What constitutes good form for barbell lifts is not a matter of opinion or up for debate, argues lifting coach Mark Rippetoe. Rather, proper mechanics are about understanding the relevant bits of human skeletal anatomy and the principles of force and physics. These are what determine the most efficient, strongest, and mechanically sound body positions for all the lifts and these are what we, as lifters and trainers, need to learn to recognize and analyze. In this video article, he explains the skeletal geometry that is the basis for the back squat in particular.
The salient parts for geometric analysis of the squat are the shin, thigh, and back and the three angles formed by them: the knee angle (formed by the tibia and the femur), hip angle (rigid back and the femur), and back angle (the back and the floor). The relationships among these--with the added point that the bar will always be directly over the mid-foot if the system is to be in balance--determine the correct position of the bar on the back and of the elements of the body under that bar. Once the pieces are in place, then the force of the bar on the spine (and other joints) and the force generated by the body are applied in appropriate planes and the lifter is poised to be efficient and correct.