Tips » Cardio and Aerobics
So what is the purpose of the intervals in HIIT (High Intensity Interval Training)?
Most people cannot sustain 90% of max HR for a prolonged period of time ... and if you target this, what happens is that your body adapts and the heart becomes more efficient so it becomes harder to raise the rate. The intervals do several things. They allow short bearts of near maximal effort, which by definition and design of your energy systems would be impossible to prolong, so the second thing is the lower intervals allow active recovery to prepare for another bout. The stimulus creates an adaptation - the heart is able to return to a normal heart rate more quickly, which is a good thing, and you create more post-EPOC consumption.
It's simply like this - you can do an all out sprint for several seconds. If you are going all out for several minutes, you are not going all out - you are doing your best for several minutes. So back to the all out sprint - this is anaerobic, high heart rate, heavy calorie burn, etc. Now, you could do that and stop. Or, you could rest a bit, and go more. Obviously, this is going to burn more calories. Then you could rest a bit, and go more.
Understand there is this continuum ... moderate cardio burns few calories, so you must do it longer. Intense cardio burns more calories, but you cannot sustain it longer. HIIT is a compromise that allows you to extend those bouts of intense cardio/high calorie burn while still allowing active recovery and keeping your heart rate from going back to resting levels.
Hope that made sense. The more you can do those near-maximal bouts, the more post-exercise energy consumption you'll experience = the more fat burned even the day after you train.