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The supinator muscle is a broad muscle in the posterior compartment of the forearm, responsible for supination of the forearm. Supination is the act of twisting the forearm into the palm-up position. The supinator muscle works with the biceps brachii to achieve this movement, by pulling on the radius. The biceps brachii is also the main supinator of the forearm, helping us rotate the palm up and down. The supinator muscle is innervated by the deep branch of the radial nerve, which becomes the posterior interosseous nerve upon exiting the muscle.
| Characteristics | Values |
|---|---|
| Location | Posterior compartment of the forearm |
| Shape | Broad and curved |
| Consists of | Two layers of fibres |
| Innervation | Deep branch of the radial nerve |
| Function | Supination of the forearm |
| Synergistic Muscles | Biceps Brachii, Brachialis, Brachioradialis |
| Elbow Position | Any position of flexion or extension |
| Power | Provides about half the power of the biceps muscle for supination |
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What You'll Learn
The supinator muscle
Supination of the forearm is the act of twisting the forearm into the palm-up position. The supinator is located just below the elbow and provides about half the power of the biceps muscle for supination. The supinator is also important as a location where the radial nerve can be entrapped. Pronation and supination are unique movements possible only in the forearms and hands, allowing the human body to flip the palm either face up or face down.
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Biceps Brachii
The biceps brachii, commonly known as the biceps, is a large, thick muscle on the front of the upper arm between the shoulder and the elbow. The muscle is composed of two heads: a short head and a long head. The short head is sometimes referred to as "caput breve", while the long head is 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 the glenohumeral, elbow, and radio-ulnar joints.
The biceps brachii is an important landmark for locating the brachial artery during physical examination and ultrasound-guided arterial cannulation. The primary arterial blood supply for the biceps brachii muscle comes from the muscular branches of the brachial artery. The musculocutaneous nerve C5, C6, and C7 provide the biceps' sensory and motor innervation.
The biceps brachii functions as a powerful supinator of the forearm, turning the palm upwards. This action is facilitated by the 90-degree rotation of the muscle as it connects to the radius. The biceps are involved in tasks such as lifting, sports involving throwing and racket use, and gesturing. As a result, biceps tendinopathy is a common condition seen in this muscle, often caused by muscle overuse or trauma.
The short head of the biceps brachii assists with horizontal adduction (bringing the arm across the body) when the arm is internally (or medially) rotated. It also assists in stabilizing the shoulder joint when carrying a heavy weight. The long head of the biceps tendon assists in holding the head of the humerus in the glenoid cavity and prevents impingement of the supraspinatus tendon.
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Forearm supination
The supinator muscle is located in the posterior compartment of the forearm. It is a broadly shaped muscle that curves around the upper third of the radius and consists of two layers of fibres. The supinator muscle has a broad origin from the ulna and humerus. The two layers of fibres originate in common from the supinator crest of the ulna, the lateral epicondyle of the humerus, the radial collateral ligament, and the annular radial ligament. The supinator always acts together with the biceps, except when the elbow joint is extended. It is the most active muscle in forearm supination during unresisted supination, while the biceps become increasingly active with heavy loading.
The supinator 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 part, potentially resulting in selective paralysis of the muscles served by this nerve.
The biceps are prone to injury, particularly the tendon of the long head and the distal tendon, which inserts into the radius. Rupture of the tendon of the long head allows the biceps muscle to sink lower in the arm, creating a “Popeye” deformity. If the distal tendon ruptures, there may be an approximate 30% loss of elbow strength and a 40% loss of supination strength.
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Elbow extension
The supinator muscle is located in the posterior compartment of the forearm, just below the elbow. It is a broad muscle that curves around the upper third of the radius bone. The supinator muscle has a broad origin from the ulna and humerus, with two layers of fibres. The superficial layer of fibres has a tendinous origin and surrounds the upper part of the radius, while the deeper layer of fibres encircles the neck of the radius above the radial tuberosity.
The supinator muscle is the prime supinator of the forearm, working with the biceps brachii to achieve powerful supination. The biceps brachii is unable to supinate when the elbow 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. 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.
The main function of the supinator muscle is to supinate the forearm, which is the act of twisting the forearm into the palm-up position. This movement can be done with the elbow in any position of flexion or extension. Supination involves rotating the radius bone in the opposite direction of the pronator muscles, moving the distal end of the radius back to its position on the lateral side of the wrist.
The triceps muscle is the only muscle in the back of the arm and is responsible for straightening the elbow, allowing us to push up out of a chair or throw a ball. It also stabilizes the elbow during forceful supination, such as when turning a screwdriver, preventing the elbow from bending with every twist.
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Supinator entrapment syndrome
The supinator muscle is a broadly shaped muscle in the superior and posterior compartment of the forearm. It is innervated by the deep branch of the radial nerve, which becomes the posterior interosseous nerve upon exiting the supinator muscle. The main function of the supinator muscle is to supinate the forearm. This can be done with the elbow in any position of flexion or extension.
The posterior interosseous nerve is located close to the shaft of the humerus and the elbow. It is the deep motor branch of the radial nerve. Proximal to the supinator arch, the radial nerve divides into a superficial branch and a posterior interosseous branch. The nerve further divides into four terminal branches that can be compressed at one of four other sites: the fibrous bands around the radial head, the recurrent radial vessels, the arcade of Frohse, and/or the tendinous margin of the extensor carpi radialis brevis. The arcade of Frohse is the most frequent site of posterior interosseous nerve entrapment.
Symptoms of nerve entrapment syndromes generally involve pain, sensory and motor changes, sensations of popping, paresthesias, and paresis. Posterior interosseous nerve syndrome is characterised by variable degrees of weakness involving ulnar deviation, with or without pain. The syndrome can be caused by a traumatic injury, tumours, inflammation, and anatomic injury. Repeated pronation and supination can create dynamic compression of the nerve in the proximal part of the forearm.
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Frequently asked questions
Supination is the act of twisting the forearm into the palm-up position.
The supinator muscle, located in the posterior compartment of the forearm, allows supination. The biceps brachii of the upper arm also supinates the forearm by pulling on the radius.
The supinator muscle provides about half the power of the biceps muscle for supination. It is innervated by the deep branch of the radial nerve, which becomes the posterior interosseous nerve upon exiting the supinator muscle.
