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Why you shouldn't rely on your fitness tracker's heart rate monitor

Charlotte Hilton Andersen is the author of the book The Great Fitness Experiment: One Year of Trying Everything and runs the popular health and fitness website of the same name, where she tries out a new workout every month, specializing...

Fitbit isn't the only fitness tracker with an inaccurate heart rate monitor

We all expect our machines to work, and work well. Call it a perk of the modern age, but this can be a problem when our tech isn't as trustworthy as we'd like it to be. Case in point: The lawsuit being brought against the über-popular Fitbit fitness trackers.

People are suing Fitbit, saying that the wristband and watch heart rate monitors don't correctly measure heart rate. They're calling them "dangerously inaccurate" and the lawsuit cites several cases where the bands registered 30 to 80 less beats per minute than other types of measurements.

It's not just a few people griping on the Internet either — there is legit science to back up the reports of inaccuracy. According to the Berkeley Science Review, "The Fitbit does tend to underestimate the amount of energy expended (EE) during certain activities, such as cycling and doing laundry, and overestimated energy expended for others (apparently carrying groceries is a real Fitbit-fooler). Another study found that the Fitbit underestimated EE during brisk walking, but was more accurate at slow speeds. Yet another study, of folks in their 60s, also found that the Fitbit underestimated calories burned."

More: Now your sports bra can measure your heart rate and fitness

It should be noted that of all the models the researchers studied, the Fitbit was still more accurate than the two other biggies in the fitness tracker market: the Jawbone Up and the Nike FuelBand.

Treadmills, stair-climbers, spin bikes and ellipticals all have similar problems (or worse) with their heart rate monitors and calorie counters. In a study by the University of California, San Francisco, researchers found that numbers varied up to 60 percent between models and types. Treadmills were the least problematic, being off by only about 13 percent, while ellipticals were the worst offenders, overestimating calories burned by nearly 50 percent.

So, while I love to see big companies get taken down as much as the next person, the Fitbit lawsuit isn't quite warranted. As a girl who has an actual heart condition, which requires I stay below a certain level, and as a fitness fanatic who's been wearing heart rate monitors of various kinds for nearly 10 years now, I feel uniquely qualified to address this. The very first thing you need to know about any type of heart rate monitor — be it on a watch, chest strap, treadmill handle, laser or even the old finger-on-the-wrist trick — is that it's only an estimate.

More: My Fitbit is making me fat — ridiculous claim or actually true?

That's right, all our cool gadgets are based off of proprietary mathematical algorithms that provide an estimate of our heart rate. Most of the time, they do a pretty decent job of it. Sometimes, they don't. There have been many occasions where I've looked down in the middle of an intense sprint interval to see my heart rate blinking at 60 beats per minute — or worse, at "--." But here's the thing: I don't believe it. I've learned to trust my body over my tech, and so when I'm sprinting and I can feel my heart beating out of my chest, I know to back off.

Fitness trackers are most useful when you see them as a guide to help you understand your own body better, rather than as an authority on what's going on inside you. Instead of focusing too much on the actual number, pay attention to how your heart rate responds to different types of activities. But, the most important thing to remember is to not trust a machine over your own judgment of how your body is feeling. You are the expert of you.

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