Many gym goers live and die by the calories burned feature of their heart rate monitors. Call me a Cynical Cindy, but I’ve always thought that some of the numbers that I’ve heard people throwing around seemed kind of, well, high. As someone who is interested in helping other people be fit, this concerns me. Could inflated numbers lead to extra eating (I burned it all off plus some, so pass me another cookie)? Will people become discouraged when weight loss is much more meager than expected? I don’t claim to be any sort of expert, but I enjoy research and this is what I’ve learned.
The best way to estimate calorie burn is by measuring oxygen consumption, but this is cumbersome outside the laboratory (and not to mention costly), so people have sought other methods. Heart rate is relatively simple to monitor and there is a relationship between higher heart rates and increased calorie expenditure. It’s not a perfect correlation, however. Heat, dehydration, stress, and caffeine can all increase heart rate even while calorie burn remains static. Unless an exercise session is short, cardiac drift also becomes a factor. As core temperature rises, the heart works harder even if oxygen uptake remains unchanged.
Heart rate zones vary from person to person and can change drastically as someone becomes more or less fit. The old “220 minus your age” method is a wide generalization and may be flat out wrong, especially for women. At the end of a hard race, I’m pegged at about 165, while a male friend who is about seven years younger will hit the 180s on a moderate run. This leads to the question “How accurate are the resting and max heart rate values that the monitor is using to perform the calculations?”, if even uses them at all. Most companies do not publish their formulas, so we don’t know.
How does weight factor in to the equation? According to recent findings, running burns approximately .75 calories per pound per mile (± 5 calories/mile). A 135 pound person running 7 miles in an hour expends around 700 calories, while a 160 pound person would burn 840, even if their heart rates were the same. Running is a good exercise for performing comparisons, because the movement is relatively standard and it’s much easier to benchmark against a measured mile than say, 15 minutes of Zumba. People who like to experiment might want to try running a mile with their heart rate monitor and compare the calorie burn against the formula above. If the numbers are extra generous (or stingy), I would expect that trend would carry over to other forms of exercise as well.
There are a few other caveats to consider. The less vigorous the activity, the less accurate the results. The numbers are virtually useless for strength training. One scholarly source listed the Polar’s “laboratory error” as 16.9–20%, and some other brands were worse (see Table 1 in the linked document). Heart rate zones and weight need to be kept up to date to for the most accurate readings, but it’s easy to forget to do it.
Even if the calorie readouts are skewed, there is still value in training with a heart rate monitor. If your goal is getting the most burn for the buck, they’re good for comparing different activities. They’re useful for tracking fitness gains. They can be help detect overtraining (is your heart rate much higher than normal during a certain activity?). Many athletes use them determine training intensity. They can provide clues about why you might be having an off day. For example, during one of my marathon training long runs, I really struggled through the last couple miles. My heart rate was low, which led me to believe it was a fueling rather than a fitness issue, and I was able to fix it the next time out. Some people find seeing a calorie burn can be highly motivating and might even go the extra minutes or mile to reach the next round number. By understanding its strengths and limitations, the heart rate monitor can be a valuable instrument for helping people achieve their fitness goals.
For more information on heart rate monitors and other methods of tracking calorie burn, check out the following article. http://www.wired.com/playbook/2012/08/fitness-trackers/