One recently published study indicates that wrist-based heart rate monitoring may not be as reliable as hoped. Researchers at the Cleveland Clinic's Heart and Vascular Institute found that four leading devices were of inconsistent accuracy, and that accuracy drops during workouts.

The study tested the Fitbit Charge HR, Apple Watch, Mio Fuse and Basis Peak wrist-worn devices. 50 healthy adults were assigned two different wrist monitors at random, along with a proven Polar H7 chest strap monitor as a control. The participants' heart rates were measured at rest, and on a treadmill at 2, 3, 4, 5 and 6 miles per hour.

Researchers found that "[i]n general, accuracy of wrist-worn monitors was best at rest and diminished with exercise." Of the devices tested, the Apple Watch and Mio Fuse were most accurate, followed by the Fitbit Charge HR and lastly, Basis Peak.

But even the top-rated devices display a wide and difficult to predict range of results. For some perspective, 95-percent of the Apple Watch and Mio Fuse readings were between -27 BPM and +29 BPM of the accurate reading. For the Fitbit Charge, 95-percent of readings were between -34 and +39 BPM. The corresponding range for the Basis Peak was between -39 and +33 BPM.

Device performance also skewed differently depending on the level of exertion. For instance, Basis Peak overestimated HR during moderate exercise, with median differences of -8.9 and -7.3 beats per minute (BPM) at 2-3 miles per hour. On the other hand, Fitbit Charge HR skewed low during more vigorous exercise, with median differences of 7.2 and 6.4 BPM at 4 miles per hour.

Although this study is limited in scope, its results are not encouraging for fitness-tracker fans, many of whom paid a premium for heart rate tracking capabilities. Findings like these could also bolster the class action lawsuit filed against Fitbit earlier this year.

More importantly, they indicate the danger of mistaking a wrist-based heart rate monitor for a medical-grade device, particularly for individuals with cardiovascular conditions. The authors of the study write that "[e]lectrode-containing chest monitors should be used when accurate HR measurement is imperative," especially for cardiac patients that rely on monitors to stay within physician-recommended thresholds.

Given the explosion of the health and fitness wearable market, studies like these could prompt greater investigation and consumer awareness. It could also spur a trend toward non-wrist technology, such as the Moov HR headband-worn heart rate sensor, or other types of heart rate monitoring altogether.

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