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The muscles of the thumb are nine skeletal muscles located in the hand and forearm. These 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 and the intrinsic hand muscles. The former group has muscle bellies located in the forearm, while the latter group has muscle bellies located in the hand. The muscles of the thumb include the abductor pollicis brevis, flexor pollicis brevis, opponens pollicis, adductor pollicis, and first dorsal interosseous muscle.
| Characteristics | Values |
|---|---|
| Number of muscles | 9 |
| Muscle groups | 2: extrinsic hand muscles, intrinsic hand muscles |
| Muscle functions | flexion, extension, adduction, abduction, opposition |
| Thenar eminence muscles | abductor pollicis brevis, flexor pollicis brevis, opponens pollicis |
| Other muscles | adductor pollicis, first dorsal interosseous muscle |
| Abductor pollicis brevis function | pulls thumb away from other fingers |
| Flexor pollicis brevis function | flexes thumb, curling it into the palm |
| Opponens pollicis function | opposes thumb, bringing it against fingers |
| Adductor pollicis function | rotation, opposition, adduction |
| First dorsal interosseous function | spreads fingers away from each other |
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What You'll Learn
The thenar eminence group of muscles
The abductor pollicis brevis is the most superficial muscle of the thenar group. It originates from the tubercles of the scaphoid and trapezium bones, as well as the flexor retinaculum. The muscle fibres extend laterally and distally, ending in a tendon that inserts onto the lateral aspect of the base of the proximal phalanx. The primary function of this muscle is the abduction of the thumb at the first carpometacarpal joint, along with its long counterpart, the abductor pollicis longus muscle. It also participates in the flexion and opposition of the thumb.
The flexor pollicis brevis is a two-headed muscle that forms the medial aspect of the thenar eminence. The superficial head originates from the flexor retinaculum, while the deep head originates from three carpal bones: the trapezium, trapezoid, and capitate. The muscle inserts onto the radial sesamoid bone of the metacarpophalangeal joint. It acts to flex, adduct, and abduct the thumb, and is therefore also able to oppose the thumb. The flexor pollicis brevis is the most medial of the thenar muscles, and its primary function is to produce flexion of the thumb at the metacarpophalangeal and carpometacarpal joints.
The opponens pollicis is the largest of the thenar muscles and lies underneath the other two. It originates from the tubercle of the trapezium and the associated flexor retinaculum, inserting onto the lateral margin of the first metacarpal. The opponens pollicis opposes the thumb by medially rotating and flexing the metacarpal on the trapezium. This movement is very important as most of human hand dexterity, including grip, comes from this action.
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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. It is the most superficial muscle in the thenar group. The abductor pollicis brevis pulls the thumb away from the index finger.
The abductor pollicis brevis originates from several locations. The majority of muscle fibres originate from the flexor retinaculum. Two smaller origins are 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. It inserts into the radial sesamoid bone and the proximal phalanx of the thumb. A thin, flat tendon inserts into the lateral side of the base of the first phalanx of the thumb.
The abductor pollicis brevis is supplied by the recurrent branch of the median nerve (Roots C8-T1). It is innervated by the median nerve (C8 and T1). The superficial palmar branch of the radial artery runs across the superficial aspect of the abductor pollicis brevis. The abductor pollicis brevis is vascularized by the superficial palmar branch that arises from the radial artery.
The main action of the abductor pollicis brevis muscle is the abduction of the thumb at the carpometacarpal and metacarpophalangeal joints. 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. Abduction of the thumb is defined as the movement of the thumb anteriorly, a direction perpendicular to the palm.
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Flexor pollicis brevis
The flexor pollicis brevis is a two-headed muscle that acts to flex, adduct, and abduct the thumb, allowing it to be opposed. 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 eminence is composed of three muscles: the abductor pollicis brevis, the flexor pollicis brevis, and the opponens pollicis. The abductor pollicis brevis pulls the thumb away from the index finger, and the flexor pollicis brevis bends the thumb toward the small finger.
The superficial head of the flexor pollicis brevis 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 two heads of the muscle 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.
The flexor pollicis brevis receives its blood supply from the superficial palmar branches of the radial artery, including the superficial palmar artery, branches of the princeps pollicis artery, and radialis indicis artery. The median nerve may innervate the flexor pollicis brevis in 35% of people due to a common interconnection between the median and ulnar nerves in the hand (known as the Riche-Cannieu interconnection).
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 is important for most human hand dexterity, including grip. The flexor pollicis brevis acts on the thumb and flexes it 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 for fine movements like precision, pinching, and power gripping.
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Opposition and flexion
The muscles of the thumb are nine skeletal muscles located in the hand and forearm. The muscles 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 in the hand. The intrinsic muscles of the thumb 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. The three muscles that compose the thenar eminence are the abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis. The abductor pollicis brevis pulls the thumb away from the index finger, and the flexor pollicis brevis bends the thumb toward the small finger. The flexor pollicis brevis is also responsible for thumb flexion, adduction, and abduction, and therefore also opposes the thumb. The opponens pollicis brings the thumb against the fingers, which is important for grip and most of human hand dexterity.
The other two muscles that influence the movement of the thumb are the adductor pollicis and the first dorsal interosseous muscle. The adductor pollicis is a large triangular muscle with two heads that lie deeper and more distal to the flexor pollicis brevis. Its main action is rotation and opposition, and it also assists in flexion. 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.
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The adductor pollicis and first dorsal interosseous muscles
The adductor pollicis is a large triangular muscle with two heads. One head originates from the third metacarpal bone, while the other originates from the capitate and adjacent areas of the second and third metacarpal bones. Both heads attach to the base of the proximal phalanx of the thumb. The adductor pollicis is responsible for adduction of the thumb, as well as assisting in opposition and flexion. It is also important for pinching, as it helps fill the first web space between the thumb and index finger.
The first dorsal interosseous muscle is one of the central muscles of the hand. It is the largest of the dorsal interosseous muscles and originates from the first and second hand bones. It extends from the base of the thumb metacarpal to the radial side of the proximal phalanx of the index finger, forming the contour between the thumb and index finger. The dorsal interosseous muscles allow us to spread our fingers away from each other, and the first dorsal interosseous muscle also pulls the thumb towards the index finger, providing strength and stability when pinching.
The function of the adductor pollicis and first dorsal interosseous muscles can be assessed through a clinical test known as Froment's sign. This test involves the patient pinching a piece of paper between the first and second metacarpals. If these muscles are weak, the patient will be unable to hold the paper securely and will instead flex the thumb to compensate.
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Frequently asked questions
The muscles of the thumb are nine skeletal muscles located in the hand and forearm. They are divided into two groups: extrinsic hand muscles and intrinsic hand muscles.
The extrinsic hand muscles originate in the forearm and include muscles like the abductor pollicis longus and the extensor pollicis longus.
The intrinsic muscles of the thumb are further divided into two groups: the thenar eminence and other muscles. The thenar eminence refers to the group of muscles at the base of the thumb, including the abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis. The other intrinsic muscles include the adductor pollicis and the first dorsal interosseous muscle.
The abductor pollicis brevis pulls the thumb away from the index finger.
The abductor pollicis longus (extrinsic) and abductor pollicis brevis (intrinsic) work together to abduct the hand and flex it towards the palm.
