Wearables can track your steps—but struggle to monitor your ticker

When it comes to tracking steps, most wearables are relatively accurate. But when monitoring a user's heart, wearables fall behind in precision, according to researchers at the Cleveland Clinic who conducted a study testing the top heart monitoring devices for accuracy when compared to electrocardiograms.

Presented at the American College of Cardiology's 66th Annual Scientific Session, the study included 50 Cleveland Clinic employees fitted with an array of heart monitors. Tested devices included chest-strap monitoring, Apple Watch, Fitbit Blaze, Garmin Forerunner 235 and TomTom Spark Cardio.

"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 Marc Gillinov, MD, with the Cleveland Clinic and 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."

Each participant wore a continuous monitoring EKG, chest monitor and armband, as well as two out of the four available testing wearables. Participant were tested at rest and again at light, moderate and vigorous exercise levels on machines including a treadmill, bike and elliptical. After 18 minutes of exercise, wearable measurements were compared to EKG measurements.  

Results showed that the chest-strap was the most accurate. Wrist wearables were less accurate when compared to EKG results. The wearables were all successful in measuring heart rate at rest and on the treadmill, but failed when used for bicycling and the elliptical. Out of all the wearables, the Apple Watch was the only one to provide accurate heart readings when used on the elliptical. Overall, wrist wearables were not accurate in measuring heart data and decreased in accuracy as fitness level increased, except for the Apple Watch.

"We are just at the beginning of a revolution in personal management of health by virtue of wearable physiological monitoring," Gillinov said. "As people take more control of their health and record their own physiological data, they need to know how accurate it is; this is especially concerning for people with heart conditions that can be exacerbated [with activity]. Cardiologists can use this data and decide which monitor they would recommend and help educate patients about their limitations."