Fibularis tertius muscle
Fibularis tertius muscle, also called peroneus tertius, is located on the lower lateral aspect of the leg. It is part of the anterior, or extensor, compartment of the leg, together with three additional muscles; extensor digitorum longus, extensor hallucis longus and tibialis anterior. Collectively, they are responsible for ankle dorsiflexion.
Fibularis tertius extends from the distal end of the fibula to the base of the fifth metatarsal bone. Therefore, it spans and acts on two main joints of the foot; the talocrural (ankle) and subtalar joints. Due to its relative small size and poor mechanical leverage, fibularis tertius is considered a weak dorsiflexor and evertor of the foot. In addition, the muscle is quite varied; it can be absent in some individuals, while in others it forms part of extensor digitorum longus, being described as the muscle’s ‘fifth tendon’.
|Origin||Medial surface (distal third) of fibula, interosseous membrane (anterior surface), anterior intermuscular septum|
|Insertion||Dorsal surface of base of metatarsal bone 5|
|Action||Talocrural joint: Foot dorsiflexion
Subtalar joint: Foot eversion
|Innervation||Deep fibular nerve (L5, S1)|
|Blood supply||Anterior lateral malleolar artery, lateral tarsal artery, metatarsal arteries, lateral plantar artery, digital arteries, arcuate artery, fourth dorsal metatarsal artery|
This article will describe the anatomy and functions of fibularis tertius.
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.
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.
Fibularis tertius receives innervation via the deep fibular nerve, a branch of the sciatic nerve. It originates from the L5 and S1 spinal nerves.
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
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.
While the movements of fibularis tertius cannot be tested in isolation, its tendon can sometimes be palpated. Firstly, the foot and toes should be dorsiflexed against resistance. The tendon can be felt laterally within the shallow depression located anterior to the lateral malleolus. A tear of the tendon, which can result during a violent inversion, will result in pain and swelling over this specific location.
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