I hate having to think too much during my cardio workouts. I like to dial in my target heart rate, set the audible alarm on my HR monitor, crank up the music and then proceeed to shut off my brain for the duration of my workout. HIIT, by comparison, is far more involved.

So, as the title says, what's the purpose of the intervals? Wouldn't it be just as productive, if not more so, to do an entire workout at 90% of Max HR?

I hate having to think too much during my cardio workouts. I like to dial in my target heart rate, set the audible alarm on my HR monitor, crank up the music and then proceeed to shut off my brain for the duration of my workout. HIIT, by comparison, is far more involved.

So, as the title says, what's the purpose of the intervals? Wouldn't it be just as productive, if not more so, to do an entire workout at 90% of Max HR?

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.

Jeremy

Thanks Jeremy. That made perfect sense.

I suspected that it might be difficult to maintain 90% max HR, but I wasn't sure. I also didn't consider the adaptation factor.

I guess I'll just have to get used to checking my HR frequently. I wish I could use the RPE scale effectively, but I have this annoying tendency to constantly second-guess myself.

Great post Jeremy! There's always some really good info coming from you. In fact I may even attempt HIIT now that I understand how it works!