I did an animation of someone breathing for a company I used to work for. It was an animation that used overlays to show how the diaphram works when you inhale and exhale. I used Poser for the base art. I used different scales in X, Y, Z to get the effect. In my animation the chest expanded side to side more than vertical. I did not have to adjust the values very much to get the effect I needed. Of course if you want someone to be gasping for air you might need to really exagerate the motions. In any case I think the side to side expands more than the up and down . I hope this helps.