Overview

The lumbricals (lumbricus, earthworm) are unique in arising from tendons: the four tendons of flexor digitorum profundus in the palm. The 1st and 2nd are unipennate, each from the radial side of one tendon. The 3rd and 4th are bipennate, from adjacent tendons. Each passes on the radial side of the MCP joint, in front of the deep transverse metacarpal ligament, and inserts into the lateral band of the extensor expansion of digits 2–5.

Their line of pull lies anterior to the MCP joint axis and posterior to the IP joint axes. They therefore flex the MCP joints and extend the PIP and DIP joints, giving the "lumbrical" or "table-top" position used in writing and precision tasks. Because they run between a flexor and an extensor tendon, they are also rich in muscle spindles and act as sensors in finger coordination. Innervation: the lateral two (index and middle) are supplied by the median nerve, the medial two (ring and little) by the deep branch of the ulnar nerve.

Parts & subdivisions

1st and 2nd lumbricals
Unipennate, from the FDP tendons of the index and middle fingers. Median nerve.
3rd and 4th lumbricals
Bipennate, from adjacent FDP tendons (middle/ring and ring/little). Deep branch of the ulnar nerve.

Anatomy

Origin: Tendons of flexor digitorum profundus in the palm.
Insertion: Lateral (radial) bands of the extensor expansions of digits 2–5.
Action: Flexion of the MCP joints with extension of the PIP and DIP joints.
Innervation: Lateral two: median nerve. Medial two: deep branch of the ulnar nerve.
Nerve roots: C8–T1
Arterial supply: Superficial and deep palmar arches; common and proper palmar digital arteries; dorsal digital arteries.

Function

Flex the MCP joints and extend the IP joints of digits 2–5
Fine control of finger posture (precision grip, writing)

Clinical significance

Claw hand

An ulnar nerve lesion paralyses the medial two lumbricals and the interossei. The ring and little fingers hyperextend at the MCP joints and flex at the IP joints (the "ulnar claw"). The index and middle fingers are spared by their median-supplied lumbricals. Combined median and ulnar or C8–T1 lesions (Klumpke palsy) cause total clawing.

Lumbrical-plus finger

After FDP tendon division distal to the lumbrical origin, pulling on FDP is transmitted through the lumbrical to the extensor expansion. The finger paradoxically extends at the IP joints when the patient tries to make a fist.

Mnemonics

Innervation

LOAF: the Lateral two lumbricals are median-supplied, like the thenar muscles. The medial two are ulnar-supplied, like the interossei.

Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.