Muscles of the hand: thenar, hypothenar, lumbricals and interossei

The hand is moved by two sets of muscles. The extrinsic muscles start in the forearm and reach the hand by long tendons. The intrinsic muscles both originate and insert within the hand. OpenStax groups the intrinsic muscles into three sets: thenar muscles that act on the thumb, hypothenar muscles that act on the little finger, and intermediate muscles (the lumbricals and the palmar and dorsal interossei) that act on all the fingers. (OpenStax Anatomy and Physiology 2e, 11.5: Muscles of the Pectoral Girdle and Upper Limbs.)

Books differ on where adductor pollicis belongs. OpenStax counts it as a fourth thenar muscle, while Kenhub lists the thenar group as only abductor pollicis brevis, flexor pollicis brevis and opponens pollicis, and places adductor pollicis in a separate adductor compartment. Both agree on what it does. (Kenhub: Thenar muscles.)

TeachMeAnatomy adds palmaris brevis, a thin muscle in the skin over the hypothenar eminence, and treats it and adductor pollicis separately from the thenar and hypothenar groups. Most hand muscles are supplied by the ulnar nerve; the median nerve supplies the thenar muscles and the two lateral lumbricals. (TeachMeAnatomy: The Intrinsic Muscles of the Hand.)

The carpals, metacarpals and phalanges these muscles attach to. Hand bones.

The thenar eminence (thenar prominence) is the fleshy mound at the base of the thumb on the palm. It is formed by three short muscles on the lateral (radial) side of the hand: abductor pollicis brevis, flexor pollicis brevis and opponens pollicis. All three arise from the flexor retinaculum and the tubercle of the trapezium; abductor pollicis brevis also arises from the tubercle of the scaphoid. (Kenhub: Thenar muscles.)

Abductor pollicis brevis inserts on the lateral side of the base of the proximal phalanx of the thumb, abducts the thumb and helps oppose it. It is supplied by the recurrent branch of the median nerve (C8 and T1). (University of Washington Musculoskeletal Atlas: Abductor pollicis brevis.)

Flexor pollicis brevis inserts on the base of the proximal phalanx of the thumb and flexes the thumb at its metacarpophalangeal joint. It has two heads. The superficial head is supplied by the recurrent branch of the median nerve; the deep head, which varies in size, is supplied by the deep branch of the ulnar nerve. (TeachMeAnatomy: The Intrinsic Muscles of the Hand.)

Opponens pollicis arises from the flexor retinaculum and the tubercle of the trapezium and inserts along the lateral side of the first metacarpal. It draws the first metacarpal across the palm and rotates it medially, so that the thumb pad can face the finger pads. This is opposition. It is supplied by the recurrent branch of the median nerve. (University of Washington Musculoskeletal Atlas: Opponens pollicis.)

Because the recurrent branch of the median nerve supplies the thenar muscles, compression of the median nerve in the carpal tunnel can, if untreated, cause weakness and wasting of the thenar eminence. (TeachMeAnatomy: The Carpal Tunnel.)

The hypothenar eminence is the smaller pad on the medial (ulnar) side of the palm. Its three muscles mirror the thenar group and act on the little finger: abductor digiti minimi, flexor digiti minimi brevis and opponens digiti minimi. All three are supplied by the deep branch of the ulnar nerve (C8 and T1) and receive blood from the ulnar artery. (Kenhub: Hypothenar muscles.)

Abductor digiti minimi arises from the pisiform and inserts on the medial side of the base of the proximal phalanx of the little finger, which it abducts (spreads away from the ring finger). (University of Washington Musculoskeletal Atlas: Abductor digiti minimi.)

Flexor digiti minimi brevis arises from the hook of the hamate and the flexor retinaculum and inserts on the medial side of the base of the proximal phalanx of the little finger. It flexes the little finger at the metacarpophalangeal joint. (University of Washington Musculoskeletal Atlas: Flexor digiti minimi brevis.)

Opponens digiti minimi, the deepest, also arises from the hook of the hamate and the flexor retinaculum but inserts along the medial border of the fifth metacarpal. It draws the fifth metacarpal forward and rotates it, bringing the little finger into opposition with the thumb. (University of Washington Musculoskeletal Atlas: Opponens digiti minimi.)

