HIIT is an acronym used to describe high-intensity interval training. It is a work out characterized by quick bursts of high effort exercise followed by short, sometimes active, periods of recovery. If you have ever completed a Tabata workout or Fuse 45™ Class, you’ve done HIIT.

But, how does HIIT actually improve your health and fitness?

Many studies on HIIT exercise use VO2 max as a measure of fitness. VO2 max is measured in terms of milliliters of oxygen consumed per minute per kilogram of body weight (ml/min/kg) – it is your body’s ability to consume oxygen. Your VO2 max, however, is not constant. It can be impacted by how much blood your heart can pump, your red blood cell count, and how adapted your muscles are to exercise, etc. In a lab, a VO2 max is measured by finding the point where a person’s oxygen consumption flat lines while running on a treadmill, where heart rate is maximized. With all else being equal, a higher VO2 max is better; a given amount of effort combined with a higher VO2 max, will allow you to work harder.

In many studies, training programs were developed to compare continuous training to interval training (or HIIT). An analysis of these studies has proven that HIIT training will in fact lead to larger increases in a person’s VO2 max, than continuous training, and such exercise “generates marked increases in VO2 max in almost all relatively young adults”. Mixing in continuous training like running or walking definitely won’t hurt you (unless of course you have knee issues), but it’s really important to mix in your HIIT to increase your fitness level and your overall health.

Resting after any type of intense exercise, allows the body to recover from an oxygen deficit created during the initial exercise. Fundamentally, there are certain times when the body is consuming more oxygen than what is available to it. After the workout is over, the body must recover this oxygen in what is known as EPOC (excess post-exercise oxygen consumption).
VO2 is your body’s ability to consume oxygen.
Studies have found that EPOC increases in a linear fashion due to increasing exercise duration. However, EPOC also increases in an exponential fashion with regard to increasing exercise intensity. Higher EPOC will be generated by higher intensity exercise as opposed to lower intensity exercise, given the same amount of calories burned in a workout. This is where HIIT is relevant. Measuring EPOC in terms of the total number of calories burned, has shown that calories burned due to EPOC are equal to about 13% of total energy used during HIIT, but only 7% of total energy used during a steady state workout.

If you are looking to increase your fitness and calorie-burn after you workout, HIIT is for you.