You may have heard of something called the afterburn effect, which is the process your body goes through post workout to burn fat. The scientific term for this process is EPOC or excess post exercise oxygen consumption. The answer to how it works is basically in the name; during exercise your body goes into an oxygen deficit, due to your body not being able to consume all of the oxygen required to make energy during exercise. Since you are in an oxygen deficit, after your workout your body will be taking in extra oxygen in order to “pay off the debt” so to speak. This process burns calories even when you are at rest. Consuming 1L of oxygen burns 5 calories and when you are at rest most or all of those calories burned will come from fat. You may be asking, since taking in more oxygen burns more calories how can I increase the amount of oxygen I consume? The answer will come towards the end of the post.
First, we’ll explore three different methods of exercise and their effect on EPOC. These are LISS, circuit training and HIIT.
One example of LISS or low intensity steady state cardio is hopping on a treadmill and going at a brisk walk or a jog that’s easy enough for you to maintain for about an hour. In the past it was widely believed that in order to burn fat you have to enter a so-called ‘fat burning zone’, for an hour or longer, at a pace that takes you to 60-70% of your max heart rate. The thought behind it is that your body uses fat as a fuel at lower intensities and when you ramp up the intensity of your workout it will need to rely on a faster burning fuel, glucose. Although this is true, it is a little misguided since your body uses fat as a fuel source when you are typing, reading, driving and even in your sleep. In light of this, LISS cardio has the lowest EPOC effect of the methodsI have listed earlier. It is not to say that there is no place for LISS in a fat burning program, but the idea that it is the key to burning fat is outdated.
Circuit training requires energy more quickly because of the higher intensity. One way to perform this type of training is doing approximately 4-6 different exercises back to back, resting for about a minute and then repeating the whole process three more times. (for maximal benefits incorporate more multi joint and combination exercises instead of isolation exercises) This keeps the heart rate elevated to around 80% of your max and will cause you to tap into the energy systems that do not use oxygen to make energy. During your rest you will be replenishing your energy stores, by consuming more oxygen, for you to attack the circuit again. This type of training causes a greater oxygen deficit than LISS. It is for that reason that the afterburn effect lasts longer than after performing a circuit training session. It is also worth noting that heavy resistance training with short periods of rest between sets can create similar effects on EPOC.
HIIT or high intensity interval training by far has the greatest effect on EPOC thus making it key to an effective fat burning program. HIIT involves performing an exercise at maximal or near maximal effort for a short period of time with brief rests in between to be repeated several times. For example getting on an exercise bike and going as fast as you can for 20 seconds and resting for 10 seconds then repeating that four or five more times. (this would be a form of HIIT referred to as Tabata) Since HIIT taxes your energy levels at a faster rate than the other methods of exercise mentioned, it creates the greatest oxygen deficit of all of them. The post workout afterburn can last up to 36 hours after a HIIT session. To top it all off, a proper HIIT session can be done in 15 minutes or less saving you time and making your workout very efficient. This goes to prove that the intensity of the workout has a greater influence on burning fat than the length of your workout.
In case you are still asking how you can increase your oxygen consumption, here is the answer. If you want a better afterburn effect from your workouts, you should prioritize HIIT and circuit training in your program.
Finally, I want to note that it would be wise to avoid performing HIIT and circuit training on back to back days since your body would require some time to recover. If you want to exercise the next day, a LISS session for 30 to 40 minutes would be a good idea as it would enhance the fat burning effect you would already be experiencing.