I am aware of that for exercising, but was ignorant of what "intense" actually means in this context. And you're right.

Looking around, the simplest wording I get:

> the intensity must be high. This means that you need to really exert yourself so you get out of breath. [https://norwegianscitechnews.com/2026/05/exercise-a-very-lit...]

So if climbing the stairs gets someone out of breath it's intense (and I also see how getting to your limits, whatever they are, can help)