One of the primary bases upon which the superiority of HIIT is based is the idea of the calorie afterburn that occurs after training. Technically this is called the Excess Post-Exercise Oxygen Consumption or EPOC. Since it relates to the topic of steady state vs. interval training, I want to examine it here.
Recovery from a bout of exercise is associated with an elevation in metabolism referred to as the excess post-exercise oxygen consumption (EPOC). A number of investigators in the first half of the last century reported prolonged EPOC durations and that the EPOC was a major component of the thermic effect of activity. It was therefore thought that the EPOC was a major contributor to total daily energy expenditure and hence the maintenance of body mass.
Investigations conducted over the last two or three decades have improved the experimental protocols used in the pioneering studies and therefore have more accurately characterized the EPOC. Evidence has accumulated to suggest an exponential relationship between exercise intensity and the magnitude of the EPOC for specific exercise durations.
Furthermore, work at exercise intensities >or=50-60% VO2max stimulate a linear increase in EPOC as exercise duration increases. The existence of these relationships with resistance exercise at this stage remains unclear because of the limited number of studies and problems with quantification of work intensity for this type of exercise. Although the more recent studies do not support the extended EPOC durations reported by some of the pioneering investigators, it is now apparent that a prolonged EPOC (3-24 h) may result from an appropriate exercise stimulus (submaximal: >or=50 min at >or=70% VO2max; supramaximal: >or=6 min at >or=105% VO2max).
However, even those studies incorporating exercise stimuli resulting in prolonged EPOC durations have identified that the EPOC comprises only 6-15% of the net total oxygen cost of the exercise. But this figure may need to be increased when studies utilizing intermittent work bouts are designed to allow the determination of rest interval EPOCs, which should logically contribute to the EPOC determined following the cessation of the last work bout.
Notwithstanding the aforementioned, the earlier research optimism regarding an important role for the EPOC in weight loss is generally unfounded. This is further reinforced by acknowledging that the exercise stimuli required to promote a prolonged EPOC are unlikely to be tolerated by non-athletic individuals. 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.
In the last year or three, exercise programs for fat loss have been geared around the concept of using certain types of training (either interval style cardio or highish rep/short rest weight training) to cause fat loss through an ‘afterburn‘ effect where calories are burned after workouts to a greater degree than following standard training styles (esp. low intensity cardio). Clearly from a real-world perspective, this type of training ‘works’.
Of course so do a lot of other kinds of training including the standard bodybuilder model of doing heavy weights to maintain muscle and using diet/low intensity cardio to lose fat but that’s neither here nor there right now. But a question is whether the high intensity interval types of programs are actually working via the mechanism of EPOC (as is usually claimed).
Put differently, there’s no doubt that there is an EPOC following training. The important question is what the actual magnitude of that calorie burn is and whether or not it’s reasonable to except amazing results based on EPOC per se. This paper looks at the topic in enormous detail.
The first topic discussed is what EPOC actually represents. An outdated concept is that the post-exercise calorie burn represented an ‘oxygen debt’ representing the difference between what the body needed and what was available, this turns out to be simplistic and wrong. Lactate metabolism, phosphate resynthesis and fatty acid cycling, along with increases in catecholamine levels are likely the cause of the post-exercise calorie burn. Ultimately, the mechanisms are less important than the fact that EPOC is the result of a metabolic perturbation that has to be ‘repayed’ afterwards.
I’m not going to detail the next section of the paper as it dealt with a bunch of boring methodological issues. Sufficed to say that accurate measurement of EPOC requires that certain methodologies be adhered to. One huge confound, which is likely the cause of the ‘exercise raises metabolism for 24 hours’ thing is due to a massive methodological flaw in early studies: they didn’t take into account the thermic effect of eating. It’s easy to mistake the thermic effect of eating with an effect of exercise. Good studies take this into account. Other issues such as taking into account baseline metabolic rate and subject characteristics are also important.
Continuous Exercise and EPOC
The next section of the paper deals with continuous exercise and the impact of both duration and intensity on EPOC. Without going into every paper detailed in the review, the picture that has developed from the research is that EPOC goes up linearly with increasing exercise duration but exponentially with increasing intensity. That is, higher intensity exercise generates the higher EPOC. This is true if the duration is the same or if the same number of calories are burned.
That is, if two people both burned 300 calories during exercise but one exercised at a high intensity and one at low intensity, the high intensity guy would get about double the EPOC. The problem is that, even under these conditions, the EPOC is still pretty minimal in an absolute sense (e.g. total number of calories burned).
In one study, subjects who exercised for 80 minutes at 70% VO2 max (about 80% of maximum heart rate) had an EPOC lasting 7 hours. But it only amounted to about 80 calories extra burned. Not to mention that only the most well trained individuals could sustain such a workload in the first place. As well, this still represented a rather small proportion of the total calorie burn from the exercise bout itself. That is, most of the calories burned were from the 80 minutes of exercise, the small EPOC only added a bit to that. Yeah, every little bit helps but which is going to contribute more to fat loss: the 700-800 calories burned during the exercise bout itself or the 80 calories burned afterwards?
The Impact of Intensity on EPOC
Additionally, it appears that there is an intensity threshold to generate any EPOC at all, compared to exercise at 30-50% VO2 max (50% VO2 is about 65% of max HR or the typical ‘fat burning’ zone), exercise at 75% generates a larger EPOC. However, the total calorie burn is still relatively small overall, averaging perhaps 7% of the total energy burned.
