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The muscles of the thumb are nine skeletal muscles located in the hand and forearm. The muscles allow for flexion, extension, adduction, abduction, and opposition of the thumb. The muscles acting on the thumb can be divided into two groups: the extrinsic hand muscles, with their muscle bellies located in the forearm, and the intrinsic hand muscles, with their muscle bellies located in the hand. The flexor pollicis longus (FPL) is a muscle in the forearm and hand that flexes the thumb. The flexor pollicis brevis (FPB) is present on the radial border of the palm and also flexes the thumb.
| Characteristics | Values |
|---|---|
| Number of muscles in the thumb | 9 |
| Muscle groups | Extrinsic hand muscles, intrinsic hand muscles |
| Intrinsic muscle groups | Thenar eminence, other muscles |
| Thenar eminence muscles | Abductor pollicis brevis, flexor pollicis brevis, opponens pollicis |
| Other intrinsic muscles | Adductor pollicis, first dorsal interosseous muscle |
| Flexor pollicis brevis origin | Scaphoid tubercle, flexor retinaculum |
| Flexor pollicis brevis insertion | Radial sesamoid bone, proximal phalanx of the thumb |
| Flexor pollicis brevis innervation | Median nerve, ulnar nerve |
| Opponens pollicis origin | Tubercle of the trapezium, flexor retinaculum |
| Opponens pollicis insertion | Radial side of the first metacarpal |
| Opponens pollicis innervation | Median nerve |
| Adductor pollicis heads | Transversal, oblique |
| Transversal head origin | Third metacarpal bone |
| Oblique head origin | Carpal bones proximal to the third metacarpal |
| Flexor pollicis longus function | Flexion of thumb, flexion of wrist |
| Flexor pollicis longus innervation | Median nerve, anterior interosseous nerve |
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What You'll Learn
Flexor Pollicis Longus
The flexor pollicis longus (FPL) is a long muscle located in the forearm and hand that is responsible for flexing the thumb. It is one of the three deep flexors of the volar compartment of the forearm and is unique to humans, being either rudimentary or absent in other primates.
The FPL arises from the volar or anterior surface of the radius, extending from below the radial tuberosity to the attachment of the pronator quadratus muscle. It also originates from the adjacent interosseous membrane and, occasionally, the medial epicondyle of the humerus or the coronoid process of the ulna. The muscle tapers into a long, flat tendon near the wrist and inserts into the base of the distal phalanx of the thumb on the palmar surface.
The FPL flexes the interphalangeal joint of the distal phalanx of the thumb, allowing for gripping. It also assists in flexing the metacarpophalangeal joint of the proximal phalanx and the carpometacarpal joint of the first metacarpal, as well as contributing to radial wrist deviation and wrist flexion. The FPL is innervated by the anterior interosseous nerve, a branch of the median nerve, and receives dual blood supply from the anterior interosseous artery (a branch of the ulnar artery) and the radial artery.
Injuries to the FPL tendon can result in a condition called trigger thumb, where the tendon cannot smoothly glide through the pulley system due to stenosis. Another type of FPL rupture is Mannerfelt's lesion, which occurs within the carpal tunnel from chronic tendon attrition and is often caused by rheumatoid arthritis.
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Flexor Pollicis Brevis
The flexor pollicis brevis is a short, broad intrinsic muscle of the hand. It is the most medial of the thenar muscles, which are a group of muscles on the palm at the base of the thumb. The flexor pollicis brevis lies medial to the abductor pollicis brevis and opponens pollicis muscles, and lateral to the adductor pollicis muscle.
The flexor pollicis brevis is a two-headed muscle, consisting of an outer portion, known as the superficial head, and an inner portion, known as the deep head. The superficial head arises on the flexor retinaculum, while the deep head originates on three carpal bones: the trapezium, trapezoid, and capitate. The muscle is inserted onto the radial sesamoid bone of the metacarpophalangeal joint.
The flexor pollicis brevis acts to flex, adduct, and abduct the thumb, and therefore also assists in opposing the thumb. It flexes the thumb at the metacarpophalangeal joint, as well as flexion and medial rotation of the 1st metacarpal bone at the
The flexor pollicis brevis receives arterial supply from branches of the radial artery, including the superficial palmar artery, branches of the princeps pollicis artery, and radialis indicis artery. The two heads of the flexor pollicis brevis usually differ in their innervation. The superficial head receives nervous supply from the recurrent branch of the median nerve, while the deep head is innervated by the deep branch of the ulnar nerve, derived from spinal roots C8 and T1.
