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The muscles of the thumb are nine skeletal muscles located in the hand and forearm. The muscles responsible for extending the thumb include the extensor pollicis longus and brevis muscles, which are located in the lower forearm. These muscles allow you to extend your thumb, for example, when hitchhiking or giving a thumbs-up. The abductor pollicis longus and brevis muscles also play a role in extending the thumb, specifically by abducting it, or moving it away from the fingers.
| Characteristics | Values |
|---|---|
| Muscles extending the thumb | Extensor pollicis longus, Extensor pollicis brevis, Abductor pollicis longus |
| Muscle group | Extrinsic hand muscles |
| Muscle belly location | Forearm |
| Muscle length | N/A |
| Innervation | Posterior interosseous nerve (C7 and C8), continuation of the deep branch of the radial nerve |
| Tendon location | Radial side of the wrist |
| Tendon sheath | Long and superficial synovial sheath |
| Tendon length | 6.7 to 9.7 centimetres (2.6 to 3.8 inches) |
| Tendon blood supply | Branches from various arteries, including the anterior interosseous artery, its muscular branches, and the radial artery |
| Synovial sheath blood supply | Posterior ramus of the radial artery |
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What You'll Learn
Extensor pollicis longus
The EPL is a deep muscle of the posterior aspect of the forearm, extending over the distal aspect of the interosseous membrane and distal radioulnar joint. It is initially deep to the extensor digitorum, medial to the abductor pollicis longus, and lateral to the extensor indicis. The tendon of the EPL is relatively long and traverses the tendons of the extensor carpi radialis longus and extensor carpi radialis brevis in the distal aspect of the hand. At the level of the radiocarpal joint, the tendon of the EPL sits medial to that of the extensor pollicis brevis, contributing to the formation of the anatomical snuff box when the thumb is extended.
The main function of the EPL is the extension of the thumb at the metacarpophalangeal and interphalangeal joints. It works in synergy with the extensor pollicis brevis muscle to achieve this extension. When the thumb reaches full extension or abduction, the EPL can also assist in the adduction of the thumb. The tendons of the EPL and the extensor pollicis brevis cross the radial artery, and the tendon of the EPL receives innervation from the posterior interosseous nerve (C7 and C8), which is a continuation of the deep branch of the radial nerve.
Inflammation or rupture of the EPL tendon, commonly referred to as "drummer's paralysis", can occur due to blunt mechanical injuries to the wrist or overuse of the muscle. This condition is commonly seen in drummers, sculptors, blacksmiths, or waiters.
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Extensor pollicis brevis
The tendon of extensor pollicis brevis, together with the tendon of abductor pollicis longus, comprises the lateral border of a triangular depression on the lateral aspect of the wrist, called the anatomical snuffbox. The radial artery can be palpated anteriorly at the wrist, specifically in this depression.
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Abductor pollicis longus
The abductor pollicis longus (APL) is a muscle in the distal half of the posterior forearm, and is one of the five deep extensors in the forearm. It is one of the extrinsic muscles of the hand, with its muscle belly located in the forearm. The muscle belly of the APL consists of three parts, resulting in two divisions of tendons. The first part of the abductor pollicis muscle is a deep part that originates from the ulna, interosseous membrane, and radius and is covered by the extensor digitorum longus. The APL tendon is present in the first extensor compartment of the wrist and forms the lateral border of the anatomical snuffbox. This is a triangular space found at the base of the thumb, distal and posterior to the styloid process of the radius.
The APL is innervated by the posterior interosseous nerve, which is a continuation of the deep branch of the radial nerve. The radial nerve is a branch of the posterior cord of the brachial plexus. The APL is supplied by the posterior interosseous artery. The blood supply to the APL comes 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 is vascularised by a perforating branch of the anterior interosseous artery.
The APL's main function is to abduct the thumb at the carpometacarpal joint, moving the thumb anteriorly. It also assists in extending and rotating the thumb, and abducting the wrist. Working together with the long and short extensors of the thumb, the APL helps to fully extend the thumb at the metacarpophalangeal joint. This action is seen in activities such as bowling and shovelling.
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Flexor pollicis longus
The flexor pollicis longus (FPL) is a long muscle located in the forearm and hand that flexes the thumb. It is one of nine skeletal muscles that allow for flexion, extension, adduction, abduction, and opposition of the thumb. The flexor pollicis longus is unique to humans, being either rudimentary or absent in other primates.
The flexor pollicis longus is a flexor of the phalanges of the thumb, and it assists in flexing the wrist when the thumb is fixed. It is the only muscle that flexes the interphalangeal joint of the thumb, making it essential for activities that require hand gripping. The muscle is supplied by the anterior interosseous (C8-T1) branch of the median nerve (C5-T1).
The flexor pollicis longus is quite variable, and in some individuals, it may be blended with the flexor digitorum superficialis, flexor digitorum profundus, or pronator teres. In a small portion of the population, the flexor pollicis longus may even be absent. It receives dual blood supply, with its medial part supplied by the anterior interosseous artery (a branch of the ulnar artery) and its lateral part receiving blood from the radial artery.
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Abductor pollicis brevis
The abductor pollicis brevis is a muscle in the hand that is responsible for the abduction of the thumb. It is a flat, thin muscle located just under the skin. It is a thenar muscle, contributing to the bulk of the palm's thenar eminence. The thenar eminence is an elevation located on the radial (lateral) aspect of the palm, formed by the abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis muscles.
The abductor pollicis brevis originates from the flexor retinaculum of the hand, the tubercle of the scaphoid bone, and sometimes the tubercle of the trapezium. It runs laterally and downward, inserting by a thin, flat tendon into the lateral side of the base of the first phalanx of the thumb and the capsule of the metacarpophalangeal joint. This insertion allows for the abduction of the thumb, which is defined as the movement of the thumb anteriorly, perpendicular to the palm.
The abductor pollicis brevis is innervated by the recurrent branch of the median nerve (C8-T1). The superficial palmar branch of the radial artery, which arises from the radial artery, vascularizes the muscle. The abductor pollicis brevis acts in synergy with the abductor pollicis longus muscle to facilitate the movement of the thumb towards the fingertips in the carpometacarpal joint (opposition) and flexion in the metacarpophalangeal joint.
The abductor pollicis brevis is the most lateral and superficial of the thenar muscles. It is located underneath the skin, with its fibres originating from several locations. The majority of its fibres originate from the flexor retinaculum, with smaller origins on the carpal bones: the tubercle of the scaphoid and the tubercle of the trapezium. These fibres then form a single muscle belly that runs distally and radially towards the thumb, ending in a flat tendon that inserts onto the radial aspect of the base of the proximal phalanx of the thumb.
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Frequently asked questions
The muscles that extend the thumb are the extensor pollicis longus, extensor pollicis brevis, and abductor pollicis longus.
The extensor pollicis longus and brevis muscles are located in the lower forearm.
These muscles allow you to extend your thumb, for example, when hitchhiking or signaling a thumbs-up.
The abductor pollicis longus and brevis are responsible for abducting the thumb.
