It’s comforting to imagine that your fitness tracker has your heart rate covered, but research conducted at the Cleveland Clinic suggests that fitness tracker wearers should take some heart rate monitors with a grain of salt, or perhaps with something else that won’t raise blood pressure. A Cleveland Clinic news release reports that researchers at the clinic randomly fitted 50 volunteers (average age 38) with two of four different wearable heart rate monitors (one on each wrist): the Apple Watch, Fitbit Blaze, Garmin Forerunner 235, and TomTom Spark Cardio. The volunteers also wore a continuous 4-lead electrocardiogram (EKG), a chest monitor and an armband (Scosche Rhythm+). Heart rates were recorded at rest and after light, moderate and vigorous exercise across three types of activities– treadmill, stationary bike and elliptical, while the volunteers exercised for 18 minutes. Unsurprisingly, readings on the chest strap monitor matched those from the EKG. The wrist worn devices were less in sync, with some off as much as 34 heart beats a minute, depending on the activity. Resting heart rates were right on target, and treadmill readings were very close, but cycling and elliptical use was problematic. Only the Apple Watch was accurate when volunteers switched to the elliptical trainer without arm levers; none gave correct measurements when they used arm levers, and the wrist and forearm monitors became less accurate the more intense the activity levels, with the exception of the Apple Watch.