If doing HIIT, what should you use to judge the intensity level? Should you use your HR, or how tired you feel?
i go by effort level, when doing the intervals, i think that it kills the intensity to be constantly monitoring your HR, so just give it 100% and kill yourself... seems to work for me
intensity level i believe is simply how much you go to pure failure and beyond (forced reps, cheat reps, etc)
though getting to those points is a mental game, and the mind lies to you to make you quit... its a tough judgement