Dominic Stone
The human hand is a complex structure, with numerous muscles, tendons, and joints working in harmony to allow for a wide range of movements. One important function is the extension of the digits, or fingers, which is primarily facilitated by the extensor digitorum muscle. This muscle, located in the forearm, is responsible for extending the medial four digits of the hand, or fingers 2-5. It works in conjunction with other muscles and tendons to allow for the extension and flexion of the fingers, as well as the separation of the fingers from each other. The extensor digitorum is just one component of the intricate musculature of the hand, which also includes muscles such as the interossei, hypothenar, and thenar muscle groups, each contributing to the hand's impressive dexterity.
| Characteristics | Values |
|---|---|
| Muscle | Extensor digitorum muscle (also known as extensor digitorum communis) |
| Location | Posterior forearm |
| Function | Extends the medial four digits of the hand |
| Innervation | Posterior interosseous nerve, a branch of the radial nerve |
| Origin | Lateral epicondyle of the humerus, by the common tendon; intermuscular septa; antebrachial fascia |
| Tendons | Four tendons that pass through the dorsal carpal ligament and insert into the middle and distal phalanges of the fingers |
| Ligaments | Dorsal ligament of the metacarpophalangeal joint |
| Other extrinsic extensor muscles of the hand | Extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), extensor digiti minimi (EDM), extensor carpi ulnaris (ECU), abductor pollicis longus (APL), extensor pollicis brevis (EPB), extensor pollicis longus (EPL), extensor indicis (EI) |
| Interossei muscles | Allow us to spread our fingers away from each other |
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What You'll Learn
Extensor digitorum muscle
The extensor digitorum muscle, also known as the extensor digitorum communis, is a muscle of the posterior forearm in humans and other animals. It is a long muscle located in the posterior compartment of the forearm. It is one of the extrinsic muscles of the hand. Extensor digitorum is a superficial muscle, and it is the most superficial muscle of the posterior forearm. It is located medial to the extensor carpi radialis brevis muscle and lateral to the extensor digiti minimi and extensor carpi ulnaris muscles.
Extensor digitorum is innervated by the posterior interosseous nerve, which is a branch of the radial nerve. It arises from the lateral epicondyle of the humerus, by the common tendon; from the intermuscular septa between it and the adjacent muscles, and from the antebrachial fascia. It divides below into four tendons, which pass through a separate compartment of the dorsal carpal ligament, within a mucous sheath. The tendons then diverge on the back of the hand, and are inserted into the middle and distal phalanges of the fingers.
The extensor digitorum extends the phalanges, the wrist, and the elbow. It extends the four medial fingers in their metacarpophalangeal and both interphalangeal joints. It also participates in the extension of the wrist. It tends to separate the fingers as it extends them. In the fingers, the extensor digitorum acts principally on the proximal phalanges, extending the metacarpophalangeal joint. Extension of the proximal and distal interphalangeal joints is mediated predominantly by the dorsal and palmar interossei and lumbricals of the hand.
The extensor digitorum is vascularized by the branches of three different arteries. The anterior and posterior interosseous arteries are the branches of the common interosseus artery, which arises from the ulnar artery. The radial recurrent artery is a branch of the radial artery.
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Interossei muscles
The interossei muscles are muscles found near the metacarpal bones that help control the fingers. They are considered voluntary muscles. There are two types of interossei muscles: dorsal interossei and palmar interossei.
Dorsal Interossei
The dorsal interossei muscles are short, bipennate, intrinsic muscles of the hand. They are found on the dorsal aspect of the hand, occupying the space between the metacarpal bones, along with the palmar interossei muscles. The dorsal interossei consist of four short muscles that attach to the adjacent sides of metacarpals 1-4. Their function is to abduct digits 2-4 and assist in flexion of these fingers at the metacarpophalangeal (MCP) joints and in extension at the interphalangeal (IP) joints. The first dorsal interosseous muscle is the largest and strongest of the dorsal interossei and is sometimes referred to as abductor indicis. It arises from the adjacent surfaces of the first and second metacarpal bones. The muscle fibres converge distally into a tendon, which inserts on the radial side of the base of the second proximal phalanx and its dorsal digital expansion. The second dorsal interosseous muscle arises from the adjacent surfaces of the second and third metacarpal bones. Like the first muscle, the muscle fibres of the second dorsal interosseous converge distally into a tendon, which inserts on the radial side of the base of the third proximal phalanx and its dorsal digital expansion. The third dorsal interosseous muscle arises from the adjacent surfaces of the third and fourth metacarpal bones. The tendon of the third dorsal interosseous inserts on the ulnar side of the base of the third proximal phalanx and its dorsal digital expansion. The fourth dorsal interosseous muscle arises from the adjacent surfaces of the fourth and fifth metacarpal bones. The tendon of the fourth dorsal interosseous inserts on the ulnar side, attaching to the base of the fourth proximal phalanx and its dorsal digital expansion. The dorsal interossei muscles are situated within the dorsal compartment of the hand, lying deep to the palmar interossei and filling the interosseous space.
Palmar Interossei
The palmar interossei muscles adduct the digits towards the third digit (towards the axial line) and are unipennate. The axial line goes down the middle of the third digit, towards the palm of the hand. Both sets of muscles are innervated by the deep branch of the ulnar nerve.
