Study Finds Fitness Trackers Not So Accurate

     (CN) – If the findings of a recent study on the accuracy of wrist-worn heart monitors is any indication, that killer workout your device says you had might not have been quite as killer as you think.
     In a study published Wednesday in the journal JAMA Cardiology, researchers tested the accuracy of four popular wrist-worn heart rate monitors during varying levels of physical exertion. The tests involved 50 healthy adults with an average age of 37 years.
     While studies have confirmed the accuracy of electrode-based heart rate monitors strapped to the chest, the accuracy of wrist-worn, optically based monitors is uncertain despite their growing popularity.
     Researchers, led by Marc Gillinov of the Cleveland Clinic, designed a study to determine whether leading wrist-worn monitors provide accurate heart rate readings. The readings are important for people who use them to guide the intensity, length and format of their workouts, as well as for doctors who analyze their patients’ readings.
     The team used the products Fitbit Charge HR, Apple Watch, Mio Alpha and Basis Peak to measure participants’ heart rates on a treadmill at rest and at 2, 3, 4, and 5 mph. The subjects exercised at each speed for three minutes to establish a steady state, and their heart rate was recorded right at the 3-minute point. Upon finishing up on the treadmill, the participants’ heart rates were then recorded at 30, 60 and 90 seconds of recovery.
     Participants were randomly assigned to wear two different wrist-worn monitors, while also wearing standard electrocardiographic limb leads and a Polar H7 chest-strap monitor. Nearly 1,800 heart rate values were recorded across the four wrist-worn devices.
     After comparing the readings with the heart rate measurements recorded by the electrocardiogram, the team found that wrist-worn monitors produced inaccurate measurements that varied at different levels of exertion.
     The Fitbit Charge HR underestimated heart rates during more vigorous exercise, while the Basis Peak overestimated heart rates during moderate exercise. The Mio Fuse and Apple Watch had 95 percent of heart rate reading variations of 27 beats per minute (bpm) under and 29 bpm over what the electrocardiogram recorded, while the Basis Peak had 95 percent of values within 34 bpm under and 39 bpm over. The Fitbit Charge HR readings were within 34 bpm under and 39 bpm over the electrocardiogram as well.
     “In general, accuracy of wrist-worn monitors was best at rest and diminished with exercise,” the authors write. “None achieved the accuracy of a chest strap-based monitor.”
     Though these readings may only impact workout intensity for healthy individuals, people who rely on wrist-worn monitors to stay within physician-recommend heart rate ranges during rehabilitation and exercise could experience serious health issues.
     “Electrode-containing chest monitors should be used when accurate heart rate measurement is imperative,” the authors write.

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