Popliteus Muscle: Function, Anatomy, and Clinical Relevance

The popliteus is a small, deep muscle in the posterior knee crucial for unlocking the flexed knee.

Anatomy

• Origin: Lateral femoral condyle (popliteal groove), lateral meniscus, fibular head (arcuate ligament).
• Insertion: Posterior proximal tibia (medial to tibial crest), above soleal line.
• Innervation: Tibial nerve (L4–S1).
• Action: Internal (medial) rotation of tibia on femur (or external rotation of femur on fixed tibia) — "unlocks" knee from full extension.
• Secondary: Weak knee flexion; stabilizes posterior knee; pulls lateral meniscus posteriorly during flexion.

Clinical Significance

• Popliteus tendinopathy: Posterolateral knee pain in runners/downhill athletes; tenderness at femoral insertion.
• Popliteus syndrome: Entrapment of popliteal artery/vein or tibial nerve by muscle hypertrophy.
• PLC (posterolateral corner) injury: Popliteus tendon + arcuate ligament + LCL + biceps femoris; causes varus/rotational instability.
• Gait unlocking: Failure → "stiff knee" gait, limited flexion initiation.

Assessment

• Resisted internal rotation of flexed knee (90°) reproduces pain.
• MRI for tendon tears, edema, PLC injury.

Though small, the popliteus is the "key" to knee flexion initiation and posterolateral stability.