The fibularis longus (also called the peroneus longus) is a muscle on the outer side of your lower leg that everts the foot, assists with pointing the toes downward, and plays a key role in supporting the arches of your foot. It runs from just below the knee all the way to the underside of the foot, giving it a uniquely long tendon that crosses the sole and influences both ankle movement and foot shape.
Where the Muscle Attaches
The fibularis longus originates from the head of the fibula (the thin bone on the outside of your lower leg) and the upper half of that bone’s outer surface. From there, it travels down the leg and becomes a long tendon that loops behind the outer ankle bone, then dives underneath the foot. It inserts on the underside of two bones near the base of your big toe: the medial cuneiform and the base of the first metatarsal.
That path is important. Because the tendon wraps under the foot from the outer edge to the inner edge, it acts like a sling. This positioning is what allows the muscle to pull the foot into eversion (tilting the sole outward) while also pressing down on the inner forefoot, supporting the arch from below.
Eversion and Plantarflexion
The fibularis longus is best known as a foot evertor, meaning it turns the sole of your foot outward. But its contribution is more specific than that. Electromyographic studies show the muscle is most active when you evert your foot while it’s pointed downward (plantarflexed). In a neutral ankle position, its shorter neighbor, the fibularis brevis, takes over as the primary evertor.
This distinction matters for activities like pushing off during a step or landing from a jump, when your ankle is naturally in a pointed position. The fibularis longus is also a secondary plantarflexor, helping the calf muscles push your foot downward, though it’s far less powerful than the main calf muscles in that role.
Arch Support
One of the fibularis longus’s most distinctive jobs is maintaining the arches of your foot. Its tendon crosses the sole diagonally, and when it pulls, it draws the inner and outer edges of the forefoot closer together. A cadaver study applying 150 newtons of force through the tendon found it reduced the width of the transverse arch (the side-to-side arch across the ball of your foot) by about 4.6 mm and compressed the medial longitudinal arch (the main arch along the inside of your foot) by about 6.8 mm.
The relationship was especially strong for the transverse arch. The angle at which the tendon crosses the sole correlated closely with how much it tightened that arch (Pearson r = .83), suggesting the fibularis longus is one of the primary dynamic stabilizers of the forefoot’s cross-arch shape. This is why weakness or injury to the muscle can contribute to forefoot splaying and altered weight distribution.
Balance and Ankle Stability
When you stand on one leg, your body constantly makes tiny corrections to keep you from tipping sideways. The fibularis longus is central to that process on the outer ankle. It fires reflexively to resist the foot rolling inward, which is exactly the motion that causes the most common type of ankle sprain.
Research on people with chronic ankle instability reveals how much this muscle matters. In healthy ankles, the nervous system adjusts the fibularis longus’s reflexive activity when shifting from lying down to standing on one foot. People with unstable ankles show significantly less of this reflex adjustment. One study found that about 33% of the variance in single-leg balance impairment could be explained by how well the fibularis longus reflex adapted to the standing position. In other words, when the muscle’s reflexes aren’t firing properly, balance suffers in a measurable way.
Its Role During Walking
During normal walking, the fibularis longus is most active during the stance phase, when your foot is on the ground and bearing weight. It stabilizes the ankle laterally as your body moves forward over the planted foot, then helps with push-off by pressing the first metatarsal into the ground.
In people with functional ankle instability, activity of the fibularis longus during stance drops substantially. One study measured it at roughly 23% of the stance phase on the injured side compared to about 38% on the healthy side. That reduced activation leaves the ankle less protected against the kind of inward rolling that causes lateral sprains, which helps explain why people who sprain an ankle once are so prone to doing it again.
Fibularis Longus vs. Fibularis Brevis
The fibularis brevis sits directly underneath the longus on the outer leg and attaches to the base of the fifth metatarsal (the outer edge of the foot) rather than crossing to the inner foot. Textbooks have traditionally called the longus the stronger evertor because it’s the larger muscle by volume. Clinical evidence tells a different story.
When the brevis tendon ruptures, patients commonly develop a noticeable inward tilt of the heel (hindfoot varus) and significant eversion weakness. A longus tendon rupture doesn’t produce the same degree of hindfoot instability. The likely explanation is leverage: the brevis has a more favorable moment arm for rotating the hindfoot, and it has a greater effect on external rotation of the midfoot bones. So while the longus handles arch support and eversion during plantarflexion, the brevis is arguably more critical for keeping the heel and hindfoot properly aligned.
Tendon Problems and Symptoms
Peroneal tendonitis, inflammation of one or both fibularis tendons, is common in runners, dancers, and people who repeatedly load the outer ankle. Symptoms include pain along the outer ankle that worsens with activity, swelling or warmth around the tendon, and sometimes a thickened or nodular feel along the tendon’s path. The pain typically flares when you push off, walk on uneven surfaces, or evert your foot against resistance.
Because the fibularis longus tendon takes such a long, winding route under the foot, it’s also vulnerable to tears at the point where it curves around the outer ankle bone. Chronic overuse or repeated ankle sprains can gradually damage the tendon, and the reduced reflex activity seen in unstable ankles may accelerate that process by changing how load is distributed.
Surgical Uses of the Tendon
Surgeons frequently harvest or reroute the fibularis longus tendon for reconstructive procedures, largely because the fibularis brevis can compensate for most of its eversion function. One of the most common applications is a peroneus longus-to-brevis transfer, where the longus tendon is attached to the brevis tendon to reinforce hindfoot eversion in people with cavovarus foot deformity (a high-arched foot that tilts inward). This procedure, often combined with bone realignment and ligament repair, has been shown to significantly improve patient-reported outcomes.
The tendon is also used in managing diabetic foot ulcers under the first metatarsal head. Because the fibularis longus presses the first metatarsal down, it can concentrate pressure on that spot. Lengthening or transferring the tendon reduces that downward force, and one approach combining this with Achilles tendon lengthening has achieved a 92% rate of successful ulcer healing.