why HIIT works

The first time I was introduced to HIIT (High Intensity Interval Training) was thru Bill Phillips’ “body for life” program.

I never knew why this program worked so well. Now I understand, and I will let you in on this secret.

I still use HIIT three times a week in my own workouts;

Why Long, Slow Cardio Doesn't Deliver Desired Results

For those who are not yet familiar with HIIT, it works because it engages your fast and super-fast twitch muscle fibers, which promotes human growth hormone (HGH), a synergistic, foundational biochemical underpinning that helps make your strength training and everything else work like a charm, and effectively burns off calories.
"Most exercise programs today are built based upon a very incomplete picture of the physiology of your body. For example, long slow cardio, "calories in, calories out," would be a perfect way to look at the body if it were all slow-twitch fiber … [but] there are three muscle fiber types: slow, fast and super-fast … both those types of fast-twitch fibers are essentially 50 percent of your muscle fibers that don't get recruited until you add a velocity of movement."
If you don't actively engage and strengthen all three muscle fiber types and energy systems, then you're not going to work both processes of your heart muscle. Many mistakenly believe that cardio works out your heart muscle, but what you're really working is your slow twitch muscle fibers. You're not effectively engaging the anaerobic process of your heart. Fortunately HIIT type exercises however, do address these fibers and metabolic systems.
Traditional cardio primarily exercise your slow twitch muscle fibers. Your body kicks in these slow twitch muscles first, in an effort to not recruit your fast twitch muscles, or work your heart anaerobically.
This is why you may not see results even though you spend an hour on the treadmill a few times a week – you're basically denying the natural physiology of your body by not working the other half of your muscle fibers; your fast-twitch muscles.

You Should NOT do Peak Fitness Every Day

I want to point out that even though HIIT is, in my impression, an essential, crucial element of any exercise training program -- it is NOT something you should do every day, as your body requires more time to heal in between sessions.
The benefit of doing HIIT three times a week comes not only from the way it works your fast and super-fast muscle fibers, but also the way it increases your growth hormone with each session.

Each HIIT Workout will Increase Your Growth Hormones

Human growth hormone is often referred to as "the fitness hormone." The higher your levels of growth hormone, the healthier and stronger you will be. Once you hit the age of 30, you enter what's called "somatopause," at which point your levels of human HGH begin to drop off quite dramatically. This decline of HGH is part of what drives your aging process, so maintaining your HGH levels gets increasingly important with age.
The longer you can keep your body producing higher levels of HGH, the longer you will likely experience more robust health and strength. Some athletes choose to inject it for this very reason, though it is a banned substance in nearly every professional sport. I do not recommend injecting HGH however, due to the potential side effects, the cost and, more importantly, it is likely to cause more long-term harm than good. Fortunately, your body produces HGH naturally when you exercise your super-fast muscle fibers during vigorous, high-intensity exercise like HIIT.
"You know, walking is a great thing, but it only works the aerobic process of your heart muscle. It doesn't work the anaerobic process. It only recruits your slow-twitch fibers. So those two other muscle fiber types are meant to be used to exercise is necessary to release growth hormones.

HIIT Also Increases Your Endurance

Another great facet of HIIT is that it gives you both aerobic and anaerobic benefits at the same time, essentially replacing the need to do long, slow training. In fact, if you're trying to build your endurance levels, replacing tedious distance workouts with HIIT is going to give you better results.

Summary of my HIIT Workout

Here's a summary of what my HIIT routine looks like:

1. Figure out your perceived exertion rate; 0 (easiest) thru 10 (hardest – can’t continue – body shuts down)
2. Warm up for two minutes level 5
3. Starting at level 6; go up every minute 1 level up to level 9 (1 minute at level 6-7-8-9)
4. Repeat the high intensity exercise and recovery 4  times
5. on the last interval go up to level 10 for 1 minute
6. recover at level 5 for 1 minute
7. takes 20 minutes
8. In real speed I would; warm-up at 6mph; interval at 7-8-9-10mph and the last interval sprint between 11 and 12mph.

Here's a what another routine might look like:

1. Warm up for three minutes
2. Exercise as hard and fast as you can for 30 seconds. You should feel like you couldn't possibly go on another few seconds
3. Recover for 90 seconds
4. Repeat the high intensity exercise and recovery 7 more times

You Can Use HIIT for Strength Training, Too

Tabata training is an excellent form of HIIT Strength training