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The muscles of the thumb are nine skeletal muscles located in the hand and forearm. These muscles are responsible 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 muscles bellies located in the hand proper. The flexor pollicis longus (FPL) is a muscle in the forearm and hand that flexes the thumb. The flexor pollicis brevis (FPB) is another muscle that flexes the thumb, curling it up in the palm.
| Characteristics | Values |
|---|---|
| Number of muscles in the thumb | 9 |
| Muscle groups | Extrinsic hand muscles, Intrinsic hand muscles |
| Intrinsic muscle groups | Thenar eminence, Adductor pollicis, First dorsal interosseous muscle |
| Thenar eminence muscles | Abductor pollicis brevis, Flexor pollicis brevis, Opponens pollicis |
| Flexor pollicis brevis origin | Flexor retinaculum, Trapezium, Trapezoid, Capitate |
| Flexor pollicis brevis insertion | Radial sesamoid bone of the metacarpophalangeal joint |
| Flexor pollicis brevis function | Flexion, Adduction, Abduction, Opposition of the thumb |
| Opponens pollicis origin | Trapezium, Flexor retinaculum |
| Opponens pollicis insertion | Radial side of the first metacarpal |
| Opponens pollicis function | Opposition, Adduction of the thumb |
| Flexor pollicis longus origin | Anterior side of the radius, Interosseous membrane |
| Flexor pollicis longus insertion | Base of the distal phalanx of the thumb |
| Flexor pollicis longus function | Flexion of the thumb, Wrist deviation, Wrist flexion |
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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 flexes the thumb. It is one of three deep flexors of the volar compartment of the forearm, along with flexor digitorum profundus and pronator quadratus.
The FPL arises from the anterior surface of the radius and adjacent structures, including the interosseous membrane and the medial epicondyle of the humerus. It crosses three joints and inserts onto the distal phalanx of the thumb, forming a flattened, long tendon. This tendon courses through the carpal tunnel, passing deep to the transverse carpal ligament and inserting at the base of the distal phalanx on the palmar surface.
The FPL is innervated by the anterior interosseous nerve, a branch of the median nerve. It receives dual blood supply, with the medial part supplied by the anterior interosseous artery (a branch of the ulnar artery) and the lateral part receiving blood from the radial artery.
The main function of the FPL is flexion of the thumb at the interphalangeal joint, which is essential for gripping. It is the only muscle that flexes this joint, making it vital for activities requiring hand grip. The FPL also assists in wrist flexion and radial wrist deviation.
An accessory head of the FPL, known as Gantzer's muscle, is present in a significant portion of the population. This accessory head may cause compression of the anterior interosseous nerve.
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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 thenar muscles include the flexor pollicis brevis, the opponens pollicis, and the abductor pollicis brevis. The flexor pollicis brevis is a two-headed muscle, with a superficial head and a 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 flexes the thumb at the metacarpophalangeal joint, as well as flexion and medial rotation of the 1st metacarpal bone at the carpometacarpal joint. This action aids in opposition of the thumb, allowing us to manipulate objects and create tools. It also plays a role in fine movements like precision, pinching, and power gripping.
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 of the flexor pollicis brevis receives nervous supply from the recurrent branch of the median nerve, while the deep head receives innervation from the deep branch of the ulnar nerve, derived from spinal roots C8 and T1.
To activate the flexor pollicis brevis muscle, place your palm facing up and your wrist in a neutral position. Bring your thumb to touch the base of your little finger and apply as much pressure as possible. Hold the pressure for a few seconds and then release it.
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Opponens Pollicis
The opponens pollicis is a small, triangular, intrinsic muscle of the hand. It is one of three thenar muscles, along with the abductor pollicis brevis and the flexor pollicis brevis. The thenar muscles are located on the radial (lateral) aspect of the hand, forming an elevation called the thenar eminence.
The opponens pollicis originates from the flexor retinaculum of the hand and the tubercle of the trapezium bone. It extends from the trapezium bone and flexor retinaculum to the first metacarpal bone, inserting onto the radial side of the first metacarpal shaft. This muscle is relatively short and slender and lies deep to the abductor pollicis brevis and lateral to the flexor pollicis brevis.
The main function of the opponens pollicis is to produce an opposition of the thumb, allowing the tip of the thumb to touch the tips of the other fingers. This opposition is a complex movement that involves flexion, adduction, and medial rotation at the first carpometacarpal joint. This movement is crucial for human hand dexterity and grip, enabling precise actions such as grasping round objects or fine gripping of a pencil.
The opponens pollicis is innervated by the recurrent branch of the median nerve (C8 and T1 roots) and, in some cases, by the deep terminal branch of the ulnar nerve. The muscle receives its blood supply primarily from the superficial palmar branch of the radial artery, with additional supply from several other arteries.
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Abductor Pollicis Brevis
The abductor pollicis brevis is a thin, flat muscle located just under the skin in the hand. It is a thenar muscle, 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 is the most lateral and superficial of the thenar 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 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 main function of the abductor pollicis brevis is the abduction of the thumb, moving it away from the other four fingers. This movement occurs at the carpometacarpal and metacarpophalangeal joints. The abductor pollicis brevis also facilitates the movement of the thumb towards the fingertips in the carpometacarpal joint (opposition) and flexion in the metacarpophalangeal joint.
The abductor pollicis brevis is innervated by the recurrent branch of the median nerve (Roots C8-T1). The superficial palmar branch of the radial artery vascularises the muscle.
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Adductor Pollicis
The adductor pollicis is a muscle in the hand that functions to adduct the thumb, bringing it back into the plane of the palm from an abducted position. It is also capable of bringing the thumb to the side of the palm and index finger. This muscle is essential for hand dexterity, facilitating pinching and gripping actions.
The adductor pollicis is the most powerful intrinsic muscle of the hand. It is larger and deeper than the thenar muscles. The radial artery passes between the two heads of the adductor pollicis, travelling from the back of the hand into the palm, where it forms the deep palmar arch. The muscle is innervated by the deep branch of the ulnar nerve (C8-T1) and vascularised by the deep palmar arterial arch.
The adductor pollicis muscle evolved as human ancestors' thumbs and big toes became opposable. It is an important anatomical landmark for the radial artery, which passes through the two heads of the muscle as it enters the hand. The development of the adductor pollicis begins during the sixth and seventh weeks of fetal development, along with the formation of other intrinsic hand muscles.
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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 flexes the thumb. It is unique to humans and is absent or rudimentary in other primates.
The flexor pollicis brevis is a two-headed muscle that flexes, abducts, and adducts the thumb. It lies within the thenar eminence, which is a group of muscles at the base of the thumb on the palm.
The opponens pollicis muscle opposes the thumb, bringing it against the fingers. This movement is crucial for human hand dexterity and grip.
Yes, the abductor pollicis brevis and the first dorsal interosseous muscles also play a role in thumb flexion. The abductor pollicis brevis moves the thumb away from the other fingers, while the first dorsal interosseous muscle assists in adduction.
