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The muscles that extend the wrist are known as the extensor muscles, which are located in the posterior compartment of the forearm. They are innervated by the radial nerve and produce extension at the wrist and fingers. The extrinsic extensor muscles of the hand include the extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), extensor digitorum (ED), extensor digiti minimi (EDM), extensor carpi ulnaris (ECU), abductor pollicis longus (APL), extensor pollicis brevis (EPB), extensor pollicis longus (EPL), and extensor indicis (EI). Thumb extension is carried out by the APL, EPB, and EPL muscles, while independent index finger extension is achieved by the EI muscle.
| Characteristics | Values |
|---|---|
| Wrist extensor muscles | Extensor carpi radialis brevis (ECRB), extensor carpi radialis longus (ECRL), and extensor carpi ulnaris (ECU) |
| Thumb extension muscles | Abductor pollicis longus (APL), extensor pollicis brevis (EPB), and extensor pollicis longus (EPL) |
| Index finger extension muscle | Extensor indicis proprius (EIP) |
| Little finger extension muscle | Extensor digiti minimi (EDM) |
| Extension of other fingers | Extensor digitorum communis (EDC) muscles |
| Innervation | Radial nerve (posterior interosseous branch) |
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What You'll Learn
Extensor carpi radialis longus (ECRL)
ECRL is in the same muscle family as the extensor carpi radialis brevis (ECRB). Both muscles need to be strong to move the arm, wrist, and hand appropriately. The tendons of ECRL and ECRB pass deep into the extensor retinaculum in a common synovial sheath. Together they groove the posterior surface of the styloid process of the radius. ECRL is partly overlapped by brachioradialis and these muscles often blend together. It is also one of the three muscles of the radial forearm group, initially lying beside the brachioradialis, but it becomes mostly tendon early on.
The best exercises to improve strength in the ECRL are exercises that require you to move your wrist, hands, elbows, and forearms. Some examples include holding your arm out straight and gently pulling back on your fingertips, holding a weight in your hand and slowly lifting it toward your chest and lowering it down, and getting down on your hands and knees, slowly bending your arms to lower your chest toward the floor and then pushing up again.
ECRL can be palpated infero-posteriorly to the elbow. With resisted wrist extension and abduction, the ECRL can be palpated in the upper lateral portion of the posterior forearm. The tendon of ECRL, in particular, can be felt in the floor of the anatomical snuffbox when these movements (wrist extension and abduction) are performed.
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Extensor carpi radialis brevis (ECRB)
The extensor carpi radialis brevis (ECRB) is a muscle in the forearm that acts to extend and abduct the wrist. It is shorter and thicker than its counterpart, the extensor carpi radialis longus (ECRL), which is found above the proximal end of the ECRB. The ECRB originates from the common extensor tendon attached to the lateral epicondyle of the humerus, as well as from the radial collateral ligament of the elbow joint. The fibres of the ECRB end in the middle of the forearm, forming a flat tendon that is closely connected to the ECRL tendon.
The ECRB and ECRL work together to extend the wrist and abduct it radially. While the ECRL typically originates from the lateral supracondylar ridge of the humerus, the ECRB originates from the common extensor tendon. Both muscles run along the length of the forearm and pass through the second compartment of the extensor retinaculum. The ECRB inserts into the dorsal base of the third metacarpal, while the ECRL inserts into the base of the second metacarpal.
The ECRB is part of the superficial layer of the posterior compartment of the forearm, also known as the extensor compartment. This layer contains seven muscles, four of which (including the ECRB) share a common tendinous origin at the lateral epicondyle. The ECRB is also involved in neutral wrist extension movements, working synergistically with the ECRL and the extensor carpi ulnaris (ECU) based on each muscle's insertion and dynamic function.
Variations in the origins of the radial extensor muscles have been reported, although they are rare. In one case, a 75-year-old female cadaver was found to have an accessory tendon arising from the ECRL that travelled with the primary tendon through the second compartment of the extensor retinaculum. This accessory tendon inserted on the base of the third metacarpal, which is typically associated with the ECRB. Additionally, bilateral agenesis of the ECRB was reported in the same study, highlighting the uniqueness of these variations.
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Extensor digitorum (ED)
ED originates from the lateral epicondyle of the humerus and runs to the medial four phalanges of the hand. It descends superficially down the posterior aspect of the forearm, ending in four tendons that enter the dorsum of the hand, passing deep into the extensor retinaculum. ED is innervated by the radial nerve, specifically the posterior interosseous branch.
