Logan Steele
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 flexor pollicis longus (FPL) is a long muscle located at the deep layer of the anterior compartment of the forearm. It is the primary flexor of the thumb, providing flexion at its metacarpophalangeal (MCP) joint and interphalangeal (IP) joint. The FPL is also involved in radial wrist deviation and wrist flexion. The flexor pollicis brevis is another muscle that flexes the thumb, curling it up in the palm.
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What You'll Learn
Flexor pollicis longus
The flexor pollicis longus (FPL) is a long muscle located in the anterior compartment of the forearm, in the same plane as the flexor digitorum profundus. It is a deep forearm flexor that flexes the thumb and contributes to flexing the wrist.
The FPL is unique to humans, being either rudimentary or absent in other primates. It is classified as an extrinsic muscle of the hand because it lies in the forearm but inserts in the hand. It originates from the mid-half of the anterior surface of the radius and adjacent half of the interosseous membrane. The muscle also takes its origin from the medial epicondyle of the humerus, coronoid process of the ulna, and from flexor digitorum superficialis. The muscle tapers into a flat tendon near the wrist and inserts into the base of the distal phalanx of the thumb on the palmar surface.
The FPL is the only muscle that flexes the interphalangeal joint of the thumb, which makes it vital for activities that require hand gripping. It also flexes the thumb at the metacarpophalangeal joint and contributes to the flexion of the wrist. The FPL is tested by holding the proximal phalanx of the thumb stationary while flexing the distal phalanx against resistance.
The FPL receives dual blood supply. Its medial part is supplied by the anterior interosseous artery, a branch of the ulnar artery, while the lateral part receives blood from the radial artery. The FPL may also receive contributions from the median artery if the muscle is well-developed. It receives nervous supply from the anterior interosseous branch of the median nerve, derived from spinal roots C7 and C8.
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Flexor pollicis brevis
The flexor pollicis brevis is a short, broad intrinsic muscle of the hand. It is one of the muscles of the thenar eminence, which is a group of muscles on the palm at the base of the thumb. The thenar eminence also includes the abductor pollicis brevis and opponens pollicis muscles. The flexor pollicis brevis is a two-headed muscle, with a superficial head and a deep head. The superficial head arises from the crest of the trapezium and flexor retinaculum, while the deep head originates from the trapezoid and capitate bones, as well as the palmar ligaments of the distal row of carpals bones.
The flexor pollicis brevis acts to flex, adduct, and abduct the thumb, allowing it to curl up into the palm. It also assists in opposition and medial rotation of the thumb. This muscle receives its blood supply from the superficial palmar branches of the radial artery. The superficial head of the muscle is innervated by the median nerve, while the deep head is innervated by the ulnar nerve.
The flexor pollicis brevis can be activated by placing the palm facing up and the wrist in a neutral position. The thumb is then brought to touch the base of the little finger, and pressure is applied for a few seconds before releasing. This muscle can also be tested and palpated on the thenar eminence when the thumb is flexed against resistance.
The flexor pollicis brevis is an important muscle for fine movements and dexterity. Along with the first dorsal interosseous muscle, it helps with tip-pinch movements that are common in daily activities such as turning a key or opening a package. Impairments in this muscle, such as numbness or motor deficits, can be caused by median nerve compression in carpal tunnel syndrome.
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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, along with the abductor pollicis brevis and flexor pollicis brevis. The thenar eminence refers to the group of muscles on the palm at the base of the thumb. The opponens pollicis 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 passes downward 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, with some innervation by the ulnar nerve in 20% of the population. 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 coordination of several other muscles at the thumb's metacarpophalangeal joint. The two opponens muscles, opponens pollicis and opponens digiti minimi, are named for the fact that they oppose each other, but their actions result in the apposition of the bones.
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Abductor pollicis brevis
The abductor pollicis brevis is a muscle in the hand that functions as an abductor of the thumb. It is a flat, thin muscle located just under the skin. It is a thenar muscle, and therefore contributes to the bulk of the palm's thenar eminence. The abductor pollicis brevis is the most lateral and the most superficial of the thenar muscles, located underneath the skin. It runs from the scaphoid and trapezium carpal bones, and from the flexor retinaculum to the proximal phalanx of the thumb.
The main function of the abductor pollicis brevis is the abduction of the thumb at the carpometacarpal joint. This action happens in synergy with the abductor pollicis longus muscle. 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 muscle fibres originate from the flexor retinaculum, with two smaller origins located on carpal bones; the first is found on the tubercle of the scaphoid, and the second is on the tubercle of the trapezium bone.
The abductor pollicis brevis is innervated by the recurrent (thenar) branch of the median nerve (root value C8 and T1). It is vascularized by the superficial palmar branch that arises from the radial artery. The abductor pollicis brevis is supplied by the recurrent branch of the median nerve (roots C8-T1). The abductor pollicis brevis is one of three muscles that comprise the thenar eminence, along with the flexor pollicis brevis and opponens pollicis.
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Adductor pollicis
The adductor pollicis is a muscle in the hand that functions to adduct the thumb. It is a unique, fleshy, flat, triangular, and fan-shaped muscle deep in the thenar compartment beneath the long flexor tendons and the lumbrical muscles at the center of the palm. It is the most powerful of the intrinsic muscles of the hand. It is also larger and deeper than the thenar muscles.
The adductor pollicis has two heads: transverse and oblique. The oblique head originates at the capitate as well as at the bases of the second and third metacarpals, while the transverse head originates from the volar aspect of the third metacarpal. These two heads then merge as the fibers travel laterally, forming the tendon of the adductor pollicis, which often contains a sesamoid bone. The muscle ends in a tendon that contains a sesamoid bone and inserts onto the base of the first proximal phalanx as well as its extensor hood.
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.
The main function of the adductor pollicis is to adduct the thumb, bringing it back into the plane of the palm of the hand from its previously abducted position. It can also bring the thumb to the side of the palm and index finger. This action is essential for functions that require pinching and
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Frequently asked questions
The muscles that flex the thumb include the flexor pollicis longus, flexor pollicis brevis, opponens pollicis, and abductor pollicis brevis.
The FPL is a long muscle located in the anterior compartment of the forearm. It is one of the three deep flexors of the volar compartment of the forearm. It is also classified as an extrinsic muscle of the hand as it contributes to thumb movement.
The FPL is the primary flexor of the thumb, providing flexion at its metacarpophalangeal (MCP) joint and interphalangeal (IP) joint. It is the only muscle that flexes the interphalangeal joint of the thumb, making it vital for activities that require hand gripping.
The flexor pollicis brevis flexes, adducts, and abducts the thumb, and therefore also assists in opposing the thumb.
The opponens pollicis opposes the thumb, bringing it against the fingers. This movement is important for human hand dexterity, including grip.
