This is a total extension of the knee joint. (Attached is an image I found on the internet of the total extension of the knee - when it is at 0º.)
The knee is a joint that articulates the thigh with the leg. In order to do this full extension, there's a knee rotation of 5° medially obliged by the leg with a lateral rotation of the tibia when off weight, or a medial rotation of the femur when standing.
