What Could Possibly Go Wrong?

I discovered something about the lunges that I do. If I do the shallower squats, where the tops of my thighs are parallel to the floor at the lowest part of the squat, then I work a part of my thighs that I don't normally work. However, when I do the squats where I go down to where my butt almost hits my heels, then I stretch the tendons in my knees, which I think is what keeps my knees from hurting so much. So, I've decided to do 20 of each. My goal is to strengthen my legs to the point where they don't complain when I try to do lunges and squats in tai chi class. Usually, I'm not able to go through the whole sequence in class without resting my legs at least once. Maybe I can add some work on my abs to my routine. They could use it.