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The muscles that extend the wrist are known as the wrist extensors or extrinsic extensor muscles of the hand. They are located in the back of the forearm and include muscles such as the extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), extensor digitorum (ED), extensor digiti minimi (EDM), and extensor carpi ulnaris (ECU). These muscles work together to produce wrist extension and finger extension, with some muscles also allowing for independent finger extension, such as the extensor indicis proprius (EIP) muscle for the index finger and the EDM for the little finger.
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What You'll Learn
Extensor Carpi Radialis Longus (ECRL)
ECRL is one of three primary wrist extensors. It is most effective as a wrist extensor when the elbow is extended and when radial deviation is balanced by the primary ulnar deviator- extensor carpi ulnaris. Functionally, the wrist extensors work strongly in the action of gripping, together with extensor carpi ulnaris. By keeping the wrist in extension, flexion of the wrist by flexors digitorum superficialis and profundus is prevented, with the result that these muscles act on the fingers. When the wrist then moves into flexion, the flexor tendons are unable to shorten enough to generate effective movement at the interphalangeal joints, resulting in a state of active insufficiency.
The ECRL has the most proximal origin of the extrinsic hand extensors. It originates just distal to the brachioradialis at the lateral supracondylar ridge of the humerus, the lateral intermuscular septum, and by a few fibres at the lateral epicondyle of the humerus. The muscle fibres end at the upper third of the forearm in a flat tendon, which runs along the lateral border of the radius, beneath the abductor pollicis longus and extensor pollicis brevis. They then pass beneath the dorsal carpal ligament, where they lie in a groove on the back of the radius common to it and the extensor carpi radialis brevis, immediately behind the styloid process.
The best exercises to improve strength in your 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 or a can of food in your hand and slowly lifting it toward your chest and lowering it down, and laying on your stomach and propping yourself up on your forearms in a plank position.
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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 namesake, 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. It also arises from the radial collateral ligament of the elbow joint and the intermuscular septa between it and its adjacent muscles. The ECRB fibres end at the middle of the forearm in the form of a flat tendon, which is closely connected with that of the ECRL.
The ECRB and ECRL form the lateral compartment, with their muscle fibres ending at the upper third and mid-forearm respectively. They continue as flat tendons along the lateral border of the radius, beneath the abductor pollicis longus (APL) and extensor pollicis brevis (EPB). The tendons then pass beneath the extensor retinaculum and dorsal carpal ligament, lying in a groove on the back of the radius, immediately behind the styloid process. The ECRB and ECRL function in unison to extend the wrist and abduct it radially.
The ECRB inserts into the lateral dorsal surface of the base of the third metacarpal bone, with some fibres inserting into the medial dorsal surface of the second metacarpal bone. This is in contrast to the ECRL, which inserts into the dorsal surface of the base of the second metacarpal bone on its radial side. The ECRB and ECRL act synergistically with the extensor carpi ulnaris (ECU) to achieve neutral wrist extension movements.
In terms of clinical pathology, lateral epicondylitis affects 1-5% of the general population. Surgical intervention may be indicated to treat this condition, aiming to debride angiofibroblastic tissue at the origin of the ECRB. Tendon repair or complete muscle release may also be necessary, depending on the extent of pathology.
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Extensor Digitorum (ED)
ED is a long muscle that runs from the lateral epicondyle of the humerus to the medial four phalanges of the hand. It is the most superficial muscle of the posterior forearm. ED is situated medial to the extensor carpi radialis brevis muscle and lateral to the extensor digiti minimi and extensor carpi ulnaris muscles. The ED tendon joins the ulnar side of one of the ED tendons along the back of the index finger. The extensor digitorum communis is supplied by the posterior interosseous artery and the radial recurrent artery.
The primary function of the ED muscle is to extend the wrist and the medial four digits (the second, third, fourth, and fifth fingers) at the metacarpophalangeal joints and secondarily at the interphalangeal joints. It generates the pull for the extension of the four medial fingers. ED is also involved in the pathology of lateral epicondylitis, which affects 1-5% of the general population. This condition causes pain on resisted extension of the middle finger.
The ED muscle is easily palpable, especially during the extension of the hand when it is contracted. It can be examined using Maudsley's test, which involves forced flexion of an extended DIP joint. ED injuries can result in varying degrees of flexion deformity of the affected finger, with an inability to actively extend the DIP joint.
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Extensor Carpi Ulnaris (ECU)
The ECU inserts at the base of the 5th metacarpal to extend and adduct the wrist. It is innervated by the posterior interosseous nerve (C7 and C8), a branch of the deep division of the radial nerve. The radial nerve stems from the posterior cord of the brachial plexus. The ECU is supplied by branches of the radial recurrent and posterior interosseous arteries, which are themselves branches of the radial and ulnar arteries, respectively.
The oblique course of the ECU means that its contraction results in a combined extension and adduction (ulnar deviation) of the hand. Working together with the extensor carpi radialis brevis and extensor carpi radialis longus, the ECU contributes to a balanced extension of the wrist without deviating the hand in the transverse plane. This is important for activities such as clenching a fist or making a power grip, as it allows for an effective grip.
The ECU holds considerable clinical relevance due to its role in wrist and forearm function and its susceptibility to injuries. Athletes engaged in activities requiring forceful and repetitive wrist movements, such as gripping, throwing, or racket sports, are particularly prone to ECU injuries. Tennis elbow is a common injury to the ECU, occurring in people who participate in activities requiring repetitive arm, elbow, and wrist movements, especially when tightly gripping an object. Symptoms include pain when shaking hands or squeezing/gripping an object, with the pain worsening when moving the wrist with force.
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Extensor Muscles
The muscles in the posterior compartment of the forearm are commonly referred to as the extensor muscles. These muscles generally originate on or near the lateral epicondyle and insert on the distal forearm or in the hand. The extensor muscles are involved in wrist extension, with some muscles dedicated to specific fingers.
The extrinsic extensor muscles of the hand, located in the back of the forearm, 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 ECRL and ECRB form the lateral compartment, with their muscle fibres ending at different points in the forearm and continuing as flat tendons along the lateral border of the radius, beneath the APL and EPB. The ED is responsible for extending the fingers and wrist by dividing into four tendons that pass through the dorsal carpal ligament. The EI tendon joins the ED tendon along the back of the index finger, while the EDM follows a similar path along the little finger.
The APL, EPB, and EPL are involved in thumb extension. The APL abducts the thumb at the carpometacarpal joint, while the EPB extends and abducts the thumb at the carpometacarpal and MCP joints. The EPL extends all joints of the thumb, using the dorsal tubercle at the wrist as a 'pulley' to increase force.
The ECU contributes to wrist extension and adduction, while the EDM extends the little finger and assists in wrist extension. The extensor muscles are susceptible to injuries due to their superficial location, and lateral epicondylitis is a common condition affecting these muscles.
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Frequently asked questions
The wrist extensor muscles are a significant component of the posterior forearm musculature. They generally originate on or near the lateral epicondyle and insert on the distal forearm or in the hand.
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) are all muscles that extend the wrist.
The extensor carpi ulnaris is located on the medial aspect of the posterior forearm. It produces both adduction and extension at the wrist.
The extensor pollicis longus extends all joints of the thumb: carpometacarpal, metacarpophalangeal, and interphalangeal.
The extensor digitorum extends the fingers and wrist.
