Popliteus muscle

The popliteus muscle in the leg is used to unlock the knee by laterally rotating the femur on the tibia during a closed chain movement (such as one with the foot in contact with the ground).

Origin and insertion
It originates from the middle facet of the lateral surface of the lateral femoral condyle and inserts onto the posterior tibia under the tibial condyles, with its tendon running into the knee capsule to the posterior lateral meniscus.

Innervation
Nerve supply is via the tibial nerve from spinal roots L5 and S1.

Action
The Popliteus assists in flexing the leg upon the thigh; when the leg is flexed, it will rotate the tibia inward.

It is especially called into action at the beginning of the act of bending the knee, in as much as it produces the slight inward rotation of the tibia, which is essential in the early stage of this movement.

When the knee is it full extension; the femur slightly medially rotates on the tibia to lock the knee joint in place. Popliteus is often referred to as the "Key" to unlocking the knee since it begins knee flexion by laterally rotating the femur on the tibia.

Popliteus is also attached to the lateral meniscus in the knee; and draws it posteriorly during knee flexion to prevent crushing the meniscus between the tibia/femur as the knee flexes.

Variations
Additional head from the sesamoid bone in the outer head of the Gastrocnemius.

Popliteus minor, rare, origin from femur on the inner side of the Plantaris, insertion into the posterior ligament of the knee-joint.

Peroneotibialis, 14 per cent., origin inner side of the head of the fibula, insertion into the upper end of the oblique line of the tibia, it lies beneath the Popliteus.