Flexor Hallucis Longus

Origin, Insertion, Action, Innervation

Origin: Distal two-thirds of the posterior surface of the fibula, the adjacent interosseous membrane, and the posterior intermuscular septum
Insertion: Plantar surface of the distal phalanx of the great toe (hallux)
Action:
Primary: Flexion of the interphalangeal (IP) joint of the great toe
Assists flexion of the MTP joint of the great toe
Assists plantarflexion and inversion of the ankle
Critical for the "push-off" phase of gait — the great toe provides the final propulsive force
Innervation: Tibial nerve (L5, S1, S2)

Palpation Guide

Client position: Prone with the foot off the end of the table.
Landmark sequence:

Locate the medial malleolus. FHL is the most posterior of the "Tom, Dick, and Harry" tendons — it passes behind the medial malleolus posterior to the tibial nerve and posterior tibial artery.
Follow the tendon from behind the medial malleolus along the plantar surface of the foot to the great toe.
The muscle belly is on the posterior fibula — it lies deep to soleus and lateral to FDL. Access the belly through the lateral posterior calf, pressing medially toward the interosseous membrane.
Behind the talus, the tendon passes through a fibro-osseous tunnel — this is the most common site of tendinopathy and is not directly palpable but is tender on deep palpation posterior to the medial malleolus.

Tissue feel: The tendon behind the medial malleolus is palpable when the great toe is actively flexed. The muscle belly on the posterior fibula is deep and difficult to distinguish from surrounding tissue without the toe-flexion confirmation test.
Confirmation test: Ask the client to flex only the great toe (curl the big toe down). The FHL tendon tightens behind the medial malleolus, and the belly contracts on the posterior fibula. Ensure the client does not curl all the toes (which activates FDL) — isolate the great toe.
Common errors:
Confusing with FDL — FDL lies anterior to FHL behind the malleolus and is activated by flexing toes 2–5. Isolate by testing great toe flexion alone.
Palpating the muscle belly on the wrong bone — FHL originates from the fibula (not the tibia like FDL). The belly is accessed from the lateral posterior leg.

Trigger Point Referral

Common TrP locations: In the muscle belly on the posterior fibular surface, deep in the distal posterior leg.
Referral pattern: Plantar surface of the great toe and the first metatarsal head region.
Clinical significance: Referral under the first metatarsal head and great toe mimics sesamoiditis or hallux rigidus. If great toe pain occurs during push-off without joint stiffness on examination, check FHL — the referral pattern localizes precisely to the area the client blames on the toe joint.

Trigger point referral diagram — coming soon

No external TrP reference link available for flexor hallucis longus.

Clinical Notes

Common conditions:

FHL tenosynovitis ("dancer's tendinitis") — the tendon becomes inflamed in its fibro-osseous tunnel behind the talus. Common in ballet dancers (repetitive relevé and demi-pointe) and runners. Presents as pain behind the medial ankle, worsened by great toe push-off. The client may report triggering or catching of the great toe ("trigger toe").
Relevant to tarsal tunnel syndrome — FHL tendon swelling in the tarsal tunnel contributes to tibial nerve compression.
Contributes to hallux valgus (bunions) mechanics — FHL bowstringing increases the valgus force on the great toe when the MTP joint is already deviated.

What you'll typically find:

In ballet dancers, FHL is one of the most commonly overloaded structures. Pain behind the medial malleolus with great toe flexion is FHL tenosynovitis until proven otherwise.
In runners, FHL overload presents as deep posterior ankle or plantar great toe pain during the push-off phase. Often coexists with Achilles tendinopathy.
The "trigger toe" phenomenon — the great toe catches or locks during flexion, then releases with a snap. This is analogous to trigger finger and indicates stenosing tenosynovitis of the FHL sheath.

Treatment effects:

Gentle cross-fiber friction on the tendon behind the medial malleolus can reduce tenosynovitis symptoms. Avoid aggressive friction if the tendon is acutely inflamed.
The muscle belly on the posterior fibula is accessed through the posterolateral calf — release soleus and gastrocnemius first. Deep longitudinal stripping along the fibular surface is effective.
Post-treatment, great toe mobility exercises (passive extension and active flexion through full range) help maintain tendon glide through the fibro-osseous tunnel.

Cautions:

The posterior tibial artery and tibial nerve lie between FDL and FHL behind the medial malleolus. Precise contact on the tendon is essential — broad pressure compresses the neurovascular bundle.
In acute trigger toe presentations, forced manipulation of the locked toe can damage the tendon sheath. Gently work the tissue around the restriction; if the toe remains locked, refer for orthopedic assessment.

Clinical pearl:

FHL and FDL cross each other at the "knot of Henry" on the plantar foot, with fibrous connections between the two tendons. A tight FHL can restrict FDL movement (and vice versa) through this interconnection. When treating either muscle, check the other — they are functionally linked at the midfoot.

Assessment

Resisted great toe flexion (IP joint):

Client supine. Stabilize the proximal phalanx of the great toe. Ask the client to flex the distal phalanx against your resistance. Pain behind the medial ankle or weakness implicates FHL.

Passive great toe extension:

With the ankle dorsiflexed, passively extend the great toe at the IP and MTP joints. Limited range or pain behind the medial malleolus suggests FHL tightness or tenosynovitis.

Muscle Groups

Deep posterior compartment (anatomical):

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

Foot invertors (functional):

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

Tibial nerve group (innervation):

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

Related Muscles

Deep posterior compartment neighbors:

Tibialis posterior — most anterior, deepest, arch support
Flexor digitorum longus — intermediate, flexes lateral toes, crosses FHL at the knot of Henry

Antagonist (great toe extension):

Extensor hallucis longus — extends the great toe and assists dorsiflexion

Key Takeaways

FHL is the powerhouse of great toe push-off — its tenosynovitis ("dancer's tendinitis") is the most common overuse injury in ballet dancers.
"Trigger toe" (catching/locking of the great toe) indicates stenosing tenosynovitis of the FHL sheath.
The tendon is the most posterior of "Tom, Dick, and Harry" behind the medial malleolus — isolate with great toe flexion.