The flexor hallucis longus is the deep calf muscle responsible for curling your big toe downward, but its influence extends far beyond that single motion. Running from the back of your lower leg bone (the fibula) down through a bony tunnel behind the ankle and along the sole of the foot to the tip of the big toe, this muscle and its long tendon play a surprisingly central role in how you walk, balance, and push off the ground. It is also one of the most commonly injured structures in ballet dancers, a reliable surgical backup when the Achilles tendon fails, and an anatomical quirk that can flex toes it is not even supposed to control.
Where It Sits and What It Does
The flexor hallucis longus (FHL) originates on the back surface of the fibula and the membrane between the two lower-leg bones, deep beneath the more prominent calf muscles. Its tendon travels behind the ankle through a groove in the talus bone, passes under a bony shelf called the sustentaculum tali on the heel bone, and then runs along the sole of the foot to insert on the base of the big toe’s last bone (the distal phalanx). That is a long route for a single tendon, and it crosses through two tight fibro-osseous tunnels along the way. These tunnels keep the tendon in place but also create friction points where problems tend to develop.
The FHL’s primary job is plantar flexion of the big toe, the motion you feel when you curl your toes into the ground. But cadaver studies show the muscle does much more than wiggle one toe. When tension was applied to the FHL tendon alone in cadaver specimens, the entire bony arch of the foot stiffened, the subtalar joint inverted, the longitudinal arch rose, and the forefoot slightly plantar-flexed and turned inward.1Journal of Dance Medicine & Science. The Role of the Flexor Hallucis Longus and Peroneus Longus in the Stabilization of the Ballet Foot In other words, pulling on this one tendon converted a flexible foot into a rigid lever, exactly what you need during the push-off phase of walking or running. The muscle also strengthens and elevates the medial longitudinal arch, helping protect against flat-foot collapse.2PubMed Central. Muscular Systems and Their Influence on Foot Arches and Toes Alignment—Towards the Proper Diagnosis and Treatment of Hallux Valgus
Its Role in Walking and Running
During normal walking, the FHL does not behave the way you might expect for a muscle named after toe flexion. Electromyography studies show it fires during the loading phase of gait, right after the heel strikes the ground, acting as a dynamic stabilizer of the rearfoot and ankle before the big toe even touches down.3PubMed. Comparison of EMG signal of the flexor hallucis longus recorded using surface and intramuscular electrodes during walking It then contributes to propulsion during the latter part of stance, when the foot is pushing off. Biomechanical modeling suggests the muscle can generate the forces needed for this push-off phase while working nearly isometrically, meaning the muscle-tendon junction barely moves even as it produces substantial force.4PubMed. Evidence of isometric function of the flexor hallucis longus muscle in normal gait
Speed matters. As walking pace increases, the peak force under the big toe rises more steeply than force under any other region of the foot.5Journal of Biomechanics. EMG and force production of the flexor hallucis longus muscle in isometric plantarflexion and the push-off phase of walking That disproportionate increase reflects the FHL’s growing importance at higher speeds. Runners, sprinters, and anyone who spends time pushing off forcefully through the big toe is loading this muscle hard.
The Knot of Henry and Why It Matters
One of the more unusual features of the FHL is its anatomical relationship with another tendon in the sole of the foot, the flexor digitorum longus (FDL), which flexes the lesser toes. The two tendons cross each other at a fibrous junction called the knot of Henry, and in most people they share a tendinous connection there. A large anatomical study of 164 cadaveric feet found three main types of interconnection, with the most common pattern present in about 85% of specimens.6The FASEB Journal. Anatomical Variation of Tendinous Interconnection between Flexor Hallucis Longus and Flexor Digitorum Longus Tendon and Its Location regarding Master Knot of Henry In roughly 6% of specimens, the FHL tendon itself split into two branches, one going to the big toe and the other merging with the FDL.
The practical consequence: in over two-thirds of the cadaver specimens studied, pulling on the FHL tendon alone produced flexion of all the toes, not just the big toe.7PubMed. Clinical significance of variations in the interconnections between flexor digitorum longus and flexor hallucis longus in the region of the knot of Henry That interconnection has real surgical implications. When a surgeon needs to cut or transfer one of these tendons, the specific pattern of interconnection in that patient determines which tendon to transect and where, making this a decision that has to be confirmed during the operation rather than assumed from textbook diagrams.
What Goes Wrong With the FHL
FHL problems can show up as pain almost anywhere from the back of the ankle to the tip of the big toe, which makes diagnosis tricky. In a clinical study of patients with confirmed FHL pathology, pain was located at the back-inside of the ankle in 40 patients, the plantar heel in 23, the plantar midfoot in 22, and at multiple locations in 16.8PubMed. Tenosynovitis of the flexor hallucis longus: a clinical study of the spectrum of presentation and treatment All patients had tenderness when the tendon was palpated directly, and about a third showed restricted FHL excursion on clinical testing.
The most common conditions affecting the FHL include:
- Tenosynovitis: Inflammation of the sheath surrounding the tendon, usually at the tunnel behind the ankle. Repetitive forced pointing of the foot (plantar flexion) is the classic trigger.
