The flexor digitorum longus is situated on the tibial side of the leg. At its origin, it is thin and pointed, but it gradually increases in size as it descends. This muscle serves to curl the second, third, fourth, and fifth toes (flexion of phalanges II-V). 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.
Anatomy of Flexor Digitorum Longus
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Origin: Posterior surface of the tibia distal to the popliteal line
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Insertion: Continues distally to split into four individual tendons which insert on the plantar surfaces of the bases of the second through fifth distal phalanges
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Action: Flex the digits two through five and may aid in plantar flexion of the ankle
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Blood Supply: Posterior tibial artery
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Innervation: Tibial nerve
Nerve Supply of Flexor Digitorum Longus
- 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 of Flexor Digitorum Longus
- 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 of Flexor Digitorum Longus
- Similar to the flexor hallucis longus and tibialis posterior muscles, the flexor digitorum longus muscle functions to plantar flex and invert the foot. The flexor digitorum longus muscle is responsible for the movement and curling of the second, third, fourth and fifth toes.
- This muscle makes it possible for the toes to grip the surface of floors, which is important when it comes to maintaining postural balance on surfaces that are rough or uneven. The other deep muscles are the flexor hallucis longus and tibialis posterior; the tibialis posterior is the most powerful of these deep muscles. All three muscles are innervated by the tibial nerve which comprises half of the 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.
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