M.E.L.T (Make Exercise Less Time- [Consuming])

All About High Intensity Interval Training

by The Precision Nutrition Team

What is high intensity interval training? High intensity interval training (HIIT) is when you alternate between high- and low- intensity exercise. For example, a short sprint up a flight of stairs followed by a walk back down is interval training....

Why is high intensity interval training so important?  It’s physiologically impossible to sustain maximal intensities during exercise for an extended amount of time. If I told you to go outside and run as fast as you can for 20 minutes, you could expect the first 10 to 20 seconds to go well. You’d be able to run very fast. That is because of phosphocreatine (a high intensity energy source).

After about 20 seconds, your phosphocreatine would be running low, and anaerobic glycolysis would predominate. At this point, more lactic acid would be produced and used as a fuel source. You would still be running as hard as you could, but your pace would be slowed and your lungs would be working overtime. If you were a member of the Canadian Olympic Hockey team, you could probably maintain this for 10 minutes. But those who are not well conditioned would need to slow down and even stop.

So why can’t you work at maximal intensity for an extended amount of time? One reason is the supply and demand of oxygen when working so hard. When you work at a lower intensity (e.g., brisk walk) aerobic metabolism predominates and uses oxygen to break down carbohydrate and fat for energy. This is very efficient. With aerobic metabolism you gain efficiency but lose intensity.

Understanding the aforementioned situation helps to comprehend HIIT. With HIIT, you can take advantage of these brief bouts of very intense exercise (e.g., 20 second sprint) and then work at a lower intensity (e.g., walk for 1-2 minutes) to allow recovery, but also utilize the aerobic energy system.

The hormones releases during exercise are secreted in proportion to exercise intensity. Norepinephrine and epinephrine are released and activate circulatory adjustments. When you approach 85 to 95% of VO2max, growth hormone, endorphins, epinephrine, norepinephrine, cortisol, and aldosterone all increase. With intense exercise, plasma testosterone can also increase.


What you should know Exercise ranges from gentle movements to maximal efforts. Yoga and tai-chi can lower stress hormones. HIIT and weights can elevate stress hormones.

High intensity exercise is a state of “crisis” in the body that endangers oxygen supply to tissues, increases body temperature, reduces body fluids and fuel stores, and causes tissue damage. The costs of intense exercise bring forth endocrine and defense reactions that are similar to those elicited by low blood oxygen, high blood carbon dioxide, acidosis, high body temperature, dehydration, low blood sugar, physical injury and psychological stresses.

In essence, high intensity exercise stresses the body so much that it's forced to adapt. That which does not kill us...

For those new to exercise, HIIT provides a more efficient way to strengthen your cardiovascular system and use fat stores for energy. For those that fall into the advanced category, HIIT offers a sport-specific way to develop energy systems and fitness.

If you have 12 minutes to fit in a workout, what do you think would benefit you more, a 12 minute walk or a 12 minute sprint?

Well, that is a trick question.

A 12 minute walk is definitely better than nothing, and a 12 minute sprint is physiologically impossible. The good news is that you can get the best of both worlds by alternating hard sprints and brisk walks. The HIIT style workout would benefit your health and body composition more than a walk.


For extra credit

--With very intense bouts of exercise, the body’s energy requirement can increase 2 to 20 fold!

--During the first 1-2 hours after intense bouts of exercise, your body’s energy requirements remain high.

--So what is high intensity? Well, during your next sprint, envision a crazy grizzly bear chasing you. That should suffice.

--HIIT will not only improve body composition, it may extend your life. The Harvard Alumni Health Study, a 4-year study of more than 17,000 men, found that only vigorous – not moderate – exercise reduced risk of death.

--Another nice side effect of HIIT is that you are challenging the fast twitch muscle fibers during workouts. These muscle fibers are great for strength, power and looking buff.

--Do an adequate warm up and cool down when performing HIIT.

Summary and Recommendations

· Pick one or two exercises of your choice (e.g., running, cycling, rowing, jumping, dumbbell swings, jump roping, swimming, etc)

· Begin with a 3-5 minute progressive warm-up

· Work at a high (work) intensity for 15 – 60 seconds

· Work at a low (recovery) intensity for 45 – 180 seconds

· Repeat for 3 – 10 bouts

· End with a 3-5 minute progressive cool-down

Note: You can use one exercise with alternating intensities (e.g., walk, sprint, walk, sprint, etc) or you can pick two exercises, one of higher intensity and one of lower intensity (e.g., jump rope, cycle, jump rope, cycle, etc)