Flexor hallucis longus and flexion of the great toe

Flexor hallucis longus is a muscle of the leg. Its name reads as the long bender of the hallux, where hallux is the Latin-derived name for the great toe. The muscle sits in the deep layer of the posterior compartment, the muscles at the back of the leg behind the tibia and fibula, and like the rest of that compartment it is supplied by the tibial nerve (a branch of the sciatic nerve).

It arises from the back of the fibula. Its tendon runs behind the medial malleolus (the bony bump on the inner side of the ankle), travels along the sole, and ends on the underside of the distal phalanx, which is the bone at the tip of the great toe. Because the tendon pulls on the underside of the toe, the muscle curls the tip downwards. That is flexion of the hallux at the interphalangeal joint (the joint near the tip of the toe), and it also contributes at the metatarsophalangeal joint (where the toe meets the foot).

The name is a useful memory aid for the look-alikes. Hallucis points to the hallux, whereas digitorum points to the digits, so flexor digitorum longus is the long flexor of the lateral four toes. Extensor hallucis longus acts on the same toe but extends it, and it lies in the anterior compartment on the front of the leg. Tibialis posterior, whose tendon passes behind the medial malleolus, inverts the foot (turns the sole inwards) and plantarflexes it (points the foot downwards) rather than moving any toe.

A deep cut behind the medial malleolus can divide the tendon and weaken great toe flexion. Normal flexion of the lesser toes (the lateral four toes) with a weak great toe points to this muscle, because flexor digitorum longus flexes the lesser toes.