Athena Vale
The human hand has an incredible capacity for manipulation, but the movement of its digits is usually not entirely independent. 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. These muscles extend, or open flat, joints in the hand. The extensor digitorum muscle, for example, extends the phalanges, wrist, and elbow. It tends to separate the fingers as it extends them. Extension of the index finger, middle finger, ring finger, and small finger occurs via the extensor digitorum communis (EDC) muscles. The extensor digitorum muscle is also known as extensor digitorum communis and is present in humans and other animals.
| Characteristics | Values |
|---|---|
| Muscles that extend your fingers | 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), extensor indicis (EI), extensor digitorum communis (EDC), lumbricals, palmar interossei, and dorsal interossei |
| Location of extrinsic extensor muscles | Back of the forearm |
| Function of extrinsic extensor muscles | Extend or open flat joints in the hand |
| Independent control of ED | Limited |
| Independent control of FDS | Limited |
| Independent control of FDP | Limited |
| Independent control of FPL | Limited |
| Average MVC values for each finger | Thumb: 19.7 N, index: 30.3 N, middle: 23.9 N, ring: 17.4 N, little finger: 19.1 N |
| 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 (EI) |
| Middle and ring finger extension muscle | Extensor digitorum (ED) |
| Little finger extension muscle | Extensor digiti minimi (EDM) |
| Function of lumbrical muscles | Allow fingers to straighten, help bend MCP joints, and flex the base of digits at the metacarpophalangeal joints while extending the fingers at the interphalangeal joints |
| Function of palmar interossei muscles | Adduct the fingers and pull them together |
| Function of dorsal interossei muscles | Abduct, or spread apart, the fingers |
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What You'll Learn
Extensor digitorum muscle
Extensor digitorum is a long muscle located in the posterior compartment of the forearm. It is one of the extrinsic muscles of the hand, located outside the hand but connected to it by long tendons. Extensor digitorum 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 vascularized by the branches of three different arteries: the anterior and posterior interosseous arteries, which are the branches of the common interosseus artery that arises from the ulnar artery, and the radial recurrent artery, which is a branch of the radial artery. The muscle is innervated by the posterior interosseous nerve, which is a continuation of the deep branch of the radial nerve.
The main function of extensor digitorum is the extension of the four medial fingers in metacarpophalangeal and proximal and distal interphalangeal joints. This muscle contributes to the opening of the hand and letting go of an object. The action of this single muscle opposes the actions of two flexors of the fingers: flexor digitorum superficialis and flexor digitorum profundus.
The independent control of the output of extensor digitorum is limited. This may be due to the ''spill-over' of motor commands to other digital extensor compartments.
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Extensor tendons
Extensor tendinitis is a common condition that affects the extensor tendons, causing inflammation and irritation. It is usually caused by repetitive motions and overuse of the tendons, especially in activities that require frequent use of the hands and feet. Symptoms of extensor tendinitis include pain and difficulty in moving the hands and feet, and it can be treated by avoiding aggravating activities, rest, and physical therapy.
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). These muscles have long tendons that connect to the bones in the hand, allowing for extension and abduction of the wrist and fingers.
The ECRL and ECRB form the lateral compartment, with their muscle fibers ending at the upper third and mid-forearm, respectively. They then continue as flat tendons along the lateral border of the radius, beneath the APL and EPB. The ED divides into four tendons that, along with the EI tendons, pass through the fourth tendon compartment of the dorsal carpal ligament. On the back of the hand, the ED and EI tendons work together to extend the index finger, while the EDM follows a similar course for the little finger.
The major extrinsic muscle that extends the fingers is the extensor digitorum (ED), which has multiple compartments and separate tendons for each finger. The ED inserts into the middle and distal phalanges to extend the fingers and wrist. The EI joins the ulnar side of the ED tendon to extend the index finger specifically.
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Lumbrical muscles
The lumbricals are a set of four intrinsic hand muscles, also known as worm-like muscles, that do not attach to bones. Instead, they attach proximally to the tendons of flexor digitorum profundus (FDP) and distally to the extensor expansions. They are located deep in the palm and are essential for the fingers' precision grip and fine motor control. Each finger attaches to two lumbricals, except for the thumb, which is associated only with the first lumbrical muscle.
