Ah, you need them to move... hmm... see girls don't have this problem as we don't have muscles, lol.
You could always incorporate some elastic or a small hinge of some kind within the structure so when you do flex, you get the movement but won't snap the band. I'm thinking of the way they do it with boots, a la this:
but then the papier mache wouldn't work then, you'd have to use coloured leather or something. Hm, this is a tricky one!