Fibularis longus (peroneus longus): the lateral leg muscle whose tendon crosses the sole

Also called

Peroneus longus

What it is

A long lateral leg muscle whose tendon crosses the sole of the foot.

Location

Origin: head and upper two-thirds of the lateral fibula. Insertion: base of the 1st metatarsal and medial cuneiform.

Function

Everts the foot, weakly plantarflexes the ankle and supports the transverse arch.

Body system

Muscular · paired (left and right)

FMA ID

FMA22552 (Foundational Model of Anatomy)

Key facts

• Innervation: superficial fibular nerve (L5–S1)
• Blood supply: fibular and anterior tibial arteries

Fibularis longus is the current anatomical name; peroneus longus is the older name, still common in clinics, radiology reports and textbooks. OpenStax gives both: the lateral compartment of the leg includes two muscles, fibularis longus (peroneus longus) and fibularis brevis (peroneus brevis). (OpenStax Anatomy and Physiology 2e, 11.6: Appendicular Muscles of the Pelvic Girdle and Lower Limbs.)

The lateral compartment is supplied by the superficial fibular (peroneal) nerve, and its two muscles share one main job: eversion, turning the sole of the foot outward. (TeachMeAnatomy: Muscles in the Lateral Compartment of the Leg.)

Fibularis longus arises from the head of the fibula, the upper half to two-thirds of the lateral surface of the fibular shaft, and the anterior and posterior intermuscular septa of the leg, the sheets of fascia that wall off the lateral compartment. (University of Washington Musculoskeletal Atlas: Peroneus longus.)

Kenhub adds that some fibres occasionally arise from the lateral condyle of the tibia. In the leg the muscle lies superficially, covering fibularis brevis. (Kenhub: Fibularis longus muscle.)

The fibres from the fibular head and those from the neck leave a small gap between them. The common fibular nerve passes through this gap and then divides into its deep and superficial branches, so fibularis longus is a useful landmark for finding the nerve. (TeachMeAnatomy: Muscles in the Lateral Compartment of the Leg.)

The tendon descends behind the lateral malleolus together with the fibularis brevis tendon. The two share a common synovial sheath in a groove held down by the superior fibular (peroneal) retinaculum. (Kenhub: Fibularis longus muscle.)

Behind the malleolus both tendons lie in the retromalleolar groove. The longus tendon sits slightly lateral to and behind the brevis tendon, covering about two-thirds of it, and their shared sheath extends from about 4 cm above to about 1 cm below the malleolus. (Bokwa-Dąbrowska K et al. Peroneus brevis split tear: a pictorial review, Part 1. Eur J Radiol Open. 2025.)

On the lateral surface of the calcaneus the tendons separate around the peroneal (fibular) tubercle: brevis runs above it, longus below. The inferior peroneal retinaculum holds both tendons against the calcaneus and sends a septum to the crest of the tubercle. (Palmanovich E et al. Peroneus longus tear and its relation to the peroneal tubercle: a review of the literature. Muscles Ligaments Tendons J. 2012.)

The tendon then turns under the foot at the cuboid. It runs in a groove on the cuboid that the long plantar ligament converts into a tunnel, and it is thickened where it turns, often containing a small sesamoid bone. From there it crosses the sole obliquely, from lateral to medial. (Kenhub: Fibularis longus muscle.)

The tendon inserts on the plantar and lateral aspect of the medial cuneiform and the lateral side of the base of the first metatarsal. (University of Washington Musculoskeletal Atlas: Peroneus longus.)

This is the same pair of bones that tibialis anterior reaches from the medial side, on the medial and plantar surfaces of the medial cuneiform and the base of the first metatarsal. Tibialis anterior dorsiflexes and inverts the foot, so the two muscles pull on one spot from opposite sides. (University of Washington Musculoskeletal Atlas: Tibialis anterior.)

Rarely, slips of the tendon also reach the base of the second metatarsal. (Kenhub: Fibularis longus muscle.)

Fibularis longus everts the foot, working with fibularis brevis, and weakly plantarflexes the ankle. It also supports the lateral longitudinal and transverse arches of the foot. Its antagonist is tibialis anterior. (Radiopaedia: Peroneus longus muscle.)

Kenhub splits the actions by joint: plantar flexion at the ankle (talocrural) joint and eversion at the subtalar joint, because the talocrural joint itself only allows flexion and extension. (Kenhub: Fibularis longus muscle.)

Because its tendon pulls on the base of the first metatarsal from below, fibularis longus also plantarflexes the first ray. A review of fibularis longus tears notes that loss of first-ray plantar flexion suggests the muscle is not working. Together with fibularis brevis it also stabilises the lateral side of the ankle during mid-stance and heel rise. (Palmanovich E et al. Peroneus longus tear and its relation to the peroneal tubercle: a review of the literature. Muscles Ligaments Tendons J. 2012.)

