Flexor hallucis longus

At a glance

English

Flexor hallucis longus

Latin (TA2)

Flexor longus hallucis

System

Muscle

Region

Lower limb

Side

Paired, left and right

About the flexor hallucis longus

Flexor hallucis longus belongs to the deep group at the back of the leg, alongside tibialis posterior and the long toe flexor. Although it acts on the big toe, on the inner side of the foot, it lies on the outer, fibular side of the leg, and it is the bulkiest of the deep group in its lower part.

It arises from the lower two-thirds of the back of the fibula, except the last few centimetres, from the lower interosseous membrane, and from the septum next to the fibular muscles and the fascia over tibialis posterior. Its fibres slope down and back onto a tendon running along nearly the whole back of the muscle, which stays fleshy almost to the ankle. The tendon grooves the back of the lower tibia, then the back of the talus between its two tubercles, then the underside of the sustentaculum tali, each groove turned into a tunnel by fibrous bands and lined by a synovial sheath. Behind the ankle it is the deepest and most lateral structure in the tarsal tunnel.

In the sole the tendon crosses above that of flexor digitorum longus, from outer to inner side, and sends it a slip. It then runs between the two heads of flexor hallucis brevis, between the sesamoids, and ends on the plantar base of the big toe's distal phalanx. It flexes both joints of the big toe and helps plantarflex and invert the foot. At the end of stance it gives the final thrust from the big toe and supports the medial arch.

Ballet dancers, gymnasts and runners load the tendon heavily at the ankle and may develop dancer's tendinitis, with pain and swelling behind the inner ankle. A nodule on the tendon can catch in its tunnel and cause a painful click or triggering of the big toe. An os trigonum behind the talus can pinch the tendon and must be ruled out by X-ray. Most cases settle with rest, physiotherapy and splints; surgery is kept for persistent tears or tunnel tightness.

Attachments, action and supply

Origin

Back of the fibula over its lower two-thirds, lower interosseous membrane, adjacent intermuscular septum and fascia over tibialis posterior.

Insertion

Plantar surface of the base of the big toe's distal phalanx.

Action

Flexes the big toe at both its joints; helps plantarflex and invert the foot; supports the medial longitudinal arch and drives push-off.

Nerve supply

Tibial nerve (S2, S3).

Blood supply

Fibular artery, with branches of the posterior tibial artery.