This is the kind of look I would be aiming for (really quick sketch!) since it seems as though you're going for an organic look, and since it's a humaniod, I'd suggest using a ribcage as reference for the upper part and then shape the bottom part accordingly. The dotted lines are to represent the joint between the two parts. If you make it as a ball joint, the upper part can move in any direction on it and thereby lean forwards, backwards and sideways...

Another way to do it would be to model a spine and then somehow attach a chest and maybe stomach part to it in someway, but that would probably require some more modelling of internal parts to seem convincing...