The controversy about deep knee bends has been going since the invention of sports medicine
. To break the two arguments up into the two schools of thought is probably the best way to deal with the subject. It should be noted here that both schools of thought offer proof that they are correct.
The first school of thought says that going beyond a 40% angle when bending your knees puts 8 times your bodyweight of stress directly onto your knees. They say when you do this with weights on your back or shoulders then the stress is amplified 8 times as much on your knees.
They say that the best way to develop the leg muscles is by doing a half squat which is basically like sitting down on a chair and then lifting yourself up again. This school of thought says that going anywhere past that point is going to cause damage.
It is obvious that bodybuilders fall into the second school of thought which says that doing a mid-range movement is even going to put more stress on the knees. They say that a squat should be performed when the femur (upper leg) is level with the floor.
It is important to note that they also say that the back should be rigidly straight and that the knees should never go past the point of the toes when bending down. There are bodybuilders that have been doing deep knee bends for years without getting any damage to their knees.
It is important to make sure that firstly no momentum is used and therefore no bouncing at the bottom of the squat. Secondly that one should not go past the point of having your legs parallel with the floor as you will then be resting on your hamstrings and putting a lot of stress on your knees.
But everyone has a different set of genes in them and there are people who start training correctly doing deep knee bends and land up with knee problems. There could be many explanations for this but one would need to look at each specific case individually to get a true assessment.
It is the opinion of the writer of this article that deep knee bends work the quads to a full range of motion and when done correctly is the only way to develop the strength in your legs. In fact it is probably the only way that you are going to get stronger.