Really nice drawings! -You might want to add a stronger sense of structure where the obliques insert to the illium. In the bottom left drawing, the muscle feels loose, like it's not attaching firmly to the illium. Being aware of the line of insertion could help. As it's squashed the muscle might spill over, but never looses its attachment. For even more solidity, you might want to make the form of the muslce, squash less like a water ballon, and more like a sandbag, or some firmer type of matter. I hope this helps :)