Flexor digitorum longus muscle

Origin and insertion

Flexor digitorum longus muscle originates from the medial part of the posterior surface of the tibia, medial to the origin of tibialis posterior muscle and inferior to soleus muscle.

From here, the muscle fibers descend through the medial aspect of the posterior compartment of the leg. A few centimeters above the ankle joint, the muscle gives off a tendon that runs posterior to the medial malleolus and deep to the flexor retinaculum.

Traversing the flexor retinaculum, the muscle enters the plantar compartment of the foot. In this compartment, the tendon divides into four smaller tendons that insert onto the bases of the distal phalanges of the lateral four digits.

Relations

Being in the deep posterior leg compartment together with flexor hallucis longus, tibialis posterior and popliteus muscles, this muscle is separated from the superficial layer by the deep fascia of the leg.

Flexor digitorum longus sits posteriorly to the tibia and tibialis posterior muscle. It is situated medial to flexor hallucis longus and deep to the superficial layer of muscles in the posterior leg compartment, specifically the gastrocnemius and soleus muscles, which collectively form the triceps surae muscle.

It is important to understand the specific relation between flexor digitorum longus and quadratus plantae muscle (flexor accessorius). The quadratus plantae is a 2 head muscle that is located in the sole of the foot. The two muscle bellies of quadratus plantae attach to the inserting tendons of flexor digitorum longus. This relation contributes to the stability and strength of the flexor digitorum longus muscle, especially when flexing the toes. A similar relation exists distally in the foot with lumbrical muscles which also insert in the tendons of the flexor digitorum longus and through this relation those muscles act synergistically to stabilize the foot.

The four tendons of flexor digitorum longus run deep to corresponding tendons of flexor digitorum brevis muscle. Before attaching onto the base of the distal phalanges they pass through the tunnel bounded by the tendons of flexor digitorum brevis muscle.

To expand your knowledge check out our article and quiz about muscles of the leg.

Innervation

Like all muscles in the deep posterior compartment of the leg, flexor digitorum longus muscle is innervated by branches of the tibial nerve (root value L5, S1 and S2) which is a branch of sciatic nerve.

Blood supply

Flexor digitorum muscle is supplied by branches of the posterior tibial artery, which is a terminal branch of the popliteal artery.

The blood from the deep posterior compartment of the leg is drained through posterior tibial veins which empty into the popliteal vein.

Function

As the name suggests, the main function of this muscle is plantarflexion of the foot at the ankle joint and the second to fifth phalanges at the metatarsophalangeal and interphalangeal joints. Plantarflexion is aided by the triceps surae muscle. Due to its attachments and course around the medial malleolus, flexor digitorum longus assists in foot inversion at the subtalar joint.

The actions of flexor digitorum longus are crucial for the gait cycle. When the foot is off the ground, flexor digitorum longus muscle flexes the four lateral toes. This action starts in the distal interphalangeal joints and is followed by flexion in proximal interphalangeal and metatarsophalangeal joints respectively. When the foot is placed on the ground, flexor digitorum longus acts in synergy with the lumbrical and interossei muscles to maintain balance by keeping the toes in firm contact with the ground.

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