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When I was new to weightlifting, I used to yawn quite often after a heavy set. So I always subscribed to the "lack of oxygen" theory. (Now that I've been doing it a _long_ time, that no longer happens.) My point is, brief intense physical activity did seen to bring on yawning in my case. So if the cause isn't lack of oxy, it was something else related to physical activity. And it was something that stopped over time as I became stronger and more used to doing the exercises.
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