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The muscles that control finger extension are called the extensor digitorum communis (EDC) or extensor digitorum muscles. These muscles originate near the elbow, run through the forearm, and insert into the fingers. The index finger, for example, has two muscles for extension: the index finger compartment of the extensor digitorum (ED2) and the extensor indicis (EI). The thumb, on the other hand, has its own set of muscles, including extensors like the extensor pollicis longus and extensor pollicis brevis, which extend and abduct the thumb. The interossei muscles, which include the dorsal and palmar varieties, also play a role in finger extension by allowing us to spread our fingers apart or pull them together.
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What You'll Learn
The extensor digitorum muscle
Each tendon attaches to its respective digit via extensor expansion (extensor hood). This extensor hood covers the dorsal surfaces of digits 2-5, extending from the metacarpophalangeal joint to the proximal interphalangeal joint of each digit. At the distal end of the proximal phalanx, the extensor hood divides into three separate slips: the central part and two collateral (lateral) parts. The central part inserts onto the dorsal aspect of the base of the middle phalanx, while the two collateral parts merge with the intrinsic muscles of the hand to form a conjoint tendon that inserts onto the base of the distal phalanx.
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The extensor indicis muscle
Extensor Indicis Muscle, also known as Extensor Indicis Propius, is a narrow, elongated skeletal muscle situated in the deep layer of the dorsal forearm. It is one of the deep extensors of the forearm, along with the supinator, abductor pollicis longus, extensor pollicis longus, and extensor pollicis brevis muscles. Extensor Indicis Muscle is responsible for the movement of the index finger.
The extensor indicis proprius inserts into the index finger on the ulnar side of the extensor digitorum. The extensor digitorum is a muscle that allows for the extension of the index, middle, ring, and small fingers. The extensor indicis tendon lies in the fourth dorsal (extensor) compartment of the wrist. It shares this compartment with the tendons of the extensor digitorum muscle, sitting medial to them in their common tendinous sheath.
The extensor indicis receives its nerve supply from the posterior interosseous nerve, a branch of the radial nerve derived from spinal roots C7 and C8. The skin overlying the muscle is supplied by the same nerve, with fibres stemming from the spinal roots C6 and C7. The superficial surface of the extensor indicis receives arterial blood supply from the posterior interosseous branch of the ulnar artery, while its deep surface receives blood from perforating branches of the anterior interosseous artery.
The extensor indicis acts at the metacarpophalangeal and interphalangeal joints to extend the index finger. The index finger has its own separate extensor, allowing it to extend independently from other fingers. Additionally, the extensor indicis muscle produces a weak extension of the wrist. The extensor indicis is a narrow muscle that originates mainly from the ulna, arising from the posterior two-thirds of its distal surface, distal to the extensor pollicis longus muscle.
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The extensor digiti minimi muscle
The extensor digitorum communis provides the ability to straighten the index, middle, ring, and small fingers. The extensor digitorum muscle is a long muscle located in the posterior compartment of the forearm. It is a superficial forearm extensor that is mainly responsible for extending fingers 2-5.
The extensor digiti minimi originates from the lateral epicondyle of the humerus via the common extensor tendon that it shares with the extensor digitorum, extensor carpi radialis brevis, and extensor carpi ulnaris muscles. The muscle courses inferiorly, however, its proximal fibres are usually indistinguishably fused with those of the extensor digitorum muscle. From approximately the middle of the forearm, the fibres of the extensor digiti minimi are clearly separated from the extensor digitorum.
The extensor digiti minimi functions primarily to extend the fifth digit at its metacarpophalangeal joint. It also contributes to the extension of the wrist and all the fifth finger joints from the ulnar side of the hand, via its attachment into the extensor expansion and by acting along with the extensor carpi ulnaris muscle. Having its own extensor muscle, the little finger can extend independently of digits 2-4.
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The extensor carpi radialis brevis muscle
The extensor carpi radialis brevis (ECRB) is a fusiform muscle found in the lateral part of the posterior forearm. It is one of seven superficial extensor muscles of the posterior forearm, including the extensor carpi radialis longus (ECRL), brachioradialis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and anconeus. The ECRB is the shortest of the two muscles, with its tendon coursing deep to the abductor pollicis and extensor pollicis brevis before passing under the extensor retinaculum on the dorsal aspect of the hand.
The ECRB originates from the lateral epicondyle of the humerus via the common extensor tendon, which is also a common origin shared with the extensor digitorum, extensor digiti minimi, and extensor carpi ulnaris muscles. Some fibres of the ECRB also originate from the lateral intermuscular septum and the radial collateral ligament. The muscle travels deep to the dorsal carpal ligament (extensor retinaculum) within the second extensor compartment of the wrist on the dorsal surface of the hand.
The ECRB tendon passes through a groove on the posterior surface of the radius, deep to the extensor retinaculum, and 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. The ECRB is supplied by a branch of the radial nerve and is vascularized primarily by the radial artery, with additional supply from the radial collateral branch of the profunda brachii.
The ECRB works in conjunction with the ECRL and the extensor carpi ulnaris to extend and abduct the wrist. This action is vital in a sequence of muscle contractions needed for clenching a fist or making a grip. The ECRB also contributes to producing hand abduction (radial deviation) when contracting together with the ECRL and flexor carpi radialis.
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The extensor carpi ulnaris muscle
The extensor carpi ulnaris (ECU) is a skeletal muscle located on the ulnar side of the forearm. It is the most medial muscle of the posterior forearm and is involved in wrist and forearm function, contributing to both extension and adduction of the hand at the wrist. The ECU also provides essential medial stability to the wrist, preventing excessive lateral deviation.
Just above the wrist, the extensor carpi ulnaris gives off a tendon that passes beneath the extensor retinaculum and through a groove on the posterior surface of the head and styloid process of the ulna. The extensor carpi ulnaris inserts onto the dorsal base of the fifth metacarpal, extending the wrist. However, when acting alone, it inclines the hand towards the ulnar side. By its continued action, it can also extend the elbow joint.
The extensor carpi ulnaris is susceptible to injuries, particularly in activities requiring repetitive arm, elbow, and wrist movements, such as tennis or golf. Tennis elbow is a common injury, causing pain when shaking hands or gripping objects, and the pain intensifies with forceful wrist movements. Treatment options include occupational therapy, physical therapy, anti-inflammatory medication, and rest from the aggravating activity.
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Frequently asked questions
The extensor digitorum communis, or extensor digitorum, is the major extrinsic muscle that extends the fingers. It extends the middle phalanx and distal phalanx. The extensor indicis proprius (EIP) muscle controls independent index finger extension, and the extensor digiti minimi (EDM) muscle controls independent small finger extension.
The extrinsic extensor muscles of the hand are located in the back of the forearm and have long tendons connecting them to bones in the hand. They extend, or open flat, joints in the hand.
The interossei and lumbricals are intrinsic hand muscles that extend interphalangeal joints. The dorsal interossei allow us to spread our fingers away from each other, and the palmar interossei pull our fingers together. The thumb has its own set of muscles, including two extensors: extensor pollicis longus and extensor pollicis brevis.
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