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I recently heard that if you breathe in during the effort of your abdominal exercises, it causes you to get a more outward six pack. Is this true and is there any evidence for this?

Well it most certainly doesn't sound reasonable. Maybe in theory, because when you breathe in your thorax expands, the diaphragm contracs and makes room for the lungs to get enough air in, thus maybe pushing the gut out a bit also. But no, I don't think there are actual ways to get "outward sixpacks". The rectus abdominis is a muscle just like any other, and acts like one too. It activates more on the exhale part anyway. What I've been recommended is to breathe out while in the consentric part of the movement. Not too sure where that is based on either, maybe it works more on the abs because of the exhaling. Or maybe it's a back-thing.

It think the outwardness is due to body fat and the actual size of the rectus abdominis.

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Prove the impossible!

I had never heard it before but I just thought that possibly the breath in could cause internal pressures that make the rectus abdominus more round (highly unlikely though). I have always been told to breathe out anyway-it makes more sense in many ways

I would guess that this is the same thinking that's involved in recommendation of doing "abdominal vacuum" to develop abs.

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Our greatest fear should not be of failure, but of succeeding at things in life that don't really matter.--Francis Chan

Another problem with that is the actual six pack. It's not just the straight abdominis muscle, the lines that make your "pacs" are fibrous structure/tissue, called linea alba. I have my doubts that the tissue would lengthen for good because of some inhaling. I think inhaling doesn't affect your gut and organs just that much Heavy exhaling affects them more, as the abs push the organs and diaphragm to create heavy internal pressure to the thorax, which causes the air to exit the lungs.

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Prove the impossible!

I think a layer of fat over the abdomen is what keeps ab muscles from popping. To master a six-pack, I'd focus on diet instead.