Peroneus Tertius: Origin, Insertion, Nerve, Action & The "Fifth" Tendon

The Peroneus Tertius (also known as Fibularis Tertius) is a small muscle of the anterior leg. Despite its name ("Peroneus"), it is located in the Anterior Compartment, not the Lateral Compartment. It is often considered a separated part of the Extensor Digitorum Longus (EDL) and is unique to humans, likely evolving to support bipedal walking.

[Image of Peroneus Longus muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Distal third of the anterior surface of the Fibula and the Interosseous Membrane.
Insertion (Distal)	Dorsal surface of the base of the 5th Metatarsal bone.
Nerve Supply	Deep Peroneal (Fibular) Nerve (L5, S1).
Blood Supply	Anterior Tibial Artery.
Primary Actions	Dorsiflexion: Lifts the foot at the ankle. Eversion: Turns the sole of the foot outward.

Deep Dive: The "Third" Peroneal

The name implies it is the third muscle of the peroneal group, but functionally and anatomically, it belongs with the extensors.

1. The Fifth Tendon

The Extensor Digitorum Longus (EDL) sends four tendons to toes 2-5. The Peroneus Tertius essentially acts as a "5th Tendon" of the EDL group. However, instead of going to a toe, it stops short and attaches to the 5th Metatarsal base.

2. Compartment Confusion

Peroneus Longus & Brevis: Lateral Compartment (Superficial Peroneal Nerve, Plantarflexors).
Peroneus Tertius: Anterior Compartment (Deep Peroneal Nerve, Dorsiflexor).
Don't let the name fool you!

Physio Corner: Clinical Relevance

💪 Functional Fact: The Peroneus Tertius is a weak dorsiflexor and evertor individually. Its true value lies in proprioception and fine-tuning foot position during the swing phase of gait to prevent the toes from dragging and ensuring the foot lands flat.

Palpation

Ask the patient to Dorsiflex and Evert the foot. Look just lateral to the EDL tendons (specifically the tendon going to the little toe). If present, the Peroneus Tertius tendon will pop up near the sinus tarsi, inserting into the base of the 5th metatarsal.

⚠️ Clinical Pathology: Absent Muscle
The Peroneus Tertius is absent in approximately 5-10% of the population. Its absence rarely causes dysfunction because the EDL and Peroneus Brevis compensate easily. However, its tendon can sometimes be used for grafts in reconstructive surgery.

Manual Muscle Testing (MMT)

To isolate it, we combine Dorsiflexion with Eversion.

Testing Tip: This test distinguishes it from the Tibialis Anterior (Dorsiflexion + Inversion) and Peroneus Longus/Brevis (Plantarflexion + Eversion).

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Sitting or Supine. Action: Patient brings the foot Up and Out (Dorsiflexion + Eversion). Resistance: Applied to the lateral-dorsal aspect of the foot, pushing down and in (into Plantarflexion + Inversion). Grade 3: Full range against gravity. Grade 4/5: Holds against resistance.
Grade 0, 1 (Palpation)	Action: Palpate lateral to the EDL tendons near the ankle crease. Cue: "Lift your foot and turn it out." Grade 1: Tendon tension felt (if muscle is present). Grade 0: No activity.

Frequently Asked Questions

Is it an Everter or Dorsiflexor?

Both! It is a unique muscle that bridges the gap. Unlike the other Peroneals (which plantarflex), the Tertius assists in lifting the foot while turning it out.

What is its insertion point relative to Peroneus Brevis?

Both insert on the base of the 5th Metatarsal. However, the Peroneus Brevis inserts on the tuberosity (styloid process) laterally, while the Peroneus Tertius inserts slightly more dorsally and medially on the shaft base.

Can it cause pain?

Yes. While rare, strain of the Peroneus Tertius can cause anterolateral ankle pain. It can also be involved in shin splints (anterior compartment syndrome) along with the Tibialis Anterior.

Test Your Knowledge: Peroneus Tertius Quiz

1. Which nerve supplies the Peroneus Tertius?

2. Which compartment of the leg is it in?

3. What are the actions of Peroneus Tertius?

4. Where does the Peroneus Tertius insert?

5. The Peroneus Tertius is often considered a part of which muscle?

6. True or False: Peroneus Tertius is present in 100% of people.

7. Where does it originate?

8. Which retinaculum does the tendon pass under?

9. To MMT the Tertius, you apply resistance towards:

10. The Peroneus Tertius tendon lies ________ to the Extensor Digitorum Longus tendons.

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.