I was just watching Marks's sprinting video. At the end he does a deep squat/sit for rest. I am unable to do this. Nor can I sit back on my heels *my knees just don't bend that far. I am reasonably fit (5'7", 160 lbs), but I've seen really overweight people sit back on their heels with no problem. The last time I remember squatting that way was in my 20's (30 years ago), and I don't remember ever being able to sit on my heels. Any orthopaedists out there who may know what the issue is? I realize no one can diagnose from an email, but if you've seen this limitation before, what is usually the root cause? Thanks.