I mentioned this to my wife (who is a doctor) and her response was basically "no duh." She said they learned about this in med school. Her explanation was that

I mentioned this to my wife (who is a doctor) and her response was basically "no duh." She said they learned about this in med school. Her explanation was that this is typically happening to people who rarely exercise and then all of a sudden are physically exerting themselves for a sustained period of time.