Lumbricals

Introduction

Introduction to the Lumbricals of the Hand

The lumbricals are four small, worm‑shaped intrinsic muscles of the hand that lie in the central compartment of the palm. They are unique because they originate from the tendons of the flexor digitorum profundus (FDP) and insert into the dorsal digital expansions (extensor hoods) of the medial four digits. The lumbricals are essential for coordinating finger movements, particularly simultaneous flexion at the metacarpophalangeal (MCP) joints and extension at the interphalangeal (IP) joints. Their dual innervation—median nerve for the lateral two and ulnar nerve for the medial two—provides important localising value in nerve injuries.

This module describes the attachments, actions, nerve and blood supply, and clinical significance of the lumbrical muscles.

Study

1. Structure and Position

The four lumbricals are numbered from lateral (radial) to medial (ulnar). They arise in the palm from the radial side of each of the four FDP tendons, at the level of the mid‑palm. They then pass distally on the radial side of the corresponding metacarpophalangeal joint, deep to the deep transverse metacarpal ligament, and insert into the radial aspect of the dorsal digital expansion (extensor hood) of digits 2–5. This course allows them to flex the MCP joints while simultaneously extending the IP joints.

2. Attachments of Individual Lumbricals

First lumbrical (most lateral): Arises from the radial side of the FDP tendon to the index finger. It inserts into the dorsal expansion of the index finger. Innervation: median nerve (C8, T1).

Second lumbrical: Arises from the radial side of the FDP tendon to the middle finger. It inserts into the dorsal expansion of the middle finger. Innervation: median nerve (C8, T1).

Third lumbrical: Arises from the adjacent sides of the FDP tendons to the middle and ring fingers (bipennate origin). It inserts into the dorsal expansion of the ring finger. Innervation: deep branch of the ulnar nerve (C8, T1).

Fourth lumbrical: Arises from the adjacent sides of the FDP tendons to the ring and little fingers (bipennate origin). It inserts into the dorsal expansion of the little finger. Innervation: deep branch of the ulnar nerve (C8, T1).

3. Nerve Supply

The lumbricals display a classic dual innervation. The first and second lumbricals (to the index and middle fingers) are innervated by the median nerve, typically by its terminal digital branches in the palm. The third and fourth lumbricals (to the ring and little fingers) are supplied by the deep branch of the ulnar nerve. This pattern is subject to anatomical variation; for example, a Martin–Gruber anastomosis may alter the origin of motor fibres.

4. Blood Supply

The lumbricals are supplied by branches from the superficial palmar arch and its common palmar digital arteries. Venous drainage follows the corresponding venae comitantes to the deep palmar venous arch.

5. Actions of the Lumbricals

The lumbricals flex the metacarpophalangeal joints and extend the interphalangeal joints of the corresponding digit. This action is fundamental to the intrinsic plus posture, where the fingers are positioned for precision tasks. Because the lumbricals attach to the dorsal digital expansion, they also contribute to the delicate balance of the extensor and flexor mechanisms of the digits. Their origin from the FDP tendons means they can act as a proprioceptive link between the long flexors and extensors.

6. Clinical Significance

Lumbrical plus finger: If the FDP tendon is divided distal to the lumbrical origin, contraction of the lumbrical can paradoxically extend the IP joint, producing a lumbrical plus deformity. This is an important consideration in flexor tendon repair.

Intrinsic minus hand (claw hand): Paralysis of the lumbricals (and interossei) leads to hyperextension at the MCP joints and flexion at the IP joints due to unopposed action of the long extensors and flexors. This occurs in combined median and ulnar nerve palsies.

Median nerve palsy: Loss of the first and second lumbricals contributes to clawing of the index and middle fingers, though this may be subtle because the interossei remain innervated by the ulnar nerve.

Ulnar nerve palsy: Loss of the third and fourth lumbricals contributes to clawing of the ring and little fingers, which is often more pronounced because both lumbricals and interossei are affected.

Summary

Summary of the Lumbricals

The four lumbricals are unique intrinsic hand muscles that arise from the FDP tendons and insert into the dorsal digital expansions. They flex the MCP joints and extend the IP joints, integrating the action of the long flexors and extensors. Their dual innervation—median nerve (first and second) and ulnar nerve (third and fourth)—provides a sensitive test for nerve lesions. Lumbrical dysfunction contributes to characteristic digital deformities such as claw hand and lumbrical plus finger.