Fibularis tertius muscle

Origin and insertion

Fibularis tertius originates from three locations:

• As a direct continuation from the attachment of extensor digitorum longus muscle from the distal third of medial surface of fibula.
• Anterior surface of interosseous membrane, which connects the tibia to the fibula. It separates the anterior from the deep posterior muscles of the leg. 
• Anterior intermuscular septum, which separates the anterior and lateral compartments of the leg. 

The muscle fibers course in an inferolateral direction, forming a tendon that travels underneath the superior extensor retinaculum. Then, it continues towards the inferior extensor retinaculum and courses through its loop, accompanied by the extensor digitorum longus muscle. Fibularis tertius finally inserts onto the dorsal surface (medial part) of base of metatarsal bone 5.

Relations

Fibularis tertius is located anterior to the fibularis brevis muscle and the anterior lateral malleolar artery. It also travels laterally to extensor digitorum longus, with their tendons sharing a common sheath.

Throughout its course, fibularis tertius passes laterally to the distal third of the tibia, posterolaterally to the talocalcaneonavicular joint and laterally to the tarsometatarsal joint of the foot.

Innervation

Fibularis tertius receives innervation via the deep fibular nerve, a branch of the sciatic nerve. It originates from the L5 and S1 spinal nerves.

Blood supply

Fibularis tertius receives arterial blood from several sources:

• Anterior tibial artery, via the anterior lateral malleolar artery
• Dorsalis pedis artery and its branches, such as lateral tarsal artery, arcuate artery, metatarsal arteries, fourth dorsal metatarsal artery and digital arteries
• Posterior tibial artery, via the lateral plantar artery

Function

Due to its poor mechanical leverage, fibularis tertius can produce only two weak movements: 

• Foot dorsiflexion around the talocrural (ankle) joint, with the help of extensor digitorum longus and tibialis anterior muscles.
• Foot eversion at the subtalar joint with the aid of fibularis longus and fibularis brevis muscles.

Through these actions, fibularis tertius has a role in the gait cycle, acting with other foot dorsiflexors during the swing phase to clear the foot and toes off the ground. In addition, it has supporting functions for the ankle joint, preventing excessive inversion of the foot during physical activities. However, it is difficult to assess the true importance of fibularis tertius because its actions can be completed efficiently by other, more powerful muscles. In fact, fibularis tertius is absent in many individuals who don’t accuse any particular movement impairment.

Learn more about the fibularis tertius and other leg muscles with our articles, video tutorials, quizzes and atlas illustrations.