Squats are by far the most productive exercise for the lower body. Because of the muscle mass involved, they also provide great overall stimulation. But they’re not without their problems, and one relates to depth.
“The danger in a full squat, a low squat is not a result of the position of your legs in relation to your torso. The danger is a result of the direction from which the force is imposed. The force is trying to bend the bones of your lower leg and pull your knee apart—the same as a leg extension.
Although the direction of force is worse in a leg extension, the amount of force is greater in a squat. Results are about the same.
There are bodybuilders perform squats to a level where their thighs were below parallel to the floor. According to the opinion of several sports trainers, squats should be carried to a point where the thighs first start to contact the backs of the calves. At that point the squat should be stopped by muscular action instead of by bouncing the thighs off the calves. Performed in the correct manner, there is no danger to the knees. On the contrary, squats can do more to prevent knee injuries than any other barbell exercise.