Fibularis longus
The muscle on the outside of the leg whose tendon crosses under the whole foot to attach on the other side.
- Type
- Muscle
- Region
- Leg
- Standard name
- m. peroneus longus
- Origin
- Head and upper lateral surface of the fibula
- Insertion
- Medial cuneiform and the base of the first metatarsal, crossing the sole of the foot
- Actions
- foot eversionankle plantarflexion
- Verify
- TA2 2652 TA98 A04.7.02.041
What it is
A long muscle on the outer side of the lower leg. Its tendon runs down behind the bony bump on the outside of the ankle, then does something no other tendon does: it turns under the foot and crosses the entire sole diagonally, from the outer edge to attach on the inner side, at the same bones tibialis anterior attaches to.
It is also called peroneus longus — perone is the Greek name for the fibula and fibula the Latin. Terminologia Anatomica prefers the Latin, so fibularis is the current standard term, but peroneus remains in wide clinical use and both are correct. This record uses the standard.
What it does
Everts the foot — turns the sole outward — and assists plantarflexion.
Because its tendon crosses beneath the foot, it also presses the first metatarsal down against the ground and helps support the transverse arch. Between it and tibialis posterior, whose tendon runs the opposite way, the two form a sling under the foot.
Its everting action is what resists the ankle rolling inward, which is the mechanism of the common ankle sprain.
What loads it
Eversion against resistance, and balancing on one foot on an unstable surface, where it works continuously to stop the ankle rolling.
The single-leg Romanian deadlift and the step-up load it continuously, because both put all your weight on one foot and something has to stop the ankle rolling.
What is commonly got wrong about it
The name. Peroneus and fibularis are the same muscle. Older textbooks, most clinicians and most search results say peroneus; the international standard says fibularis. Neither is wrong and the inconsistency confuses people constantly.
That ankle sprains are a ligament problem only. The ligaments on the outside of the ankle are what tear, but the fibularis muscles are the active defence that would have prevented the roll, and their reaction time and strength are part of the picture. That said, a sprained ankle is an injury, and injuries are outside what this site covers.
That it everts and therefore is on the "outside" of the foot. Its muscle is; its attachment is on the inner side, having crossed underneath.