Peroneus tertius muscle

Last revised by Dr Daniel J Bell on 02 Aug 2021

The peroneus tertius muscle, also known as fibularis tertius muscle, is a muscle of the anterior compartment of the leg, despite its name suggesting it is in the lateral compartment. It aids in dorsiflexion and eversion of the foot.

  • origin: inferior 1/3rd of anterior surface of fibula and interosseous membrane below the extensor digitorum longus 
  • insertion: dorsal surface of the 5th metatarsal base and with a so-called falciform extension to the superior surface of that bone
  • innervation: deep peroneal nerve
  • action: dorsiflexion and foot eversion

Its tendon passes anterior to the lateral malleolus (unlike peroneus brevis and longus which pass posterior), deep to the inferior extensor retinaculum to become enclosed in a common tendon sheath with the tendons of the extensor digitorum longus muscle.

It may be absent or associated with other peroneus accessory muscles.

The prevalence of the peroneus tertius muscle, determined by cadaveric dissection, has been found to be 93.2% 4.

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Cases and figures

  • Figure 1: peroneal tendons
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  • Figure 2: Gray's anatomy illustration
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