Popliteus
Musculus popliteus
Also known as: key of the knee
A thin, triangular muscle in the floor of the popliteal fossa. It "unlocks" the fully extended knee by laterally rotating the femur on the tibia.
Overview
Popliteus arises inside the knee joint capsule by a cord-like tendon from the popliteal groove on the lateral femoral condyle, with fibres from the lateral meniscus and the arcuate popliteal ligament. The tendon runs between the lateral meniscus and the fibular collateral ligament. Its triangular belly then fans out over the back of the tibia to insert above the soleal line.
In the last degrees of knee extension, the femur rotates medially on the tibia, the "screw-home" mechanism that locks the knee for standing. To begin flexion, popliteus laterally rotates the femur on the fixed tibia (or medially rotates the tibia when the foot is free), unlocking the knee. It also pulls the lateral meniscus backward during flexion, protecting it from being crushed, and it resists posterolateral instability. It is supplied by the tibial nerve, which gives the branch as it crosses the muscle.
Anatomy
- Origin
Lateral surface of the lateral femoral condyle (popliteal groove) and the lateral meniscus.
- Insertion
Posterior surface of the tibia, above the soleal line.
- Action
Lateral rotation of the femur on the fixed tibia (unlocking the knee); weak knee flexion.
- Innervation
Tibial nerve.
- Nerve roots
L4–S1
- Arterial supply
Inferior medial and lateral genicular arteries; posterior tibial artery (muscular and recurrent branches).
- Relations
Forms the lower part of the floor of the popliteal fossa. Its tendon is intracapsular but extrasynovial and passes deep to the fibular collateral ligament.
Function
- Unlocks the extended knee (lateral rotation of the femur on the tibia)
- Retracts the lateral meniscus during flexion
- Stabilises the posterolateral corner of the knee
Clinical significance
Posterolateral corner injury
Popliteus tendon tears often accompany ACL or PCL ruptures. Unrecognised injury causes persistent posterolateral rotatory instability and failure of cruciate reconstruction.
Test yourself
Which muscle "unlocks" the fully extended knee by laterally rotating the femur on the fixed tibia?
- Popliteus
- Plantaris
- Biceps femoris
- Gracilis
Answer: Popliteus
Popliteus (tibial nerve) reverses the terminal "screw-home" medial rotation of the femur, so knee flexion can begin.
Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.