So if you burn 600 calories with high intensity continuous exercise, you might burn an additional 45 afterwards. While this certainly adds up over long periods of time, it’s still relatively insignificant compared to the total energy expenditure of the exercise bout. Again, which is more important for fat loss:the 45 extra calories you burned via EPOC or the 600 calories you burned with the exercise bout itself?
The next section of the paper dealt with supramaximal work, intervals basically. Interestingly, the data available here finds that relatively short amounts of intervals can generate EPOCs comparable to much longer bouts of continuous exercise. Several studies measured EPOCs from relatively short interval workouts on par with studies using much longer (>50 minutes) of moderate intensity work.
Still, the total magnitude (total calories burned) of the EPOC was relatively small, equal to roughly 13% of the total energy used during the exercise bout (yes, about double the 7% of steady state but still small in absolute terms). So while the relative amount of calories burned after interval training is larger, the total amount is still small.
Let’s put this into real-world perspective. In one study, subjects ran 20X1 minute intervals above VO2 max with a 2′ rest between. While the EPOC was about double that found in subjects who performed 30′ at 70% Vo2 max, the total EPOC was only about 32 calories (135 kJ). This is less than the calories found in about half an apple. You’ll be ripped in about 15 years at that rate.
The next section of the paper dealt a little more with the issue of exercise duration as studies have identified an increase in EPOC with increasing durations. However, the effect is only significant for exercise performed at intensities greater than 50-60% VO2 max (60-72% max heart rate). However, unless folks are willing to do 60-90 minutes+ of training, this still doesn’t amount to very much in absolute terms. This is especially true of lower intensity exercise where prolonged durations of 90′ or more are necessary to generate a prolonged EPOC. Even there the absolute magnitude of calories burned is still small.
Weight Training and EPOC
Finally the paper examines the impact of resistance training on EPOC. A number of studies have been performed and found fairly prolonged durations of EPOC (15-38 hours) and an increase in metabolic rate of 9-11% over that time period (so someone with a basal metabolic rate of 1600 calories per day might burn 160 calories extra).
However, many of the studies used horribly unrealistic numbers of sets (60 sets of 8-12 in one study, 30 sets in another) and that certainly doesn’t represent the types of “metabolic” workouts I’ve seen recommended. Interestingly, a study of women found a much shorter duration of EPOC (60-90 minutes); the reasons for this are unknown. The paper points out that the average trainee is unlikely to be able to sustain either the volumes (30-60 sets) or intensities used in these studies.
Finally, perhaps the most interesting study was the one using a relatively low volume of training (4 exercises for 4 sets of 10 each) in experienced lifters; in that study metabolic rate was significantly elevated for nearly 48 hours after lifting burning something like 700 calories extra over that time period (this was thought to represent the energy cost of protein synthesis and repairing muscle damage).
This study has never been replicated and the other studies examining the topic have not found nearly the same effect. Perhaps only experienced lifters can train hard enough to make EPOC significant. Perhaps the study was simply a fluke and the measured increase in metabolic rate didn’t actually occur. Without replication, nobody can really say.
The paper concludes that, despite the variability in studies, the intensity of exercise appears to be of the utmost importance in terms of generating an EPOC. No argument there. However, most studies indicate that the total magnitude of the EPOC is unlikely to be very large no matter what is done.
With interval type training, EPOC may approach 14% of the total energy expended but, generally speaking, interval training doesn’t burn as many calories during the bout so while the relative amount may be larger, the total EPOC is still small as noted above. For submaximal work, an EPOC of 7% is roughly the average. Even though the EPOC as a %age is smaller, the absolute magnitude of calories burned will still be larger. As well, odds are that the longer, less intensive steady state sessions burned more total calories during exercise. Steady state may still come out ahead here, I’ll come back to this in tomorrow’s follow-up blog post to put some concrete numbers to things.
As well, outside of trained individuals, most folks couldn’t sustain the durations (90’+) or intensities (80% maximum heart rate for steady state work or supra-maximal intervals) required to generate much of an EPOC in the first place. I would note that even beginners can work up to that level with a properly set up progressive program. One beef I tend to have with many exercise and fat loss studies is that the intensity or duration of the exercise is never increased as the folks become fitter. But that’s a separate topic for another day.
The paper suggests that focusing on maximizing the calorie burn of the exercise bout itself and issues of compliance should be the primary goal (e.g. beginners + high-intensity training tend to equal burnout, injuries and quitting exercise). Because even if you burn a few extra calories after the exercise bout, if you increase how many calories you burn with exercise by a couple of hundred, that couple of hundred will have a much larger impact than the 15 extra you burn because of it. Regardless of what you do.
Summary on Exercise and EPOC
There is absolutely no doubt that higher intensity activity generates a larger EPOC, as measured by the percentage contribution. But like the fact that low intensity cardio burns a greater percentage of fat than higher intensity, this is misleading. 14% of a smaller calorie burn may still be smaller than 7% of a much larger burn. At the end of the day, outside of extremely unrealistic levels of exercise, the basic fact is that the absolute magnitude of the EPOC simply doesn’t amount to very much in the first place. One interval study, which found a 14% increase in metabolic rate via EPOC measured an irrelevant 32 calorie afterburn. Yayyy.
And while some weight training studies are suggestive of higher EPOC’s, the volumes used are typically absurd; the one study which showed a big afterburn from a low volume of training has never been replicated and there are more questions than answers here.
The simple fact is that the calories burned during activity are going to contribute the most to calorie burn, not EPOC and focusing on increasing that value is going to have a much larger impact on calorie balance (all other things equal) than worrying about EPOC.
Having looked at the issue of EPOC from a technical standpoint, I now want to look at Steady State vs. Intervals and EPOC: Practical Application