Starting Strength, 3rd Edition - Mark Rippetoe [34]
To recap: The complete concept of the correct use of the hips in the squat is best understood as the use of both an actively locked lumbar extension and actively shoved-out knees, resulting in a below-parallel squat that incorporates a stretch reflex, using all the muscles of the posterior chain in the most optimal way possible. This movement pattern gets the thighs out of the way of the pelvis so that good depth can be more easily obtained. At the same time, it makes the squat stronger because the active use of the external rotators holds the femurs in a position that enables both the external rotators and the adductors to contribute to hip extension. This hip extension produces a more effective use of more muscles over a wider range of motion.
Knees
In a correct back squat of the style advocated here, there is one correct place for the knees: directly in line with the feet so that the femurs and the feet are parallel. This position will, for most people, be slightly out in front of the toes, with the exact distance being determined by the anthropometry of the individual. This basically means that the femur and the foot should be in a straight line as seen from directly above, so there is no twisting of the knee. Depending on your femur, tibia, and trunk dimensions, your knees could be anywhere from very slightly behind your toes, with short femurs and long tibias, to 3 or 4 inches in front of the toes, with long femurs and short tibias.
Figure 2-45. The differences that anthropometry can produce in the appearance of the bottom position of the squat. Both are correct, but both are different due to variations in leg and trunk length.
Since your knees will be directly in line with your toes, the angle of your feet in your stance will determine the angle of the knees as well. As shown in Figure 2-12, an angle of about 30 degrees out from the perpendicular works for most people, although this varies as well. This angle allows the hips to function as discussed above.
By far, the two most common knee errors are 1) knees caved in too much, and 2) knees too far forward, either early in the descent or at the bottom. It is actually unusual to see novices not make one or both of these errors the first time they squat. Both errors are related to hip function and positional awareness.
If you allow your knees to come together at any time during the squat, you dilute the function of the muscles both medial and lateral to the femurs. But this problem cannot be corrected if it is not identified. When you squat, look down even more than usual, to a point on the floor right between your toes, where you can clearly see your knees, and check your position. If your knees move toward each other at any point during the squat, shove them out. You will probably have to exaggerate this shoving-out in order for it to put your knees in the correct position, since you thought they were in the right position when they were coming in. When you get them back out to parallel with your feet and keep them there for a couple of sets, you will notice later that your adductors, and perhaps your most lateral glutes, get sore. From our previous discussion, you know why.
Figure 2-46. (A) The knees-in position most people will assume unless coached to do otherwise. (B) The way to coach knees-out.
Letting your knees travel too far forward presents a different challenge. The problem with this position is not so much that it destroys the knees (although it is not particularly good for them), but that it has a detrimental effect on hip drive out of the bottom. A knees-forward position produces a more acute knee angle, and the resultant distally shortened hamstrings have less room to contract from the other end. When the hamstrings are already contracted, their contribution to hip extension is much less efficient than it would be with longer, stretched-out hamstrings. It also means that there is more moment force against the ankle/mid-foot balance point because of the more horizontal angle