When I went to a pilates class a few times, the ab work was a bit hard for me, but there was a shoulder-upper back thing that I found ridiculously easy...and the rest of the class seemed to really struggle with it.
Maybe that strength comes from coughing like Diane suggested.
(You lie on your stomach and lift your arms and upper body off the floor--like you are doing the breaststroke--no touching the floor.)
I think that my upper body looks more muscular than it really is because I am skinny up top But you'd never know if there were muscles under my puffy butt.