By James Fell

Interval training causes a mega-caloric "after burn"

I'll make it simple: When you exercise, you burn more calories than you do sitting on the couch. The more intensely you exercise the more calories you burn.

Despite this reality, many look for tricks to increase resting metabolism and burn extra calories while doing nothing. Bill Phillips, author of Body for Life, endorsed High-Intensity Interval Training (HIIT), asserting, "Research indicates that not only does high-intensity training burn fat more effectively than low-intensity exercise ... It also speeds up your metabolism and keeps it revved up for some time after your workout."

Bill makes numerous references to "research" and "studies" in his book, yet doesn't actually name them, and there's no bibliography. Regardless, his exalting of HIIT as the ultimate fat-burner caught on with a vengeance. Personal trainers, websites, books, and magazines (including the highly popular Men's Health magazine) often recommend this training method for people seeking to accelerate weight loss. Anecdotally, I regularly see people at the gym constantly changing speeds between fast and slow on treadmills, bikes and elliptical trainers to get some perceived lasting metabolic boost from interval training.

It makes me shake my head.

Through an assistant, Phillips told me that he based his claims on a 1994 study in the journal Metabolism that examined exercise intensity and fat loss. But in that study, neither the people in the lower-intensity training group nor the HIIT group lost much weight at all, and measurements of fat loss in the torso were very similar for both groups. Members of the HIIT group had 20% more body fat to start with, and those in the lower-intensity group gained fat in their calves for some reason, which also skewed the data.

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Good for speed, but not for fat burning

Don't get me wrong; interval training is a useful method for improving athletic performance by enhancing both maximum oxygen uptake and anaerobic metabolism, and competitive runners, rowers, cyclists, and other athletes do use it to get faster. If you’re interested in speed, I do recommend engaging in it a couple of times a week. But know that it doesn't burn calories any better than exercising at a steady pace.

HIIT is mistakenly lauded by fat-loss "gurus" because of its alleged potential to dramatically boost Excess Post-Exercise Oxygen Consumption (EPOC). This is the caloric after-burn of a metabolism that stays elevated -- or "revved up," in Phillip's words -- and consumes extra calories after completion of an intense exercise session.

EPOC does exist, but it's been vastly overblown.

In a 2006 article in the Journal of Sports Sciences, researchers from the School of Pharmacy and Medical Sciences at the University of South Australia did find that more intense exercise creates a higher EPOC, but even so: "EPOC comprises only 6-15% of the net total oxygen cost [calories burned] of the exercise." For example, if someone burns 1,000 total calories via intense and prolonged exercise, then at most an additional 150 calories result from EPOC.

What's more, the better shape a person is in, the lower the EPOC because the body's metabolism returns to normal sooner. In 1990 researchers from Flinders University's Exercise Physiology Laboratory studied nine men and reported in the Journal of Applied Physiology that in well-trained people, EPOC could be as low as 1%, and the mean was 4.8%. I'm in good shape, so if I fit into that "mean," then after my usual eight-mile run, EPOC calorie burning earns me about one-third of a beer. Don't wait up.

The modesty of EPOC in trained people appears logical, because the first time I tried running, my heart was ready to explode out of my chest, my lungs felt like I'd inhaled a bunch of fire ants, and for a long time afterward, I was ready to barf up my toenails. Now that I'm "well trained," I recover quickly, meaning reduced EPOC.

So what about HIIT? As I've already mentioned, EPOC is of little consequence, even for intense exercise, so does engaging in intervals somehow change this dynamic?

No, it doesn't. In reality, if you do the same amount of exercise "work" (i.e. distance traveled) using either a steady pace or via HIIT, you burn close to the same number of calories, including the minimal EPOC.

In 1997, the aforementioned Flinders University researchers took another look at EPOC and how interval training affected it. Using eight men, they compared 30 minutes of running at 70 percent maximal oxygen uptake vs. 20 rounds of one-minute running at 105 percent. The high-intensity sessions involved two-minute rest periods after each one-minute round, where I assume participants used the breaks to say things like "gasp" and "hack" and "please let me die."

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Caloric after-burn: Not all it's cracked up to be

Because of the rest intervals, the HIIT took twice as long to complete the exercise sessions, but it did pay off. And by "pay off," I mean running all the numbers of calories burned both during and in the nine hours after the exercise, the high-intensity training burned about half an Oreo cookie more worth of calories, which hardly seems worth it. I mean, I'm not even talking Double Stuff. Oh, and EPOC was again shown to be only a small portion of total calories burned.

Eight years later, researchers from the Faculty of Physical Education and Recreation at the University of Alberta published a study in the International Journal of Sport Nutrition and Exercise Metabolism examining 12 cyclists and asserted that "The magnitude and duration of EPOC were similar" for both interval and continuous training of the same total work. In other words, the minimal caloric after burn of exercise was the same.

Interval training is not some holy grail of fat loss; it is a tool for getting faster. As any teenage boy whose girlfriend's parents came home early will tell you, being quick is a useful skill, but the National Strength and Conditioning Association asserts that interval training is not for the out-of-shape and should be used sparingly because of its punishing intensity.

If fat loss is your goal, then the best pace to perform aerobic exercise at is the one you can sustain for long periods and therefore maximize caloric burn during the exercise. It helps if it's also a pace that doesn't make you hate your workout.

Has this myth blown your mind? Then find out why muscle mass doesn't burn as many calories as you think.