Peroneus Longus: Origin, Insertion, Nerve, Action & The "Stirrup"

The Peroneus Longus (also known as Fibularis Longus) is the larger and more superficial muscle of the lateral leg compartment. It is famous for its long tendon that travels all the way under the foot to the opposite side, acting as a sling for the arches of the foot.

[Image of Peroneus Longus muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Head and proximal 2/3 of the lateral surface of the Fibula.
Insertion (Distal)	Base of the 1st Metatarsal and the Medial Cuneiform (Plantar surface).
Nerve Supply	Superficial Peroneal (Fibular) Nerve (L5, S1).
Blood Supply	Peroneal (Fibular) Artery.
Primary Actions	Eversion: Turns the sole of the foot outwards. Plantarflexion: Assists in pointing the foot down. Support: Supports the Transverse Arch of the foot.

Deep Dive: The Anatomical "Stirrup"

The Peroneus Longus works in a unique partnership with the Tibialis Anterior to stabilize the foot.

1. The Long Journey

The muscle originates on the lateral side of the leg, but it inserts on the medial side of the foot. Its tendon travels behind the lateral malleolus, crosses the sole of the foot obliquely, and attaches to the base of the big toe (1st metatarsal).

2. The Stirrup Effect

The Tibialis Anterior comes down the medial side to attach to the 1st Metatarsal. The Peroneus Longus comes from the lateral side to attach to the 1st Metatarsal. Together, they form a "stirrup" under the foot, balancing inversion and eversion and maintaining the arches.

Physio Corner: Clinical Relevance

💪 Functional Fact: During the "Push-Off" phase of walking, the Peroneus Longus pulls down on the 1st Metatarsal. This keeps the big toe firmly on the ground, providing a rigid lever for propulsion. If this muscle is weak, the foot may supinate (roll out) excessively.

Palpation

Palpate the head of the fibula just below the lateral knee. The muscle belly is found immediately distal to this head. Ask the patient to evert their foot. You can trace the tendon running behind the lateral malleolus (it is the superficial tendon; the Brevis lies deep/anterior to it).

⚠️ Clinical Pathology: Os Peroneum
The Os Peroneum is a small sesamoid bone found within the Peroneus Longus tendon near the cuboid bone (outer foot) in about 20% of people. It can become inflamed (Painful Os Peroneum Syndrome) or fractured, causing lateral foot pain that mimics a sprain.

Manual Muscle Testing (MMT)

We test eversion with plantarflexion to engage the Peroneus Longus fully.

Testing Tip: To differentiate from the Extensor Digitorum Longus (which also everts), ensure the foot is Plantarflexed.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Supine or side-lying. Ankle in plantarflexion. Action: Patient everts the foot (turns sole out) while keeping toes pointed down. Resistance: Applied to the lateral border of the foot, pushing inward and upward (into Inversion and Dorsiflexion). Grade 3: Full range against gravity. Grade 4/5: Holds against resistance.
Grade 2 (Gravity Eliminated)	Position: Supine, heel resting on table. Action: Patient slides the forefoot laterally into eversion. Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate the tendon behind the lateral malleolus or muscle belly at proximal fibula. Cue: "Turn your foot out." Grade 1: Tendon tension felt. Grade 0: No activity.

Frequently Asked Questions

Why is it called "Longus"?

Because its tendon is much longer than the Peroneus Brevis. It traverses the entire width of the sole of the foot to reach the medial side.

Does it help with flat feet?

Yes. By pulling the 1st Metatarsal and Cuneiform down towards the Fibula, it increases the curvature of the Transverse Arch, helping to maintain the foot's structure.

What is the difference between Peroneus Longus and Brevis?

Longus: Originates higher, inserts on 1st metatarsal (medial side), supports arch.
Brevis: Originates lower, inserts on 5th metatarsal (lateral side), stabilizes lateral column.

Test Your Knowledge: Peroneus Longus Quiz

1. Where does the Peroneus Longus insert?

2. Which nerve supplies the Peroneus Longus?

3. What is the primary action of Peroneus Longus?

4. The Peroneus Longus forms a "stirrup" with which muscle?

5. Where does the Peroneus Longus originate?

6. The Os Peroneum is a ________ bone found in the tendon.

7. During gait, the Peroneus Longus helps to:

8. Which compartment of the leg contains the Peroneus Longus?

9. The tendon of Peroneus Longus lies ________ to the Peroneus Brevis tendon at the ankle.

10. True or False: Peroneus Longus crosses the sole of the foot.

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• Brukner, P., & Khan, K. (2017). Clinical Sports Medicine. 5th ed. McGraw-Hill Education.