Fibularis longus !

The fibularis longus is a muscle on the outer side of the leg; it steadies the ankle, turns the foot outward and supports the arch.

Official name: Fibularis longus muscle

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What it is

A muscle on the outer side of the leg.

What it does

Turns the foot outward and steadies the ankle.

Key facts

Origin: Head and upper two-thirds of lateral surface of fibula (some texts: also lateral condyle of tibia)
Insertion: Plantar base of first metatarsal and medial cuneiform (tendon crosses the sole in cuboid groove)
Action: Everts foot; weak plantar flexion; supports transverse arch
Nerve: Superficial fibular nerve (L5–S2)
Blood supply: Anterior tibial and fibular arteries

Relations

Course: From the upper lateral fibula it runs down the lateral leg, behind the lateral malleolus under the superior fibular retinaculum, below the fibular trochlea of the calcaneus, through the groove of the cuboid and across the sole
Deep: Fibula and fibularis brevis, which lies closer to the bone behind the malleolus; at the fibular neck, the common fibular nerve between muscle and bone
In front: Anterior intermuscular septum, then extensor digitorum longus in the anterior compartment
Behind: Posterior intermuscular septum, then soleus and flexor hallucis longus
Below: In the sole it runs in the deepest (fourth) layer, in a tunnel roofed by the long plantar ligament

How to find it

With the foot pushed outward against resistance, the belly firms on the upper lateral leg below the fibular head, and its tendon is felt behind the lateral malleolus, more superficial than that of fibularis brevis.

Examination

Resisted eversion: weakness suggests a superficial or common fibular nerve lesion or an L5–S1 root problem
Fibular tendon subluxation test: resisted eversion and dorsiflexion of the ankle; a tendon snapping forward over the malleolus suggests a torn superior fibular retinaculum

On imaging

Ankle and foot radiographs may show an os peroneum beside the cuboid; a fractured one, or one displaced up the leg, suggests a tendon rupture. Ultrasound (with dynamic views) and MRI show tears, tenosynovitis and tendon subluxation behind the lateral malleolus, where the longus lies behind and superficial to the brevis. MRI also shows split tears of the brevis next to it.

Clinical notes

Common fibular nerve palsy at the fibular neck (fracture, tight cast, prolonged leg crossing, knee dislocation): foot drop with weak eversion and numbness over the dorsum of the foot
Fibular tendon dislocation after a forced dorsiflexion injury, as in skiing or football, with painful snapping behind the lateral malleolus
Painful os peroneum syndrome: lateral foot pain from a fractured sesamoid or a tear of the tendon around it

Anatomical variants

It always reaches the base of the first metatarsal; most also attach to the medial cuneiform, and extra slips to other metatarsal bases are common
An os peroneum is found in a minority of feet

Development

The fibular muscles come from the dorsal muscle mass, like the anterior compartment, and are supplied by the superficial branch of the same common fibular nerve.

Test yourself

Think of the answer, then tap the question to see it.

What is the course of the fibularis longus tendon in the foot?

Behind the lateral malleolus, round the cuboid groove and across the sole to the first metatarsal base and medial cuneiform, forming a stirrup with the tibialis anterior.

Which nerve supplies the fibularis longus, and what is its main action?

The superficial fibular nerve (L5–S2); it everts the foot and supports the transverse arch.

Which nerve winds round the fibular neck deep to the upper fibularis longus fibres, and what does its palsy cause?

The common fibular nerve; palsy causes foot drop with weak eversion and numbness of the dorsum of the foot.

What does a tendon snapping forward over the lateral malleolus on resisted eversion and dorsiflexion suggest?

Fibular tendon dislocation from a torn superior fibular retinaculum, typically after a forced dorsiflexion injury.

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): musculus fibularis longus TA2 2652
Official Spanish (DeCS): Músculo fibular largo
English (TA2): Fibularis longus muscle
Also called: musculus peroneus longus · peroneus longus muscle

Sources: Terminologia Anatomica (TA2); Anatomy, Bony Pelvis and Lower Limb: Calf Peroneus Longus Muscle.

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Representative model, for educational purposes only.

Fibularis longus highlighted on the Durgical 3D model