Dominic Stone
The forearm is pronated by the pronator teres muscle, which is located mainly in the forearm and has two origins, at the medial humeral supracondylar ridge and the medial side of the coronoid process of the ulna. It inserts near the middle of the radius. The pronator teres has two heads: the humeral and the ulnar. The former is larger and more superficial, arising from the medial supracondylar ridge, while the latter is thin and arises from the medial surface of the coronoid process. The median nerve passes through the two heads to reach the forearm, and the muscle contracts to cause pronation. The pronator quadratus is another muscle that pronates the forearm, especially when the elbow is flexed.
| Characteristics | Values |
|---|---|
| Muscles involved | Pronator teres, Pronator quadratus, Brachioradialis, Flexor digitorum profundus |
| Location | Forearm |
| Innervation | Median nerve, Radial nerve, Anterior interosseus nerve (branch of Median nerve) |
| Function | Pronation of the forearm (turning it so the palm faces down) |
| Attachments | Originates from the medial epicondyle, attaches to the flexor retinaculum of the wrist |
| Actions | Pronation, Flexion at the wrist, Abduction at the wrist |
| Additional Notes | Pronator teres syndrome can cause numbness in median nerve distribution with repetitive pronation/supination |
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What You'll Learn
The pronator teres muscle
The pronator teres is a muscle located mainly in the forearm. It is one of two muscles that serve to pronate the forearm, turning it so that the palm faces backward. The other muscle that does this is the pronator quadratus. The pronator teres is a long, round muscle that is found in the anterior forearm. It is the most lateral muscle of the superficial flexors of the forearm.
The pronator teres has two points of origin: the humeral head and the ulnar head. The humeral head is larger and more superficial, arising from the medial supracondylar ridge of the humerus. The ulnar head is deeper and joins the humeral head at an acute angle, forming one muscle belly. It arises from the coronoid process of the ulna. The two heads eventually fuse into a single muscle belly that inserts via a flat tendon onto the lateral surface of the radius.
The median nerve commonly passes between the two heads of the pronator teres, where it is at risk of compression. The median nerve enters the forearm between the two heads of the muscle and is separated from the ulnar artery by the ulnar head. The pronator teres is innervated by the median nerve and nerve roots C6 and C7. To stimulate the pronator teres, a signal begins in the precentral gyrus in the brain and goes down through the internal capsule.
The pronator teres syndrome is a condition where patients experience numbness in median nerve distribution with repetitive pronation/supination of the forearm, not flexion and extension of the elbow. Early fatigue of the forearm muscles is seen with repetitive stressful motion, especially pronation.
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The pronator quadratus muscle
The muscle is part of the deep group of forearm flexors, along with the flexor digitorum profundus and flexor pollicis longus. These three muscles are overlaid by the superficial group of forearm flexors. Pronator quadratus is the deepest muscle in the anterior (flexor) compartment of the forearm, located underneath the remaining deep forearm flexors. It is a flat, short, quadrilateral muscle that originates from the anterior surface of the distal shaft of the ulna and an aponeurosis that partially covers the muscle.
The pronator quadratus is innervated by the anterior interosseous nerve, a branch of the median nerve. When the muscle contracts, it pulls the lateral side of the radius towards the ulna, pronating the hand. This action is aided by the pronator teres and brachioradialis muscles. The lateral corticospinal tract is responsible for the motor pathway of the pronator quadratus.
The pronator quadratus also serves a protective role in the forearm. When upward pressure is applied during weight-bearing activities, the muscle holds together the distal ends of the radius and ulna, protecting and stabilising the distal radioulnar joint. It also protects the interosseous membrane during forced and rapid forearm rotations by dissipating the forces pulling on the membrane.
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Median nerve innervation
The median nerve is a sensory and motor nerve of the upper limb that provides sensory and motor functions to the forearm, wrist, and hand. It is one of five nerve branches of the brachial plexus, a complex network of nerves that helps with the movement of the shoulders, arms, and hands. The median nerve starts at the armpit and stimulates muscles in the forearm, allowing for the bending and straightening of the wrists, thumbs, and fingers. It also enables the rotation of the forearm and hand, turning the palm downward.
The median nerve innervates the majority of the muscles in the anterior forearm, and some intrinsic hand muscles. In the forearm, the median nerve directly innervates muscles in the superficial and intermediate layers. The superficial layer includes the pronator teres, flexor carpi radialis, and palmaris longus, while the intermediate layer includes the flexor digitorum superficialis. The median nerve also gives rise to the anterior interosseous nerve, which supplies the deep flexors. The deep layer includes the flexor pollicis longus, pronator quadratus, and the lateral half of the flexor digitorum profundus (the medial half of the muscle is innervated by the ulnar nerve). These muscles are involved in pronating the forearm, flexing the wrist, and flexing the digits of the hand.
