The popliteus muscle in the leg is used for unlocking the knees when walking, by laterally rotating the femur on the tibia during the closed chain portion of the gait cycle (one with the foot in contact with the ground).

In open chain movements (when the involved limb is not in contact with the ground), the popliteus muscle medially rotates the tibia on the femur.

It is the only muscle in the posterior (back) compartment of the lower leg that acts just on the knee and not on the ankle.

Origin is 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.

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.