The flexor hallucis longus (FHL) is the muscle responsible for bending your big toe downward. It runs from the back of your lower leg all the way to the tip of your big toe, and it plays a surprisingly large role in walking, balance, and maintaining the arch of your foot. While its name sounds complex, its job is straightforward: it powers the push-off phase of every step you take.
Where It Sits and How It Connects
The FHL is one of four muscles in the deep back compartment of your lower leg, tucked behind your calf. It starts on the back surface of the fibula (the thinner bone on the outside of your shin) and sends a long tendon down through the ankle, under a bony shelf on your heel bone called the sustentaculum tali, and between two smaller muscles on the sole of your foot before attaching to the bottom of your big toe’s tip.
That’s a long path for a tendon, and it passes through a tight tunnel of bone and fibrous tissue along the way. This routing matters because it’s where problems tend to develop, especially in athletes who repeatedly point their feet. The muscle gets its nerve supply from the tibial nerve, primarily through the S1 and S2 spinal roots with some contribution from L5.
Its Primary Actions
The FHL flexes every joint of the big toe. That means it curls the big toe downward at both the joint where the toe meets the foot (the metatarsophalangeal joint) and the joint near the tip (the interphalangeal joint). When your foot is off the ground, this is easy to visualize: it’s the muscle that lets you curl your big toe.
But the FHL does more than move one toe. Because of where it crosses the ankle, it also helps point your foot downward (plantarflexion) and tilt the sole inward (inversion and supination). These secondary actions make it a contributor to overall ankle stability, not just toe movement.
Its Role in Walking and Push-Off
The FHL’s most important real-world job happens during walking. Research using fine-wire sensors placed directly in the muscle confirms that the FHL is mainly active during push-off, the phase of your stride where your foot propels your body forward. At a normal walking speed, the muscle fires most intensely between roughly 65% and 71% of the stance phase, right when your weight shifts forward over your big toe.
This timing makes sense. As you roll forward and prepare to lift your foot off the ground, the FHL grips the ground through your big toe and helps generate the force that pushes you into the next step. Without it, push-off becomes weaker and less efficient. At faster walking speeds, the muscle activates slightly earlier (around 60% of stance), reflecting the greater demand for propulsive force.
Supporting the Foot’s Arch
The FHL also functions as a dynamic bowstring for the medial longitudinal arch, the main arch running along the inside of your foot. As the tendon courses beneath the arch from heel to big toe, its tension during weight-bearing pulls the two ends of the arch closer together, preventing it from collapsing. This is different from the passive support provided by ligaments. The FHL provides active, adjustable support that responds to how much force your foot is absorbing at any given moment.
Why It Matters in Ballet and Sports
The FHL is sometimes called the “Achilles tendon of the foot” in dance medicine because of how heavily ballet dancers rely on it. Going up on pointe or demi-pointe demands extreme plantarflexion and big-toe control, placing enormous repetitive stress on the FHL tendon as it slides through its bony tunnel behind the ankle.
This repetitive stress can cause a condition called hallux saltans, or “trigger toe.” The tendon becomes inflamed, develops small partial tears, and forms nodules that thicken it. When the thickened tendon tries to glide through the narrow tunnel at the sustentaculum tali, it catches and then snaps through with an audible crack, similar to a trigger finger. Symptoms include pain and swelling behind the inner ankle bone, a locking sensation in the big toe, and painful clicking during movement. The condition is most common in ballet dancers, football players, and tennis players.
Tendon Transfer Surgery
One of the FHL’s most notable clinical applications has nothing to do with the big toe. Surgeons sometimes harvest the FHL tendon and use it to replace a damaged posterior tibial tendon, the tendon primarily responsible for supporting the foot’s arch. The FHL is chosen for this job because it closely matches the strength of the posterior tibial muscle and is stronger than other available donor tendons in the area.
In a study of 19 patients who received an FHL tendon transfer combined with a heel bone repositioning procedure for posterior tibial tendon dysfunction, hindfoot function scores improved from 62 out of 100 to nearly 84 out of 100 at an average follow-up of 18 months. Sixteen of 17 returning patients reported satisfaction with the outcome. The trade-off is some loss of big-toe flexion strength, though most people compensate well because the shorter flexor hallucis brevis muscle still provides partial toe flexion.
How Strength Is Tested
If your doctor or physical therapist wants to check your FHL specifically, the test is simple. You sit or lie down with your foot supported, and you’re asked to curl your big toe while the examiner applies light resistance under the tip of the toe. The examiner stabilizes the rest of your foot so only the FHL is doing the work. Full strength means you can hold the curled position against resistance. If you can curl the toe but not hold against any pressure, or if there’s no visible movement at all, that points to weakness or nerve involvement at the L5 through S2 levels. The tendon itself can be felt on the bottom of the big toe’s first segment during the contraction.