I'm sorry that I can't really give you a formal explination, but only a anecdotal responce. When i'm trianing abs especially, it becomes much easier to preform a rep, and finish my set if I don't hold in my breath mid-rep.
They will probably tell you it is done to keep blood pressure down.
In reality if your lifting big weights and you exhale during the repetition your IAP (inner abdominal pressure) will drop. Low IAP can lead to missed lifts with near max weights,95-100% 1RM.
Don't belive the fitness mags. The Valsalva manuver is a natural responce. You do it every morning when you go to the bathroom.
Exhaling strengthens the mid section and lower back. Everything compresses making it stronger. These will come int play in most movements. Try taking a punch to the gut when you are breathing in as apposed to breathing out and you will see how much stronger it is.
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