High Intensity Interval Training (HIIT)


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

Prior to Exercise

Before you begin an exercise program, take a fitness test, or substantially increase your level of activity.  The physical activity readiness questionnaire (PAR-Q) will help determine if you’re ready to begin an exercise routine.  If otherwise than an healthy individual, prior to beginning any exercise program, individuals should seek medical evaluation and clearance to engage in activity.  Not all exercise programs are suitable for everyone, and some programs may result in injury.  Activities should be carried out at a pace that is comfortable for the exerciser. 


Prior to beginning HIIT training, a person should establish a foundational level of fitness or “base fitness level”.  A base fitness level is consistent aerobic training (3 to 5 times per week for 20 to 60 minutes per session of moderate to intense levels) for several weeks to produces muscular adaptations, which improve oxygen transport to the muscles.  Establishing appropriate exercise form and muscular strength are import before engaging in HIIT, to reduce the risk of musculoskeletal injury.  Regardless of age, gender and fitness level, one of the keys to safe participation of HIIT training is for all people to modify the intensity of the work intervals to a preferred level.  Safety in participation should always be the primary priority, and people should focus more on finding their own optimal training intensities as opposed to keeping up with other individuals.

HIIT workouts are more exhaustive than steady state workouts.  A longer recovery period is often needed. Start with one HIIT training workout per week with your other steady state routines.  As progression allows, add a second HIIT workout per week, making sure to spread them out evenly throughout the week.  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.


1 – Phosphocreatine

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.

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 highly conditioned individual, 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, you might even consider throwing up, due to the change in blood pH levels.

3 – Aerobic metabolism

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

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.  When you work at a higher intensity (such as sprinting), anaerobic metabolism predominates.  We have these two systems, both of which have their own advantages and disadvantages.

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.


Hormone release during exercise depends on exercise intensity.

Low intense movements can lower stress hormones, but when you approach 85 to 95% of your maximal heart rate (MHR), growth hormone, testosterone, endorphins, epinephrine (adrenaline), norepinephrine (noradrenaline), cortisol, and aldosterone all increase. These hormones all have effects on body composition and anabolis.

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.

High intensity exercise stresses the body so much that it’s forced to adapt.


HIIT training has been shown to improve:

·       aerobic and anaerobic fitness

·       blood pressure

·       cardiovascular health

·       insulin sensitivity (which helps the exercising muscles more readily use glucose for fuel to make energy)

·       cholesterol profiles

·       abdominal fat and body weight while maintaining muscle mass.

Tabata Study

One of the most famous studies of HIIT is known as the Tabata study. In this study, subjects performed rowing intervals: 20 seconds of fast rowing alternated with 10 seconds of 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.

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

High-intensity exercise of any type brings with it a higher risk of musculoskeletal injury and cardiac events. But along with healthy subjects, HIIT has been studied as a training method for people with heart disease and congestive heart failure. Under clinical supervision, subjects were able to tolerate high-intensity intervals without negative effects. Most importantly, they experienced bigger improvements in cardiovascular function compared to those undergoing continuous moderate-intensity training.

Choose an aerobic exercise—like stationary bicycling. Warm up for 5 minutes, and perform just a few alternating speed and recovery intervals; 3-4 of each should be plenty and will give you a feel for it; finish with an easy cool down.


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HIIT protocols vary widely.

Gradually work up to 8-10 or more speed intervals, depending on your fitness goals. The most common mistake made with interval training is making the recovery intervals too short. Perform HIIT workouts 1-2 times a week at most to reduce your risk of injury.

When developing a HIIT program, consider the duration, intensity, and frequency of the work intervals and the length of the recovery intervals. Intensity during the high intensity work interval should range ≥ 80% of your estimated maximal heart rate. As a good subjective indicator, the work interval should feel like you are exercising “hard” to “very hard”. Using the talk test as your guide, it would be like carrying on a conversation, with difficulty. The intensity of the recovery interval should be 40-50% of your estimate maximal heart rate. This would be a physical activity that felt very comfortable, in order to help you recover and prepare for your next work interval.