Hallo!

 Something is really bugging me as I am currently trying to animate a V4 character. It has to do with breathing in general..

 I'm debating whether or not your belly pushes out or sinks in a bit when you inhale. I've heard conflicting reports from family I've asked. I do both.. I think... lol.. (Or I can't really decide.. because every time I experiment it's like my mind isn't letting me breathe naturally, if that makes any sense.)

Also I'm a guy.. with a not too slim belly, and I'm animating a slender female character.. not sure if that makes a difference!

  Currently I'm just using the "Inhale" morph of V4, which raises up and expands the chest a bit.. but it looks weird without the belly expanding/contracting.

Does anyone have any suggestions to animate natural breathing?

Thanks!

If you focus on breathing by expanding your chest , the belly pulls tight, If you just breathe it happens lower, just above the navel. Its best to watch someone else when they are just acting naturally , but be subtle , or ya get slapped , or they call the police. "Honest Officer , I wasnt perving, I was watching how the hips move for my characters walk cycle "   -    :(

If it looks wrong to you it probably is , there are lots of tiny things you see without realizing it and you notice when they are missing. Try a search for character animation breathing    and see what you find.

Your stomach does nothing.. your chest expands, and you back bends back slightly. I've done a few "breathe" options in some of my PLUS sets.

I usually include a morph, bend the chest back slightly, and then bend the neck foreward slightly to compensate bending the chest back, and the effect is VERY good.

Both the stomach and the chest expand, and the ribs also rotate upward
from an axis on the spine.   Broadly, males tend to expand the stomach more;
females tend to put more of the action into the chest.  

Here's a schematic of the internal operation.  When you inhale, two separate muscular actions are involved.  The diaphragm tightens up, so that its center pushes down on the abdominal innards and pulls up on the ribs.  At the same time, the muscles between the ribs contract at an angle, pivoting the ribs upward and outward.   The downward push on the innards forces them to expand the belly, unless you're also tensing your abdominal muscles.

That's an excellent demonstration of the mechanism ockham, thanks a lot for sharing !