Flexor digitorum longus muscle
Flexor digitorum longus is a thin muscle that belongs to the deep posterior muscles of the leg. It runs from the posterior surface of the tibia, across the posterior compartment of the leg to the phalanges of the foot. On its course, the muscle receives functional support from the quadratus plantae muscle.
Due to its attachments, flexor digitorum longus performs its action in four different joints; talocrural (ankle joint), talocalcaneal (subtalar joint), metatarsophalangeal and interphalangeal joints. The main function of this muscle is the flexion of the foot and toes.
In this article, we will discuss the anatomy and function of the flexor digitorum longus muscle.
|Origin||Posterior surface of tibia (inferior to soleal line)|
|Insertion||Bases of distal phalanges of digits 2-5|
Metatarsophalangeal and interphalangeal joints 2-5: toe flexion
Talocrural joint: foot plantar flexion
|Innervation||Tibial nerve (L5, S1, S2)|
|Blood supply||Posterior tibial artery|
Origin and insertion
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.
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.
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.
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.