Lumbrical muscles of foot

At a glance

English

Lumbrical muscles of foot

Latin (TA2)

Musculi lumbricales pedis

System

Muscle

Region

Lower limb

Side

Paired, left and right

About the lumbrical muscles of foot

The lumbricals are four thin muscles in the second layer of the sole. Unusually, they have no bony origin: each grows out of the tendons of flexor digitorum longus after that tendon has fanned out into its four slips for the lesser toes. They are counted from the big-toe side, so the first lies most medially and the fourth sits next to the little toe. Their name comes from the Latin word for an earthworm, which their small rounded bellies resemble.

The first lumbrical arises from just one side of the tendon to the second toe and is therefore unipennate. The other three are bipennate, each taking fibres from the facing sides of two neighbouring tendons. Each muscle narrows to a fine tendon that runs forwards beneath the deep transverse metatarsal ligament on the medial side of its toe, from the second to the fifth. It then turns up onto the back of the proximal phalanx and blends with the dorsal hood formed by the extensor tendons.

This course explains their double action. Passing below the metatarsophalangeal joint, they flex it; joining the extensor hood above, they extend the two interphalangeal joints. The effect is to keep the toes straight while their bases press down, which helps the foot grip the ground and prevents the toes from buckling into a claw as the heel lifts during walking. Weakness of these small muscles is one contributor to clawing and hammering of the lesser toes.

Their nerve supply follows the split seen in the hand. The first lumbrical is served by the medial plantar nerve and the second to fourth by the deep branch of the lateral plantar nerve. Blood reaches them from the plantar metatarsal arteries arising from the plantar arch. Variation is common: one or more may be missing, the third or fourth may be doubled, and an insertion may go partly into the proximal phalanx itself.

Attachments, action and supply

Origin

Tendons of flexor digitorum longus in the sole: the first from the inner edge of the second-toe tendon, the others from adjacent sides of two tendons.

Insertion

Medial side of the dorsal extensor expansions of the second to fifth toes, over the proximal phalanges.

Action

Flex the metatarsophalangeal joints and extend the interphalangeal joints of the lesser toes, helping to keep the toes flat against the ground.

Nerve supply

First lumbrical: medial plantar nerve; second to fourth: deep branch of the lateral plantar nerve (S2, S3).

Blood supply

Plantar metatarsal arteries from the plantar arch, with branches of the lateral plantar artery.