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.