Not only do heart rate monitors now clean your house, do your dishes, flush the toilet and make your bed, but they can apparently tell you how many calories you've burnt. Many people simply accept this as a mysterious magical power created by companies like Polar and Suunto. Other people go through life and exercise constantly wondering if they *really* burnt as many calories as their little wristwatch is telling them.
So when it comes to estimating caloric expenditure, are these monitors really accurate? A recent study in the Journal of Strength & Conditioning Research, titled “Validation of Heart Rate Monitor-Based Predictions of Oxygen Uptake and Energy Expenditure” asked this very same question.
Traditionally, energy expenditure (AKA caloric burn) is measured in a laboratory, via a test in which you breathe in and out of a mask while running on a treadmill or riding a bicycle. The accuracy of this test is well established, and is based on a direct correlation between oxygen consumed, carbon dioxide produced, and calories burnt.
But you obviously aren't breathing into a tiny hole in your heart rate monitor. Nor did it come equipped with a special mask for you to wear (note to Polar: please don't get any ideas). So how does the heart rate monitor calculate how many calories you've burnt?
Basically, special software built into the monitor is estimating your maximum oxygen consumption and energy expenditure via the length of time between each of your heart beats. I don't know the specifics of the actual equation used in this software, but I imagine it would likely take up a good portion of this blog post if I were to reproduce it.
In the study that I mentioned above, researchers used the “gold-standard” of measuring calories burnt (a calorimetry test used the mask) and compared it to the values the heart rate monitor was spitting out (in this case, a Suunto monitor). The results of the study showed that during both submaximal and maximal intensity treadmill running, the Suunto estimates of VO2 (maximum oxygen consumption) varied by about 6% when compared to traditional laboratory measurements.
But more significantly, there was substantial uncertainty in the calculation of calories burnt, with the Suunto system underestimating caloric utilization by about 13%. So if your heart rate monitor says you burnt 300 calories, it's possible that you actually burnt closer to 350 calories. And those measly 50 calories could be the difference between extra chocolate in your mocha, or not.
However, at very low intensities, like the type of intensity involved with the daily tasks of brushing your teeth, typing on your computer, or doing 12 oz curls with a Kokanee, the system was fairly accurate. And that's good news for those of us who obsessively wear out heart rate monitors to squeeze every extra ounce of data from the super-computer on our wrists.