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The thumb is moved by nine skeletal muscles in the hand and forearm, which allow for flexion, extension, adduction, abduction, and opposition of the thumb. The extensor pollicis longus (EPL) is a skeletal muscle located on the dorsal side of the forearm, which extends the thumb. The abductor pollicis brevis, adductor pollicis, and extensor pollicis brevis muscles also play a role in extending the thumb.
| Characteristics | Values |
|---|---|
| Muscle Extending the Thumb | Extensor pollicis longus (EPL) |
| Length of Tendon | 6.7 to 9.7 centimetres |
| Muscle Group | Extrinsic thumb muscles |
| Innervation | Posterior interosseous nerve |
| Blood Supply | Anterior interosseous artery, posterior interosseous artery, radial artery, and ulnar artery |
| Variants | Rare, with a prevalence of 1% |
| Pathologies | Stenosing tenosynovitis, spontaneous tendon rupture |
| Associated Medical Conditions | Rheumatoid arthritis, distal radius fractures |
| Other Extensor Muscles | Extensor pollicis brevis, abductor pollicis longus, extensor indicis, supinator |
| Intrinsic Thumb Muscles | Abductor pollicis brevis, opponens pollicis, adductor pollicis, flexor pollicis brevis |
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What You'll Learn
Extensor pollicis longus
The main function of the EPL is to extend the thumb at the metacarpophalangeal and interphalangeal joints. It works in synergy with the extensor pollicis brevis to achieve this extension. When the thumb reaches full extension or abduction, the EPL can also assist in adducting the thumb. The EPL is the only muscle that can achieve full hyperextension at the interphalangeal joint. The long tendon of the EPL crosses the tendons of the extensores carpi radialis longus and brevis and is separated from the extensor pollicis brevis by a triangular interval, forming the anatomical snuff box in which the radial artery is found.
The EPL tendon is associated with various pathologies, including stenosing tenosynovitis (an inflammatory irritation of the synovial sheath) and spontaneous tendon rupture. Tenosynovitis is relatively common in the EPL tendon after repetitive activities such as drum playing, and inflammation or rupture of the tendon is known as "drummer's paralysis". This condition is commonly caused by blunt mechanical injuries to the wrist or overuse of the EPL muscle and is seen in occupations such as drumming, sculpting, blacksmithing, and waiting.
The EPL is part of the deep extensors of the forearm, along with the extensor pollicis brevis, abductor pollicis longus, extensor indicis, and supinator muscles. These muscles contribute to the complex movements and stability of the thumb, allowing for flexion, extension, adduction, abduction, and opposition of the thumb.
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Extensor pollicis brevis
The extensor pollicis brevis arises from the ulna distal to the abductor pollicis longus, from the interosseous membrane, and from the dorsal surface of the radius. Its tendon passes through the same groove on the lateral side of the lower end of the radius as the abductor pollicis longus, inserting into the base of the first phalanx of the thumb. The extensor pollicis brevis is closely related to the abductor pollicis longus, and together, they extend and abduct the thumb at the carpometacarpal and metacarpophalangeal joints.
The extensor pollicis brevis is innervated by the posterior interosseous nerve, a continuation of the deep branch of the radial nerve (C7 and C8). It receives its blood supply from the posterior interosseous artery and perforating branches of the anterior interosseous artery, which are branches of the common interosseous artery. The common interosseous artery arises from the ulnar artery, just below the tuberosity of the radius.
The extensor pollicis brevis is an important muscle for grip function, as it enables the letting go of objects by extending the thumb in the first metacarpophalangeal joint and the carpometacarpal joint. As the muscle crosses the wrist, it also participates in the extension and abduction of the wrist joint.
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Abductor pollicis longus
The abductor pollicis longus (APL) is an extrinsic muscle of the hand. It is one of the five deep extensors in the forearm, along with the supinator, extensor pollicis brevis, extensor pollicis longus, and extensor indicis. The abductor pollicis longus muscle is innervated by the posterior interosseous nerve, a continuation of the deep branch of the radial nerve. The muscle belly of the abductor pollicis longus is located in the distal half of the posterior forearm, deep to the extensor digitorum and lateral to the extensor pollicis longus muscle. Its tendon is found lateral to the tendon of the extensor pollicis brevis.
