Fibularis tertius muscle

The fibularis tertius (a.k.a. the peroneus tertius) is a small, variable muscle located in the anterior compartment of the leg. It is often regarded as the lateral and distal continuation of the extensor digitorum longus (EDL).

Although absent in approximately 5–10% of individuals, when present, it assists in dorsiflexion and eversion of the foot.

Origin

• Distal one-third of the anteromedial surface of the fibula (immediately below the origin of extensor digitorum longus)

• Adjacent part of the interosseous membrane

• Anterior intermuscular septum

The fibularis tertius typically arises as a separate slip from the most lateral portion of the extensor digitorum longus.

Insertion

The tendon of fibularis tertius muscle passes beneath the superior and inferior extensor retinacula, lying lateral to the tendons of extensor digitorum longus, before inserting onto the dorsum of the base of the fifth metatarsal bone (little toe).

Innervation

Deep fibular (peroneal) nerve; Root value: L5–S1

Blood Supply

Anterior tibial artery and its branches

Actions

• Dorsiflexes the foot at the ankle joint

• Everts the foot at the subtalar and midtarsal joints

• Provides dynamic stabilization to the lateral side of the foot during walking and running