What Happens To Your Body When You Do A HIIT Workout? The science behind High Intensity Interval Training.

May 19, 2016


Have you ever read the Heart Rate chart on a cardio machine at a gym? Or chosen a cardio machine program that says "fat burning?" It sounds like just what we're all looking for, right? I mean, we all want to lose a little fat. So why choose a tough HIIT workout, when you can do a lower intensity cardio routine that specifically burns fat? The real truth lies in the details. Here's what's really happening in your body during and even more importantly, after, 3 different types of 30 minute workouts for a 35 year old woman of average weight.


Workout #1- “Fat Burning” Zone 

This represents a typical low to moderate intensity, steady state cardio workout, like walking on the treadmill at 3.5mph for 30 minutes


Total calories burned during workout= 126

% of calories from fat = 84%

Total calories burned 24 hours after workout = 0


Workout #2- “Cardio Training” Zone

Moderate intensity, steady state cardio workout, like running at 5 mph for 30 minutes.


Total calories burned during workout= 228

% calories  from fat= 66%

Total calories burned 24 hours after workout= 0


Workout #3- High Intensity Intervals (HIIT)

Varying intensity workout, in this example running at 5 mph for 40 sec. (low) and 9 mph for 20 sec. for a total of 30 min.


Total calories burned during workout= 298

% calories from fat= 35 %

Total calories burned 24 hours after workout= 200

Total calories burned = 498


So what’s the reason for the additional calorie burn after you’ve finished the HIIT workout?  It’s called EPOC, which stands for Excess Post-exercise Oxygen Consumption.  In simple terms, it’s the additional energy your body uses to return to equilibrium or homeostasis.  This process includes replacing muscle glycogen (stored energy within the muscles), repairing muscle fibers, returning the body temperature to normal and restoring oxygen levels in the blood and other body systems - the higher the intensity of a workout, the great the EPOC effect.  HIIT workouts provide your body with multiple opportunities for your body to work anaerobically, increasing the body’s need to replace energy stores and increasing the EPOC required to bring the body back to it’s normal resting state.  

So what’s the bottom line?  HIIT workouts are great for burning lots of calories in a short amount of time, as well as improving cardiovascular and muscular endurance and strength.  It’s important to start each workout with a proper warm up to prepare your body to work at a high intensity.  To get optimal results, HIIT workouts should be performed no more than 3-4 days per week to allow for recovery time in between.  It is beneficial to perform low to moderate intensity workouts on the days in between HIIT workouts.  The bottom line is that if you’re going to spend any amount of time working out, you might as well get the biggest bang for your buck and do HIIT!

Please reload

Featured Posts

I Grocery Shop on Facebook...

April 27, 2018

Please reload

Recent Posts

October 4, 2017

September 5, 2017

April 16, 2017

Please reload