Fibularis tertius muscle

At a glance

English

Fibularis tertius muscle

Latin (TA2)

Musculus fibularis tertius

System

Muscle

Region

Lower limb

Side

Paired, left and right

About the fibularis tertius muscle

Fibularis tertius, also called peroneus tertius, sits in the front compartment of the leg, at the lower outer edge of extensor digitorum longus, with which it is often partly fused. Despite its name, it does not belong to the lateral compartment that holds fibularis longus and brevis. Terminologia Anatomica prefers the word fibularis to peroneus; both refer to the fibula.

It arises from the lower third or so of the front of the fibula, from the interosseous membrane beside it, and from the septum between it and fibularis brevis. Its tendon descends under both extensor retinacula in the same sheath as the long toe extensor, so it can easily be taken for a fifth tendon of that muscle. Instead of going to a toe, it ends on the upper surface of the fifth metatarsal, at the inner part of the shaft near its base.

The muscle helps dorsiflex the ankle and evert the foot, lifting the outer border of the foot. In walking it is thought to raise the lateral side of the foot during the swing phase, helping the toes clear the ground, and to set the foot level before heel strike. It is useful rather than essential, and it is absent in a fair share of people, the proportion differing between populations. Because it is seldom found in other primates, it has been linked with efficient upright walking.

Like the other muscles of the front of the leg, it receives the deep fibular nerve, not the superficial nerve that serves the lateral compartment. It can be strained or, rarely, torn when the ankle is forced into extreme inversion and plantarflexion, and its tendon can be seen with ultrasound. In horses a muscle of the same name is a largely tendinous cord linking the movements of hock and stifle.

Attachments, action and supply

Origin

Lower third of the anterior surface of the fibula, the adjoining interosseous membrane and the septum next to fibularis brevis.

Insertion

Dorsal surface of the fifth metatarsal, on the medial part of the shaft near its base.

Action

Dorsiflexes the ankle and everts the foot; lifts the lateral border of the foot in the swing phase of walking.

Nerve supply

Deep fibular nerve (L5, S1).

Blood supply

Anterior tibial artery, with the perforating branch of the fibular artery.