Flexor Hallucis Longus Tendinopathy: Behind-Ankle Pain in Runners and Dancers

Medically reviewed by Dr. Tom Biernacki, DPM — Board-Certified Podiatric Surgeon — Balance Foot & Ankle, Howell & Bloomfield Hills, MI. Last updated April 2026.

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What Is the Flexor Hallucis Longus Tendon?

The flexor hallucis longus (FHL) is a deep calf muscle whose tendon runs behind the ankle in a fibro-osseous tunnel behind the medial malleolus, under the plantar surface of the foot, and inserts into the base of the distal phalanx of the big toe. It is the primary toe flexor for the hallux and provides significant plantarflexion power during push-off — making it arguably the most important single tendon for athletic performance involving running, jumping, and dynamic forefoot loading. FHL tendinopathy causes pain behind the ankle and along the inner arch that significantly limits these activities.

At Balance Foot & Ankle, we diagnose and treat FHL tendinopathy in athletes throughout Southeast Michigan, with particular experience in runners and dance athletes in whom this condition is most common.

Why Ballet Dancers Are Particularly Vulnerable

The FHL tendon is often called the Achilles tendon of the ballet dancer because of its critical role in en pointe and demi-pointe work and the frequency with which it is injured in this population. Repeated extreme plantarflexion loads the FHL tendon against a stenosing zone (a narrowing) where it passes between the medial and lateral tubercles of the posterior talus. In dancers who perform sustained en pointe work, the tendon eventually develops tendinosis (degenerative change) and may trigger — intermittently catching and releasing as it passes through the fibro-osseous tunnel, producing a painful snap.

FHL Tendinopathy in Runners

Distance runners develop FHL tendinopathy from the cumulative loading of push-off during high mileage training. The condition is more common in runners who increase mileage rapidly, run significant hills, or have a rigid forefoot that forces the FHL to work harder for toe-off. Pain behind the medial ankle that increases with high mileage and improves with rest is the typical presentation. The pain is often mistaken for Achilles tendinopathy or posterior tibial tendinopathy because of its posterior ankle location.

Diagnosis: Clinical and Imaging

Pain and tenderness posterior to the medial malleolus along the FHL tendon course, worsening with resisted big toe plantarflexion, is the hallmark clinical finding. The FHL stress test — passive maximum dorsiflexion of the ankle while the big toe is held in dorsiflexion to tension the FHL — reproduces posterior ankle pain. Triggering of the big toe (intermittent locking of the hallux in flexion or extension) strongly suggests FHL tenosynovitis. MRI evaluates tendon degeneration, sheath fluid, and excludes posterior ankle osseous pathology. Ultrasound allows dynamic assessment of tendon gliding and tendon caliber in real time.

Treatment

Conservative treatment with activity modification to reduce extreme plantarflexion, anti-inflammatory medication, and physical therapy for eccentric FHL strengthening and calf stretching is effective for most cases of FHL tendinopathy. A corticosteroid injection into the FHL sheath reduces acute tenosynovitis and can break a pain cycle that prevents rehabilitation. Custom orthotics with a rigid carbon fiber plate under the hallux reduce FHL demand during push-off for runners.

Surgical release of the FHL fibro-osseous tunnel (endoscopic or open) is effective for refractory cases with true tendon triggering or significant stenosing tenosynovitis. The procedure provides immediate relief from triggering and allows rapid return to activity. Contact Balance Foot & Ankle for evaluation of posterior ankle or big toe tendon pain. We serve Southeast Michigan with same-week appointments.

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Frequently Asked Questions

What is FHL tendinopathy?

FHL tendinopathy is inflammation or degeneration of the flexor hallucis longus tendon behind the medial ankle. It causes pain with big toe push-off and is common in dancers and runners.

Why does my big toe lock when I point my foot?

Triggering or locking of the big toe indicates stenosing tenosynovitis of the FHL tendon — the tendon catches as it passes through its tunnel, often requiring surgical release.

How is FHL tendinopathy treated?

Treatment includes activity modification, targeted physical therapy, soft tissue mobilization, orthotic support, and occasionally injections. Surgery is reserved for triggering or severe chronic cases.

FHL Tendon Treatment in Michigan

Flexor hallucis longus tendinopathy causes behind-ankle pain in runners and dancers. Our podiatrists diagnose and treat FHL conditions at our Howell and Bloomfield Hills offices.

Learn About Tendon Treatment | Book Your Appointment | Call (810) 206-1402

Clinical References

1. Hamilton WG, et al. Stenosing tenosynovitis of the flexor hallucis longus tendon and posterior impingement upon the os trigonum in ballet dancers. Foot Ankle. 1982;3(2):74-80.
2. Michelson J, Dunn L. Tenosynovitis of the flexor hallucis longus: a clinical study of the spectrum of presentation and treatment. Foot Ankle Int. 2005;26(4):291-303.
3. Coull R, et al. Flexor hallucis longus tendinopathy: review of the literature. Foot Ankle Surg. 2016;22(4):224-232.

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Dr. Tom Biernacki, DPM

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Dr. Tom Biernacki, DPM

Dr. Tom Biernacki, DPM is a double board-certified podiatrist and foot & ankle surgeon at Balance Foot & Ankle Specialists in Southeast Michigan. With over a decade of clinical experience, he specializes in heel pain, bunions, diabetic foot care, sports injuries, and minimally invasive surgery. Dr. Biernacki is a member of the APMA and ACFAS, and his patient education content on MichiganFootDoctors.com and YouTube has reached over one million views.