- Stenosing tenosynovitis: A more advanced form where the tendon sheath thickens enough to catch or lock. This can produce “trigger toe” (hallux saltans), where the big toe snaps during flexion and extension. Ultrasound can reveal the focal thickening and demonstrate the snapping in real time.9PubMed. Hallux saltans due to stenosing tenosynovitis of flexor hallucis longus: dynamic sonography and arthroscopic findings
- Os trigonum syndrome: An extra bone (the os trigonum) at the back of the ankle can pinch the FHL tendon during forced plantar flexion.10Clinics in Sports Medicine. Posterior Ankle Impingement and Flexor Hallucis Longus Pathology The anatomy of the FHL itself plays a role: people whose muscle belly extends lower than average into the ankle tunnel are more likely to develop symptoms from an os trigonum.11PubMed Central. The Effect of Flexor Hallucis Longus Morphology on Os Trigonum Syndrome
- Functional hallux limitus: A condition in which big-toe dorsiflexion is restricted during weight-bearing. Cadaveric work has shown that adhesion of the FHL tendon behind the ankle can increase joint pressures at the big toe’s base joint, and releasing that constraint normalizes those pressures.12PubMed. Impact of Flexor Hallucis Longus Retrotalar Pulley Release on First Ray Joint Pressures in Functional Hallux Limitus: A Cadaveric Study
Dancers and Runners at Particular Risk
Ballet dancers load the FHL in ways almost no other population does. Going en pointe or demi-pointe demands extreme plantar flexion of the ankle and powerful toe flexion simultaneously, forcing the tendon through its bony tunnels under high load, thousands of times per rehearsal. Ultrasound comparisons found that dancers clinically diagnosed with FHL tendinopathy had significantly thicker tendons than both healthy dancers and non-dancers. Interestingly, healthy dancers’ tendons were not significantly different in thickness from those of non-dancers, suggesting the thickening is a sign of injury rather than a normal adaptation to dance training.13PubMed Central. Flexor hallucis longus tendon morphology in dancers clinically diagnosed with tendinopathy
Runners face a different pattern. In amateur marathon runners with FHL tendon injury, plantar pressure measurements showed a shift of force from the medial (big-toe) side of the foot to the lateral (little-toe) side. Specifically, the contact area under the first metatarsal dropped, load-bearing under the second metatarsal decreased, and the contact area under the fifth metatarsal increased.14PubMed Central. Injury of flexor halluics longus tendon in amateur marathon runners results in abnormal plantar pressure distribution: observational study That lateral shift may explain why some runners with vague medial foot pain eventually develop problems on the outside of the foot or lower leg as well, as their gait compensates for the weakened medial push-off.
How FHL Problems Are Diagnosed
The most useful bedside test is the FHL stretch test. You dorsiflex the ankle (pull the foot up) while simultaneously trying to extend the big toe upward. A healthy FHL allows at least 30 degrees of big-toe extension at maximum ankle dorsiflexion. Less than 30 degrees suggests a tight or shortened FHL.15Journal of Korean Physical Therapy. The Reliability of Flexor Hallucis Longus Stretch Test in Subjects with Asymmetric Hallux Valgus Angles This test works because the ankle and big toe share the same tendon. Dorsiflexing the ankle tensions the tendon from one end, so any tightness or scar tissue along its route limits how much range of motion remains for the toe at the other end.
For deeper assessment, MRI can reveal the specific cause of FHL entrapment, whether that is an enlarged os trigonum, a calcaneal fracture, or soft-tissue scarring around the tendon.16American Journal of Roentgenology (AJR). MR imaging findings of entrapment of the flexor hallucis longus tendon Ultrasound is particularly good at demonstrating triggering and tenosynovitis in real time, since the examiner can watch the tendon snap through a thickened sheath as the patient moves the toe.17PubMed. Hallux saltans due to stenosing tenosynovitis of flexor hallucis longus: dynamic sonography and arthroscopic findings
Conservative Treatment
Most FHL problems are managed without surgery, at least initially. The cornerstone is a specific stretching protocol targeting the FHL, often combined with activity modification and, in more severe cases, temporary immobilization. In one cohort focused on nonoperative management, tenderness along the tendon, most commonly at the fibro-osseous tunnel behind the ankle, was the key diagnostic finding. Following a dedicated FHL stretching program, 44% of patients improved enough to decide surgery was unnecessary.18PubMed Central. What Is the Efficacy of a Nonoperative Program Including a Specific Stretching Protocol for Flexor Hallucis Longus Tendonitis?
A systematic review of stenosing tenosynovitis treatment found that specific FHL stretching exercises were successful in about 64% of cases. Injection of the FHL tendon sheath with a local anesthetic to inflate and break up adhesions had a lower success rate of about 33%.19Journal of ISAKOS. Lower extremity Good results for treatment of flexor hallucis longus (stenosing) tenosynovitis: a systematic review For a pre-professional ballet dancer with chronic recurrent stenosing tenosynovitis, a multimodal conservative program including soft tissue mobilization, laser therapy, kinesiology tape, and rehabilitative exercise over four months produced an 8-point drop in pain rating and a 15-point improvement on a functional scale.20PubMed Central. Conservative management of a chronic recurrent flexor hallucis longus stenosing tenosynovitis in a pre-professional ballet dancer: a case report
The stretch itself is straightforward: with the ankle held in dorsiflexion, you gently extend the big toe upward and hold for 20 to 30 seconds. The goal is to restore full excursion of the tendon through its tunnels. When performed consistently over weeks, this can reduce the mechanical catching and pain that come from a tight or inflamed tendon sheath.
