All About High Intensity Interval Training (HIIT)

By Ryan Andrews, MS, MA, RD, RYT, CSCS

Searching for the most efficient way to get lean, get conditioned, and get tough? Look no further than HIIT. We can’t promise it’s easy…but it sure gets results.

What is high intensity interval training?

High intensity interval training (HIIT) is when you alternate between high and low intensity exercise(s) or between high intensity exercise and a short period of rest.

For example, a short sprint up a flight of stairs followed by a walk back down is interval training. Or a set of burpees followed by bodyweight rows.

Example of HIIT on a Treadmill

Example of HIIT with Bodyweight

If you’ve ever participated in HIIT, you know that alternating bodyweight conditioning exercises for 15 minutes can be a lot more challenging than a walk around the block.

Why is high intensity interval training so important?

It’s physiologically impossible to sustain maximal intensities during exercise for an extended amount of time. This is because of how our bodies use fuel.

Let’s say I tell you to go outside and run as fast as you can for 20 minutes.

Stage 1 – Phosphocreatine

OK! The first 10 to 20 seconds are going great! You’re sprinting like the wind! That’s because you’re using a high-intensity energy source known as phosphocreatine.

Stage 2 – Lactic acid and anaerobic glycolysis

After about 20 seconds, your phosphocreatine start to run low, and anaerobic glycolysis would predominate. At this point, more lactic acid would be produced and used as a fuel source.

You’re still be running as hard as you can, but you’d be slowing down, and your lungs are working overtime.

If you were a member of the Canadian Olympic Hockey team or an elite speed skater, you could probably maintain this for up to 10 minutes. But those who are not well conditioned would need to slow down and even stop. If this is your first time off the couch, you might even consider throwing up, thanks to the change in blood pH levels.

Well, it looks like the spirit is willing but the flesh is weak. 20-minute sprint challenge: FAIL.

So why can’t you work at maximal intensity for an extended amount of time?

Oxygen: The molecule that makes the magic

One reason is the supply and demand of oxygen when working so hard.

Nature is full of trade-offs. In this case, we trade efficiency for intensity.

When you work at a lower intensity (such as during a brisk walk), aerobic metabolism predominates.

Your body uses oxygen to break down carbohydrate and fat for energy. This is very efficient, but you can’t work at top speed. With aerobic metabolism, you gain efficiency but lose intensity. Evolutionarily, this would be useful for traveling long distances while foraging for food or water.

On the other hand, when you work at a higher intensity (such as sprinting), anaerobic metabolism predominates.

Your body can’t get oxygen to where it needs to go fast enough. This is very inefficient, but it lets you produce short bursts of speed or high energy — very handy when you’re running away from a sabre-toothed tiger or a rock-wielding Grok.

So, we have these two systems, both of which have their own advantages and disadvantages. What if we could have our cake and eat it too? (Or, in this case, lose the fat we gained by eating the cake in the first place.)

Enter HIIT.

With HIIT, you alternate short bursts of very intense exercise (such as 10-20 sec of sprinting) with periods of lower intensity (such as 1 min of walking).

The higher intensity periods create a metabolic demand that is very effective for long-term fat loss and overall conditioning.
The lower intensity periods let you recover and use the aerobic energy system.

Contribution of different energy systems to exercise, depending on how long the exercise takes

Hormones

In addition, hormone release during exercise depends on exercise intensity.

Gentle movements such as yoga, tai-chi, or a pleasant stroll outside can lower stress hormones.

But when you approach 85 to 95% of VO2max, growth hormone, testosterone, endorphins, epinephrine (adrenaline), norepinephrine (noradrenaline), cortisol, and aldosterone all increase. These hormones all have effects on body composition and anabolism.

What you should know about HIIT

Exercise can range from gentle movements to maximal efforts. HIIT and heavy weights can elevate stress hormones.

Most every high intensity physical activity is a state of “crisis” in the body. It endangers oxygen supply to tissues, increases body temperature, reduces body fluids and fuel stores, and causes tissue damage.

Intense exercise creates 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.

Hormonally, your body basically freaks out. Then it brings out the big guns to deal with the problem. High intensity exercise stresses the body so much that it’s forced to adapt.

As Nietzsche gasped during a 20-rep squat set, “That which does not kill me makes my quads bodacious.” (It makes more sense in German.)

Why HIIT?

HIIT is excellent for:

losing body fat (while retaining lean body mass)
strengthening the cardiovascular system
developing sport-specific energy systems (e.g. training for that Olympic hockey team)
developing “work capacity” (i.e. the ability to tolerate a high level of intensity for a longer period)
improving fat and carbohydrate oxidation in skeletal muscle
developing “mental toughness”
making you a badass
challenging the fast twitch muscle fibres — the fibres that are great for strength, power and looking buff

HIIT is extremely efficient. It lets you get a bigger training effect with less time spent. And compared to a 45-minute jog, 5 min of HIIT is a lot easier on the joints.

How to do HIIT

There are many ways to do HIIT. All you need to remember is the basic principle: Alternate short bursts of very high intensity with periods of recovery/low intensity.

The longest 4 minutes of your life: The Tabata study

One of the most famous studies of HIIT is known as the Tabata study. In this study (see abstract below), subjects performed rowing intervals: 20 sec of ultra-fast rowing alternated with 10 seconds of relaxed recovery rowing, for a total of 8 intervals, or 4 minutes.

At the end of the study, participants showed a 28% increase in anaerobic capacity along with a 14% increase in V02max. Pretty impressive!

The “Tabata protocol” — 20 seconds on, 10 seconds off — has become one of the most common methods of doing HIIT.

Weighted circuits

Using resistance exercises can be a very effective method of doing HIIT.

To use resistance training, choose compound exercises that are good “oxygen suckers”, such as:

burpees
kettlebell swings or snatches
jumping squats

Combo exercises are also good choices. For instance:

do a pullup, drop down, do a pushup, jump back up to the pullup bar for another pullup, and so on
hang clean + front squat + overhead press
dumbbell snatch + wrestler’s sprawl

You can also combine resistance exercises with “cardio” type exercises.

For example

alternate a set of dumbbell swings with 100 m sprints
sprint up a hill, then do a fast set of pushups at the top
carry a heavy sandbag for speed

Customizing HIIT for your goals

You can mix up your HIIT choices to avoid overtraining and overuse injuries, and to keep things fresh and interesting.

If you’re a competitive athlete who needs energy systems work for your sport, incorporate some sessions of sport-specific HIIT work. For example:

sprints for soccer players
jumps for volleyball and basketball players
heavy bag intervals for boxers
etc.

You can also vary the length of your intervals.

High intensity intervals can last anywhere from 10 to 60 seconds
Low intensity recovery periods can last anywhere from 10 seconds to a minute or more

If you’re new to HIIT, opt for shorter high intensity periods and longer low intensity periods.

And note: “high intensity” means “high intensity for YOU”. If you’re a beginner, a fast jog or uphill walk for 10 seconds is a better start than trying to handle an all-out sprint workout.

Don’t forget: Perform an adequate warm up and cool down when performing HIIT.

For extra credit

The benefits of HIIT are exercise specific. If you do squat jumps and running, you get better at squat jumps and running.

You can maximize your HIIT performance by using non-competing exercises. Instead of doing split squat jumps alternated with squat jumps, which use the same muscles, you could pair up bodyweight rows and squat jumps.

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

During the first 1-2 hours after intense 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.

The Gymboss is a handy timer for interval workouts.