ED is responsible for extending the second (index finger), third (long finger), fourth (ring finger), and fifth (small finger) digits. It also participates in the extension of the wrist and contributes to the opening of the hand and letting go of objects. Any injury to the ED muscle can be debilitating and take significant time to heal, as it is essential in many daily activities that require wrist and hand extension, such as waving.
The extensor digiti minimi (EDM) muscle is thought to originate from the ED muscle, and in some people, these two muscles are fused together. The EDM muscle is responsible for independent small finger extension.
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Extensor digiti minimi (EDM)
The extensor digiti minimi (EDM), also known as the extensor digiti quinti proprius, is a slender muscle in the forearm. It is situated on the ulnar side of the extensor digitorum communis, with which it is generally connected. The extensor digitorum is one of the muscles that extend the index finger. The EDM is a muscle of the superficial layer of the posterior compartment of the forearm. It is part of a group of medial superficial extensor muscles.
The EDM arises from the common extensor tendon, attached to the lateral epicondyle of the humerus. It passes under the extensor retinaculum near the wrist and through the fifth extensor compartment of the wrist. The tendon of the EDM divides into two as it crosses the dorsum of the hand, and finally joins the extensor digitorum tendon. All three tendons attach to the dorsal digital expansion of the fifth digit (little finger). There may also be a slip of tendon connecting to the fourth digit, or ring finger.
The EDM is one of the extrinsic muscles of the hand. It extends the little finger, moving it from a clenched to a straight position. It also contributes to extension at the wrist, which means it moves the back of the hand towards the back of the forearm.
The EDM is innervated by the posterior interosseous nerve (C7 and C8), a continuation of the deep branch of the radial nerve (C7, C8). The radial nerve innervates all muscles in the extensor compartment of the forearm.
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Extensor carpi ulnaris (ECU)
ECU originates from the lateral epicondyle of the humerus and the posterior border of the ulna. It crosses the forearm to the ulnar (medial) side to insert at the base of the fifth metacarpal. The muscle acts to extend and adduct at the wrist. When acting alone, it inclines the hand toward the ulnar side. ECU is the only forearm extensor that lies in its own fibro-osseous tunnel at the level of the wrist with its own subsheath as it passes through the sixth extensor compartment of the wrist. At the level of the ulna, it is covered by a subsheath, extensor retinaculum, and the linea jugata.
ECU is innervated by the posterior interosseous nerve (C7 and C8), a continuation of the deep branch of the radial nerve. The arterial supply is provided by the radial recurrent artery and the posterior interosseous artery.
ECU injuries are common in people who participate in activities requiring repetitive arm, elbow, and wrist movements, especially when tightly gripping an object. Symptoms of an ECU injury include pain when shaking hands or squeezing/gripping an object. The pain worsens with forceful wrist movement. Treatment options depend on the severity of the injury and can range from conservative treatments such as immobilization and stabilization to invasive procedures like steroid injections and, in severe cases, surgical repair or reconstruction.
The instability of the ECU tendon can be challenging to diagnose due to normal changes in tendon position during wrist motion. Ultrasound imaging and MRI are useful tools for evaluating ECU tendon displacement and detecting abnormalities.
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Frequently asked questions
The muscles that extend the wrist are called the extensor muscles, and they are located in the posterior forearm. They include the extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), extensor digitorum (ED), extensor digiti minimi (EDM), extensor carpi ulnaris (ECU), abductor pollicis longus (APL), extensor pollicis brevis (EPB), extensor pollicis longus (EPL), and extensor indicis (EI).
The primary function of the extensor muscles is to produce extension and abduction of the wrist and fingers. They work together to allow for movements like extending the fingers, thumb, and wrist.
The extensor muscles originate on or near the lateral epicondyle of the humerus. They insert into the dorsal surface of the base of the metacarpal bones and the phalanges to facilitate wrist and finger extension. For example, the ECRL inserts into the base of the second metacarpal, while the ECU inserts at the base of the fifth metacarpal.
Yes, the extensor muscles can be classified into two groups: extrinsic and intrinsic. The extrinsic extensor muscles are located in the back of the forearm and have long tendons connecting them to the bones in the hand. The intrinsic extensor muscles are found deeper within the hand and help with finer movements of the fingers and thumb.
