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Pronation is the movement that defines the orientation of the palm, forearm, or foot in space. In the upper limb, pronation is the rotatory movement during which the palm and forearm face downward. The main muscles enabling pronation of the upper limb are the pronator teres, pronator quadratus, and brachioradialis muscles. The pronator teres is a fusiform forearm muscle found in the superficial layer of the anterior compartment of the forearm. Its primary function is to pronate the forearm and assist in forearm flexion. The pronator quadratus is a square-shaped muscle located adjacent to the wrist in the deep region of the anterior compartment. The supinator and biceps brachii muscles, on the other hand, facilitate supination, which is the opposite of pronation.
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Pronator Teres
The pronator teres is a long, round muscle that is located on the anterior aspect of the forearm. It is a fusiform muscle, meaning it is thick in the middle and tapered at the ends. The pronator teres is the most lateral muscle of the superficial flexors group of the forearm, which also includes the flexor carpi radialis, palmaris longus, flexor digitorum superficialis, and flexor carpi ulnaris muscles.
The pronator teres has two heads, each named after their origin sites: the humeral head and the ulnar head. The humeral head is the larger and more superficial of the two, originating on the medial supracondylar ridge of the humerus, just superior to the medial epicondyle. The ulnar head, on the other hand, is deeper and joins the humeral head at an acute angle, forming a single muscle belly. It originates from the coronoid process of the ulna, located between the attachments of the brachialis and flexor digitorum superficialis muscles.
The two heads of the pronator teres join and travel distally, deep to the brachioradialis in the superficial layer of the anterior compartment of the forearm. They eventually fuse and insert via a flat tendon onto the lateral surface of the radius, specifically at the rough area in the middle of its shaft called the pronator tuberosity. The median nerve, which innervates the pronator teres, enters the forearm between these two heads, and the ulnar head separates the nerve from the ulnar artery.
The primary function of the pronator teres is to pronate the forearm, or rotate it so that the palm faces downward. It achieves this by pulling the radius medially, causing the radial head to rotate around the proximal ulna. This movement also brings the hand from an upward palm position (supination) to a downward-facing palm (pronation). In addition to pronation, the pronator teres also plays a role in flexion of the elbow, particularly when there is strong resistance.
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Pronator Quadratus
The pronator quadratus is a thin, short, flat, and quadrilateral muscle found in the forearm. It is the deepest muscle in the anterior (flexor) compartment of the forearm, located underneath the remaining deep forearm flexors. It is a small muscle, compacted in a small closed compartment, and covered by the interosseous membrane dorsally and by its own fascia volarly.
The pronator quadratus is the only muscle that attaches to the ulna at one end and the radius at the other. It extends across the distal parts of the radius and ulna. The muscle fibres run perpendicular to the direction of the arm, from the distal quarter of the anterior ulna to the distal quarter of the radius. The muscle has two heads: the superficial head originates from the anterior distal aspect of the diaphysis (shaft) of the ulna and inserts into the anterior distal diaphysis of the radius and its anterior metaphysis. The deep head has the same origin, but inserts proximal to the ulnar notch.
The main function of the pronator quadratus is forearm pronation. It works in conjunction with the pronator teres to pronate the radioulnar joint. When the muscle contracts, it pulls the lateral side of the radius towards the ulna, causing the hand to pronate, or turn so that the palm faces downwards. This movement is essential for daily activities such as writing or typing.
The pronator quadratus is innervated by the anterior interosseous nerve of the forearm, with contributions from the C7 and C8 spinal nerves. The anterior interosseous nerve is a branch of the median nerve, which arises from the brachial plexus. The muscle receives arterial blood from the anterior interosseous artery, a branch of the common interosseous artery.
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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. The muscle is used to stabilize the elbow during rapid flexion and extension while in a midposition, such as in hammering. It is one of the first muscles to recover following radial nerve injury.
The brachioradialis is a fusiform muscle on the lateral side of the forearm. It originates proximally from the proximal two-thirds of the lateral humeral supracondylar ridge and inserts distally on the base of the lateral radial styloid process. It is often fused proximally with the brachialis. It has a thin belly that descends in the mid-forearm, where its long, flat tendon begins, and then the tendon continues to the radius. It is also known as supinator longus.
The brachioradialis is considered a posterior or extensor-compartment muscle, though it functions as a flexor. It is one of two forearm extensor-compartment muscles that do not cross the wrist, the other being the supinator. The brachioradialis flexes, pronates, and supinates the forearm but is innervated by the radial nerve. It is one of only four muscles that receive innervation from the radial nerve.