The median nerve passes through the two heads of the pronator teres muscle to reach the forearm, and it is separated from the ulnar artery by the ulnar head. The muscle inserts onto the lateral surface of the radius distal to the supinator, causing pronation when it contracts. The anterior interosseus nerve, a branch of the median nerve, innervates the square-shaped muscle in the anterior compartment of the forearm. This muscle arises from the distal anterior surface of the ulna and inserts onto the distal anterior shaft of the radius, causing pronation when contracted.
The median nerve is responsible for cutaneous innervation of part of the hand through two branches: the palmar cutaneous branch and the palmar digital cutaneous branches. The palmar cutaneous branch arises in the forearm, travels into the hand, and innervates the lateral aspect of the palm. It does not pass through the carpal tunnel and is spared in carpal tunnel syndrome. The palmar digital cutaneous branches arise in the hand and innervate the palmar surface and fingertips of the lateral three and a half digits.
Damage to the median nerve can result in carpal tunnel syndrome or other nerve compression issues, causing pain, weakness, or numbness that can affect daily life. Pronator teres syndrome is a condition where the pronator teres muscle near the elbow pinches the median nerve, leading to dull, aching pain in the forearm.
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Forearm supination
The ability to supinate the forearm is made possible by several muscles and connective tissues working together. One of the key muscles involved in forearm supination is the supinator muscle. The supinator muscle arises from the lateral supracondylar ridge and lateral intermuscular septum of the arm. It inserts into the radial tuberosity and crosses the elbow joint, acting as an elbow flexor. When the forearm is in a mid-position between supination and pronation, the supinator muscle plays a crucial role in elbow flexion.
Another important muscle group associated with forearm supination is the deep anterior forearm muscles, which include the flexor digitorum profundus, flexor pollicis longus, and pronator quadratus. These muscles originate from the ulna and interosseous membrane, and they contribute to the complex movements of the wrist and fingers.
The median nerve also plays a significant role in forearm supination. It innervates the muscles involved in this movement, including the supinator muscle and the deep anterior forearm muscles. The median nerve has two heads, the ulnar head, and the humeral head, with the latter being larger and more superficial, arising from the medial supracondylar ridge.
Additionally, a tough membrane of connective tissue called the triangular fibrocartilage connects the ulna and radius bones, ensuring they remain stable during forearm supination. This membrane is thicker at its periphery and thinner at its centre, providing the necessary support for the forearm's rotational movements.
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Brachioradialis muscle
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. It is attached to the distal styloid process of the radius by way of the brachioradialis tendon, and to the lateral supracondylar ridge of the humerus. The muscle is innervated by the radial nerve and is one of only four muscles that receive direct input from this nerve. It is considered a posterior or extensor-compartment muscle, though it functions as a flexor.
The brachioradialis is a superficial, fusiform muscle on the lateral side of the forearm. It originates proximally on the lateral supracondylar ridge of the humerus and inserts distally on the radius, at the base of its styloid process. It forms the lateral limit of the cubital fossa, or elbow pit, and is often fused proximally with the brachialis. The muscle has a thin belly that descends in the mid-forearm, where its long, flat tendon begins. It is also known as supinator longus.
The brachioradialis is one of seven muscles in the forearm's superficial layer that attach to the lateral epicondyle and supracondylar ridge of the humerus. The other six muscles originating from these regions include the extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and anconeus. Several anatomic variations of the brachioradialis have been reported, including one variant with two superficial radial nerves and two brachioradialis muscle bellies with a common origin and insertion point. The two muscle bellies form superficial and deep layers.
The brachioradialis is important in the recovery of radial nerve injury. It is typically one of the first two muscles to recover innervation following injury to the radial nerve in the radial groove, along with the extensor carpi radialis longus. It is also vital in hand defect reconstruction, forearm tendon transfers, and carpal tunnel release during surgery.
The muscle is effective in stabilising the elbow during rapid flexion and extension while in a midposition, such as in hammering. It is synergistic with the brachialis and biceps brachii, and the triceps brachii and anconeus are antagonistic.
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Frequently asked questions
The pronator teres and the pronator quadratus are the muscles that pronate the forearm.
The pronator teres is a long, round muscle that pronates the forearm and assists in flexion of the elbow joint.
The pronator quadratus is the main pronator of the forearm, especially when the elbow is flexed.
The pronator teres has two points of origin: the humeral head and the ulnar head.
The pronator quadratus arises from the ulna and inserts into the radius.
