Interesting thread. One possible theory (and this is stretching it) is that when you exercise endorphines are released. Endorphine stands for endogenous morphine, your body naturally makes pain killers. Anyway, morphine and drugs like it (codeine, which has an active metabolite of morphine), have a tendency to be a cough suppressant. So, one could infer that during exercise endorphines are released, suppressing the cough and as they wear off the cough returns and you are able to expel mucus.
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<br />That being said, I think a good exercise trumps any other form of airway clearance.
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<br />That being said, I think a good exercise trumps any other form of airway clearance.