The first and second lumbricals (the most radial two) are innervated by the median nerve. The third and fourth lumbricals (the most ulnar two) are innervated by the deep branch of the ulnar nerve. This is the most common innervation pattern of the lumbricals, occurring in 50-60% of individuals. However, there are variations, with 20-30% of individuals having different innervation patterns.
The lumbricals flex the metacarpophalangeal (MCP) joints and extend the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints. By coordinating with other hand muscles, the lumbricals help control the fine and delicate movements of the fingers. The actions of these muscles are crucial for gripping objects, typing, and performing various dexterous hand movements.
The lumbrical muscles contain many muscle spindles and have a large fibre length, indicating that they play a role in proprioception and sensory feedback. This is important for precision pinch movements and precise manipulation of objects. The lumbricals are also relevant in carpal tunnel syndrome and can be damaged in crush injuries to the hand.
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Interossei muscles
The interosseous muscles of the hand 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 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. The dorsal interossei consist of four short muscles that attach to the adjacent sides of metacarpals 1-4. Their function is to abduct the digits 2-4, moving them away from each other, as well as to 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 that inserts on the radial side of the base of the second proximal phalanx and its dorsal digital expansion.
Palmar Interossei
The palmar interossei 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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Flexor muscles
The muscles that extend the fingers are known as extrinsic extensor muscles. These muscles are located in the back of the forearm and have long tendons that connect them to bones in the hand. The extrinsic extensor muscles 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).
Now, onto the flexor muscles. The muscles that flex the fingers are called long flexor muscles. Six long flexor muscles originate in the forearm and have tendon attachments in the hand. These muscles are:
- Flexor carpi radialis muscle: This muscle originates from the medial epicondyle of the humerus and travels diagonally down the arm to attach to the base of the second and third metacarpal bones. Its contraction causes the hand to flex laterally.
- Flexor carpi ulnaris muscle: This muscle is one of two flexor muscles that is not exclusively supplied by the median nerve. Its contraction causes the hand to flex medially.
- Flexor pollicis longus muscle: This muscle is responsible for the action of the thumb. When it contracts, it causes the thumb to flex towards the palm.
- Flexor digitorum profundus muscle: This muscle is also known as flexor digitorum communis profundus. It is considered an extrinsic hand muscle because it acts on the hand while its muscle belly is located in the forearm. It fans out into four tendons, one for each of the second to fifth fingers, and allows the distal phalanges to flex.
- Flexor digitorum superficialis muscle: This muscle has long tendons that run down the arm and through the carpal tunnel, attaching to the palmar side of the phalanges of the fingers. Its contraction enables the flexion of the fingers up to the middle phalangeal joint.
- Palmaris longus muscle: This is one of the six long flexor muscles that either flex the fingers or flex the hand at the wrist joint.
The tendons of the long flexor muscles can become thickened or affected by fibrosis, which can result in a condition called "Dupuytren contracture". This condition causes the irreversible pathological flexion of the fingers, typically affecting the third and fourth digits. Another condition associated with the flexor tendons is "trigger finger", which results in the fingers presenting in rigid flexion at rest and an inability to extend the fingers.
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Frequently asked questions
The extensor digitorum muscle, also known as the extensor digitorum communis, is the major extrinsic muscle that extends the fingers. The extensor carpi radialis longus, extensor carpi radialis brevis, and extensor carpi ulnaris muscles also extend the hand at the wrist.
Extensor muscles are long and thin and extend into the hand via long tendons. Extrinsic extensor muscles are located in the back of the forearm and have long tendons connecting them to bones in the hand.
The extensor digitorum muscle extends the phalanges, the wrist, and the elbow. It separates the fingers as it extends them and acts principally on the proximal phalanges, extending the metacarpophalangeal joint.
The four palmar interossei muscles extend from the metacarpals and insert into each of the phalanges to adduct the fingers and pull them together. The four dorsal interossei muscles work against the palmar interossei muscles to abduct, or spread apart, the fingers.
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