From the anatomical position only a few degrees of eversion are possible. In practice the lateral compartment fixes the medial edge of the foot during running and prevents the foot from rolling into too much inversion. (TeachMeAnatomy: Muscles in the Lateral Compartment of the Leg.)

The superficial fibular (peroneal) nerve supplies fibularis longus. The University of Washington atlas gives its roots as L5, S1 and S2 and notes that the muscle may also receive branches from the common or deep fibular nerves. Its arterial supply comes from the anterior tibial and fibular (peroneal) arteries. (University of Washington Musculoskeletal Atlas: Peroneus longus.)

Radiopaedia gives the roots as L5 and S1 and names the fibular artery as the arterial supply. Venous blood drains through the venae comitantes and the small (short) saphenous vein. (Radiopaedia: Peroneus longus muscle.)

Fibularis brevis arises lower, from the lower two-thirds of the lateral fibula and the intermuscular septa, and inserts on the tuberosity (styloid process) at the base of the fifth metatarsal, on the outer edge of the foot. Like longus it everts the foot and plantarflexes the ankle and is supplied by the superficial fibular nerve. (University of Washington Musculoskeletal Atlas: Peroneus brevis.)

The main difference is where the tendons end. Brevis stops on the lateral border of the foot, while the longus tendon dives under the cuboid and crosses the whole sole to the medial side. In the leg longus is the more superficial of the two; behind the malleolus its tendon lies outside brevis, which therefore sits against the bone. (Radiopaedia: Peroneus longus muscle.)

The shorter lateral leg muscle that inserts on the fifth metatarsal. Fibularis brevis.

Known variants of fibularis longus include a double (duplex) muscle belly, fusion with fibularis brevis, and an os peroneum, a sesamoid bone that forms in the fibrocartilage of the tendon where it presses against the cuboid. (Radiopaedia: Peroneus longus muscle.)

The os peroneum is present in about 20% of people and can predispose to stenosing tenosynovitis inside the cuboid tunnel. The peroneal tubercle is present in most calcanei (90.4% in one study of 114 bones) and is enlarged in roughly 20 to 24% in older studies; an accessory peroneus quartus muscle was found in 21.7% of 124 legs in one series. (Palmanovich E et al. Peroneus longus tear and its relation to the peroneal tubercle: a review of the literature. Muscles Ligaments Tendons J. 2012.)

Reported rates of peroneus quartus differ between studies: a 2025 imaging review quotes about 10%. Such accessory muscles crowd the tunnel behind the malleolus and are discussed as one possible contributor to fibular tendon problems. (Bokwa-Dąbrowska K et al. Peroneus brevis split tear: a pictorial review, Part 1. Eur J Radiol Open. 2025.)

Peroneal tenosynovitis is fluid and inflammation in the shared tendon sheath, causing localised swelling and tenderness behind the lateral malleolus. Kenhub lists causes including stress behind the malleolus, inversion sprains, fractures of the lateral malleolus or calcaneus, and tears of the calcaneofibular ligament. (Kenhub: Fibularis longus muscle.)

If the superior peroneal retinaculum is stripped from the fibula, the tendons can dislocate over the lateral malleolus. This typically follows sudden dorsiflexion with a forceful contraction of the fibular muscles and causes bruising, swelling and pain along the malleolus. (Kenhub: Fibularis longus muscle.)

Tears of the longus tendon happen most often where it turns around the cuboid notch, a high-stress zone. Patients usually describe gradual pain, swelling and warmth along the lateral ankle, recurrent giving way and weakness. A cavovarus (high-arched) foot and an enlarged peroneal tubercle both increase friction on the tendon. (Palmanovich E et al. Peroneus longus tear and its relation to the peroneal tubercle: a review of the literature. Muscles Ligaments Tendons J. 2012.)

Radiopaedia lists painful os peroneum syndrome (POPS) among the related conditions, along with tendinopathy, tenosynovitis and tendon instability. On MRI the tendon can also look abnormal because of the magic angle artefact where it curves, an imaging effect rather than real damage. (Radiopaedia: Peroneus longus muscle.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Anterior tibial artery · cardiovascular
• Tibialis anterior · muscular
• Fibula · skeletal
• Tibialis posterior · muscular
• Interosseous membrane of leg · skeletal
• Tibia · skeletal
• Posterior tibial artery · cardiovascular
• Posterior tibial vein · cardiovascular
• Small saphenous vein · cardiovascular
• Extensor digitorum longus · muscular