Fibularis Tertius

Leg · Lower Limb

The Fibularis Tertius is found among the Leg muscles of the Lower Limb. It arises from Distal third of the anterior surface of the fibula and interosseous membrane and attaches to Dorsal surface of the base of the fifth metatarsal. Functionally, it dorsiflexes and everts the foot. It is innervated by the Deep fibular nerve (L5–S1).

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)

OIANS stands for Origin, Insertion, Action and Nerve, the four facts that uniquely define every skeletal muscle. To learn the Fibularis Tertius, work through them in order: picture its origin, trace the muscle to its insertion, reason out the action that shortening between those two points must create, then add the nerve that drives it.

Most students remember the Fibularis Tertius fastest by linking its action back to its attachments rather than memorising each fact in isolation. Once the origin and insertion make sense, the action usually follows logically.