Lumbrical muscles of the foot

Origin and insertion

The lumbricals originate from the tendons of flexor digitorum longus muscle, which is why they are sometimes referred to as the accessory muscles of the flexor digitorum longus. From medial to lateral, they are numbered I-IV.

The first lumbrical muscle originates from the medial side of the first flexor digitorum longus tendon, i.e. the one intended for the second toe. The rest of lumbricals have two origin points, arising from the facing surfaces of two adjacent tendons of the flexor digitorum longus, respectively.

From the point of origin, each lumbrical muscle passes anteriorly, plantar to the deep transverse metatarsal ligaments. The muscles bellies then turn obliquely upwards to attach on the base of the proximal phalanges of second to fifth digits and their extensor expansions.

Relations

Upon originating from the tendons of flexor digitorum longus muscle, the lumbricals pass from the flexor to the extensor compartment of the foot. After providing the origin sites for them, the flexor digitorum longus tendons cross the inferomedial and lateral surfaces of the lumbricals before passing through the flexor sheaths of the second to fifth toe.

The inserting fibers of each lumbrical muscle are in relation to plantar surfaces of deep transverse metatarsal ligaments, which are the wide fibrous bands that connect the plantar ligaments of adjoining metatarsophalangeal joints.

Innervation

Lumbrical muscles are supplied by the two terminal branches of the tibial nerve;

• The first lumbrical muscle is supplied by the medial plantar nerve (S1, S2).
• The lateral three lumbricals are innervated by the lateral plantar nerve (S2, S3).

Blood supply

Lumbrical muscles are supplied by branches of an anastomotic network between the lateral plantar and dorsalis pedis arteries called the plantar arch. The convexity of the plantar arch gives off the lateral plantar artery and four plantar metatarsal arteries that supply the lumbrical muscles.

Function

By pulling the medial base of the proximal phalanx, the lumbrical muscles flex and adduct the toes at the metatarsophalangeal (MTP) joints. Conversely, by pulling the extensor expansions of the phalanges, the lumbricals extend the toes at the interphalangeal (IP) joints.

These actions of the lumbrical muscles provide a balancing function to the foot. Flexion and adduction at the MTP joints oppose the extension of the long and short extensor of the toes and prevent hyperextension of the toes during the propulsive phase of gait. The extension at IP joints prevents clawing during gait as it opposes to the flexion produced by the long and short flexors of the toes.

To understand the functions of the lumbricals and other plantar foot muscles, check out our learning materials!