Training Reference?

[RonP]

I need a training reference. The one where you only do something for about 20 minutes. Or more specifically, 14 to 18 minutes. I see this over and over in my training.
Please forgive me for being too unorganized to refer back to it. I know the name of the theory or principle has been mentioned. Would like to read a book or paper on it in order to organize all of my personal practice more on it.

[Joseph David]

Ron,
I think your referring to the time it takes to reach cardio-respiratory distress, CRD, and step through the neuro endocrine immuno response, NEI membrane. This is like shifting gears and the nervous system adapts to the activity to become more efficient at accomplishing the movement.