Flexor digitorum longus

At a glance

English

Flexor digitorum longus

Latin (TA2)

Musculus flexor digitorum longus

System

Muscle

Region

Lower limb

Side

Paired, left and right

Inserts on

The distal phalanx of fifth finger of foot, distal phalanx of fourth finger of foot and distal phalanx of third finger of foot

About the flexor digitorum longus

Flexor digitorum longus belongs to the deep group of the posterior compartment of the leg, with tibialis posterior and flexor hallucis longus. It lies against the back of the tibia, slender and pointed above and growing thicker as it descends. Its main work is to bend the lesser toes, and it also helps point the foot and turn the sole inwards.

It arises from the back of the tibial shaft, from just below the soleal line to a few centimetres above the ankle, on the inner side of where tibialis posterior arises from the tibia, and from the fascia over that muscle. A tendon runs along almost the whole length of its back surface. Above the ankle it crosses superficial to the tibialis posterior tendon, so that behind the medial malleolus it lies just behind it, each tendon in its own synovial sheath in a shared groove. It then passes through the tarsal tunnel under the flexor retinaculum, curves around the sustentaculum tali and runs forwards and outwards into the sole.

In the sole, near the navicular, its tendon crosses below that of flexor hallucis longus and receives a strong slip from it; this crossing is the master knot of Henry. Quadratus plantae then pulls on the tendon from the side, straightening its oblique line of pull. The tendon divides into slips for toes two to five; each slip runs through the split in the matching flexor digitorum brevis tendon and ends on the base of the distal phalanx.

The muscle flexes all the joints of the lesser toes and helps plantarflex and invert the foot. By curling the toes it lets them grip the ground, which helps balance on rough or uneven surfaces and stiffens the forefoot at push-off. It can be strained in a stumble when the toes fail to grip, or overused in barefoot running on sand. It is supplied by the tibial nerve and the posterior tibial artery.

Attachments, action and supply

Origin

Posterior surface of the tibial shaft below the soleal line, medial to tibialis posterior, and the fascia covering tibialis posterior.

Insertion

Bases of the distal phalanges of the second to fifth toes.

Action

Flexes the lesser toes at all their joints; helps plantarflex and invert the foot and supports the longitudinal arch.

Nerve supply

Tibial nerve (S2, S3).

Blood supply

Posterior tibial artery.