Dominic Stone
Extensor muscles are those that increase the angle between members of a limb, causing straightening movements. Extension is usually directed backward, with the exception of the knee joint. Extensor muscles include those in the hand, wrist, foot, leg, thigh, back, and arm. For example, the straightening of the elbow, knee, or bending of the wrist and spine backward are all examples of extension. The opposite of extensor muscles are flexor muscles, which decrease the angle between bones on two sides of a joint, such as bending the elbow and knee.
| Characteristics | Values |
|---|---|
| Definition | Muscles that initiate extension in the body |
| Direction of Movement | Backward, except in the case of the knee joint |
| Function | Increase the angle between members of a limb |
| Examples of Extension | Straightening of the elbow, knee, wrist or spine |
| Wrist Extensor Muscles | Extensor carpi radialis brevis, extensor carpi radialis longus, extensor carpi ulnaris |
| Thumb Extensors | Extensor pollicis brevis, extensor pollicis longus |
| Finger Extensors | Extensor digitorum, extensor indicis, extensor digiti minimi |
| Toe Extensors | Extensor hallucis longus, extensor digitorum longus, extensor digitorum brevis, extensor hallucis brevis |
| Extensor Tendons | Enter the dorsum of the hand via 6 compartments covered by the extensor retinaculum |
| Intertendinous Connections | Junctura A, B, and C connect extensor tendons of different fingers |
| Functions of Intertendinous Connections | Stabilization of joints, force distribution during extension, coordination of tendon movements |
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What You'll Learn
Extensors in the foot
Extensor muscles are those that increase the angle between members of a limb, causing a straightening movement. In the foot, extensors include the extensor digitorum longus and extensor digitorum brevis. These muscles originate in the upper and lower parts of the lower leg and act on the toes through long tendons. The extensor digitorum longus is a pennate muscle, situated at the lateral part of the front of the leg. It passes under the superior and inferior extensor retinaculum of the foot, dividing into four slips that run forward on the dorsum of the foot and are inserted into the second and third phalanges of the four lesser toes. The extensor digitorum brevis is also inserted into the second phalanx, with its tendon joined to that of the extensor digitorum longus by a tendon on the lateral side.
The foot also contains intrinsic and extrinsic muscles. There are ten intrinsic muscles located in the plantar aspect (sole) of the foot. They collectively stabilise the arches of the foot and individually control the movement of the digits. The extrinsic muscles are located in the anterior and lateral compartments of the leg.
The extensor hallucis longus and brevis muscles are also present in the foot and are targeted by exercises such as the 'Big Toe Lift' and 'Big Toe Extension with Heel Raises'. The extensor hallucis longus extends the metatarsophalangeal and interphalangeal joints of the big toe and assists in the inversion of the foot and dorsiflexion of the ankle.
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Wrist extensor muscles
Extensor muscles are those 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 movement is usually directed backward, with the knee joint being a notable exception.
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 muscles are situated on the lateral aspect of the posterior forearm. Due to their position, they can produce abduction as well as extension at the wrist. The extensor digitorum is the main extensor of the fingers.
The supinator muscle, abductor pollicis longus muscle, extensor pollicis brevis muscle, extensor pollicis longus muscle, and extensor indicis muscle comprise the deep compartment. The supinator muscle originates at the superficial lateral epicondyle of the humerus, radial collateral ligament, and annular ligament. Its insertion point is the lateral proximal third of the radius. The abductor pollicis longus originates from the posterior proximal surface of the ulna and radius. Its insertion point is the lateral base of the first metacarpal. The extensor pollicis brevis originates from the posterior proximal surface of the radius (distal to the abductor pollicis longus). It inserts into the dorsal surface of the base of the thumb. The extensor pollicis longus originates from the posterior surface of the ulna and interosseous membrane. It attaches to the distal phalanx of the thumb. The extensor indicis originates from the posterior surface of the proximal ulna.
The extensor tendon zones are a helpful way to identify the region where injuries to the extensor tendons occur in the hand and wrist. Zone I covers the fingertip to the distal interphalangeal joint.
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Extensor tendons
Extensor muscles are those that increase the angle between members of a limb, resulting in movements like straightening the elbow or knee or bending the wrist or spine backward. The movement is usually directed backward, except in the case of the knee joint. Extensor tendons are a crucial component of the extensor mechanism, facilitating the extension of various body parts.
The extensor tendons enter the back of the hand through six extensor tendon compartments covered by the extensor retinaculum, a structure that spans from the pisiform and triquetrum bones to the radiopalmar radius. This arrangement allows for coordinated movement and force distribution during finger extension. The extensor tendon zones in the hand and wrist are important for identifying the region where injuries to the extensor tendons occur. For example, Zone I covers the fingertip to the distal interphalangeal (DIP) joint.
