Tiger Woods with hyperflexed knees and partial body weight load on knee joint fulcrum.
The knees are potentially harmed by a full down weight-bearing squat, as seen during preparing for a putt. When the end range posterior pelvic tilt occurs in a deep squat or crouch, and the power or force production capability is lost from the slacked hip extensors (or hamstrings), the force to lift the body upward must come from the quadriceps (knee extension) and the low back muscles (torso extension) — compensating for the lack of force output from the hip extensors for hip extension. The force output necessary from the quadriceps is transmitted through the knee and especially to the patella and the patellar tendon (ligament), causing unnecessary stress on the knees.
The deep squat puts a great deal of force transmission from the ground to the feet, lower legs, across the knees, and to the force output of the quadriceps. The knee acts as a fulcrum takes a beating internally on the meniscus, and on the anterior and posterior cruciate ligaments. Compared to standing, consider the force level that is increased on the knees when the thigh or femur is 90-degrees to the tibia and fibula — and held in the position statically. Consider also the energy that is wasted, for example, when a golfer crouches and maintains a static position while inspecting the hole before a putt.
Tiger Woods on Putting.