In the research review, Effects of exercise intensity and duration on the excess post-exercise oxygen consumption, I threw out a lot of data regarding the actual impact of exercise on the post-exercise calorie burn (called EPOC which stands for Excess Post-exercise Oxygen Consumption, essentially the ‘extra’ calories you burn after training). Since that piece was on the long side to begin with, I decided to save some more concrete examples for today’s follow-up blog.
Semi-recapping from yesterday, the paper conclude that high intensity training can generate larger EPOC’s at least in terms of the percentage contribution. The paper suggested that values of a 7% EPOC for steady state work but 14% for interval work were approximately correct values so that’s what I’m going to use.
As I mentioned yesterday, and want to look at in more detail today, although 14% sounds impressively larger than 7%, this can be terribly misleading. 7% of a large number can still be more than 14% of a much smaller number even if the percentage contribution is higher in the second case. And no matter how you cut it, the majority of calories burned come during the workout, not afterwards. As you’ll see, the EPOC doesn’t amount to jack for any realistic amount of activity.
What I want to look at today is how those values might apply in the real world in terms of determining how different types of exercise affect energy balance. This is going to be a lead in to next week’s series of articles where I make some slightly more concrete statements about the role of either intervals or steady state exercise for fat loss. Which of course leads into comments on how to practically implement either one.
Now, in the illustrations below, I’m going to be using some values that may seem pulled out of thin air; they are not and I want to describe briefly where they are coming from.
I have a Power Meter based bike. That is, a bike that has special cranks to measure how much mechanical power I am putting out.
In addition to measuring my power output on the bike, it tells me how much total work I’ve done at the end of the ride in kilojoules; this can be converted to calories by multiplying roughly by 1. So 450 kJ expenditure is about 450 calories. Before I continue, let me explain this since anybody who knows math/physiology will realize that it’s not right.
Converting Power to Calories
To accurately calculate calories from kilojoules, you actually divide by 4.2 (so 420 kJ = 100 calories). Obviously it’s not one to one unless you’re one of the trainers on the Biggest Loser (bit of an in-joke there, you’d have to watch the show).
However, exercise efficiency while cycling varies from 20-25%, meaning that only that much of the actual caloric expenditure goes into power output. The rest is lost as heat.
So to figure total calories burned you re-divide by anywhere from 4 to 5 (depending on efficiency). If you assume 25% efficiency, you re-divide by 4. Essentially you divide kJ by 4 and then multiply back by 4. So you end up being able to roughly estimate calorie burn by simply taking kJ as the calorie burn at a 1:1 ratio. Ok, back to the article.
As well, I’ve checked the value on the powerbike using the Bodybugg (the little armband thingie that estimates energy expenditure based on body heat, movement and a couple of other things).
I’ve also recently checked the bike against the caloric expenditure from my new Polar RS800 watch and it is basically identical to the powerbike’s kJ reading with the Bugg being in close shooting distance of both (the value isn’t identical). My point being that the values I’m going to use, I consider accurate. I’m not making them up so that my math will support what I’m trying to argue.
Now, semi-tangentially, a lot of people on my forum have been using the Bugg and one thing that keeps coming up (so I might as well address it) is disappointment that an interval workout invariably ends up burning either about the same or less total calories than a similar amount of moderate steady state.
That is to say that 30 minutes of intervals may burn roughly 300 calories. But so does 30 minutes of moderate intensity cardio (for trained folks). And the intervals are a hell of a lot tougher.
How Does that Even Make Sense?
The reason of course is that the interval workout is alternating between very high caloric expenditures and very low expenditures such that the average expenditure still ends up coming out about the same. That is, say I do 1 minute intervals with 1 minute rest, alternating between 15 cal/min during the hard bit and 5 cal/min during the recovery. That’s an average of 10 cal/min. I can achieve that same 10 cal/min consistently with moderate intensity cardio. The second workout will be far easier to complete.
