i have a question about hiit training. i understand that it is a matter of interval training, but the high intensity intervals involve max heart rates or more. when you do a 20sec or however long high intensity interval then bring it down for the break, do you need to bring your heart rate down to a certain percentage before you begin the next high intensity interval? specfically what percentage of max heart rate should a person go for during the high and low intensity intervals?

The High Intensity Interval should be 15-30 seconds of maximum 100% all out effort, with out regard to heart rate (although at the end of the interval you should be a or near your maximum heart rate, assuming you are not limited in your effort by fatigued muscles instead of lack of oxygen). The "low" intensity interval should only be long enough to allow you to perform at maximum intensity for the next high intensity interval (usually no more than 30-120 seconds).