[2] Together with the fibularis longus and the tibialis posterior, it extends the foot downward away from the body at the ankle (plantar flexion).It opposes the tibialis anterior and the fibularis tertius, which pull the foot upward toward the body (dorsiflexion).Fibularis brevis split tears are not uncommon a source of lateral ankle pain.In medical terminology, the word refers to being of or relating to the fibula or to the outer portion of the leg.This article incorporates text in the public domain from page 487 of the 20th edition of Gray's Anatomy (1918)