Most personal trainers and general fitness sites will tell you that HIIT is effective because it allows you to burn calories long after the workout has finished. This is often referred to as “the afterburn effect”. The scientific term is EPOC: Excess Post-exercise Oxygen Consumption. The idea is that although HIIT workouts may be short, you burn calories long after the workout is complete.
I USED to believe this was the case.
A game-changing paper came out which studied all of the various EPOC studies over the past 30 years. LaForgia J et. al. Effects of exercise intensity and duration on the excess post-exercise oxygen consumption. J Sports Sci. 2006 Dec;24(12):1247-64.
“…the earlier research optimism regarding an important role for the EPOC in weight loss is generally unfounded…The role of exercise in the maintenance of body mass is therefore predominantly mediated via the cumulative effect of the energy expenditure during the actual exercise.”
This paper came to these conclusions:
EPOC = 6-15% of the calories burned during the actual exercise.
Steady-state cardio averages close to 7% EPOC.
Intense intervals can approach 14% EPOC.
What does this mean in actual calories burned?
If you walk for an hour on a treadmill at a steady pace and burn 600 calories, you will burn an additional 42 calories after the workout is over (7% of 600).
If you perform HIIT on a stair climber for 20 minutes and burn 400 calories? You will also burn an additional 56 calories after the workout is over (14% of 400). These aren’t exact numbers or anything. The main point is that focusing on calories during the ACTUAL workout is more important than any small afterburn effect benefit. This doesn’t mean that HIIT is useless. There certainly are benefits outside of calories burned after the workout.
The intense portion of each interval typically is spent above your Lactate Threshold.
*I will explain the lactate threshold in the next section.
This is the IDEAL level of intensity for rapid calorie burning and glycogen depletion.
You can’t train at this level for very long, which is why the rest portions of the interval are important. But…
Spending a portion of your workout in this rapid calorie-burning zone helps immensely if fat loss is a goal. High-Intensity Interval Training also has these benefits:
Releases Free Fatty Acids from Fat Cells
Improves VO2 Max
Increases Lactate Threshold
Increases HGH Release
In short, it is a way to rapidly burn calories and stored glycogen while improving overall fitness. Improving VO2 Max is especially interesting because people with a high VO2 Max burn more calories doing the EXACT same activity as someone who is less fit.
Another benefit of HIIT is improving the stroke volume of your heart.
During the recovery period of intervals, your heartbeat slows down faster than blood flow. Because of this, the heart pumps more blood per beat (higher stroke volume). There is even a recent study showing that interval training slows down the ageing process on a cellular level. How Interval Training Slows the Aging Process So although HIIT doesn’t have as high of an afterburn effect as we once thought. It is extremely effective for fat loss, heart health and possibly a bit of anti-ageing as a bonus.