Palmaris brevis is a small, thin muscle that lies in the superficial tissue over the hypothenar eminence. It arises from the palmar aponeurosis and the flexor retinaculum and inserts into the skin on the medial side of the palm. Kenhub places it in the hypothenar region but not in the hypothenar muscle group, because it does not move the little finger. (Kenhub: Hypothenar muscles.)

Palmaris brevis wrinkles the skin over the hypothenar eminence and deepens the hollow of the palm. It is the only hand muscle supplied by the superficial branch of the ulnar nerve. It is not modelled in BodyParts3D, so it does not appear in the 3D view. (TeachMeAnatomy: The Intrinsic Muscles of the Hand.)

Adductor pollicis lies deep in the palm and has two heads. The oblique head arises from the capitate and the bases of the second and third metacarpals; the transverse head arises from the front of the body of the third metacarpal. Both insert on the medial side of the base of the proximal phalanx of the thumb. (University of Washington Musculoskeletal Atlas: Adductor pollicis.)

Adductor pollicis pulls the thumb towards the index finger (adduction). It is supplied by the deep branch of the ulnar nerve, not the median nerve. The radial artery passes between its two heads to form the deep palmar arch. (TeachMeAnatomy: The Intrinsic Muscles of the Hand.)

Weakness of adductor pollicis is tested with Froment's sign. The patient holds a sheet of paper between the thumb and the side of the index finger. With an ulnar nerve lesion, the patient cannot adduct the thumb strongly and instead bends the thumb tip using flexor pollicis longus, which is supplied by the median nerve. (Kenhub: Hypothenar muscles.)

The four lumbricals are small muscles in the palm, numbered 1 to 4 from the index finger side. Unusually, they arise from tendons rather than bone: from the tendons of flexor digitorum profundus. Lumbricals 1 and 2 are unipennate and arise from one tendon each; lumbricals 3 and 4 are bipennate and arise from two neighbouring tendons. (Kenhub: Lumbrical muscles of the hand.)

Each lumbrical inserts on the radial (lateral) side of the extensor expansion of a finger. Together they flex the metacarpophalangeal joints and extend the interphalangeal joints at the same time, the position used to hold a pen. Lumbricals 1 and 2 are supplied by the median nerve and 3 and 4 by the ulnar nerve; a short mnemonic is '1, 2 me; 3, 4 u'. (Kenhub: Lumbrical muscles of the hand.)

The interossei lie between the metacarpal bones. All of them are supplied by the deep branch of the ulnar nerve. Finger abduction and adduction are measured from the long axis of the middle finger: abduction spreads the fingers away from it and adduction brings them back. The rule to remember is PAD and DAB: Palmar interossei ADduct, Dorsal interossei ABduct. (Kenhub: Palmar interossei muscles.)

The University of Washington atlas describes three palmar interossei. They are unipennate and arise from the palmar surfaces of the second, fourth and fifth metacarpals, and insert into the extensor expansions and proximal phalanges of the index, ring and little fingers. They adduct these fingers towards the middle finger and help the lumbricals flex the metacarpophalangeal joints and extend the interphalangeal joints. (University of Washington Musculoskeletal Atlas: Palmar interossei.)

Some books count four palmar interossei by adding a small first palmar interosseous to the thumb, which arises from the base of the first metacarpal. Kenhub notes that this muscle is sometimes rudimentary or absent. The middle finger never has a palmar interosseous, because it is the axis that the other fingers move towards. (Kenhub: Palmar interossei muscles.)

The four dorsal interossei are bipennate: each arises from the facing sides of two neighbouring metacarpals. They insert into the extensor expansions and proximal phalanges of the index, middle and ring fingers and abduct them away from the axis of the middle finger. (University of Washington Musculoskeletal Atlas: Dorsal interossei.)

The first dorsal interosseous is the largest and strongest of the group and is sometimes called abductor indicis. It can be felt in the first web space, between the thumb and the index finger. The middle finger receives two dorsal interossei, one on each side, so it can be moved either way. The thumb and little finger have their own abductors (abductor pollicis brevis and longus, and abductor digiti minimi), so they have no dorsal interosseous. (Kenhub: Dorsal interossei muscles of the hand.)

