Fran, in a timely answer to your squat question, Alwyn Cosgrove published an article in the 10/19/07 issue of Testosterone mag called
Leg Training Myths Exposed. One of the issues he examines is keeping your knees behind your toes during squats and lunges and he concludes:
Quote:
Keeping your knees behind your toes when squatting or lunging is a little better for your knees, but much, much worse for your hips and lower back.
It's worth reading the whole article and checking out the study he cites.