Is the Peroneal Tendon a Flexor or Extensor?

The peroneal tendons do not fit neatly into the flexor-or-extensor binary that applies to many other muscles. Their primary action is eversion, meaning they roll the sole of the foot outward, and they serve as the main lateral stabilizers of the ankle. That places them in their own functional category rather than cleanly in the flexor or extensor camp. But the full picture is more interesting, because different peroneal tendons have secondary actions that pull in opposite directions along the sagittal plane, and one member of the peroneal family genuinely qualifies as an extensor.

Why the Flexor-Extensor Question Doesn’t Quite Apply

When people ask whether a muscle is a “flexor” or “extensor,” they usually mean whether it bends a joint or straightens it. At the ankle, flexion and extension translate to plantarflexion (pointing the toes down) and dorsiflexion (pulling the toes up). But the peroneal tendons sit on the lateral side of the leg, running behind and below the outer ankle bone. Their line of pull is aimed sideways more than it is aimed forward or backward. The result is that their dominant action is eversion of the foot, and they are classified as the primary evertors, not as plantarflexors or dorsiflexors.¹

That said, muscles rarely do just one thing. A tendon’s action depends on where it crosses the joint’s axis of rotation, and the peroneal tendons cross more than one axis. So while eversion is the headline action, each individual peroneal tendon also contributes a secondary movement in the sagittal plane. And those secondary contributions differ depending on which tendon you are talking about.

Peroneus Longus and Its Plantarflexion Role

The peroneus longus is the larger and more superficial of the two main peroneal tendons. It originates on the upper part of the fibula, runs behind the lateral ankle, then takes a distinctive turn underneath the foot to attach on the plantar surface of the first metatarsal and the medial cuneiform bone.² That deep, plantar insertion is the key to its secondary action. Because the tendon crosses below the ankle joint’s transverse axis, it assists with plantarflexion, the same motion you use when pressing a gas pedal or rising onto your toes.

Research looking at the neuromuscular compartments within the peroneus longus confirms this dual role. When scientists measured muscle activity during maximum-effort contractions, they found that the portions of the muscle closer to the front of the leg were significantly more active during plantarflexion, while the muscle as a whole activated more uniformly during eversion.³ In other words, different regions of the same muscle are recruited differently depending on whether the foot needs to push down or roll outward. Eversion is the whole muscle’s job; plantarflexion is a secondary contribution driven by specific compartments.

So if you absolutely had to pick a sagittal-plane category for the peroneus longus, it falls on the flexor (plantarflexor) side. But calling it a flexor would badly misrepresent what it does. It is an evertor that also contributes to plantarflexion, not a plantarflexor that happens to evert.

Peroneus Brevis and Lateral Stability

The peroneus brevis sits deeper than the longus and takes a shorter path. Instead of diving under the foot, it inserts on the base of the fifth metatarsal, the bony bump you can feel on the outside of your foot. This more lateral and less plantar insertion means the brevis has a weaker plantarflexion moment arm than the longus. Its real strength is in eversion and in controlling rotation of the leg over the planted foot.

Moment-arm measurements confirm this distinction. The peroneus longus has the largest eversion moment arm of any muscle crossing the ankle, averaging about 31 millimeters, while the peroneus brevis has the largest moment arm for internal and external rotation, averaging roughly 20.5 millimeters.⁴ That rotational control matters because when your foot is planted and your body is turning or shifting weight, the brevis acts as a brake that keeps the lower leg from spinning too far over the foot.

Gait studies reinforce this picture. During walking, the peroneus longus fires after heel contact to evert the foot, then switches to stabilizing the forefoot once the heel rises. The peroneus brevis, meanwhile, fires later in the stance phase, where its primary role seems to be restraining lateral rotation of the leg.⁵ Neither tendon is doing classic flexion-extension work during walking. Both are working in the frontal and transverse planes, keeping the ankle from collapsing inward or letting the leg rotate out of control.

The Peroneus Tertius Exception

There is one member of the peroneal family that genuinely qualifies as an extensor, and it is the odd one out in almost every way. The peroneus tertius (also called fibularis tertius) is a small, variable muscle that sits in the anterior compartment of the leg rather than the lateral compartment where the longus and brevis live. Its tendon inserts on the dorsal surface of the fifth metatarsal, and because it crosses in front of the ankle joint’s axis, it dorsiflexes the foot. It also everts, giving it one thing in common with its lateral-compartment relatives.⁶

The peroneus tertius is anatomically and functionally more like an extension of the extensor digitorum longus (the muscle that lifts the toes) than it is like the peroneus longus or brevis. It shares the anterior compartment, it dorsiflexes, and it is innervated by the deep peroneal nerve rather than the superficial peroneal nerve that supplies the longus and brevis. The “peroneus” in its name comes from the fibula (the older anatomical name for the fibula is “perone”), not from a shared function with the other peroneals.

Not everyone has one, either. Anatomical studies show considerable variation in its presence and size across populations, and when it is absent, the anterior compartment muscles cover its function without obvious problems. Its existence is thought to be connected to the demands of upright walking on two legs, but it is far from a universal human feature.

The Naming Confusion

Part of the confusion around the peroneal tendons stems from inconsistent naming. The older anatomical terminology calls these muscles peroneus longus, peroneus brevis, and peroneus tertius, after the Greek word for fibula. The more current international anatomical nomenclature prefers “fibularis” (longus, brevis, tertius), derived from the Latin word for the same bone. Both naming systems are still in active clinical use, which means you will see “peroneal” and “fibularis” used interchangeably in medical literature, imaging reports, and physical therapy notes.