The abductor pollicis longus originates on the dorsal sides of the ulna and radius and the interosseous membrane. It passes through the first tendon compartment and inserts into the base of the first metacarpal bone. The abductor pollicis longus tendon joins the tendon of the extensor pollicis brevis muscle and passes beneath the extensor retinaculum. The abductor pollicis longus is responsible for abducting the thumb at the carpometacarpal joint, moving the thumb anteriorly. It also assists in extending and rotating the thumb. By abducting the wrist and flexing the hand, the abductor pollicis longus helps to move the thumb away from the palm.
The abductor pollicis longus receives its blood supply from the interosseous branches of the ulnar artery. The proximal part of the muscle is supplied by the lateral branch of the posterior interosseous artery, while the distal part receives blood from a perforating branch of the anterior interosseous artery. The abductor pollicis longus is associated with the anatomical snuff box, a triangular space found at the base of the thumb. The tendon of the abductor pollicis longus forms the lateral or anterior border of this space, which is traversed by neurovascular structures of the hand.
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Flexor pollicis longus
The flexor pollicis longus is a muscle in the forearm and hand that flexes the thumb. It is a long muscle located at the deep layer with flexor digitorum profundus and pronator quadratus in the anterior compartment of the forearm. It is unique to humans, being either rudimentary or absent in other primates.
The flexor pollicis longus is supplied by the anterior interosseous branch of the median nerve, derived from spinal roots C7 and C8. It receives dual blood supply. Its medial part is supplied by the anterior interosseous artery, an indirect branch of the ulnar artery, while the lateral part receives blood from the radial artery.
The main function of the flexor pollicis longus is flexion of the thumb at the interphalangeal joint, which is essential for gripping. It is the only muscle that flexes the interphalangeal joint of the thumb, making it vital for activities that require hand gripping.
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Intrinsic thumb muscles
The thumb has nine skeletal muscles located in the hand and forearm, which enable flexion, extension, adduction, abduction, and opposition of the thumb. These muscles can be divided into extrinsic hand muscles and intrinsic hand muscles. The intrinsic hand muscles are located in the hand, while the extrinsic hand muscles are located in the forearm.
The intrinsic thumb muscles can be further divided into two groups: the thenar eminence and other muscles. The thenar eminence refers to the group of muscles on the palm at the base of the thumb, which includes the abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis. The abductor pollicis brevis originates on the scaphoid tubercle and the flexor retinaculum, inserting into the radial sesamoid bone and the proximal phalanx of the thumb. It is innervated by the median nerve (C8 and T1) and brings the thumb away from the other four fingers. The flexor pollicis brevis is a two-headed muscle that arises from the flexor retinaculum and three carpal bones: the trapezium, trapezoid, and capitate. It inserts onto the radial sesamoid bone of the metacarpophalangeal joint and is innervated by the median nerve (superficial head) and the ulnar nerve (deep head). The opponens pollicis originates from the tubercle of the trapezium and the flexor retinaculum and inserts onto the radial side of the first metacarpal. It is innervated by the median nerve and assists in thumb opposition.
The other two muscles influencing thumb movement are the adductor pollicis and the first dorsal interosseous muscle. The adductor pollicis has two heads and lies deeper and more distal to the flexor pollicis brevis. Its main action is rotation and opposition, and it adducts the thumb while assisting in opposition and flexion. It is innervated by the deep branch of the ulnar nerve (C8-T1). The first dorsal interosseous is one of the central muscles of the hand, extending from the base of the thumb metacarpal to the radial side of the proximal phalanx of the index finger.
The intrinsic muscles of the hand arise from the brachial plexus, a network of nerves from ventral rami between the C5 to T1 nerve roots. The median and ulnar nerves are the nerves of the brachial plexus that supply the intrinsic hand muscles. The median nerve is composed of C5 to C8 nerve roots and enters the hand through the carpal tunnel, where it divides into a recurrent motor branch and a cutaneous branch.
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Frequently asked questions
The extensor pollicis longus (EPL) muscle extends the thumb.
The EPL muscle is an extrinsic thumb muscle that extends and adducts the thumb metacarpophalangeal (MCP) and interphalangeal (IP) joints.
The EPL muscle originates from the dorsal surface of the ulna and the interosseous membrane.
The EPL tendon is approximately 6.7 to 9.7 centimetres (2.6 to 3.8 inches) in length.
The thumb muscles allow for flexion, extension, adduction, abduction, and opposition of the thumb.
