We can use a link diagram to describe the body's motion during a squat. Each link in the diagram is a line segment that represents an unbendable part of the human body. The links are connected at their endpoints by joints that are allowed to rotate.
As you can see, our link diagram is a simplified, movable ``stick figure'' of the human body. For our purposes, we can represent the squatter by a link diagram in the X-Y plane.
There are three links (line segments) in our diagram: one for the shins, one for the thighs, and one for the torso. All of the line segments lie in the X-Y plane. The lengths of the segments are shown in the link diagram.
The X-Y position of the ankle joint is fixed--in other words, the X-Y position of the squatter's ankle does not move. The bottom endpoint of the shin link is attached at the ankle joint. The shin can rotate around the ankle joint, but the shin must always be attached to the ankle. Otherwise--ouch!
And of course, the thigh link is connected to the shin link, and the torso link is connected to the thigh link.