The name switch has no functional significance, but it can compound the classification confusion. Someone reading about the “fibularis tertius” might not immediately realize it is the same muscle as the “peroneus tertius,” or might assume all three fibularis muscles belong to the same compartment and do the same thing. They do not. The tertius lives in a completely different compartment and pulls the foot in the opposite sagittal direction from the longus.

What the Peroneus Longus Does to the First Ray

Beyond its role at the ankle, the peroneus longus has an important effect on the architecture of the foot itself. Because its tendon crosses the sole of the foot to attach on the first metatarsal and first cuneiform, it actively pulls the first ray (the big-toe column) downward toward the ground. This plantarflexion of the first ray is what locks the medial arch of the foot into a rigid lever during push-off in walking and running.²

When the peroneus longus is overactive or the foot’s structure allows too much first-ray plantarflexion, the result can be a cavus (high-arched) foot. In the opposite direction, when the peroneus longus is weak or disrupted, the first ray fails to stabilize properly during gait, potentially contributing to forefoot instability and altered weight distribution across the ball of the foot. This is one reason why peroneal tendon injuries can produce symptoms that seem unrelated to the lateral ankle, including pain or dysfunction under the big toe.

Peroneal Tendons and Ankle Instability

Because the peroneals are the primary lateral stabilizers of the ankle, problems with these tendons often show up as chronic ankle instability. People who repeatedly roll their ankles inward are not just stretching ligaments; they are also challenging the peroneal tendons’ ability to fire quickly enough to prevent the ankle from giving way.

Studies of people with chronic ankle instability have found that a delayed reaction time in the peroneus brevis is one of the strongest predictors of ongoing problems. In one analysis, peroneus brevis reaction time and balance-test performance together accounted for about a third of the variability in ankle instability scores.⁷ This makes intuitive sense: the brevis is ideally positioned to counter an inversion force, and if it fires too slowly, the ankle tips before the muscle can catch it.

This is worth knowing because rehabilitation for chronic ankle instability often includes peroneal strengthening exercises, which are essentially eversion exercises against resistance. If you think of the peroneals as flexors or extensors, you might train the wrong movement pattern. They need to be trained as evertors, with exercises like resisted ankle eversion using a band, standing on one leg on an unstable surface, or controlled lateral stepping drills that challenge the peroneals’ ability to stabilize the ankle in real time.

How the Peroneals Behave During Walking

During a normal walking stride, the peroneal muscles follow a specific activation pattern that illustrates why the flexor-extensor label misses the point. Early in the stance phase, as the foot hits the ground and weight shifts onto it, peroneal activity is relatively low compared to other muscles. The tibialis anterior, which dorsiflexes the ankle and inverts the foot, is doing most of the work at that stage to control the foot’s descent to the ground.⁸

As the stride progresses, peroneal activity ramps up. The peroneus longus activates to evert the foot after heel contact, helping the foot pronate enough to absorb shock. Then, once the heel lifts off the ground and the body’s weight shifts forward over the forefoot, the longus transitions to its role as a first-ray stabilizer. It locks the big-toe side of the foot down so the foot can act as a rigid lever for push-off.⁵ At no point in this cycle are the peroneals doing the kind of straightforward plantarflexion or dorsiflexion that would justify calling them flexors or extensors.

In people with flat feet, the timing of this activation pattern shifts. Peroneal activity early in stance tends to be lower than in people with normal arches, which may reflect the different demands placed on lateral stabilizers when the foot is already in a pronated position at ground contact.⁸

Anatomical Variations That Complicate the Picture

The peroneal tendon group is more variable than most people realize. Beyond the standard longus, brevis, and occasional tertius, some individuals have a fourth peroneal muscle called the peroneus (or fibularis) quartus. This accessory muscle, when present, can insert on the cuboid bone or the calcaneus, and its clinical importance is that it occupies space in the already tight tunnel behind the lateral ankle. That crowding can cause lateral ankle pain, tenosynovitis (inflammation of the tendon sheath), or even subluxation of the other peroneal tendons out of their normal groove.⁹

These variant muscles matter for anyone undergoing imaging or surgery for lateral ankle problems. An unexpected extra tendon behind the ankle can be misidentified as a torn or split peroneus brevis on MRI. Surgeons encountering it during a procedure need to decide whether to leave it in place or remove it, depending on whether it is contributing to the patient’s symptoms. The functional role of the quartus, when present, seems to be a minor contribution to eversion, keeping it in the same functional family as the longus and brevis rather than adding any meaningful flexion or extension capacity.

When the Peroneus Longus Insertion Varies

Even the peroneus longus itself does not always insert exactly where the textbooks say. Cadaver studies have documented variations in how the tendon fans out across the sole of the foot, with some specimens showing a broader attachment that reaches beyond the first metatarsal and cuneiform to adjacent structures.² These variations can influence how effectively the tendon plantarflexes the first ray and how it distributes force across the forefoot.

For most people, this variation is clinically invisible. But in someone with a foot deformity or a peroneal tendon injury requiring surgical repair, the exact insertion pattern determines where the tendon needs to be reattached and how much first-ray stabilization can be restored. A surgeon expecting the classic textbook insertion and finding something different intraoperatively may need to adjust the reconstruction plan on the spot. This is one of the practical reasons why the peroneus longus gets so much attention in foot and ankle surgery relative to its modest role as a secondary plantarflexor.