Four small worm-like muscles of the hand. Numbered 1st to 4th, lateral to medial. One for each of the medial four digits.

Origin, Insertion and Nerve Supply

Insertion (all four): Radial (lateral) side of MP joint → lateral side of dorsal digital expansion of digits 2–5

Unique: arise from flexor tendons (FDP) → insert into extensor expansion — only muscles connecting flexor and extensor apparatus of digits.

Actions and the Lumbrical Grip

• Flex MP joints + simultaneously extend IP joints (PIP and DIP) of digits 2–5
• Produces lumbrical grip / lumbrical posture — pen-holding, tray-holding position
• Also maintain tension in long extensor tendons during finger flexion → prevent bowstringing

Applied Anatomy

• Claw hand: Paralysis of lumbricals (+ interossei) → loss of MP flexion/IP extension → hyperextension at MP + flexion at IP
  • Medial two lumbricals: ulnar nerve injury
  • All four lumbricals: combined median + ulnar nerve injury
• Lumbrical canals: Each lumbrical passes through a fascial canal connecting midpalmar/thenar spaces to web space → relevant to spread of palmar space infections

The lumbricals are four small, slender, worm-like muscles of the hand (L. lumbricus = earthworm), numbered 1st to 4th from lateral to medial, one for each of the medial four digits.

Origin, Insertion and Nerve Supply

Insertion (all four): The tendon of each lumbrical passes on the radial (lateral) side of the corresponding metacarpophalangeal (MP) joint and inserts into the lateral side of the dorsal digital expansion of digits 2–5.

Lumbricals are unique in arising from flexor tendons (FDP, in the flexor compartment) and inserting into the extensor expansion (extensor compartment) — they are the only muscles that connect the flexor and extensor apparatus of the digits.

Actions and the Lumbrical Grip

• Lumbricals flex the metacarpophalangeal (MP) joints while simultaneously extending the interphalangeal (PIP and DIP) joints of digits 2–5.
• This combined action — MP flexion with IP extension — produces the “lumbrical grip” or “lumbrical posture”: the position of the hand seen when holding a pen, a tray, or a plate flat on the palm with extended fingers.
• Because of their attachment to the extensor expansion, lumbricals also help maintain tension in the long extensor tendons during finger flexion, preventing “bowstringing.”

Applied Anatomy

• Claw hand: Paralysis of the lumbricals (along with the interossei) removes the MP-flexion/IP-extension action, contributing to the clawed posture (hyperextension at MP joints with flexion at IP joints) seen in ulnar nerve injury (medial two lumbricals) and combined median-ulnar nerve injury (all four lumbricals).
• Lumbrical canals: Each lumbrical passes through a fascial canal connecting the midpalmar/thenar spaces to the corresponding web space — relevant to the spread of palmar space infections (see Palmar Fascial Spaces).

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