How Interval Training Boosts Fat Loss


I talk about different forms of interval training and circuit training on this blog on a regular basis.

How Interval Training Boosts Fat Loss

I tend to shy away from discussing things like lactic acid and oxygen debt because these are “zone out” terms.

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Many people tend to skim when blogs begin to read like Biology text-books. I want to explain how intervals work and go into more details, without making this a mind-numbing technical article.

Why Do We Push, Then Rest A Little, Then Push Hard Again?

Since intense exercise burns a lot of calories, why don’t we just push hard the whole time instead of mixing in periods of active rest?

I mean if you have 15 minutes to do some form of training aimed at fat loss, why not spend the whole time running hard on a treadmill or exercise bike?

It will make sense when I explain lactic acid, oxygen debt, and their roles in fat loss.

Increase Oxygen Debt For Maximum After-Burn Effects

When you do a sprint or an intense circuit, your body can’t supply oxygen at a rate fast enough to fuel the muscle.

Another energy system (anarobic) has to kick in to “assist” with providing energy to the muscle.

After the intense effort is completed, your body has to basically repay that “borrowed energy” it owes oxygen to get those muscles back to their normal state.

The more energy your body borrowed during an intense effort the more oxygen it owes this is called Oxygen Debt.

A Large “Oxygen Debt” is Key in Boosting the Metabolism

So after an intense effort like a sprint, you continue to breath hard and your heartbeat is raised this is your body’s way of repaying the oxygen debt.

Does that makes sense?

Your metabolism is operating at a higher rate (breathing and heart rate) to repay the oxygen it borrowed. I’m simplifying this a bit, but this is the basic principle.

The larger the oxygen debt created by your workout the longer it will take to repay it…with the benefit of more calories burned for a longer period of time after you are done exercising.

The Best Way to Create a Large Oxygen Debt

To create an oxygen debt, the effort has to be intense enough to switch over to “borrowed energy” -or- anaerobic mode.

Sprinting or proper resistance training is the way to accomplish this.

The reason things like intervals, kettlebell circuits, and other forms of resistance intervals work so well is that you are performing the next intense effort before “catching your breath”.

The combo of intense efforts while already breathing hard, is the best way to ensure maximum oxygen debt. This is one of the reasons why interval training is ideal for boosting the metabolism.

The New Way of Looking at Lactic Acid

Lactic acid is the result of intense effort and coaches used to tell athletes to avoid training in a way that produced too much lactic acid.

Professor Thomas D. Fahey of California State University thinks differently…

“The body converts glucose, a substance removed from the blood only sluggishly, to lactate, a substance removed and used rapidly…Using lactic acid as a carbohydrate middleman helps you [metabolize] carbohydrates from your diet, without increasing insulin or stimulating fat synthesis.”

Lactic acids helps your body burn more carbohydrates and ultimately more calories…assisting your weight loss efforts.

The Best Way to Increase Lactic Acid

An intense effort that can’t be sustained for long is what it takes to increase lactic acid.

It makes sense you would want to do a few of these intense efforts.

So, you would do an intense effort, rest a bit, then do another intense effort… sound familiar?

This is an exact description of interval training.

Doing the Intense Part of the Interval With Little Rest

We already talked about why pushing hard before catching your breath all the way was a way to increase oxygen debt, it is also a way to increase lactic acid.

After an intense effort you accumulate a bit of lactic acid in the muscle and then your body begins to remove it.

You want to perform another intense effort before the body is recovered to assist the body in accumulating more lactic acid in your muscles.

With enough intervals you will have a buildup of lactic acid…and because of this, your body will be able to burn more calories than normal after your workout is finished.

Proper Intervals Increase Lactic Acid and Oxygen Debt

So the reason why a trainer like Craig Ballantyne recommends short rest after his Crazy 8 Body Weight Circuit is to ensure that you increase both lactic acid and oxygen debt.

This is the same reason that you don’t rest 5 minutes in between intense efforts when doing HIIT.

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A proper interval training routine should be just enough rest to be able to push hard for the next intense effort and you should be breathing pretty damn hard when the whole series of intervals is finished!

Note: This article was written 10 years ago and now we have a slightly greater understanding of how HIIT works. I wrote a detailed article on it here: The Afterburn Effect Revisited



-Rusty Moore

As a former fitness coach to fashion models, I can teach you how to increase muscle definition without adding size.

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