Limitations of HRM Calorie Counts

Azdak

If you have paid any attention to my posts, I fully admit that I have a certain amount of skepticism about the calorie readings you get from a HRM.

It's not that I don't think they are valuable tools (I have often stated that I never work out w/out one and have done so for many years), or that the Polar owncal feature is bogus. I actually think that Polar probably represents "state of the art" for this technology. And I think that, given the varied types of exercise activities many people participate in, a HRM is probably the best overall tool.

But, by nature, I don't like absolutist statements, and when I see things like "the only way to be sure is to buy a HRM" or "a HRM is the gold standard", my contrarian instinct kicks in and compels me to offer an alternate viewpoint.

The underlying weakness in HRM calorie counters is that they do not measure calorie expenditure directly, but assume a direct relationship between aerobic intensity and heart rate. While in general that relationship does exist, there are many variables that affect individual readings. When you develop algorithms based on larger population studies, those variables tend to even out, so one can claim some pretty impressive "correlations", however when you go back to the individual, they become more pronounced.

I am not going into all the issues in this post, but I do want to relate some individual data that illustrates my point. I suffered an injury 9 or 10 days ago that has required me to cut way down on my workouts--from 6-7 days/ wk to 3 and from 6000-6500 calories per week to 1500-2000. I am working out less often, less intensely and for shorter periods of time. As a result, I have seen some short term fitness losses that allow me to make a comparison.

On 8/15/09, I did 45 min on my stair climber.
Avg HR was 123 (peak 138)
Total machine calories: 763
Total Polar calories: 611

Today, I did another 45 min workout on the stairclimber:

Avg HR was 129 (peak 151)
Total machine calories: 706
Total Polar calories: 655

You can see the loss of fitness: my avg HR was 5% higher, yet my total aerobic work was 7.5% lower
I did an interval workout, ranging from level 5 to 8. The peak intervals (level 8) drove my peak exercise HR up 9.4% higher than before.

YET--my Polar calories were 7% higher than before (even though I did 7.5% less work). (And everything else was the same--weight, environment, posture, etc).

Why? because Polar counts heart rate, not the energy cost of the activity. Because my fitness level is down, I was working at a higher % maximum, so Polar assumed I was working at a higher intensity. Yet the actual intensity was less than before.

The energy cost of activities does not change--in other words, the intensity of running 6 mph, or example is 10-11 METs. It's the same whether you do it, I do it, or an Olympic marathoner does it. The difference is that effort might represent 75% of your maximum, 50% of my maximum, and 35% maximum of an elite marathoner.

Now, Polar allows you to compensate for this--I could have (should have) gone into the setup and lowered my VO2max number. Then, even though I was working at a higher relative intensity, it would have been a percentage of a smaller number and the total calories would have been less. But not only is it impractucal to go in and change that number all the time, the VO2 max number is an estimate to begin with, so it just introduces another error factor.

Again, none of this is meant to trash Polar products or the owncal feature, just to keep it in perspective. I would estimate that the error range for HRM calorie counts is 5%-20% (and that's for aerobic work--the counts are not valid at all for strength training). That error range is as good as anything else available (MFP estimates, machine readings etc) and HRMs are the ONLY way to give you any estimate at all during non-machine exercise (classes, etc). I just caution against getting too fixated on those numbers or overreacting over small differences.

sjmay

bump

Chenoachem

Thank you for that information.

gc2052

So? I do not believe we are looking for perfection. I think if we get an estimate we can go from there. Is there a device that is exact?

Phoenix_Rising

I agree 100%. HRMs give you a BETTER estimation of calories burned.

I also think food calorie intake is all pretty estimated too.

And when you through our estimated BMRs in the mix... :laugh:

Good post!!

ivykivy

Thank you for posting this.:flowerforyou: I am not a fan of heart rate monitors (there are some medical conditions that make HRM useless) but realize they can be useful tools.