is the deepest muscle in the anterior (flexor) compartment of the forearm and is located underneath the remaining deep forearm flexors. The median nerve supplies this muscle, allowing it to perform its functions.
The pronator quadratus plays a protective role in the distal forearm. When upward pressure is applied during weight-bearing activities, this muscle holds together the distal ends of the radius and ulna, thereby protecting and stabilising the distal radioulnar joint. Additionally, it protects the interosseous membrane during forced and rapid forearm rotations by dissipating the forces acting on the membrane.
In summary, the pronator quadratus muscle acts as the antagonist to the supinator muscle by producing the opposite movement of pronation. It has important functions in forearm rotation, protection, and stabilisation.
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Frequently asked questions
The supinator is a broad muscle in the posterior compartment of the forearm. It curves around the upper third of the radius and consists of two layers of fibres.
The supinator muscle supinates the forearm at the radioulnar joints. It rotates the radius laterally, producing a movement known as forearm supination.
The deep branch of the radial nerve pierces the supinator muscle.
Supinator entrapment syndrome is a nerve syndrome that can be caused by compression by various soft-tissue masses surrounding the nerve or stress caused by repetitive supination and pronation. It can result in selective paralysis of the muscles served by the deep radial nerve.
Supination is a movement where the palm faces upwards, while pronation is a movement where the palm faces downwards.
