Your fitness tracker can count your steps, but it’s not that good at monitoring your heart rate

Researchers from University of Wisconsin in Madison and Loras College in Dubuque, Iowa, had 40 healthy adults strap on four popular activity trackers, two on each arm. Study participants, ages 30 to 65, also were rigged up to an electrocardiograph, which uses leads in a chest strap to detect the wearer’s heart rate.