The very extreme poses are due to a number of factors. Muscle flexibility, joint alignment / flexibility, and the number of years practicing. You don't achieve some of these poses with out practicing every day. I've been practicing a couple of days per week for almost 8 years and have noticed a marked change in my flexibility, posture, attitude. However, there are some simple poses that I still am not able to achieve. And at my age I will more than likely not get there. But, I try every time.