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Your Fitness Tracker Might Be Wildly Inaccurate

Heart rate monitors didn't fare well in a recent test.

Denny Watkins

Luca Sage / Getty IMages

Fitness trackers are more popular than ever. Although the top manufacturer, Fitbit, recently reported a 19 percent drop in revenue in the last quarter of 2016, shipments of wearables over the whole year was actually up by 25 percent, with more than 102 million devices hitting stores, according to market research firm IDC. 

So it's reasonable to ask: How well do these gadgets really work? It turns out that wrist-mounted heart rate monitors can be wildly off the mark, according to a new Cleveland Clinic study being presented this weekend at a meeting of the American College of Cardiology. In some instances (depending on how hard you're working out), the heart rate count could be over or under your actual heart rate by as much as 34 beats per minute. Chest strap monitors, on the other hand, turned out to be accurate no matter what kind of exercise you're doing. 

The researchers had 50 people wear an Apple Watch, a Fitbit Blaze, Garmin Forerunner 235, TomTom Spark Cardio, or Scosche Rhythm+, and a Polar R7 chest strap while performing light, moderate, and high-intensity cardio. That included time on a treadmill, an exercise bike, and on an elliptical. At the same time, they were wired up to an electrocardiogram (EKG) to determine their actual heart rate. The researchers had previously given poor marks to the Apple Watch, plus the less popular Fitbit Charge HR, Mio Alpha, and Basis Peak, only while the participants were on a treadmill.

Here's how the fitness trackers fared:

Apple Watch ($360 and up): Accurate while running, cycling, and on the elliptical as long as you don't grab the levers. Less than 80 percent accurate if you hold on to the elliptical levers. Researchers also noted that the Apple Watch was the only tracker that didn't become less accurate as the exercise intensity level increased.

Garmin Forerunner 235 ($330): Accurate while running and cycling, but not accurate on the elliptical whether you grabbed the handles or not.

Scosche Rhythm+ ($80): Performed similar to the Garmin; accurate running and cycling but not on the elliptical.

TomTom Spark Cardio ($130): Accurate while running, but not during cycling or on the elliptical.

Fitbit Blaze ($200): Was never accurate during either running, cycling, or on the elliptical.

The problem? A heart rate sensor on your wrist measures blood flow through the veins it can sense in your arm, and it might not be able to always nab a good reading depending on how much you're sweating, the pigmentation in your skin, or how much it's shaking around. Even during light exercise, the readings were off by as much as 15 beats per minute. A chest strap, on the other hand, works exactly like the EKG by sensing electrical activity in your heart. As a result, the chest strap was more than 99 percent accurate when compared to the EKG readings.

If you're spending $200 or more for a fitness tracker, you want it to give you heart rate numbers that are at least close. But if you're counting on your heart rate monitor to help with your training (like in some endurance training plans that have you maintain a certain heart rate), you're not helped if the readings swing up and down, or if they're off by 30 beats per minute. 

Of course, if you have a medical reason to keep an eye on your heart rate, you definitely can't risk a faulty reading. If that's the case, you'll want to wear the chest strap.