Flexor Digitorum Longus Muscle: Anatomy, Function, Imaging

Topic

Flexor digitorum longus muscle

The flexor digitorum longus (FDL) is a slender, long muscle of the deep posterior compartment of the leg. It lies medial to the tibialis posterior and deep to the soleus and gastrocnemius muscles. The FDL is responsible for flexion of the lateral four toes, contributes to plantarflexion of the ankle, and supports the longitudinal arch of the foot.

Its tendon passes behind the medial malleolus, enters the sole of the foot, and divides into four slips that insert into the distal phalanges of toes 2–5. It is a critical muscle for balance, toe grip, and propulsion during gait.

Synonyms

Long flexor of toes
Toe flexor muscle (FDL)

Origin, Course, and Insertion

Origin: Posterior surface of the tibia, below the soleal line, and from the fascia covering tibialis posterior
Course: Descends obliquely down the posterior compartment of the leg → tendon passes posterior to the medial malleolus (within tarsal tunnel) → enters sole of foot → crosses tibialis posterior tendon and quadratus plantae
Insertion: Bases of the distal phalanges of the lateral four toes (digits 2–5)

Tendon Attachments

Enters sole of foot deep to flexor retinaculum
Divides into four separate tendinous slips
Inserts onto the distal phalanx of each of toes 2–5
Connections with lumbricals and quadratus plantae muscles stabilize action

Relations

Superiorly: Soleus, tibialis posterior, posterior tibial vessels and tibial nerve
Inferiorly: Flexor retinaculum, quadratus plantae, lumbricals
Medially: Tibia and tarsal tunnel
Laterally: Tibialis posterior and flexor hallucis longus

Nerve Supply

Tibial nerve (S2–S3), a branch of the sciatic nerve

Arterial Supply

Posterior tibial artery and its muscular branches

Venous Drainage

Posterior tibial veins, draining into the popliteal vein

Function

Toe flexion: Flexes distal phalanges of toes 2–5
Plantarflexion: Assists gastrocnemius and soleus at the ankle
Arch support: Maintains medial longitudinal arch of the foot
Propulsion: Provides grip and push-off strength during walking and running

Clinical Significance

Tendinopathy: Overuse injuries common in athletes and dancers
Tarsal tunnel syndrome: Compression of tibial nerve can affect FDL function
Weakness/paralysis: Impairs toe flexion and arch stability, contributing to flatfoot deformities
Surgical relevance: FDL tendon often harvested for tendon transfers (e.g., repairing posterior tibial tendon dysfunction)
Imaging importance: MRI helps detect FDL tendon tears, tenosynovitis, and degenerative changes

MRI Appearance

T1-weighted images:
- Normal muscle: intermediate signal intensity
- Normal tendon: low signal (dark band) along posterior tibia and ankle
- Tears/tendinopathy: focal hyperintense signal within tendon
- Fatty infiltration of muscle in chronic denervation: bright signal
T2-weighted images:
- Normal muscle: intermediate-to-dark, slightly darker than on T1
- Normal tendon: very low signal (black)
- Pathology: tendonitis or tears show bright hyperintense signal
- Fluid or edema around tendon sheath is hyperintense
STIR:
- Normal muscle: intermediate-to-dark
- Pathology: bright signal indicates edema, inflammation, or partial tear
- Excellent for detecting tibial tunnel synovitis and tenosynovitis
Proton Density Fat-Saturated (PD FS):
- Normal muscle: intermediate-to-dark
- Normal tendon: consistently low signal
- Pathology: bright hyperintense signals within tendon substance or sheath, excellent for subtle tears and synovitis
T1 Fat-Sat Post-Contrast:
- Normal muscle: homogeneous intermediate signal with minimal enhancement
- Normal tendon: no enhancement
- Pathology: tendon sheath inflammation, synovitis, or granulation tissue enhances; complete tears show enhancing fluid-filled gap