Flexor digitorum longus
Musculus flexor digitorum longus
Also known as: FDL
A deep posterior-compartment muscle that flexes the lateral four toes. Its tendon crosses tibialis posterior in the leg and FHL in the sole.
Overview
FDL arises from the posterior surface of the tibia below the soleal line, medial to tibialis posterior. In the lower leg its tendon crosses superficial (posterior) to tibialis posterior (the crural chiasm) and passes behind the medial malleolus. In the sole it crosses superficial to the FHL tendon (the plantar chiasm, the "knot of Henry"), receives the insertion of quadratus plantae, gives origin to the four lumbricals, and divides into four tendons. These pass through the split tendons of flexor digitorum brevis to the bases of the distal phalanges of toes 2–5.
It flexes the lateral four toes, plantarflexes the ankle and inverts the foot. It helps grip the ground in walking, especially on uneven surfaces, and supports the longitudinal arch.
Anatomy
- Origin
Medial part of the posterior surface of the tibia, below the soleal line.
- Insertion
Plantar bases of the distal phalanges of the lateral four toes.
- Action
Flexion of toes 2–5; plantarflexion of the ankle; inversion of the foot.
- Innervation
Tibial nerve.
- Nerve roots
S2–S3
- Arterial supply
Posterior tibial artery.
- Relations
Crosses tibialis posterior above the ankle and FHL in the sole (knot of Henry).
Function
- Flexes the lateral four toes
- Assists plantarflexion and inversion
- Supports the longitudinal arch
Clinical significance
Tendon transfer
FDL is transferred to the navicular to replace a failed tibialis posterior tendon in flatfoot reconstruction.
Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.