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The supinator muscle is a broad muscle in the posterior compartment of the forearm, responsible for forearm supination. This muscle works with the biceps brachii to achieve 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 muscle. Forearm supination refers to the action of turning the palm upwards, which is opposed by the pronator teres and pronator quadratus muscles during the action of pronation.
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What You'll Learn
Pronator teres and pronator quadratus
The pronator teres and pronator quadratus are the two muscles that work to neutralise forearm supination. Forearm supination is the movement of turning the palm of the hand upwards. The action of pronation, which involves turning the palm downwards, counteracts this movement.
Pronator Teres
The pronator teres is a long, round muscle located on the anterior aspect of the forearm. It has two points of origin: the humeral head and the ulnar head. The humeral head is larger and more superficial, while the ulnar head is deeper and joins the humeral head at an acute angle, forming one muscle belly. The primary function of the pronator teres is to pronate the forearm, or turn the palm downwards. It also assists in flexion of the elbow joint. When the elbow is fully flexed, the muscle fibres of the pronator teres are shortened, reducing their ability to produce force.
Pronator Quadratus
The pronator quadratus is a flat, short, quadrilateral muscle that originates from the anterior surface of the distal shaft of the ulna. It is the deepest muscle in the anterior (flexor) compartment of the forearm and is part of the deep group of forearm flexors. The pronator quadratus is innervated by the anterior interosseous nerve of the forearm, with contributions from the C7 and C8 spinal nerves. The main function of this muscle is forearm pronation, which allows for movements such as writing or typing.
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Triceps and anconeus
Forearm supination refers to the action of turning the palm of the hand upwards. The muscles that neutralise this action are the pronator teres and pronator quadratus, which turn the palm downwards.
The triceps and anconeus muscles do not contribute to pronation. Instead, they are primarily involved in extending the elbow. The triceps muscle is the principal agonist in elbow extension, and the anconeus assists in this movement. The anconeus is a small, triangular muscle located in the posterior region of the elbow. It is considered by some to be a continuation of the triceps brachii muscle, as their fibres are often partially or completely blended together. The anconeus also abducts the ulna and stabilises the elbow joint.
The anconeus muscle receives its major blood supply from the recurrent posterior interosseus and medial collateral arteries. It is innervated by a branch of the radial nerve (cervical roots 7 and 8) from the posterior cord of the brachial plexus. Trauma to the nerve supply of the anconeus muscle can result from a shoulder dislocation or fractures of the upper humerus or around the olecranon. This can lead to paralysis of the anconeus muscle and other extensors of the elbow and wrist.
Electromyographic data shows that the anconeus is active in all positions of the elbow and is, therefore, considered a dynamic joint stabilizer. It aids in the terminal 15 degrees of elbow extension and forearm supination.
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Biceps Brachii
The biceps brachii (BB), commonly known as the biceps, is a large, thick muscle on the front of the upper arm between the shoulder and the elbow. It is composed of a short head and a long head. The short head is sometimes referred to as "caput breve", while the long head is also called "caput longum". The long head is located on the lateral side of the biceps brachii, while the short head is located on the medial side.
The biceps brachii works across three joints and is able to generate movements in glenohumeral, elbow, and radio-ulnar joints. It is involved in tasks such as lifting, sports involving throwing and racket use, and gesturing. The biceps are a powerful supinator of the forearm, i.e. it turns the palm upwards. This action requires the elbow to be at least partially flexed. If the elbow is fully extended, supination is primarily carried out by the supinator muscle. The biceps brachii is also an important landmark for locating the brachial artery during physical examination and ultrasound-guided arterial cannulation.
The biceps brachii functions as a forearm flexor when extended but becomes the forearm's most powerful supinator when flexed. The main functions of the biceps are the flexion and supination (outward rotation) of the forearm. This is facilitated, in part, by the 90-degree rotation of the muscle as it connects to the radius. The biceps brachii can also contribute to abduction (bringing the arm out to the side) when the arm is externally (or laterally) rotated.
The biceps brachii works with the supinator muscle to achieve powerful supination. However, the biceps brachii is unable to supinate when the elbow is extended. The supinator muscle is located in the posterior compartment of the forearm and is the prime supinator of the forearm. It curves around the upper third of the radius and consists of two layers of fibres.
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Brachialis and brachioradialis
Forearm supination is the movement of turning the palm of the hand upward. The muscles that act on the forearm to neutralize this supination are the pronator teres and pronator quadratus. These muscles rotate the forearm to turn the palm downward, counteracting the supination movement.
While the brachialis and brachioradialis muscles are involved in forearm movement, they are not primarily associated with pronation. The brachialis is an important flexor muscle of the forearm at the elbow. It is considered a pure flexor of the forearm at the elbow and provides elbow flexion at all physiologic positions. It does not provide any supination or pronation of the forearm.
The brachioradialis is the most superficial muscle on the radial side of the forearm. It is often fused proximally with the brachialis. The muscle comprises the lateral wall of the cubital fossa and presents the border between the anterior and posterior forearm compartments. The brachioradialis flexes the forearm at the elbow and can tend to move the hand to neutral depending on the position of the hand during flexion.
The brachialis and brachioradialis work in synergy with the biceps brachii to flex the forearm at the elbow. The brachialis provides pure forearm flexion at the elbow in all positions, while the brachioradialis is a powerful forearm flexor when the forearm is semi-pronated, meaning that the palm is perpendicular to the ground.
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Supinator
The supinator is a spiral muscle located in the posterior compartment of the forearm. It is a broadly-shaped muscle that curves around the upper third of the radius. The supinator consists of two layers of fibres, or planes, originating from the supinator crest of the ulna, the lateral epicondyle of the humerus, the radial collateral ligament, and the annular radial ligament. The deeper layer of fibres encircles the neck of the radius above the radial tuberosity.
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 muscle, which can lead to entrapment and compression of the deep branch, potentially resulting in selective paralysis of the muscles served by this nerve. This nerve syndrome is known as supinator entrapment syndrome and can be caused by compression by various soft tissues surrounding the nerve and stress caused by repetitive supination and pronation.
The main function of the supinator is to supinate the forearm, rotating the radius laterally to produce this movement. Supination can be done with the elbow in any position of flexion or extension. The supinator always acts together with the biceps, except when the elbow joint is extended. The biceps become increasingly active during heavy loading. For quick, strong, and forceful supination movements, or when the movement happens against resistance, the supinator is assisted by the biceps brachii muscle. However, the biceps brachii cannot act as a supinator when the forearm is fully extended. Therefore, the most powerful supination occurs when the elbow joint is flexed to 90 degrees, such as when turning a screwdriver or opening a bottle of wine.
The antagonist of the supinator is the pronator quadratus muscle, which rotates the radius medially, producing the opposite movement of pronation. The muscles that neutralize forearm supination are the pronator teres and pronator quadratus, which are responsible for pronation, or the movement of turning the palm of the hand downward.
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Frequently asked questions
Forearm supination is counteracted by the pronator teres and pronator quadratus muscles, which are responsible for pronation, or the movement of turning the palm of the hand downward.
The supinator muscle is a broad muscle in the posterior compartment of the forearm that curves around the upper third of the radius. Its function is to supinate the forearm, or turn the palm upward.
The biceps brachii acts as a powerful supinator when the forearm is pronated. It is unable to supinate when the elbow is extended, unlike the supinator muscle.
