One of the reasons people contact me is because not enough air in the podium, in the webinar, or perhaps in a sales presentation. So I use a question to suit your needs.

Do you ever experience breathlessness a lot more conversation with family, friends or colleagues?

More than likely, your answer is No unless you happen to be very excited.
So what's happening in public speaking that causes one to gasp for air and always feel like you happen to be running on empty? There are three possibilities.

You are nervous. Nervousness can sap your oxygen.
You still believe whatever you learned in elementary school: you're not in a position to breathe unless you visit some type of punctuation.
You do not be sure you breathe.

Nervousness may become the perfect best friend or perhaps your worst enemy in public areas speaking. If it is in charge, then it's definitely not assisting you given it increases your panic. This, therefore, places you inside flight or freeze mode. In either one of these states, your upper chest breathing feeds your panic as opposed to alleviating it.
While your 2nd grade teacher could have said not to require a breath and soon you found the conclusion in the sentence, she had her reasons for saying so. The problem came later in class if you had several lines of sentence with no punctuation. You have something referred to as 'speaker's license.' Similar to the 'writer's license,' the first kind ensures that you are able to breathe almost anywhere is any sentence. Just as you supplement your air supply in normal conversation, so too should you supplement your air in public areas speaking. What happens at the lectern, for instance, is always that we hold back until were totally spent before gasping for any quite a bit of air which actually creates more tension than relief. The secret? Supplement your air supply before you run out of it. Another way to see this is always to keep your 'balloon' filled instead of empty.
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