Dominic Stone
Extensor muscles are those that increase the angle between bones, usually resulting in straightening. For example, extension occurs when a flexed (bent) elbow is straightened. Extensors in the foot include the extensor digitorum longus and extensor digitorum brevis, which originate in the lower leg and extend the toes. The extensor hallucis brevis and extensor hallucis longus extend the big toe. The extensor carpi ulnaris, extensor carpi radialis longus, and extensor carpi radialis brevis extend the wrist. The extensor digitorum extends the fingers. The thumb is extended by the abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus.
| Characteristics | Values |
|---|---|
| Definition | Any of the muscles that increase the angle between members of a limb, as by straightening the elbow or knee or bending the wrist or spine backward |
| Direction of movement | Usually backward |
| Exception | Knee joint |
| Extensor muscles of the hand | Extensor carpi ulnaris, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor indicis, extensor pollicis longus, extensor pollicis brevis, extensor medii proprius, extensor digitorum communis, extensor indicis proprius, extensor digiti minimi |
| Extensor muscles of the foot | Extensor digitorum longus, extensor digitorum brevis, extensor hallucis brevis, extensor hallucis longus |
| Extensor muscles of the forearm | Brachioradialis, anconeus, supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, extensor indicis |
| Extensor muscles of the thigh | Quadriceps femoris (rectus femoris, vastus intermedius, vastus lateralis, vastus medialis) |
| Innervation | Branches of the radial nerve (C6-C8) |
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What You'll Learn
Extensors in the foot
Extensor muscles are those that increase the angle between bones, usually by directing movement backward, with the exception of the knee joint. Extensors in the foot include the extensor digitorum longus and extensor digitorum brevis, which originate in the upper and lower parts of the lower leg and extend the toes through tendons. The extensor hallucis brevis and extensor hallucis longus extend the big toe. The longus muscles of the foot also aid upward flexion of the foot at the ankle.
Extensor tendinopathy is a condition that involves the tendons that run along the top of the foot. These tendons attach to muscles that lift the foot up and help clear the ground when walking. If overworked, these tendons may become painful and swollen. Treatment for extensor tendinopathy usually begins with exercises that can be done at home. These exercises include big toe lifts, big toe extensions with heel raises, short foot exercises, towel curls, and resistance band big toe abduction.
Claw toe deformities in a foot with sensation are quite painful. In a foot without sensation, the individual is at risk of skin breakdown due to increased pressure under the metatarsal heads and between the dorsal surfaces of the toes and the shoe.
The muscles acting on the foot can be divided into two groups: extrinsic and intrinsic muscles. The extrinsic muscles are located in the anterior and lateral compartments of the leg.
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Extensors in the hand
Extensors are muscles that increase the angle between members of a limb, such as by straightening the elbow or knee or bending the wrist or spine backward. The muscles that initiate extension in the body are known as 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, where they exert their action. Extensor muscles help in straightening or extending, whereas flexor muscles help in bending our elbows, knees, and fingers. The extrinsic extensors 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).
The extensor expansion of the hand is a specialised connective tissue structure by which the extensor tendons insert onto the phalanges. It is a complex structure that acts to balance the forces from a large number of muscles that act on the digits. The extensor expansion is triangular in shape and begins at the level of the metacarpophalangeal joint. It is formed from the extensor digitorum tendon (fingers) and extensor pollicis longus tendon (thumb). As the long extensor tendons approach the metacarpophalangeal joint, they begin to flatten and form the extensor hood.
The extensor tendons maintain their central orientation at the level of the metacarpophalangeal joints via the sagittal bands, which are a bundle of dense circular fibres originating from the volar plate and the deep metacarpal ligaments. The sagittal bands also prevent hyperextension in the metacarpophalangeal joint. Beyond the sagittal band, the extensor tendon flares slightly into three portions: a central, more robust slip, and two lateral slips.
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Extensors in the forearm
Extensor muscles are those that increase the angle between members of a limb, such as straightening the elbow or bending the wrist backward. In the forearm, extensor muscles include the extensor carpi ulnaris, extensor carpi radialis longus, and extensor carpi radialis brevis, which run from the humerus along the backside of the forearm to the metacarpal bones and extend the wrist. The extensor digitorum extends the fingers, and the extensor indicis specifically extends the index finger. The extensor pollicis brevis and extensor pollicis longus extend the thumb.
The extensor tendons arise from the corresponding extensor muscle bellies that coordinate extension of the wrist and fingers. All of the extensor muscles are innervated by branches of the radial nerve (C6–C8). The extensor muscle bellies arise from two main anatomic origins: a superficial and a deep layer. The deep layer of muscles originates in the mid-portion of the ulna and radius. The deep muscles include the extensor muscles of the thumb and fingers (dorsal compartment 1, 3, and 4): extensor policis brevis (EPB), abductor pollicis longus (APL), extensor pollicis longus (EPL), extensor digitorum communis (EDC), and extensor indicis proprius (EIP).
