"Kreighbaum (1996) illustrate the safe position of a deep squat with the knees extending beyond the toes. Kreighbaum explains how a deep squat can be performed little chance of injury to the knee. The variables of concern:
* speed of descent
* size of calves and thighs
* strength of the controlling muscles
The primary danger to the knee occurs when the tissues of the calf and thigh press together altering the center of rotation back to the contact area creating a dislocation effect. The danger of knee injury in this situation may be prevented if either of the following factor are present:
* center of gravity of the body system is keep forward of the altered center of rotation
* muscles of the thigh are strong enough to prevent the body from resting or bouncing on the calves.
Kreighbaum conclude the deep squat is of little danger to the knees unless these variables and factors are disregarded. Certainly only a limit type of athletes may have a sports specific need to perform a full squat. Olympic weightlifters commonly bounce out of a full front squat with near maximum resistances during both the Clean & Jerk and Snatch. Incidentally, the wide stance during an Olympic style squat further reduces knee torque forces.
During the lower portions of the deep squat the lower back may flex if hip flexibility is inadequate. The risk of injury is increased if the muscles of the lower back are not strong enough to support the flexed spine or the joint structures have not progressively adapted to such a stress.
Flexibility exercises can be performed if hip flexibility is insufficient for deep, or full squats. See Full Squat Flexibility."