The thumb's movements are named in its own planes. Abduction and adduction of the thumb happen in a plane perpendicular to the palm: abduction lifts the thumb forwards, away from the palm, and adduction brings it back. Flexion and extension happen in the plane of the palm. Opposition combines abduction and flexion with an automatic rotation of the thumb. (University of Oklahoma Health Sciences Center: Upper extremity anatomy and function (wrist and hand).)

Thumb abduction at the first carpometacarpal joint is produced mainly by abductor pollicis brevis with abductor pollicis longus from the forearm. Strong thumb adduction is produced by adductor pollicis. Opposition is produced by opponens pollicis, helped by the other thenar muscles and by adductor pollicis. (Kenhub: Thenar muscles.)

A thumbs up is mainly thumb extension, done by forearm muscles rather than hand muscles. Extensor pollicis longus extends all the joints of the thumb, including the tip; its tendon winds around Lister's tubercle and forms the ulnar border of the anatomical snuffbox. Extensor pollicis brevis extends the thumb at its base and abductor pollicis longus abducts it. All three are supplied by the posterior interosseous branch of the radial nerve. (TeachMeAnatomy: Muscles in the Posterior Compartment of the Forearm.)

Extensor pollicis longus is the only thumb extensor that inserts on the base of the distal phalanx of the thumb, so it is the muscle that straightens the thumb tip into the upright 'thumbs up' curve. The curled fingers are held by the finger flexors. (University of Washington Musculoskeletal Atlas: Extensor pollicis longus.)

The ulnar nerve (C8 and T1) supplies most intrinsic hand muscles. Its deep branch supplies the hypothenar muscles, the medial two lumbricals, adductor pollicis and all the palmar and dorsal interossei; its superficial branch supplies palmaris brevis. An ulnar nerve injury at the wrist affects only these intrinsic hand muscles. (TeachMeAnatomy: The Ulnar Nerve.)

The median nerve supplies the thenar muscles through its recurrent branch and the lateral two lumbricals through its palmar digital branches. A median nerve injury at the wrist paralyses the thenar muscles, so thumb opposition is lost, and weakens the lateral two lumbricals. (TeachMeAnatomy: The Median Nerve.)

Long-standing deep ulnar nerve damage weakens finger abduction and wastes the interossei. It can produce an ulnar claw hand, with the metacarpophalangeal joints hyperextended and the interphalangeal joints flexed, mainly in the ring and little fingers, because their lumbricals (3 and 4) are ulnar-supplied while the lumbricals of the index and middle fingers are supplied by the median nerve. (Kenhub: Dorsal interossei muscles of the hand.)

The ulnar nerve can also be compressed in Guyon's canal at the wrist, for example by long pressure on bicycle handlebars. This causes pain and altered sensation on the little-finger side of the palm and, in advanced cases, weakness and wasting of the hypothenar and other ulnar-supplied hand muscles. (Kenhub: Hypothenar muscles.)

Abductor digiti minimi of hand

The muscle forming the ulnar (little-finger) side of the hypothenar eminence.

Abductor pollicis brevis

A thenar muscle on the lateral side of the thenar eminence.

Flexor digiti minimi brevis of hand

A small hypothenar muscle.

Flexor pollicis brevis

A thenar muscle medial to abductor pollicis brevis.

Oblique head of adductor pollicis

The larger head of adductor pollicis in the palm.

Opponens digiti minimi of hand

A hypothenar muscle that deepens the hollow of the palm on the little finger side.

Opponens pollicis

A thenar muscle lying deep to abductor pollicis brevis that allows the thumb to oppose.

Dorsal interossei of hand

Four bipennate muscles between the metacarpals that spread the fingers.

Lumbricals of hand

Four small muscles of the palm that arise from the flexor digitorum profundus tendons.

Palmar interossei of hand

Small unipennate muscles in the palm that bring the fingers together.

Superficial head of flexor pollicis brevis

The larger, lateral head of flexor pollicis brevis in the thenar eminence.

Transverse head of adductor pollicis

The transverse head of adductor pollicis in the palm.