Muscle guide · Lower legs
Also called FHL, big toe flexor
The flexor hallucis longus is a deep calf muscle that bends the big toe. It starts on the back of the fibula, the thin outer bone of the shin, and its long tendon runs behind the inner ankle bone and along the sole to the tip of the big toe.
It does its biggest job at push-off, pressing the big toe into the ground, and it helps hold up the inner arch. Dancers know it best: pointe work strains its tendon behind the ankle, and that pain, like pain under the heel, is easily mistaken for something else.
Its tendon runs behind the inner ankle bone as the rearmost structure in the tarsal tunnel, the passage that carries the main nerve and blood vessels into the sole. It slides in a groove at the back of the ankle, beside the spot where some people have a small extra bone (an os trigonum), then under a ledge of the heel bone, past the long flexor tendon of the other toes, and between the two halves of the short big toe flexor.
It is the largest flexor of the big toe. In a study using electrodes on the skin, its activity rose with the effort of pointing the foot and with walking speed, and the force under the big toe rose faster than under the rest of the foot as walking sped up. It also helps hold up the inner arch, though the tibialis posterior is the arch's main support.
In over-60s, stronger toe flexors went with better balance in every study a review found. Among 312 older people, those who fell over the following year had weaker big toes. Toe-strength training builds strength, but no trial has yet shown that it prevents falls.
Pointe and demi-pointe work hold the foot fully pointed while the big toe bends, which overloads this muscle. In one study, dancers relied more on their toe flexors than non-dancers to balance and to rise onto their toes.
The first two rest on one study of 17 young women with bunions, in which towel curls and calf raises drew the most flexor hallucis longus activity of nine foot exercises. Its electrodes sat on the skin over a deep muscle, so neighbouring muscles may have been picked up. The last two follow from the muscle's actions. Activity is not growth, but a 12-week progressive toe-strength programme did raise big toe strength by up to 36% in older people. The most direct move, pressing the big toe into the floor (the big toe press in our big toe arthritis guide), has no 3D demo yet.
Bending the big toe back while the ankle is bent up stretches it. A big toe that won't bend back in that position is the clinical test for a tight or stuck flexor hallucis longus. Ease off with a bunion or big toe arthritis, and stop for sharp pain behind the inner ankle.
Irritation of the tendon and its sheath behind the inner ankle is common in ballet dancers, and dancer's tendinitis is its everyday name. It causes pain behind the inner ankle, sometimes with creaking or a big toe that catches. Rest and physiotherapy come first, and in one series 64% of people treated without surgery did well.
When it doesn't settle, surgery to free the tendon got all 13 ballet dancers in one series back to dancing, within 5 months on average. It isn't only dancers: in a series of 81 people with it, only 27 were athletically active at all, and partial tears of the tendon were found in both dancers and non-dancers treated for it.
Some people have a small extra bone at the back of the ankle (an os trigonum), right beside this tendon. Repeated or sustained full pointing can pinch it, and in professional and pre-professional dancers with this problem, the tendon was also irritated in 63% to 85%. Treatment starts without surgery.
Source 7
In 81 people with irritation of this tendon, the pain was behind the inner ankle in 40, under the heel in 23 and under the middle of the foot in 22, and 45 had already had treatment that failed, usually for plantar fasciitis. A clue is pain or tightness when the big toe is bent back with the ankle bent up.
A thickening on the tendon can make the big toe catch or pop as it bends, and if it grows, the toe can end up stuck in a bent position.
One idea is that a tendon that can't glide freely overloads the big toe joint. That is a hypothesis from one clinical series and one cadaver study: 10 people with symptoms of a stiff big toe but no significant bone spurs improved when the tendon's gliding was restored, and in cadavers holding the tendon back raised the load on the joint.
Separately, big toe flexor strengthening was part of a programme that eased pain in 20 people with a stiff big toe.
When an old, retracted Achilles rupture can't be repaired directly, this tendon is the one most often moved to replace it, because it lies right beside the Achilles and pulls the same way. In 7 patients the muscle grew by about half afterwards; the trade-off can be a weaker big toe.
Each grade rates the evidence for the correction.
Irritation of the flexor hallucis longus tendon can cause pain under the heel or arch. In one series, more than half of the patients had already had treatment that failed, usually for plantar fasciitis.
Towel curls drew some of its highest activity in one study, but scans show that toe-grip strength comes mainly from the small muscles inside the foot. Toe curls work both, and no exercise has been shown to isolate it.
In one series of 81 people, two-thirds were not athletically active at all, and non-dancers made up nearly half of another series.
Weaker big toes went with falls in older people, toe strength tracks balance, and a progressive programme raised toe strength by up to 36%. Whether that prevents falls is still untested.
A 15-minute barefoot routine for stronger toes, arches, calves and ankles, finishing with balance. You need only a towel, a wall and a resistance band.
15 minBeginner11 movesA wall, Other / check instructionsPlantar fascia and calf stretches, a gentle foot roll and the towel heel raise: the exercises the 2023 heel pain guideline supports.
8 minBeginner7 movesA bench or box, A massage ballFoot, calf, balance and hop exercises taken from the running trials with positive results, in a 25-minute home routine for 3 or 4 days a week.
25 minBeginner11 movesOther / check instructions, A bench or boxIt bends the big toe at both of its joints, helps point the foot and helps hold up the inner arch. It is the largest flexor of the big toe, and it works harder the faster you walk and the harder you push off.
Deep in the calf on the outer side, starting from the back of the fibula. Its tendon runs behind the inner ankle bone, under a ledge of the heel bone and along the sole to the tip of the big toe.
The everyday name for irritation of the flexor hallucis longus tendon. Ballet dancers get it because pointe and demi-pointe work hold the foot fully pointed while the big toe bends. It causes pain behind the inner ankle, sometimes with creaking or a big toe that catches.
They can overlap. In 81 people with irritation of this tendon, 45 had already had treatment that failed, usually for plantar fasciitis, and the pain was behind the inner ankle in 40, under the heel in 23 and under the middle of the foot in 22. A clue is pain or tightness when the big toe is bent back with the ankle bent up.
Pull the big toe back towards the shin with the ankle bent up, as in the seated sole stretch. A big toe that won't bend back in that position is the clinical stretch test for this muscle, and stretching was part of the care that helped 64% of people treated without surgery in one series.
Towel toe curls and calf raises drew the most activity from it of nine foot exercises in one small study, though that shows activity, not strength gained. A 12-week progressive toe-strength programme raised big toe strength by up to 36% in older people.
Treatment usually starts with rest, physiotherapy and stretching, and 64% did well without surgery in one series. When it doesn't settle, freeing the tendon got all 13 ballet dancers in one series back to dancing, within 5 months on average.
It lies right beside the Achilles and pulls the same way, so it is the tendon most often moved when an old rupture can't be repaired directly. In one small study the muscle grew by about half afterwards; the trade-off can be a weaker big toe.
In older people, those who fell over the following year had weaker big toes, and toe strength tracks balance. Toe-strength training builds strength, though no trial has yet shown it prevents falls.
Anatomy and movement education, not a diagnosis or treatment plan. If you have pain that follows an injury, is severe or getting worse, or comes with numbness, weakness or swelling, get it assessed. Health disclaimer.
