"Because the knees goes over the toes".
But why does the public still believe that squatting with knees projecting past the toes is a bad thing? It's a myth that began years ago around Step Aerobic and Jazzercise circles and has stuck around for years. Without getting into a discussion on knee tracking under loads coupled with sheer force, how can we understand this myth in simplistic terms? Easy...and I will do my best.
When we think in terms of squatting, many visualize the squat as a one-dimensional movement pattern that resembles a folding lawn chair.
1.) Path of Motion
2.) Sequential Pattern of Movement
3.) Displacement of Body Mass during Path of Motion
A folding lawn chair is somewhat "robotic" if you will. All the parts are moving in a one plane of motion. There is no "give and take" in a lawn chair metal frame. The only "give and take" is within the lawn chair fabric. The human body differs from this analogy. The human body actually has a little more "give and take" when it comes to movement patterns in the form of compensations. When the body senses a heavy load or sudden changes in its environment; or needs to adjust because of an obstacle, collision, or change in direction--it does so through compensatory means. In corrective exercise, we think of compensation as bad things or things that we need to address when working with the musculature of the body. This is true...however, the body is a fascinating machine that uses compensations to execute whenever the brain tells it. Let's go back to the squat. Most people can either squat flawlessly (although I haven't seen many), can squat with some compensation, or simply cannot squat at all. When the knees project over the toes it is a compensation that reveals that body mass has been displaced. And that displacement is caused by a heavy load on one's back.
For instance, in order to avoid our knees translating past our toes when we walk up stairs, we would have to keep our torso completely erect with each step. This is un-natural and not an automatic mass displacement. Our bodies need to regulate its center of mass/gravity during any activity.
So the squat is not a one dimensional movement pattern that resembles a lawn chair. The squat resembles more of a compression--whereas the body is compressed between the ground and the load.
And during the squat movement, the body DOES compensate because of this unique "vice grip". Does this mean that this is acceptable in weak, unhealthy individuals that have not progressed adequately in the loaded squat? No. This spring-like compression is produced through a system of levers that vary in function, tension, and strength. In order to safely compress during a squat, you must have adequate tissue quality, mobility, and flexibility.
The "knees over the toes" myth is a great example of compression during a squat. The problem lies in that we believe that sheer forces only travel down femurs [as in a waterfall] causing harm to the patella and quadriceps tendon. However, because of this compression, sheer forces disperse and are "absorbed" by the antagonistic muscles--mainly glutes and hamstrings--providing that the squat technique is coached and executed correctly.