The fibularis longus is a long, rope-like muscle on the outside of your lower leg that helps control your ankle and support your foot. You may also see it called the peroneus longus. It begins on the upper portion of the fibula, the thinner bone on the outer side of the lower leg, and runs downward behind the bony bump on the outside of your ankle. Its tendon then travels under the foot and attaches near the base of the first metatarsal and the medial cuneiform, close to the big toe side.
Its main actions are eversion and plantarflexion. Eversion means it helps roll the sole slightly outward, which matters for stability when you land, change direction, or walk on uneven ground. Plantarflexion means it assists when you point your toes or push off the floor during running and jumping. A key job that many people overlook is arch support. The fibularis longus helps lift and stiffen the midfoot, spreading force across the foot and reducing the tendency for the arch to collapse under load.
In training, a well functioning fibularis longus supports solid squats, lunges, deadlifts, sprinting, and jumping by keeping the foot stable and helping the knee track smoothly. If it is weak, overworked, or irritated, you might feel outer ankle or outer foot discomfort, notice frequent ankle sprains, or see your foot roll inward when you lift or run. Strengthening it usually involves slow calf raises with even pressure through the big toe and little toe, band resisted foot eversion, single leg balance drills, and gradual increases in impact work like hops. Strong, stable feet also make it easier to brace and transfer force, which helps your whole body training feel more efficient. Build it patiently, and keep shoes and running volume appropriate for your current tolerance.