The "screw-home" mechanism, considered to be a key element to knee stability for standing upright, is the rotation between the tibia and femur. It occurs at the end of knee extension, between full extension (0 degrees) and 20 degrees of knee flexion. The tibia rotates internally during the swing phase and externally during the stance phase. External rotation occurs during the terminal degrees of knee extension and results in tightening of both cruciate ligaments, which locks the knee. The tibia is then in the position of maximal stability with respect to the femur.