You've probably heard and read the myth that squats are bad for your knees. I wrote on this subject before, but now Mark Rippetoe covers it at PJ Media.
Squats are regarded as the basic lower-body exercise by strength athletes because nothing else compares to its ability to strengthen the structure of the knee — the muscles, tendons, bones, and ligaments that form the knee anatomy.
Fortunately, I still have both ACLs, but I'm missing part a meniscus in my left knee, and squats like the one the lady in the photo is performing were a vital part of my recovery, and continue to be a vital part of my training. I firmly believe they helped prevent further injury.
The muscles on the front of the thigh are the quadriceps. They attach below the knee to the “tibial tuberosity” — the bump at the top of the shin bone — just below the kneecap. When they pull this bone forward, the knee extends and the force at the tendon attachment is directed forward relative to the joint. In contrast, the hamstrings pull backwards on either side of the knee at their attachments, which balances the forward force from the quads. This happens in a correct squat when the hips move back and the torso leans forward. The balance of forces is optimum at a position just below parallel, and protects the joint so well that a correct squat can be safely performed even without an ACL. (I don’t have one.)
(image credit: Thomas Campitelli, The Aasgaard Company 2013)
As always with Coach Rippetoe's articles, read the whole thing.