With heart rate monitors, where you place them may determine how accurate they are, according to data published in the Journal of the College of Cardiology (2017; 69 [11], 336).

Researchers fitted 50 subjects with an electrocardiogram (EKG) device, a chest monitor and an armband, plus any two of four wrist-worn monitors (one for each wrist). The wrist monitors were the Apple Watch, Fitbit Blaze™, Garmin Forerunner® 235 and TomTom Spark Cardio. Study subjects wore the devices while exercising for 18 minutes at varying intensities on an indoor bike, a treadmill and an elliptical machine. EKG heart rate data (the gold standard) was assessed at rest and after each exercise protocol.

The EKG readings were then compared against the data produced by each of the wearable devices.

Turns out, the chest strap’s readings were closely aligned with the EKG data in all scenarios. However, the wrist monitors were less accurate.

“Even though all these wrist-worn monitors work by the same general principles, there is considerable variation among them,” said lead author Marc Gillinov, MD, cardiac surgeon at the Cleveland Clinic. “Overall, they were most accurate when someone was using the treadmill at low intensity and [least accurate] when exercising on the elliptical at high intensity.”

Gillinov added that wrist monitors are viable choices for some individuals who don’t require high levels of accuracy. But those with specific conditions or requirements should be careful when selecting a device.

“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,” he said. “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.”