So if there’s a huge benefit to intervals, one place it might come is from the EPOC (note: there are other potential benefits of intervals that I’ll be addressing in future blog posts).
So I’m going to be focusing ONLY on EPOC here. Again, I’m going to assume a 7% EPOC for steady state cardio and a 14% EPOC for intervals and put those into some real world perspective.
Say I do 20 minutes of intervals and burn 200 calories. I get a 14% EPOC which is 28 whole calories. Total calorie burn = 228 calories.
Let’s say I do 20 minutes of steady state cardio and burn the same 200 calories. 7% EPOC which is 14 calories = 214 calories.
So, for an equivalent duration workout, the interval workout comes out a whopping 14 calories ahead due to the impact of EPOC. That will net me an extra pound of fat loss every 250 days (3500 calories / 14 calories per day = 250 days). Hooray. Clearly, for any equivalent length workout the interval training will always come out slightly ahead.
Except that you wouldn’t expect someone to do longer and and longer and longer interval workouts; the whole point of intervals (or one point) is that they are more time efficient, that you get all you need in somewhere between 4 and 20-30 minutes (depending on which expert you’re listening to and what they’re selling).
So it’s more useful to compare that 20 minute interval workout to longer steady state workouts which is what most would do in the real world.
Here are calculations for different length steady state workouts based on an average burn of 10 cal/min and a 7% EPOC.
30 minutes = 300 calories + 7% EPOC = 21 calories = 321 calories.
40 minutes = 400 calories + 7% EPOC = 28 calories = 428 calories.
50 minutes = 500 calories + 7% EPOC = 35 calories = 535 calories.
60 minutes = 600 calories + 7% EPOC = 42 calories = 642 calories.
Now lemme be generous and assume I’m doing 30 minutes of intervals with a 14% EPOC
30 minute interval session = 300 calories + 14% EPOC = 42 calories = 342 calories.
While this is certainly a few more (21) calories than the 30 minute steady state session, it pales in comparison to the longer sessions. Sixty minutes of steady state cardio burns 642 calories, compared to 342 from the interval training. Looking purely at energy balance (and, again, there are other issues to consider) and fat loss, which will get me lean faster?
But you say, I don’t have time to do an hour of cardio, I get bored doing cardio, etc, etc. Those are practical considerations (which are, of course, important), not physiological ones and I’ll be coming back to those in next week’s continuation of the series. Purely physiologically speaking, the EPOC argument just doesn’t hold up. Not only is the EPOC from any realistic amount of intervals or steady state cardio irrelevant, short interval sessions still burn far LESS calories than longer steady state sessions.
The intervals only come out a TINY bit ahead if you compare workouts of identical length and even there the difference is absolutely insignificant.
But between now and Monday, here’s a question for my readers (or the pro-interval crowd) to ponder:
Let’s say I want or need to train daily for fat loss (most athletes train every day, as do most dieters).
Which am I more likely to do on a day-in day-out basis? Which is more likely to lean me out faster? Which am I more likely to BE ABLE to do daily (from a recovery standpoint)?
- 30 minutes of intervals: burning 342 calories including EPOC.
- 60 minutes of moderate teady state cardio: burning 642 calories including EPOC.
Note: Yes, I’m making it sound like it’s an either/or choice here which it’s clearly not. But just consider that one issue in isolation for now.
Note again: I’d note again that this series of articles is absolutely NOT meant to be against interval training as one tool in the fat loss arsenal. My issue is simply with the uncritical claim being made that intervals are always superior to steady state (or that steady state is somehow detrimental to fat loss) as well as some of the arguments being made to support the contention that interval training is any of these things.
Next up in the series, Steady State vs. Intervals: Explaining the Disconnect
- Steady State vs. Intervals: Explaining the Disconnect Part 2
- Steady State vs. Intervals in Real World Training – Q&A
- Steady State vs. HIIT: Explaining The Disconnect
- Explaining Exercise Efficiency
- EPOC: Excess Post-Exercise Oxygen Consumption