Fibularis Tertius
Leg · Lower Limb
The fibularis tertius (peroneus tertius) is a small muscle of the anterior compartment of the leg, essentially a partially separated lateral part of the extensor digitorum longus. It arises from the distal third of the anterior surface of the fibula and the interosseous membrane, and its tendon crosses the front of the ankle to insert on the dorsal surface of the base of the fifth metatarsal. From this attachment it dorsiflexes and everts the foot, assisting the transfer of weight during walking. It is supplied by the deep fibular nerve (L5, S1). Although often small or absent, it is unusual in that it is an everter driven by the deep fibular nerve rather than the superficial fibular nerve.
Origin, Insertion, Action & Nerve
Origin
Distal third of the anterior surface of the fibula and interosseous membrane
Insertion
Dorsal surface of the base of the fifth metatarsal
Action
Dorsiflexes and everts the foot
Nerve
Deep fibular nerve (L5–S1)
Attachments explained
The Fibularis Tertius is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.
Origin
Distal third of the anterior surface of the fibula and interosseous membrane
Insertion
Dorsal surface of the base of the fifth metatarsal
Action & function
When the Fibularis Tertius contracts, it produces the following movement: Dorsiflexes and everts the foot.
As part of the Leg group of the Lower Limb, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.
Nerve supply & clinical relevance
The Fibularis Tertius receives its nerve supply from the Deep fibular nerve (L5–S1). This nerve carries fibres from spinal segment(s) L5, S1.
Because a muscle can only contract when its nerve is intact, injury to the Deep fibular nerve or to its spinal roots (L5, S1) can weaken or paralyse the Fibularis Tertius, impairing the movements it normally produces (dorsiflexes and everts the foot). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the Fibularis Tertius (the OIANS method)
Fix the fibularis tertius as the odd everter of the anterior compartment: unlike the fibularis longus and brevis, which evert and plantarflex from the lateral compartment, the tertius everts and dorsiflexes and is really a slip of the extensor digitorum longus. Tracing its tendon to the dorsal base of the fifth metatarsal shows why it combines dorsiflexion with eversion.
Anchor the key exam point that it is supplied by the deep fibular nerve (L5, S1), the anterior compartment nerve, not the superficial fibular nerve that serves the other fibularis muscles. Grouping it with the extensor digitorum longus by shared origin, insertion region, and nerve, while distinguishing its everting action, makes this small and variable muscle easy to place.
Frequently asked questions
Where is the Fibularis Tertius located?
The Fibularis Tertius is a muscle of the Leg group, located in the Lower Limb.
What is the origin of the Fibularis Tertius?
Distal third of the anterior surface of the fibula and interosseous membrane
What is the insertion of the Fibularis Tertius?
Dorsal surface of the base of the fifth metatarsal
What movements does the Fibularis Tertius produce?
Dorsiflexes and everts the foot
What nerve supplies the Fibularis Tertius?
Deep fibular nerve (L5–S1)
Is the Fibularis Tertius free to study in OIANS?
The Fibularis Tertius is always free to browse. Its full origin, insertion, action and nerve details are open to everyone in the Muscle Directory. Quiz and Flashcard practice for the Lower Limb is part of the one-time Lifetime upgrade, though; only the Upper Limb decks are free to test yourself on.
Related muscles
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