The
popliteus muscle in the leg is used for unlocking the
knees when walking, by medially rotating the
tibia during the closed chain portion of the
gait cycle (one with the foot in contact with the ground). It is also used when sitting down and standing up. 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. The
gastrocnemius muscle acts on both
joints.