This is a major annoyance that sometimes plagues me also so i have a special interest in understanding it. Endorphins are released into the blood spine and brain during extreme exertion because you are putting your body under stress, similar to the related and also natural fight or flight response. These Endorphins interact mainly with opoid receptors in the brain similar to opiates. For those of us, and man am I one, who are sensitive to opiates, such as oxycontin for example, we feel a similar response, nausea.
It's unclear why the body produces chemicals that cause that response but the same release that causes a high, can cause nausea. I often find myself in between vomits feeling absolutely invincible.... A very strange predicament, especially since it mostly happens while I have ducked away in the stall at the gym trying to keep the vomit noise down. I have learned for me, the warning signs and try to pull up before over doing it... It just takes too long to recover once the "launch sequence" has been started.
Also, I noticed that it typically only shows up if I work both lower and upper body. If I go from big chest blast to a squat smack down then onto pull ups followed by the leaper, I know my next set will involve me wrapped around the porcelain.
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