Do you mean leg extensions? Those are exercises you do in physical therapy to protect your knees. By strenthening your quads, you help stabilize your knees.
There is a difference between light therapy to rebuild a joint and using a machine to get strong and build muscle. I've heard bad things about the leg extension with regard to the stress it puts on the knee. I wouldn't touch it regardless though; squats are much more effective. "In a leg extension machine, there is a pad against the front of your shin or ankle, and you press against it to move the weight. The foot swings upward in an arc. Thus, as you can see in the figure on the right, the pressure is coming across the shin bones, not along their length. This creates the problem in the knee joint as the shin is pressed backwards." "Leg extensions do have their place, usually in rehab. If the leg extension machine is used, it is wise to use a smaller range of motion, perhaps the top third of the movement (from slightly bent to fully straight leg), and light weight."
And this doesn't make sense. If I squat to parallel, I'm only there a slit second until I go back up. If I pass by parallel both on the way up and down, I'm probably going to be in that position twice as long. If you go all the way down, you are flexing your knee joint past its normal flexion point, which puts tremendous strain on it. Not to mention the tendency to bounce just a bit, which contributes to this hyperflexion.
"While biomechanical research does support the fact that forces on the connective tissues of the knee increase with the knee angle, particularly on the posterior cruciate ligament, there is no evidence
that these increased forces actually lead to injury. There is no direct evidence that full squatting causes or even exacerbates knee pain nor damage. I do not know of a single documented case where full squatting led directly to knee injury.
Not one!" "No study, short or long term, has ever shown an increase in knee laxity from deep squatting." "...knee injury usually results from varus or valgus force (twisting of the joint in either direction), inappropriate loading, or forcible shear across the joint. It does not occur simply from taking the knee joint through a full range of motion, using correct technique, and using a weight which is appropriate to the abilities of the trainee." http://www.stumptuous.com/cms/displayarticle.php?aid=52