Take Heart: Fitness Trackers Not Always Accurate

Bon Secours Heart & Vascular Institute, heart attack, preventionFitness trackers can be great motivators to get active. Trouble is, some of them are not always accurate when it comes to measuring heart rate.

In fact, if you’re relying on a fitness tracker to keep your heart rate at a safe limit set by your doctor, researchers at Cleveland Clinic say you may want to consider wearing an old-fashioned chest strap monitor instead.

“If you need to know your heart rate with accuracy when exercising—either because you are training for a marathon or have safe heart rate limits set by your doctor, perhaps due to coronary artery disease, heart failure or other heart conditions—wrist-worn monitors are less accurate than the standard chest strap,” said Dr. Marc Gillinov, MD, the study’s lead author in a news release. “We found these devices can equally over- and underestimate heart rate. The error ranged from +/-34 beats per minute to +/-15 beats per minute, depending on the type of activity.”

The problem with fitness trackers is how they calculate heart rate. Unlike a chest strap type of device, which measures electrical activity in the heart, wrist-worn devices rely on optical sensors and lights to measure blood flow, according to the news release.

For people with heart problems, an inaccurate heart rate reading could have serious consequences, the researchers warned. For people trying to lose weight, the readings could skew how many calories they believe they’re burning during their workouts.

Researchers focused on five popular fitness trackers for their study. The Apple Watch, Fitbit Blaze, Garmin Forerunner 235, and TomTom Spark Cardio. Researchers recorded volunteers’ heart rates at rest and after light, moderate and vigorous exercise across three types of activities, including the treadmill, stationary bike and elliptical (with and without hand levers).

They compared the measurements on the wearable devices to readings from the chest strap and EKG. Participants exercised for a total of 18 minutes; one dropped off at the final stage due to fatigue.

The chest strap monitor closely matched readings from the electrocardiogram (EKG), which is the gold standard for measuring the heart’s activity.  The wrist-worn devices were less accurate on average. While the watch-style heart rate monitors may accurately report heart rate at rest, and most were acceptable on the treadmill, they were fairly inaccurate while bicycling or using the elliptical.

Of the wrist-worn heart rate monitors, only the Apple Watch provided accurate heart rate readings when participants switched to the elliptical trainer without arm levers; none gave correct measurements when they used arm levers. The wrist and forearm monitors also became less accurate the more intense the activity levels, with the exception of the Apple Watch.

“Even though all these wrist-worn monitors work by the same general principles, there is considerable variation among them,” Gillinov said. “Overall, they were most accurate when someone was using the treadmill at low intensity and worst when exercising on the elliptical at high intensity.”

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