Flexor Hallucis Longus Tendinopathy: Deep Posterior Ankle Pain in Athletes

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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Flexor hallucis longus (FHL) tendinopathy is a condition that disproportionately affects ballet dancers, gymnasts, and runners — athletes who perform repetitive plantarflexion and toe-off loading. Often misdiagnosed as Achilles tendinopathy, ankle sprain, or posterior ankle impingement, FHL problems produce a characteristic deep posteromedial ankle and big toe pain that responds to specific treatments quite different from other ankle conditions.

Anatomy of the Flexor Hallucis Longus

The FHL originates in the posterior compartment of the lower leg and runs through a fibro-osseous tunnel (the “master knot of Henry” distally, and a groove in the posterior talus proximally) behind the medial malleolus before inserting into the distal phalanx of the hallux. It is the primary plantarflexor of the big toe and an important contributor to propulsion and push-off strength.

The narrow posterior talar groove creates a mechanical compression point — particularly vulnerable to tenosynovitis (inflammation of the tendon sheath), triggering (triggering hallux — catching or locking of the hallux in flexion), and longitudinal tears when the tendon is repeatedly loaded against the tight fibro-osseous channel.

Causes and Mechanisms

FHL tendinopathy occurs in predictable athletic contexts:

• Ballet dancers: En pointe and demi-pointe work require extreme plantarflexion and push-off through the FHL — the highest incidence of FHL tendinopathy of any athletic group
• Runners: Repetitive push-off loading, particularly with flexible footwear, increases FHL demand
• Posterior ankle impingement: Os trigonum and Stieda process compress the FHL sheath during plantarflexion — FHL tenosynovitis is frequently part of the posterior impingement syndrome complex
• Hypertrophied FHL muscle belly: A low-lying accessory muscle belly within the narrow talar groove creates direct mechanical compression of the tendon

Symptoms

Classic presentation: deep posteromedial ankle pain reproduced with resisted big toe plantarflexion and passive big toe dorsiflexion. The “triggering hallux” — a palpable or audible snap/click of the FHL within its sheath, with intermittent locking of the hallux in flexion — is pathognomonic. Symptoms often localize to the posterior ankle (behind the medial malleolus) and may extend into the plantar surface of the hallux.

Distinguishing FHL from Achilles tendinopathy: FHL pain is posteromedial and reproduced by hallux motion; Achilles pain is posterior midline and reproduced by ankle plantarflexion without hallux movement.

Diagnosis

Clinical examination demonstrating posteromedial tenderness along the FHL course, triggering, and pain with resisted hallux plantarflexion is usually sufficient for diagnosis. Diagnostic ultrasound identifies tenosynovitis (fluid in the FHL sheath), hypoechoic areas within the tendon (tendinosis), and low-lying muscle belly impingement. MRI characterizes tenosynovitis extent, partial tears, and associated posterior ankle impingement.

Treatment

Conservative: Activity modification (reducing en pointe work or push-off loading), anti-inflammatory treatment (NSAIDs, ultrasound-guided FHL sheath injection), physical therapy focusing on eccentric FHL loading, and footwear modification to reduce FHL demand.

Surgical: When conservative management fails (typically after 3–6 months), FHL release — opening the fibro-osseous tunnel to decompress the tendon and resect any associated os trigonum or Stieda process — is highly effective. Dancers typically return to full performance within 3–4 months.

Flexor Hallucis Longus Tendinopathy — Posterior Ankle Pain

FHL tendinopathy causes pain behind the inner ankle and under the big toe, especially in dancers and runners. Our podiatrists diagnose this often-missed condition with clinical examination and MRI, then provide targeted treatment from immobilization and therapy to surgical release.

View All Tendon Treatment Options | Book Your Appointment | Call (810) 206-1402

Clinical References

1. Hamilton WG. 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 International. 2005;26(4):291-303.
3. Kolettis GJ, et al. Flexor hallucis longus tendinitis and rupture: a review. Foot and Ankle Clinics. 2005;10(3):427-440.

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When to See a Podiatrist

If foot or ankle pain has been bothering you for more than a few weeks, home care alone may not be enough. Balance Foot & Ankle offers same-week appointments at our Howell and Bloomfield Hills clinics — no referral needed in most cases. Bring your current shoes and a short list of symptoms and we’ll build you a treatment plan in one visit.

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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.