What is the Fibularis Tertius?
The fibularis tertius is a small muscle in the lower leg that helps control your foot and ankle. It sits on the front outer side of your lower leg and its tendon runs down to attach on the top of the foot, near the base of the little-toe side. When it contracts, it helps lift the front of your foot upward at the ankle and it also helps turn the sole of your foot slightly outward. Those actions matter most when you walk, run, change direction, or land from a jump, because they help position the foot so it clears the ground during the swing phase and stays stable when it contacts the floor.