Upper limb

First Lumbrical

Musculus lumbricalis I manus

Origin

Radial side of the tendon of flexor digitorum profundus to the index finger (no bony attachment: unipennate from the tendon of the parent muscle)

Insertion

Radial side of the extensor hood and proximal phalanx of the index finger

Function

Flexion of the MCP joint with simultaneous extension of the IP joints of the index finger

Innervation

Median nerve (unlike the 3rd and 4th lumbricals, which are innervated by the ulnar nerve)

Movement & chain

Together with the interossei enables the 'writing position' of the finger (MCP flexed, IP joints extended), essential for fine motor tasks such as writing and typing.

Palpation

Palpable along the radial side of the 2nd metacarpal during MCP flexion with simultaneous IP extension.

Clinical

Unipennate muscle innervated by the median nerve; weakness in severe carpal tunnel syndrome impairs precision pinch grip.

Segmental innervation

Median nerve, C8-T1.

In clinical practice

Because this muscle originates from a tendon rather than bone, the bony origin shown in this model is an approximation (nearest metacarpal bone); the actual origin is the FDP tendon in the palm.

Region in the model

Upper limb