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Extensor muscles are muscles that increase the angle between bones or body surfaces at a joint, resulting in straightening. For example, extending a flexed (bent) elbow or bending the wrist or spine backward. In the hand, extensor muscles include the extensor carpi radialis brevis, extensor carpi radialis longus, and extensor carpi ulnaris, which extend the wrist. In the foot, extensors include the extensor digitorum longus and extensor digitorum brevis, which act on the toes. The extensor tendons are connected by juncturae, which stabilise the metacarpophalangeal joints, distribute force during finger extension, and coordinate tendon movements.
| Characteristics | Values |
|---|---|
| Definition | Muscles that increase the angle between members of a limb, as by straightening the elbow or knee or bending the wrist or spine backward |
| Function | Produce extension at the wrist and fingers |
| Innervation | Radial nerve |
| Compartments | Six extensor tendon compartments covered by the extensor retinaculum |
| Juncturae | Junctura A connects the extensor tendons of the index and long finger, junctura B connects the long and ring finger, and junctura C connects the ring and small finger |
| Intertendinous connections | Stabilization of the metacarpophalangeal joints, force distribution during finger extension, and coordination of tendon movements |
| Sagittal bands | Prevent hyperextension in the metacarpophalangeal joint |
| Lumbrical muscles | Initiate metacarpophalangeal joint flexion before contributing to the extension of the PIP and DIP joints |
| Brachioradialis | A paradoxical muscle that originates and is innervated like an extensor muscle but is actually a flexor at the elbow |
| Extensor carpi radialis longus | A muscle in the posterior compartment of the forearm that acts as a wrist extensor |
| Extensor carpi radialis brevis | A muscle that produces wrist extension and abduction |
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What You'll Learn
Extensor muscles in the forearm
Extension is a movement of a joint that increases the angle between two bones or body surfaces at a joint. Extension usually results in the straightening of the bones or body surfaces involved. For example, extending a flexed (bent) elbow or bending the wrist or spine backward.
The extensor muscles in the forearm include the extensor carpi radialis longus, the extensor carpi radialis brevis, the extensor carpi ulnaris, and the extensor digitorum. These muscles run from the humerus (the bone of the upper arm) along the back of the forearm to the metacarpal bones at the back of the hand, and they extend the wrist. The extensor digitorum extends from the humerus to a common tendon attached to all the fingers and extends the fingers. The extensor indicis acts upon the index finger. The extensor pollicis brevis and extensor pollicis longus act upon the thumb and run from the bones of the forearm (radius and ulna, respectively).
The extensor muscles in the forearm are also associated with extrinsic extensor muscles, including EDC, EIP, EDM, EPL, and brevis. The intrinsic muscles, including palmar and dorsal interosseous muscles and lumbrical muscles, also contribute to finger extension. The lumbrical muscles contribute to the flexion of the MCP joints and to the extension of the IP joints via lateral bands. The interosseous muscles abduct and adduct the fingers and contribute to the extension of the IP joints via lateral bands. Forearm extensor muscles and intrinsic muscles work together to provide optimal digital extension.
The extensor tendons at the wrist level form six extensor compartments as they pass under the extensor retinaculum. These compartments prevent bowstringing of the extensor tendons. The extensor tendon does not insert on the proximal phalanx. Beyond the sagittal band, the extensor tendon flares slightly into three portions: a central, more robust slip, and two lateral slips. A contiguous extensor expansion or hood is created. 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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Extensor tendons in the hand
Extensor muscles are those that increase the angle between members of a limb, resulting in movements like straightening the elbow or bending the wrist or spine backward. Extensor tendons, meanwhile, begin in the forearm and continue to the backside of the fingers and thumb. They are located just underneath the skin.
Extensor tendons allow your fingers and thumbs to straighten and perform fine, coordinated movements. They are attached to the phalanges via a complex system. As the extensor tendons reach the fingers, they become flat and thin.
There are six extensor tendon compartments in the hand, covered by the extensor retinaculum, which spans from the pisiform and triquetrum to the radiopalmar radius. The first compartment on the radiodorsal aspect of the wrist includes the tendons of the EPB and APL. The second compartment contains the ECRB and ECRL tendons. The third compartment consists only of the EPL tendon bending around the Lister tubercle. The fourth dorsal compartment has five tendons: the four EDC tendons and the deeper EIP tendon. The fifth and sixth dorsal compartments also only contain one tendon each: the EDM tendon and the ECU tendon, respectively.