The brachioradialis is located in the mobile wad compartment together with the extensor carpi radialis longus and extensor carpi radialis brevis muscles. It primarily acts as an elbow flexor and is active during elbow flexion, whether the forearm is supinated, neutral, or pronated. The brachioradialis is a stronger elbow flexor when the forearm is in a midposition between supination and pronation at the radioulnar joint.
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Supinator
The supinator is a muscle in the posterior compartment of the forearm that rotates the radius laterally at the proximal radioulnar joint. This movement is known as supination, which brings the hand into a supine position (facing anteriorly, palm up). The supinator is the prime supinator of the forearm and always acts together with the biceps, except when the elbow joint is extended. Its famous antagonist is the pronator quadratus muscle, which produces the opposite movement of pronation.
The supinator muscle has a broad origin from the ulna and humerus. It curves around the upper third of the radius and consists of two layers of fibres, or planes of fibres. The deeper layer of fibres encircles the neck of the radius above the radial tuberosity. The two layers originate in common from several osteofibrous structures of the elbow, including the lateral epicondyle of the humerus, the radial collateral ligament, the annular radial ligament, the supinator crest of the ulna, and the adjacent part of the ulnar fossa. The superficial layer of fibres has a tendinous origin and surrounds the upper part of the radius.
The supinator is innervated by the posterior interosseous nerve (C7, C8), a branch of the radial nerve. The 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 and potentially result in selective paralysis of the muscles served by this nerve. This nerve syndrome is known as supinator entrapment syndrome, which can be caused by compression by various soft-tissue masses surrounding the nerve and stress caused by repetitive supination and pronation.
Supination strength decreases by 64% if the supinator is disabled, for example, by injury. However, supination can be assisted by the biceps brachii muscle, especially for quick, strong, or forceful supination movements, or when the movement happens against resistance. The biceps brachii is unable to supinate when the elbow is extended, so the most powerful supination occurs when the elbow joint is flexed to 90 degrees.
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Biceps Brachii
The biceps brachii (BB), commonly known as the biceps, is a large, thick muscle found on the ventral portion of the upper arm. It is composed of two heads: a short head, located on the medial side, and a long head, located on the lateral side. The short head originates from the apex of the coracoid process of the scapula, while the long head originates from the supraglenoid tubercle of the scapula.
The biceps brachii works across three joints: the glenohumeral, elbow, and radio-ulnar joints. It is involved in flexion and supination (outward rotation) of the forearm, facilitated by a 90-degree rotation of the muscle as it connects to the radius. While the biceps are well-known for their role in "bicep curls," their primary function is to support and stabilize the deeper and stronger brachialis muscle during lifting or lowering of the forearm. The brachialis muscle is the strongest flexor of the elbow, generating up to 50% more power than the biceps brachii.
The biceps brachii also plays a role in shoulder flexion, although it is not the primary mover in this action. Instead, it acts as a secondary contributor and stabilizer, and tension or shortening in this muscle can affect shoulder movements. The muscle is involved in various 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 occurring due to physical trauma or repetitive activity.
In terms of pronation, the biceps brachii have a biomechanically disadvantaged role. During elbow flexion, the biceps brachii's activity remains constant across pronated, supinated, and neutral hand positions. In contrast, the brachioradialis muscle, which also assists in pronation and supination, exhibits increased activity in the pronated hand position. This means that the brachioradialis takes on a more significant role in elbow flexion when the forearm is pronated, while the biceps brachii are more powerful in the supinated position.
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Frequently asked questions
Pronation is the movement that defines the orientation of the palm, forearm, or foot in space. In the upper limb, it is the rotatory movement during which the palm and forearm face downward. In the foot, it is the natural movement of the foot when it rolls inward.
The main muscles that enable pronation of the upper limb are the pronator teres, pronator quadratus, and brachioradialis muscles. The supinator and biceps brachii muscles facilitate supination and flexion.
The pronator teres is a fusiform forearm muscle found in the superficial layer of the anterior compartment of the forearm. Its primary function is to pronate the forearm and assist in forearm flexion. It pulls the radius medially, causing the radial head to rotate around the proximal ulna.
Pronator teres syndrome is a rare condition caused by compression of the median nerve between the humeral head and ulnar head of the pronator teres. It results in pain and numbness in the forearm, with symptoms of weakness and neuropathic dysfunction.
