An Introduction to High Intensity Interval Training and Its Many Benefits

I’m a big fan of intense exercise. More specifically, I am fan of high intensity interval training.  It makes the brain work better, makes the heart work more efficiently, and makes me feel great after. This article will serve as a basic introduction to HIIT, or high intensity interval training.

HIIT are short sessions of exercise (4-30 minutes) where you are switching between high intensity exercise and low intensity exercise (or rest).  Examples of high intensity interval exercises include:

  • 30 seconds of sprints followed by 60 seconds of rest for 10 minutes
  • 60 seconds of running the treadmill at 10mph followed by 60 seconds of running the treadmill at 4mph for 12 minutes
  • 30 seconds on a stationary bike at maximal speed with resistance followed by 30 seconds of rest for 8 minutes total
  • Hitting the punching bag as hard as you can for 1 minute followed by one minute of rest for a total of 15 minutes
  • 15 seconds of running on a treadmill at max incline followed by 15 seconds of rest for a total of 4 minutes

There are infinite examples, but you get the drill.

***In this article high intensity interval training is also referred to as HIIT, interval training, intense exercise, high intensity training.  Low intensity cardio in this article is also refereed to as steady-state cardio, endurance training, and low-impact cardio.

Anaerobic and Aerobic Systems

Anaerobic and aerobic are metabolic systems that recruit different muscle fibers, use fuel differently, and use oxygen differently.  Steady-state cardio works the aerobic system because it requires oxygen (to reach the muscles) and is mostly fueled by stored fats.  High intensity interval training relies on your anaerobic system, the system that does not require free oxygen (to reach the muscles) in order to perform and relies on stored carbohydrates as fuel.  Although a mile run done in under 4 minutes can still be considered anaerobic , the aerobic system usually kicks in we reach the two minute park of continuous exercise.  When doing interval training, you are training the muscles to work efficiently without free oxygen.  You are training your muscles to work effectively in short but highly intense durations.

A 100 meter sprint is a function of your anaerobic system and a marathon is a function of your aerobic system.  The anaerobic system is not built for endurance; an Olympic sprinter will not beat a Olympic marathoner in a marathon.  But the opposite is also true.  Marathon runners and sprinters train different systems; the former primarily trains the aerobic system and the latter primarily trains the anaerobic system.

The difference in how the muscles function between the two systems can also explain why people who do interval training are more muscular than people who do regular endurance training.  When doing interval training, the body needs more muscles because it works without free oxygen reaching it.  Because oxygen doesn’t have time to reach the muscles, the muscles must already have the energy needed to function anaerobically.  This then leads to a buildup of type II muscle fibers, which are also called fast twitch muscles.  Fast twitch muscles are bulkier than slow twitch muscles fibers (type I) because they already have the necessary ingredients to work and don’t rely on drawing fuel from elsewhere.  Thus if you want a muscular physique (like that of a sprinter), it is better to do interval training than steady-state cardio.

The Benefits of High Intensity Interval Training


The benefits of interval training are many.  Below are some of the benefits, verified by science.

  • Research from Stockholm, Sweden by Nicolas Place, Niklas Ivarsson, et al, showed that ryanodine receptor fragmentation and sarcoplasmic reticulum leak after just one session of high intensity interval training.  In layman’s terms, free radicals produced during high intensity interval exercise stress muscle cells, which teach the muscles to improve for the next set of free radicals, thereby improving muscle endurance and boosting the production of mitochondria.  This happens just after one session of high intensity training.  Free radicals are normally associated with cancer but it doesn’t mean free radicals in and of itself is bad.  In order to get the most out of your exercise, you have to allow to your body to produce free radicals.  Antioxidants prevent the production of free radicals.  To have a healthy body, it is important to have a healthy balance of free radicals and antioxidants.
  • Highly intense exercise also does a better job of suppressing hunger better than traditional cardio.  This is because the body has significantly lower levels of ghrelin immediately after hard exercise.  Ghrelin is known to be an appetite stimulant.  The body has elevated levels of blood lactate and blood sugar, which also decrease the drive to eat. The blunted appetite is said to have a spillover effect onto the next day.
  • Intense exercise increases your VO2max, which has been strongly correlated with telomere health.  Telomeres are DNA at the end of our chromosomes that affect how and when our cells age.  They protect our genetic data, it enables our cells to divide, and hold important information (yet undiscovered) as to how we age and how we develop cancer.  But scientists do know that telomere health is important to keeping your cells young and preventing many age-related diseases, or delaying them.  Protected telomere equals younger biological age.
  • High intensity exercise may be more effective at burning subcutaneous fat and abdominal fat than traditional steady-state cardio.  There may be a few mechanisms at play in order to cause this accelerated fat loss.  The mechanisms underlying the fat reduction induced by intense exercise are undetermined but may include accelerated fat oxidation during and after exercise and the reduction in ghrelin, which suppresses appetite. The increased fat oxidation is accompanied by what is called the afterburn effect, or scientifically called excess post-exercise oxygen consumption. In response to the increase in oxygen intake, body fat storage is broken down and fatty acids are released into the bloodstream as fuel.
  • Other than the obvious health benefits that come with high intensity exercise, you can save quite a bit of time over traditional cardio.  One study showed that 2.5 hours of high intensity exercise a week was equal to 10 hours of traditional cardio in terms of endurance benefits.