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Hypothenar muscle group
The hypothenar muscles are a group of three or four muscles of the palm that control the motion of the little finger. They form the hypothenar eminence, a muscular protrusion on the medial side of the palm, at the base of the little finger. The hypothenar eminence is one of two "fleshy" mounds that can be observed on the palmar side of the human hand, the other being the thenar eminence at the base of the thumb.
The hypothenar muscles are intrinsic muscles of the hand, meaning their origin and insertion are within the carpal and metacarpal bones and are surrounded by ligaments and fascia of the hand. They help with fine motor movements of the hand. The ulnar nerve innervates the hypothenar muscles, and damage or compromise to the nerve leads to atrophy of the hypothenar eminence. This can be used as an identifying marker for proximal hand ulnar nerve compromise.
Hypothenar hammer syndrome is a vascular occlusion of the hypothenar region, which can be caused by arterial injury to the ulnar artery due to insufficient collateral blood flow. Compression at the Guyon's canal, where the ulnar nerve passes through the hook of the hamate and pisiform bone, can lead to atrophy, numbness, tingling, and pain in the hypothenar eminence, as well as the fourth and fifth digits.
The hypothenar muscles can be remembered using the mnemonic "A OF A OF A" for Abductor pollicis brevis, Opponens pollicis, Flexor pollicis brevis (the three thenar muscles), Adductor pollicis, and the three hypothenar muscles, Opponens digiti minimi, Flexor digiti minimi brevis, and Abductor digiti minimi.
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Extrinsic extensor muscles
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 muscles included in this group are 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 EPB inserts into the base of the first phalanx of the thumb to extend and abduct the thumb at the carpometacarpal and MCP joints. The EPL inserts on the base of the distal phalanx of the thumb and uses the dorsal tubercle on the radius as a fulcrum to help the EPB with its action, as well as extending the distal phalanx of the thumb. The index finger and little finger have separate extensors, allowing them to move more independently than the other fingers.
The ECU is supplied by the ulnar artery, while the APL, EPB, EPL, EI, ED, and EDM are supplied by the Posterior interosseous artery, a branch of the ulnar artery. The ECRL and ECRB receive blood from the radial artery. The ECRL is supplied by the radial nerve and the ECRB by its deep branch. The remaining extrinsic hand extensors are supplied by the posterior interosseus nerve, another branch of the radial nerve.
The ED divides into four tendons which, with the EI tendons, pass through the fourth tendon compartment of the dorsal carpal ligament. On the back of the hand, the ED tendons diverge to follow the fingers and the EI tendon joins the ulnar side of one of the ED tendons along the back of the index finger. The EDM takes a similar course as the EI, except it follows the ED tendon along the little finger.
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Flexor digitorum
The flexor digitorum muscle is primarily responsible for flexing the digits or fingers. There are two types of flexor digitorum muscles: the flexor digitorum profundus and the flexor digitorum superficialis.
The flexor digitorum profundus is a muscle in the forearm of humans that flexes the fingers or digits. It is considered an extrinsic hand muscle because it acts on the hand while its muscle belly is located in the forearm. The muscle is named from the Latin 'deep bender of the fingers'. The flexor digitorum profundus is the most powerful and most bulky muscle of the forearm. It acts as the main gripping power of the hand. The muscle fans out into four tendons, one for each of the second to fifth fingers, to the palmar base of the distal phalanx. It is a hybrid or composite muscle as it is innervated by two different nerves. The upper three-fourths of the anterior and medial surface of the shaft of the ulna, the medial surface of the olecranon, and the coronoid processes of the ulna are all parts of the flexor digitorum profundus.
The flexor digitorum superficialis, formerly known as the flexor digitorum sublimis, is the largest of the extrinsic flexors of the forearm. It forms the intermediate muscle layer between the superficial and deep muscle groups of the forearm. It contains four digital components, with a tendon that inserts onto each corresponding finger. The flexor digitorum superficialis is the primary flexor of the proximal interphalangeal (PIP) joints of the middle phalanges, but it also assists in flexing the metacarpophalangeal (MCP) joints. The flexor digitorum superficialis courses along the volar aspect of the forearm, superficial to the flexor digitorum profundus and flexor pollicis longus muscles. As it courses down the forearm, the flexor digitorum superficialis separates into two planes of muscular fibres, superficial and deep. The superficial plane further divides to supply tendons for the middle and ring fingers. The deep plane provides a muscular slip to join the part of the superficial plane associated with the tendon of the ring finger and then divides to supply tendons for the index and little fingers.
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Frequently asked questions
The extensor digitorum muscle is a muscle in the posterior forearm that extends the medial four digits of the hand.
The extrinsic extensor muscles of the hand are located in the back of the forearm and 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 extensor digitorum muscle is responsible for extending fingers 2-5 and generating a pull for the extension of the four medial fingers in their metacarpophalangeal and interphalangeal joints.
The interossei muscles are four dorsal and three palmar muscles that begin between the bones of the hand. They allow us to spread our fingers away from each other.
The hypothenar muscle group is formed by three muscles: the abductor digiti minimi, the flexor digiti minimi, and the opponens digiti minimi. The abductor allows the small finger to pull away from the ring finger, the flexor allows the small finger to bend at the MCP joint, and the opponens allow us to cup our hands by bringing the small finger towards the thumb.