Injuries to extensor tendons in the hand are common and require careful assessment and management. Extensor injuries may involve intra-articular damage, especially in "fight bites," which can result in infection and poor outcomes if missed. Treatment for extensor tendon injuries includes splinting and hand therapy, and in some cases, surgery.
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Extension at the wrist
The primary muscles responsible for wrist extension include the extensor carpi radialis brevis, extensor carpi radialis longus, and extensor carpi ulnaris. These muscles originate from the humerus (upper arm bone) and attach to the metacarpal bones at the back of the hand. By contracting, they generate the force necessary to extend the wrist.
To bias the extensor carpi radialis longus and brevis, one can position the wrist in a slightly extended and radially deviated position. The patient is then instructed to extend the wrist, leading with the thumb side. Resistance is applied over the radial side of the second metacarpal in the direction of flexion and ulnar deviation.
Similarly, to bias the extensor carpi ulnaris, the wrist is prepositioned in slight extension and ulnar deviation. The patient is instructed to extend the wrist, leading with the fifth digit or "pinky" side. Resistance is applied over the dorsal surface of the ulnar side of the fifth metacarpal, moving in the direction of flexion and radial deviation.
The extensor tendons play a crucial role in wrist extension as well. They enter the back of the hand through six extensor tendon compartments covered by the extensor retinaculum. The retinaculum spans from the pisiform and triquetrum bones to the radiopalmar radius. The specific arrangement of these tendons allows for the complex movement of the wrist joint during extension.
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Extension at the elbow
The primary muscles involved in elbow extension are the triceps, which are located on the back of the upper arm. These muscles act as antagonists to the biceps brachii, brachialis, and brachioradialis, which are the prime movers of elbow flexion (bending).
Elbow extension is an important movement to understand in clinical settings, especially in predicting and diagnosing elbow injuries. The elbow extension test is a valuable tool in this regard, where a patient is asked to flex their shoulders to 90 degrees and then fully extend both elbows. The injured side is then compared to the uninjured side to determine the presence of any significant injury. This test is highly effective in ruling out fractures, with high sensitivity and negative predictive values reported in both adult and pediatric patients.
Additionally, understanding elbow extension is crucial in physical therapy, especially when dealing with conditions such as quadriplegia or muscle contractures. For example, individuals with C5 or C6 quadriplegia may have functioning biceps (elbow flexors) but lack functioning triceps (elbow extensors), leading to a fixed position of elbow flexion. To prevent this, therapists may include stretching of the biceps as a regular component of therapy, which involves placing the arm in positions opposite to its natural movements, including elbow extension.
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Extension at the knee
The knee is a hinge joint that allows movement back and forth in a single plane. The movement at the knee is largely controlled by two groups of muscles: the quadriceps and the hamstrings. These muscles work in tandem to facilitate movement and provide stability.
The rectus femoris is the only one of the quadriceps muscles that attaches to the ilium, which is part of the pelvis. This means it can also act on the hip joint and is responsible for movements such as kicking and jumping. The other three quadriceps muscles, the vastus group, are responsible for stabilizing the knee during extension and absorbing shock during activities like running.
While the hamstrings are not directly responsible for knee extension, they play a crucial role in this movement by providing stability and balance. The hamstrings are a group of three muscles located at the back of the thigh: semitendinosus, semimembranosus, and biceps femoris. These muscles attach to the ischium, another part of the pelvis, and to the tibia and fibula, the smaller bone of the lower leg. During knee extension, the hamstrings work to stabilize the joint and control the movement, preventing the lower leg from extending too far back.
Strong and flexible extensor muscles at the knee are essential for injury prevention and maintaining mobility. Exercises that target the quadriceps and hamstrings, such as squats, lunges, and leg presses, can help strengthen these muscles and improve knee stability. Stretching these muscle groups can also help improve flexibility and range of motion at the knee joint.
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Frequently asked questions
Extensor muscles are muscles that increase the angle between two bones or body surfaces at a joint, usually resulting in the straightening of the bones or body surfaces involved.
Examples of extensor muscles include the extensor carpi radialis brevis, extensor carpi radialis longus, and extensor carpi ulnaris. These muscles run from the humerus (upper arm bone) to the metacarpal bones at the back of the hand and are responsible for extending the wrist.
Extensor muscles are involved in various functions, including gripping and finger extension. They also play a role in stabilization, force distribution, and coordination of tendon movements.
