Popliteus: Origin, Insertion, Nerve & The "Key to the Knee"

The Popliteus is a small, triangular muscle located deep in the posterior knee (popliteal fossa). Despite its small size relative to the hamstrings and calves, it plays a critical biomechanical role. It acts as the "key" that unlocks the knee joint from full extension to allow flexion to begin.

[Image of Popliteus muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Lateral surface of the Lateral Condyle of the Femur and the Lateral Meniscus.
Insertion (Distal)	Posterior surface of the proximal Tibia (above the soleal line).
Nerve Supply	Tibial Nerve (L4, L5, S1).
Blood Supply	Medial Inferior Genicular Artery (from Popliteal Artery).
Primary Actions	"Unlocking" the Knee: Internally rotates the tibia on the femur (or externally rotates the femur on the tibia). Knee Flexion: Weak assistance. Meniscal Retraction: Pulls the lateral meniscus backward during flexion to prevent impingement.

Deep Dive: The Screw-Home Mechanism

The Popliteus is famous for its role in the "Screw-Home Mechanism" of the knee.

1. Locking (Extension)

When you fully extend your knee (stand up straight), the tibia rotates laterally (externally) slightly to "screw" into place, locking the joint for stability. This requires no muscle energy.

2. Unlocking (Flexion)

To bend the knee again, you must "unscrew" it. The Popliteus contracts to internally rotate the tibia (if foot is free) or externally rotate the femur (if foot is planted). This "unlocks" the joint, allowing the hamstrings to flex the knee.

Physio Corner: Clinical Relevance

💪 Functional Fact: The Popliteus is a major stabilizer of the Posterolateral Corner (PLC) of the knee. It helps prevent the tibia from sliding forward (anterior translation) during flexion, assisting the PCL.

Palpation

Palpation is difficult as it is deep to the Gastrocnemius and neurovascular bundle.
Technique: Have the patient lie prone with the knee flexed to 90 degrees. Palpate the proximal posterior tibia, just medial to the fibular head. Resist internal rotation of the tibia to feel the muscle tense deep in the fossa.

⚠️ Clinical Pathology: Popliteus Tendinopathy
Pain at the posterolateral aspect of the knee is often misdiagnosed as LCL strain or lateral meniscus pathology. Popliteus pain is common in runners who run downhill frequently (eccentric load to prevent rotation) or on banked surfaces (cambered roads).

Manual Muscle Testing (MMT)

We test internal rotation of the tibia on the femur.

Testing Tip: Sit on the edge of the table with legs dangling. This puts the hamstrings at a mechanical disadvantage for rotation, isolating the Popliteus slightly better.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Sitting with knees flexed to 90° over the edge of the table. Action: Patient rotates the lower leg medially (turns toes in) without moving the thigh. Resistance: Applied at the medial ankle, pushing outward (into external rotation). Grade 3: Completes active motion. Grade 4/5: Holds against resistance.
Grade 0, 1 (Palpation)	Action: Palpate deep in the popliteal fossa. Cue: "Turn your toes in." Grade 1: Deep contraction felt (hard to distinguish from medial hamstrings). Grade 0: No activity.

Frequently Asked Questions

Is it an intra-articular muscle?

Yes! The tendon of the Popliteus passes inside the fibrous capsule of the knee joint (intra-capsular) but stays outside the synovial membrane (extra-synovial), similar to the Long Head of the Biceps in the shoulder.

Why does it attach to the Meniscus?

It attaches to the posterior horn of the Lateral Meniscus. When the knee flexes, the Popliteus pulls the meniscus backward, preventing it from getting crushed between the femur and tibia.

Does it flex the knee?

Technically yes, but it is extremely weak compared to the Hamstrings and Gastrocnemius. Its main role is rotation ("unlocking"), not power flexion.

Test Your Knowledge: Popliteus Quiz

1. What is the nickname for the Popliteus?

2. Where does the Popliteus originate?

3. What action does it perform on the tibia to unlock the knee?

4. Which nerve supplies the Popliteus?

5. The Popliteus tendon passes inside the joint capsule. This is called:

6. It attaches to which meniscus?

7. Where does the Popliteus insert?

8. Which ligament runs over the Popliteus muscle?

9. Popliteus tendinopathy is common in athletes who:

10. True or False: The Popliteus forms the floor of the Popliteal Fossa.

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.