Fibularis Quartus
Leg & Ankle
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The fibularis quartus, also called the peroneus quartus, is an extra muscle that not everyone has. Studies have reported it in roughly 10% or more of examined ankles, and it is considered a human-associated variant that may relate to the mechanical demands of bipedal walking.
Function
Primarily assists with eversion of the foot and stabilization of the lateral ankle. Depending on its insertion it may also assist slightly with plantarflexion and provide additional support to the subtalar joint and the fibularis tendons.
Origin
Variable. Most commonly arises from the fibularis brevis, particularly its distal muscular portion. It may also arise from the distal fibula or, less commonly, the fibularis longus.
Insertion
Highly variable. Common insertions include the lateral calcaneus, particularly around the fibular trochlea/retrotrochlear region, the base of the fifth metatarsal, or occasionally the cuboid or neighbouring fibularis tendons.
Innervation
Superficial fibular nerve, matching the lateral compartment of the leg, primarily L5–S1.