Flexor Digitorum Longus

Origin, Insertion, Action, Innervation

Origin: Middle third of the posterior surface of the tibia (below the soleal line)
Insertion: Plantar surfaces of the distal phalanges of toes 2–5
Action:
Primary: Flexion of the distal interphalangeal (DIP) joints of toes 2–5
Assists flexion of the proximal interphalangeal (PIP) and metatarsophalangeal (MTP) joints of toes 2–5
Assists plantarflexion and inversion of the ankle
Contributes to grip of the toes on the ground during the push-off phase of gait
Innervation: Tibial nerve (L5, S1)

Note on the "Tom, Dick, and Harry" mnemonic: Behind the medial malleolus, from anterior to posterior: Tibialis posterior, Flexor Digitorum longus, posterior tibial Artery, tibial Nerve, Flexor Hallucis longus. FDL is "Dick."

Palpation Guide

Client position: Prone with the foot off the end of the table, or supine with the medial ankle accessible.
Landmark sequence:

Locate the medial malleolus. FDL tendon passes posterior to it, between tibialis posterior (anterior) and flexor hallucis longus (posterior).
Palpate immediately posterior to the tibialis posterior tendon behind the medial malleolus. FDL is thinner than tibialis posterior.
Follow the tendon distally along the medial plantar foot. It crosses deep to FHL at the "knot of Henry" (chiasma of the plantar tendons) in the midfoot before continuing to the toes.
The muscle belly on the posterior tibia is deep to soleus and not directly palpable through the superficial calf.

Tissue feel: The tendon behind the medial malleolus is thin and cordlike — smaller than tibialis posterior but palpable when the toes are actively flexed.
Confirmation test: Ask the client to curl toes 2–5 (flex the toes without moving the ankle). The FDL tendon tightens behind the medial malleolus. If the client also inverts or plantarflexes, you may be feeling tibialis posterior instead.
Common errors:
Confusing with tibialis posterior — tibialis posterior lies anterior to FDL behind the malleolus. Asking for inversion activates tibialis posterior; asking for toe flexion isolates FDL.
Confusing with flexor hallucis longus — FHL is the most posterior of the three tendons and is activated by flexing the great toe specifically.

Trigger Point Referral

Common TrP locations: In the deep muscle belly on the posterior tibial surface, accessible through the medial tibial border by pressing posterolaterally through soleus.
Referral pattern: Plantar surface of the forefoot under the metatarsal heads (ball of the foot) and into the corresponding toes.
Clinical significance: Forefoot plantar pain mimics metatarsalgia. If the client reports pain "under the ball of the foot" without MTP joint pathology or Morton's neuroma signs, check FDL — the referral overlaps precisely with the metatarsal head region.

Trigger point referral diagram — coming soon

No external TrP reference link available for flexor digitorum longus.

Clinical Notes

Common conditions:

FDL tendinopathy presents as pain behind the medial malleolus, worsened by active toe flexion and push-off during gait. Often coexists with tibialis posterior tendinopathy because both tendons share the tarsal tunnel.
Contributes to the pathology in tarsal tunnel syndrome — tendon swelling within the tarsal tunnel can compress the tibial nerve.
Relevant to plantar fasciitis differentials — FDL overuse can mimic plantar fascial pain because its tendon crosses the plantar foot and its TrPs refer to the sole.
Involved in hammer toe and claw toe deformities — chronic FDL tightness flexes the DIP joints, contributing to fixed toe deformities over time.

What you'll typically find:

Overloaded in clients who grip the ground with their toes — common in barefoot runners, martial artists, and clients with chronic ankle instability (the toes grip to compensate for poor balance).
Tenderness behind the medial malleolus in the FDL position (between tibialis posterior and FHL). Differentiate by having the client selectively flex the lateral toes.
In clients with hammer toes, FDL is chronically shortened — passive toe extension may be limited.

Treatment effects:

The tendon behind the medial malleolus responds to gentle cross-fiber friction. Because it shares the tarsal tunnel with the tibial nerve and artery, use moderate pressure.
The muscle belly is accessed via the medial tibial border, pressing posterolaterally. Release soleus first to improve access to the deep compartment.
Stretching involves passive extension of toes 2–5 with the ankle dorsiflexed — this puts maximum stretch on FDL.

Cautions:

The tibial nerve and posterior tibial artery run between FDL and FHL in the tarsal tunnel. Sustained deep pressure in this region can compress neurovascular structures. Monitor for sole numbness or tingling.

Clinical pearl:

FDL and FHL tendons cross each other at the "knot of Henry" on the plantar foot — a fibrous connection between the two tendons in the midfoot. Adhesion or restriction at this crossing can limit independent toe flexion and contribute to plantar pain. Gentle mobilization of the midfoot plantar surface can address this.

Assessment

Resisted toe flexion (DIP):

Client supine. Stabilize the proximal phalanx and middle phalanx of toes 2–5. Ask the client to flex the distal phalanges against your resistance. Pain behind the medial malleolus or weakness implicates FDL.

Passive toe extension:

With the ankle dorsiflexed, passively extend toes 2–5 at the DIP joints. Limited range or pain in the posterior leg or medial ankle suggests FDL tightness.

Muscle Groups

Deep posterior compartment (anatomical):

Tibialis posterior
Flexor digitorum longus (this article)
Flexor hallucis longus

Toe flexors (functional):

Flexor digitorum longus (this article)
Flexor digitorum brevis — flexes PIP joints of toes 2–5
Quadratus plantae — corrects FDL's oblique pull
Plantar interossei — assist MTP flexion

Tibial nerve group (innervation):

Gastrocnemius
Soleus
Popliteus
Tibialis posterior
Flexor digitorum longus (this article)
Flexor hallucis longus

Related Muscles

Synergists for toe flexion:

Flexor digitorum brevis — flexes PIP joints (plantar layer 1)
Quadratus plantae — attaches to FDL tendon, corrects the oblique line of pull to a direct posterior pull

Deep posterior compartment neighbors:

Tibialis posterior — anterior neighbor behind the medial malleolus
Flexor hallucis longus — posterior neighbor, crosses FDL at the knot of Henry

Antagonists (toe extension):

Extensor digitorum longus — extends toes 2–5
Extensor digitorum brevis — assists extension of toes 2–4

Key Takeaways

FDL flexes the DIP joints of toes 2–5 and is the "Dick" in the "Tom, Dick, and Harry" mnemonic behind the medial malleolus.
Forefoot referral mimics metatarsalgia — check FDL when ball-of-foot pain lacks joint pathology.
Shares the tarsal tunnel with the tibial nerve — tendon swelling can contribute to tarsal tunnel syndrome.