[Aoife]

I think the difference between simple interval training and "HIIT" is the intensity. Interval training doesn't describe any sort of intensity, just that you have intervals of faster and slower. HIIT - High Intensity Interval Training has the idea of high intensity built into it, making it more descriptive than HIT. However, High doesn't necessarily mean Highest. I think I'd say anything above 80-85% of max as "High" but if you're talking all-out... Highest intensity that can be managed for say... 10 seconds, you're looking at closer to the 95%+ range I'd guess.

My definition of HIIT has been that your work intervals are at least 80%, preferably 85% of max in intensity, rest is usually twice work interval in time, and one can't do more than 20 minutes (or 30 if one includes the warmup/cooldown). That's the way I've seen it described, that's the way I understand it to be.

I think things like tabatas are a subset of HIIT the same way that HIIT is a subset of HIT. Each is more specific a protocol than the previous. But each still has some vaguery in its definition, because there seems to often be varying times/reps in which to complete the assignment, and therefore that dictates the intensity one can do. At least, in my view.

I've had specs to do 12, 15, 18, or 20 minutes of HIIT and I just adjust my intensity accordingly so that I am able to complete the prescription tanked out at the end. I don't know that it needs to be much more specific than that.