Muscle · Lower leg

Peroneus longus

Musculus fibularis longus

A long muscle running down the outside of the lower leg whose tendon does something unusual: it hooks behind the ankle bone on the outside, then dives under the sole of the foot and crosses all the way to the inside edge, anchoring near the base of the big toe. That path makes it two things at once. It tips the foot outward, which is the body's main defence against rolling the ankle. And because it slings under the foot like a stirrup, it presses the first toe down and helps hold the arch across the width of the foot. Ankle sprains and unstable ankles almost always involve this muscle.