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Opponens Pollicis
The opponens pollicis is a small, triangular muscle in the hand, which functions to oppose the thumb. It is one of the three thenar muscles, which are located on the radial (lateral) aspect of the hand, forming an elevation called the thenar eminence. The opponens pollicis is a relatively short and slender muscle, situated on the radial aspect of the hand, and lies deep to the abductor pollicis brevis and lateral to the flexor pollicis brevis.
The opponens pollicis originates on the tubercle of the trapezium and the flexor retinaculum. It is inserted onto the radial side of the first metacarpal. The muscle belly courses dorsally and laterally, inserting into the whole length of the metacarpal bone of the thumb on its radial side. The opponens pollicis is innervated by the recurrent branch of the median nerve, and occasionally by the deep terminal branch of the ulnar nerve. The muscle receives its blood supply from the superficial palmar branch, which arises from the radial artery.
The main function of the opponens pollicis is to produce opposition of the thumb, which is a complex movement that combines flexion, adduction, and medial rotation at the first carpometacarpal joint. This movement allows the tip of the thumb to touch the tips of the other fingers, enabling precise and efficient hand movements such as grasping round objects or fine gripping of a pencil. Opposition of the thumb is a combination of actions that allows the thumb to appose, or touch, the fingers. While the opponens pollicis is responsible for the flexion of the thumb's metacarpal at the first carpometacarpal joint, true apposition also requires the actions of several other muscles at the thumb's metacarpophalangeal joint.
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Abductor Pollicis Brevis
The abductor pollicis brevis is a flat, thin muscle located just under the skin in the hand. It is the most lateral and superficial of the thenar muscles, contributing to the bulk of the palm's thenar eminence. The thenar eminence is a group of muscles on the palm at the base of the thumb.
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 downwards, 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. The muscle is supplied by the recurrent branch of the median nerve (Roots C8-T1).
The main function of the abductor pollicis brevis is the abduction of the thumb, which is the movement of the thumb anteriorly, perpendicular to the palm. This occurs at the carpometacarpal joint and the metacarpophalangeal joint. The muscle also facilitates the movement of the thumb towards the fingertips in the carpometacarpal joint (opposition) and flexion in the metacarpophalangeal joint. These functions are important for the proper functioning of the hand, such as grasping round objects or carrying out precise tasks like writing and sewing.
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Adductor Pollicis
The adductor pollicis is an intrinsic muscle of the hand, and the only muscle of the adductor compartment of the hand. It is a unique, fleshy, flat, triangular, and fan-shaped muscle with a two-headed structure. The two heads are the oblique head and the transverse head. The oblique head originates at the capitate and the palmar bases of the second and third metacarpals, while the transverse head originates from the lower two-thirds of the palmar surface of the third metacarpal bone. The two heads then merge as the fibres travel laterally, forming the tendon of the adductor pollicis, which often contains a sesamoid bone. The muscle extends from the third metacarpal and capitate bones to the base of the first proximal phalanx.
The adductor pollicis is the most powerful of the intrinsic muscles of the hand. Its main function is the adduction of the thumb, which is the movement of the thumb towards the index finger from an abducted position. This action is essential for functions that require pinching and gripping. Additionally, the adductor pollicis aids the later stages of opposition of the thumb. This movement is a combination of actions, namely adduction, medial rotation, flexion, and adduction of the thumb so that it can touch each fingertip of the same hand.
The adductor pollicis is innervated by the deep branch of the ulnar nerve (C8-T1). The deep palmar arterial arch is the main blood supply to the adductor pollicis. The radial artery passes between the two heads, travelling from the back of the hand into the palm, where it forms the deep palmar arch.
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Frequently asked questions
The muscles that flex the thumb include the flexor pollicis longus, flexor pollicis brevis, and opponens pollicis.
The FPL is a long muscle located in the forearm and hand that is responsible for flexing the thumb. It is one of the three deep flexors of the volar compartment of the forearm.
The FPL originates on the anterior side of the radius distal to the radial tuberosity and from the interosseous membrane. It forms a flattened tendon that attaches at the base of the distal phalanx of the thumb.
The FPB is responsible for flexing the thumb, curling it up into the palm. It is one of the intrinsic muscles of the thumb and is part of the thenar eminence group of muscles located at the base of the thumb.
The opponens pollicis muscle opposes the thumb, bringing it against the fingers. This movement is crucial for human hand dexterity and grip.
