If you loaded up a backpack with 100 lbs, and wore it every day, you would quickly develop back pain and would probably hunch. Thats basically what is happening with fatties. Their body is trying to compensate and move itself, but its not working like a well oiled machine because there is too much weight. So it compensates but isn't fluid, and eventually it breaks down because legs and arms and spines and bones and tendons weren't meant to carry the load of extra hundreds of pounds.