The anatomy of the extensor tendons at the level of the fingers consists of a complex arrangement of extrinsic extensor tendons and the intrinsic muscles of the hand. The extensor tendons maintain their central orientation at the level of the metacarpophalangeal joints via the sagittal bands, dense circular fibres originating from the volar plate and deep metacarpal ligaments. The sagittal bands also prevent hyperextension in the metacarpophalangeal joint.
The extensor tendon does not insert into the proximal phalanx. Beyond the sagittal band, the extensor tendon flares into three portions: a central, more robust slip, and two lateral slips, creating a contiguous extensor expansion or hood. The central slip inserts onto the base of the middle phalanx, while the lateral slips pass to the lateral aspect of the proximal interphalangeal (PIP) joint.
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Extensors in the wrist
Extensor muscles are those that increase the angle between members of a limb, usually by straightening joints such as the elbow or knee, or bending the wrist or spine backward. The muscles that initiate extension in the body are known as extensor muscles.
The wrist extensor muscles make up 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 tendons enter the dorsum of the hand via six extensor tendon compartments, which are covered by the extensor retinaculum. These compartments serve as tunnels for tendons to pass from the forearm to the wrist.
The first extensor compartment is located on the lateral (radial) aspect of the wrist and transmits two tendons: the abductor pollicis longus (APL) and extensor pollicis brevis (EPB). The APL inserts into the radial side of the base of the first metacarpal bone to abduct the thumb at the carpometacarpal joint and may continue to abduct the wrist. 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 second extensor compartment contains the tendons of the extensor carpi radialis longus (ECRL) and extensor carpi radialis brevis (ECRB). The third extensor compartment conducts the extensor pollicis longus (EPL) tendon, which forms the medial border of the anatomical snuff box. The fourth extensor compartment transmits the tendons of the extensor digitorum and extensor indicis. The fifth compartment contains the extensor digiti minimi tendon, which travels into the little finger. The sixth compartment is located on the medial (ulnar) aspect of the wrist and conducts the tendon of the extensor carpi ulnaris (ECU). The ECU inserts at the base of the 5th metacarpal to extend and adduct the wrist.
The extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), and extensor carpi ulnaris (ECU) muscles work together to achieve neutral wrist extension movements.
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Extensors in the elbow
Extensors are muscles that increase the angle between members of a limb, such as straightening the elbow or bending the wrist or spine backward. The elbow extensors are primarily the triceps brachii and the anconeus. The triceps brachii is a long muscle that runs from the scapula to the olecranon of the ulna. The anconeus is a much smaller muscle that begins at the distal end of the humerus near the elbow and also ends at the olecranon. Together, these muscles increase the angle between the humerus and the ulna and radius, straightening the arm.
The elbow flexor muscles are innervated by three different nerves. Total paralysis of all elbow flexor muscles requires damage to all three nerves, which is uncommon. In contrast, total paralysis of the elbow extensor muscles (the triceps) occurs with damage to the radial nerve alone. The prime movers of elbow flexion are the biceps brachii, the brachialis, and the brachioradialis. These muscles have a line of force that passes anterior to the elbow’s axis of rotation. The biceps brachii is also a shoulder flexor, so a shoulder extensor muscle like the posterior deltoid must be active to neutralise unwanted shoulder flexion.
The forearm can be divided into two compartments by deep fascia: the anterior compartment containing the flexor muscles, and the posterior compartment containing the extensor muscles. The posterior compartment can be further divided into superficial and deep extensors. Superficial extensors consist of seven muscles: brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and the anconeus. Deep extensors include five muscles: supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis.
The forearm helps the shoulder and arm in force application and the precise placement of the hand in space, with the help of the elbow and radioulnar joints. The elbow is a common site for injury, with lateral epicondylitis (tennis elbow) being a common condition caused by soreness of the forearm extensor muscles attached to the lateral epicondyle of the humerus. This is caused by repetitive strenuous contraction of the muscles, such as in tennis backhands, and results in pain.
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Frequently asked questions
Extensors are muscles that increase the angle between members of a limb, usually by straightening the elbow or knee or bending the wrist or spine backward.
Examples of extensor muscles in the human body include the extensor carpi ulnaris, extensor carpi radialis longus, and brachioradialis.
The extensor digitorum is the main extensor of the fingers. The extensor digitorum communis (EDC) muscles are responsible for extension of the second (index finger), third (long finger), fourth (ring finger), and fifth (small finger) digits.
