High intensity interval training, known as HIIT for short, is the technique of alternating 30 to 120 second periods of very high intensity aerobics (sprints, also known as the "work interval") with 30 to 120 second periods of low to moderate intensity (the recovery interval). During the work interval, you actually push yourself outside of your target heart zone (above 85%) to the point where you begin to lose your breath. You then reduce the intensity enough during the recovery interval so you reclaim the oxygen debt just in time to do another work interval.
HIIT has received a lot of press lately as being superior to steady state exercise. In some ways, it IS superior: HIIT burns a lot of calories during the workout, but where it really shines is after the workout. Your metabolic rate stays elevated longer after the workout is over than steady state cardio. This increase in the metabolism is called excess post-exercise oxygen consumption or EPOC for short. That's right - this means you burn calories all day long after your workout is over (Imagine burning extra fat as you sit at your desk at work!)
That's the good news. The bad news is, the degree of EPOC is not as great as most people think. It's a myth that your metabolism stays elevated for 24 hours after a regular aerobic workout. That only happens after extremely intense and/or prolonged exercise such as running a marathon.
After low intensity exercise, the magnitude of the EPOC is so small that its impact on fat loss is negligible. Somewhere between 9 and 30 extra calories are burned after exercise at an intensity of less than 60-65% of maximal heart rate. In other words, a casual stroll on the treadmill will do next to nothing to increase your metabolism.
However, EPOC does increase with the intensity (and duration) of the exercise. According to Wilmore and Costill in "Physiology of Sport and Exercise," the EPOC after moderate exercise (75-80%) will amount to approximately .25 kcal/min or 15 kcal/hour. This would provide an additional expenditure of 75 kcal that would not normally be calculated in the total energy expended for that activity. An extra 75 calories is definitely nothing Earth shattering, but it adds up over time. In a year that would mean (in theory) you would burn an extra 5.2 lbs of fat from the additional calories expended after the workout.
Studies on the effects of HIIT have demonstrated a much higher EPOC for interval training than steady state training, which can add substantially to the day's calorie expenditure. In one study, scientists from the University of Alabama compared the effects of two exercise protocols on 24-hour energy expenditure. The first group cycled for 60 minutes at a moderate intensity. The second group performed HIIT, cycling for two minutes at high intensity followed by two minutes at a low intensity. The group that performed the HIIT burned 160 more calories in 24 hours than the low intensity group. That means the HIIT group would burn an extra 11.8 pounds of fat in one year if they did HIIT five days a week instead of conventional training.
Ironically, weight training has a much higher magnitude of EPOC than aerobic training. Studies have shown increases in metabolic rate of as much as 4-7% over a 24hour period from resistance training. Yes - that means weight training does burn fat -albeit through an indirect mechanism. For someone with an expenditure of 2500 calories per day, that could add up to 100 - 175 extra calories burned after your weight training workout is over. The lesson is simple: Anyone interested in losing body fat who isn't lifting weights should first take up a regimen of weight training, then - and only then -start thinking about the HIIT!
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