When Surgery Is Needed
If conservative treatment fails, surgical release of the FHL tendon sheath (tenolysis) with removal of inflamed tissue (tenosynovectomy) is the usual procedure. In a series of 63 ankles operated on in dancers, roughly two-thirds had triggering before surgery and none had it afterward. Pain scores dropped from an average of 7.0 preoperatively to 1.2 postoperatively, and 97% of dancers returned to dance without symptoms. Nearly all considered the operation a success and would recommend it to others.21The Journal of Foot and Ankle Surgery. Flexor Hallucis Longus Tenolysis and Tenosynovectomy in Dancers
Endoscopic approaches to FHL release have been attempted but carry meaningful risks. In one cadaveric evaluation of endoscopic FHL decompression, three of eight tendons were damaged during the release, and no sheath was completely freed down to the sustentaculum.22PubMed. Endoscopic flexor hallucis longus decompression: a cadaver study The tight quarters behind the ankle make this a technically demanding procedure. The neurovascular bundle, including the tibial nerve and posterior tibial artery, runs very close to the FHL tendon. MRI measurements have placed the average distance between the FHL and the tibial nerve at only about 2 mm, and the distance to the posterior tibial artery at roughly 5 mm.23PubMed. A magnetic resonance imaging safe distance beyond the flexor hallucis longus tendon and the application of the safe arthroscopic approach to prevent neurovascular injury in posterior ankle arthroscopy Those tight margins help explain why the FHL tendon is used as a landmark during posterior ankle arthroscopy: surgeons stay lateral to it to avoid nerve and artery injury.
The FHL as an Achilles Tendon Replacement
One of the most interesting clinical uses of the FHL has nothing to do with the big toe. When the Achilles tendon ruptures and is not repaired promptly, the gap between the torn ends can become too large to bridge directly. Surgeons have long used the FHL tendon as a transfer graft to reconstruct these chronic Achilles defects, and the results are consistently good.
The FHL is an ideal candidate for several reasons. It lies right next to the Achilles tendon behind the ankle, so rerouting it requires minimal surgical dissection. Its muscle belly provides a blood supply to the graft site. And it fires in the same phase of gait as the calf muscles, so the brain does not need to learn a new activation pattern. A study of 28 patients with chronic Achilles ruptures and tendon gaps averaging about 7 cm followed for a mean of five years found that none of the transferred tendons re-ruptured. Functional scores improved substantially, from about 61 preoperatively to roughly 90 at final follow-up. All patients returned to their pre-injury daily activities.24PubMed Central. Long-term outcome of flexor hallucis longus tendon transfer for chronic Achilles tendon rupture with large defect: A retrospective series Despite some weakening of hallux flexion, no patient reported a noticeable functional problem in daily life.
Similar results appear across multiple studies. In another series, ankle-hindfoot scores improved from roughly 42 preoperatively to about 90 postoperatively, and no patient had significant functional deficit or deformity of the big toe after the FHL was harvested.25PubMed. Flexor hallucis longus transfer for chronic Achilles tendonosis The trade-off is a persistent strength deficit. Dynamometer testing at six years after surgery showed an average 24% reduction in ankle plantar-flexion strength and roughly 40% reduction in hallux plantar-flexion strength compared with the non-operated side.26PubMed. The functional and dynamometer-tested results of transtendinous flexor hallucis longus transfer for neglected ruptures of the Achilles tendon at six years’ follow-up That sounds dramatic, but in practice patients rarely notice the big-toe weakness because the interconnections at the knot of Henry and the intrinsic foot muscles partially compensate. The ankle strength loss is more relevant for athletes, but most patients with chronic Achilles ruptures are not returning to competitive sport.
The FHL and Bunions
The relationship between the FHL and hallux valgus (bunions) is a newer area of investigation. In bunion deformity, the big toe drifts laterally, and it turns out the FHL tendon follows it. Cadaveric measurements found that in feet with hallux valgus, the FHL tendon’s long axis was displaced further laterally compared with non-bunion feet, both at the big-toe joint and further up toward the ankle. The tendon’s insertion on the distal phalanx was also shifted about 6% more laterally.27PubMed Central. Anatomy of the Flexor Hallucis Longus and Its Relationship With Hallux Valgus: A Cadaveric Study This lateral displacement may mean the FHL actually contributes to maintaining or worsening the bunion deformity rather than correcting it, since its line of pull shifts from a neutral toe-flexing direction to one that also tugs the toe sideways. Whether surgeons should address the FHL’s position during bunion correction is still an open question, but the anatomical evidence suggests ignoring it might leave a deforming force in place.