This is very true. You're only going to get an approximation. It might be pretty accurate for a uniform mass like jello, but the human body, with bones, BMI, tone, who knows what all, would presumably be a lot more challenging. Time to renew my call for 'Smart Figures' that incorporate information about themselves, weight, BMI etc. to interact with software simulations. A 300 lb. person and an olympic sprinter climbing stairs course are going to behave differently. You could probably take into account factors like fatigue as well. If it happens, it will probably show up in games first strangely enough.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken