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Fitness Tracker: Not Always Accurate

Posted: March 10, 2017

fitness trackerIf you feel like you’re working out harder than your fitness tracker indicates, you may be right.

That said, you might also be working out not as hard as your device says. It turns out that some wrist-worn fitness trackers aren’t very accurate when gauging heart rate during exercise. Many people use these devices to figure out how many calories they burn in an hour at the gym.

Researchers at Cleveland Clinic say the problem is how fitness trackers work. A wrist-worn fitness tracker uses optical sensors and lights to measure blood flow.

If you want really want to know how hard your heart is working, you might want to try an old fashioned heart rate monitor that uses a chest strap. These measure electrical activity in the heart.

“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. “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.”

Underestimating or overestimating the number of calories you burn could sabotage your weight loss goals. However, for people with heart problems, an inaccurate heart rate reading could have serious consequences, the researchers warned.

The study tested five popular fitness trackers. Based on their findings, the old-fashioned chest strap monitor is best, particularly for people who need to keep tabs on how quickly or slowly their heart is beating during activity, according to the study team. The standard chest strap was the most accurate regardless of the intensity of the workout or whether someone was using the treadmill, elliptical or stationery bike.

The fitness tracker devices chosen for testing included 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). Measurements on the wearable devices were compared 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 (level of agreement with EKG, rc=.996; 1 being perfect agreement); however, the wrist-worn devices were less accurate on average (level of agreement with EKG, rc=.67-.92). 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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