I started doing HIIT last week but I've been seeing conflicting advice on what to aim for when doing it. Should I be concentrating on my heart rate? Usually I go out all for 30 seconds and my HR goes up to about 90% max. After that I cool down and wait until my HR drops down to 75% then go again. Should I be paying more attention to my HR than maintaining the 1:2 ratio of sprint/jog? I've noticed that it sometimes takes longer than 60 seconds to get my HR down.