In the wrist, the extensor carpi radialis brevis, extensor carpi radialis longus, and extensor carpi ulnaris muscles extend from the upper arm (humerus) down the back of the forearm to the metacarpal bones at the back of the hand, enabling wrist extension. The extensor digitorum muscle, which runs from the humerus to a common tendon attached to all fingers, is responsible for extending the fingers. The extensor indicis specifically acts upon the index finger, while the extensor pollicis brevis and extensor pollicis longus muscles control the thumb.
The extensor digitorum longus and extensor digitorum brevis are extensors in the foot, originating in the lower leg and using long tendons to act upon the toes. The extensor tendons in the foot play a crucial role in facilitating movement and maintaining stability during various activities, such as walking or running.
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Extensor tendon compartments
Extensor muscles are those that increase the angle between members of a limb, such as straightening the elbow or knee or bending the wrist or spine backward. The movement is usually directed backward, except in the case of the knee joint. Extensor tendon compartments refer specifically to the wrist, where six tunnels transmit the long extensor tendons from the forearm into the hand. These tunnels are lined internally by a synovial sheath and are separated from one another by fibrous septa.
The first extensor compartment is located on the lateral (radial) aspect of the wrist and transmits two tendons: the extensor carpi radialis longus and the extensor carpi radialis brevis. These tendons form the lateral border of the anatomical snuffbox. De Quervain's tenosynovitis is an inflammation of the tendons within the first extensor compartment of the wrist, resulting in pain and swelling near the base of the thumb. This condition is caused by repetitive trauma, leading to thickening of the tendon sheath and subsequent movement restriction.
The second extensor compartment is separated from the first by Lister's tubercle, a bony prominence of the distal radius. Intersection syndrome, commonly affecting weightlifters and athletes in similar sports, can be caused by direct trauma to this compartment or repetitive wrist flexion and extension.
The third compartment conducts the extensor pollicis longus tendon, forming the medial border of the anatomical snuffbox. The fourth 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. This tendon can suffer recurrent dislocation due to a tear in the ulnar side of the compartment, with those engaging in racket sports and golf at the highest risk.
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Extensors and the hamstrings
Extensors are 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. The movement is usually directed backward, with the notable exception of the knee joint. In the human body, certain muscles of the hand and foot are named for their function as extensors. Extensor muscles in the hand include the extensor carpi radialis brevis, extensor carpi radialis longus, and extensor carpi ulnaris, which extend the wrist. The extensor digitorum extends the fingers, the extensor indicis acts upon the index finger, and the extensor pollicis brevis and extensor pollicis longus act upon the thumb. Extensors in the foot include the extensor digitorum longus and extensor digitorum brevis, which act upon the toes.
The hamstrings are a group of three muscles at the back of the thigh that includes the semitendinosus, semimembranosus, and biceps femoris muscles. The hamstrings act as knee flexors and hip extensors. The semitendinosus and semimembranosus are categorized as medial hamstrings, while the biceps femoris is a lateral hamstring. During hip flexion-extension, the medial and lateral hamstrings stabilize the hip joint. In the straight leg raise (SLR) position, the hamstrings can act as hip extensors when the leg is lowered by applying resistance to the ankle joint.
Studies have compared the muscle activity of the hamstrings as knee flexors and hip extensors. One study found that hamstring activity during knee flexion with tibial internal and external rotation was significantly higher than during hip extension with hip internal and external rotation. However, hip rotation during maximal isometric hip extension can effectively activate the medial and lateral hamstrings.
The balance between flexors and extensors is essential for achieving good posture and movement patterns. For example, when the thighs lean forward of the pelvis, the quadriceps become overstretched, and the flexors at the front of the body become overly long, throwing the relationship between flexors and extensors out of balance.
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Frequently asked questions
Extensor muscles are the muscles that increase the angle between members of a limb, causing extension or straightening of the elbow, knee, wrist, or spine.
An example of an extensor muscle in the hand is the extensor carpi ulnaris, which runs from the upper arm along the back of the forearm to the metacarpal bones at the back of the hand, extending the wrist.
An example of an extensor muscle in the foot is the extensor digitorum longus, which originates in the lower leg and acts on the toes through long tendons.
Flexor muscles decrease the angle between bones on two sides of a joint, causing movements like bending the elbow or knee. On the other hand, extensor muscles increase the angle, resulting in extension or straightening movements.
Wrist extensor muscles include the extensor carpi radialis brevis, extensor carpi radialis longus, and extensor carpi ulnaris. These muscles run from the upper arm to the back of the hand, allowing for wrist extension.
