More HRM/Calorie data

This is another in a series of my intermittent observations about the accuracy of HRM calorie counts. One of these days, I would like to put together a more substantial review article, but for now I am still collecting my own personal data and am just posting random bits of information that I find interesting.

Again, my usual disclaimer: I love heart rate monitors. I like lots of data about my workouts, and I think HR feedback is essential to a quality training program. I think the Polar products are excellent--I currently use an F11 and would not think of working out w/out it. I have been using HR feedback in my workouts since before HRMs were even invented.

As an exercise physiologist, I like to make sure that everyone has the most accurate information possible in order to use these tools most effectively.

OK, on to today's post. HRMs do not actually measure calories expended--they measure heart rate, and they assume a fixed relationship between HR response to exercise and oyygen uptake (the true measure on intensity). Exercise heart rate response is not always that regimented, so there is a built-in error factor, that I will illustrate.

Yesterday I did 45 minutes on my stairclimber (14 yr old Stairmaster 4200 PT, with the "upright" handles). I checked my calorie expenditure and aerobic work volume at the halfway point and the end and compared the data with my Polar HRM.

In the first 22:30, the machine calorie readout was 381 calories, my Polar HRM read 290

For the second 22:30, machine readout was 391 calories, Polar HRM was 357.

So, while I only did 2.8% more work in the last 22:30, the HRM counted 23% more calories.

Why? Because an HRM counts heart rate, not actual work performed. And over a 45 min workout, most people experience something called "cardiovascular drift". That means the heart rate increases, even with no increase in workload--mostly due to increased core temperature and decreased plasma volume. So the HRM "measured" my calorie expenditure as being greater because my avg HR was higher--even though the workload was essentially constant. I wasn't working any harder, but my heart was beating faster.

Other studies I have read suggest that Polar HRMs underestimate calorie expenditure when intensity is 65% and below. That certainly agrees with my observations in my own workouts.

Now is this significant? Depends on your perspective. For the sake of argument, if we assume the Polar calorie reading for my first 22:30 was accurate, that means total calories for the second half were off by about 20%. That percentage seems high, but it is still only 50 calories for a 45 min workout, so no big deal.

The difference for the total workout was 16% between the machine and my HRM, or 125 calories. Assuming the machine probably overestimated total calories (I try not to hold on much, but I am sure that accounts for something), and the Polar underestimated my actual count, the real difference is probably even less. So, in this case I would feel comfortable splitting the difference for my "real" number--the older Stairmaster machines were pretty accurate in measuring workload.

I would just keep a couple of things in mind: 1) assume a 10%-20% error in the calorie readings you obtain from a Polar HRM; 2) chances are, lower-intensity workouts and longer-duration workouts (greater than 60 min) will tend to be more inaccurate.