Lumbricals

Lumbricals are small intrinsic muscles found in the hand and, in the foot, the sole. In the hand there are four lumbricals; in the foot there are also four. They originate from the tendons of the flexor digitorum profundus (FDP) and insert into the extensor expansions of the digits or toes.

In the hand, the first and second lumbricals arise from the FDP tendons to the index and middle fingers, while the third and fourth lumbricals originate from adjacent sides of the FDP tendons to the middle, ring, and little fingers. In the foot, the lumbricals arise from the FDP tendons to digits 2–5 and insert into the extensor expansions of toes 2–5.

Functionally, lumbricals flex the metacarpophalangeal (MCP) or metatarsophalangeal joints and extend the proximal and distal interphalangeal joints via the extensor mechanisms. They help coordinate digit and toe movements and contribute to grip strength and postural control of the hand and foot.

Innervation differs between the upper and lower limbs. In the hand, the first and second lumbricals are supplied by the median nerve, while the third and fourth are supplied by the deep branch of the ulnar nerve. In the foot, the first and second lumbricals are innervated by the medial plantar nerve, and the third and fourth by the lateral plantar nerve. Blood supply comes from the respective palmar metacarpal arteries in the hand and plantar metatarsal arteries in the foot.

Clinical relevance centers on the role of nerve injuries in impairing lumbrical function, which can affect fine motor control and contribute to characteristic hand or toe deformities in advanced cases.