What Is This VO2max Everyone Keeps Talking about?

When talking about HIIT, the term VO2max gets thrown around a lot.  VO2max just means the maximum amount of oxygen your body is capable of utilizing (or taking in) in one minute.  The more oxygen you can take in, the more aerobic capacity your heart can withstand.  It’s a measure of your capacity to do aerobic work and a predictor of your potential as an endurance athlete.  When you are doing true HIIT, you are working at near VO2max (85-95%) during your high intensity intervals.  That means you are exerting maximum or near maximum effort.

To accurately measure your VO2max, you will need pretty expensive equipment that runs in the thousands of dollars.  But you can get a fairly good estimate of your VO2max by using the Astrand Test.  It’s a test using a stationary bicycle for 6 minutes.  You can see the instructions for the Astrand Test here.

VO2max and heart rate are related, but not the same.  There is a positive correlation between heart rate and oxygen consumption—the closer you are to your maximal heart rate, the closer you are to your VO2max.  So when you are doing interval training, you are going at (near) maximum heart rate and (near) maximum VO2max during the high intensity intervals.

Tips for Doing Interval Training Properly

Because high intensity interval training stresses the muscles a lot more than low intensity exercise, you need to be sure to give the muscles adequate rest.  It is recommended to give your body 48 hours of rest before doing another session of HIIT.  So if you an HIIT session at 7am on Monday, you shouldn’t be doing another HIIT session until 7am Wednesday.  And be sure to stretch and warm up before every session.  If not, you are leaving yourself open for injuries.

If you are interested in better endurance, train in short intervals (30 seconds to 5 minutes) at maximal speeds.  This is the most time efficient.

Don’t consume a lot of antioxidants right before a session.  Antioxidants reduces the (muscle) stress from exercise, meaning there will be less improvements.  If you are going to consume antioxidants, consume them after your HIIT session.

The most popular form of HIIT in testing environment is the Wingate tests; 30 seconds of maximal cycling with 7 percent of the subject’s body weight as resistance.  This is repeated 3-6 times, with 4 minutes rest in between.  So if you want to perfectly mirror the studies, use this regimen.

Another known effective form of HIIT is to do a combination of low intensity continuous exercise with high intensity intervals; for instance, treadmill intervals of sprinting at 8mph for 1 minute followed by jogging at 4pm for 1 minute, repeated several times.

Wind sprints are great for interval training because there are no speed constraints like there are on treadmills—you can go as fast as you want.

Are You Doing HIIT Correctly?

If you are new to interval training, you may be asking yourself if you are doing it correctly.  If you are 80 to 100 percent max effort during the high intensity periods, then you are more or less doing it correctly.  However, the anaerobic system fully kicks in at 90-100 percent of maximal effort.

If you are doing 90-100 percent of maximal effort with 15 to 30 seconds rest in between, you should feel like crap in less than 10 minutes.  I consider myself relatively fit but I can’t go past 6 minutes of interval training if it involves wind sprints.  I feel like death at the end.  I feel less tired after doing an hour of steady-state cardio than I do with 6 minutes of interval training.

Figuring Out Your Target Heart Rate

Your heart rate is an important factor in assessing the intensity of your workouts.  If you are wearing your heart monitor while exercising, you can check to see if you are between 75-90 percent of your maximum heart rate during interval training (high zone).  While resting, your heart rate should be at 65-75 percent (low zone) of your maximum heart rate.  Here is a good formula from the Journal of the American College of Cardiology for figuring out your ideal heart rate during interval training during both low intensity and high intensity intervals:

high zone and low zone heart rate

The equation is set at 70 percent for low zone and 85 percent for high zone, but you can change that.  The (208 -(age*.07)) part of the equation is your maximum heart rate.  And remember when multiplying percentage, you want to move the decimal places two places to the left. So 70 percent becomes .70 and 85 percent becomes .85.

So say I’m 30 years old and trying to figure out my target heart rate in the high zone.  Here are the steps:

  1. 30*.7=21
  2. 208-21=187
  3. 187*.85=158.95

My high zone target heart rate, using the 85 percent, is 158.95.

Final Thoughts

Although interval training can last up to 30 minutes per session, non-athletes should not be lasting that long.  Non-athletes should push themselves to the point where they are unable to go on after 20 minutes.  For HIIT, more does not mean better.  A 4 minute interval session can beat a 30 minute interval session if the 4 minute interval session pushed your body harder.

High intensity for everyone is different.  Running 10mph may be high intensity for your friend but 4mph may be high intensity for you.  That’s okay.  What matters is that you push your VO2max and your heart rate up so that it’s near its maximum threshold.

So what’s better?  High intensity interval training or steady-state cardio?  The answer lies somewhere in the middle—you should be training both your aerobic system and anaerobic system for maximum benefit.  Do HIIT 2-3 times a week and then do steady-state cardio with the days in between your interval training days.  Remember, you shouldn’t be doing HIIT on consecutive days.  This way, you can train both for endurance and explosiveness.

There you have it.  Now that you’ve gotten a basic introduction to HIIT, it’s time to go out and do it.

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