Wearable devices that can track heart rates aren’t too far off in their results, but their accuracy goes down when people would need them the most.

"We have patients come in who have different kinds of monitors like these. Many of them are very concerned because they've recorded values that seem way outside of the normal range," Marc Gillinov, MD, a cardiac surgeon at the Cleveland Clinic told NPR.

Gillinov was the lead author of a study published in JAMA Cardiology which found wrist-worn fitness trackers became less accurate with more vigorous exercise. They weren’t off by much—Apple Watch and Mio Alpha wristbands had about 91 percent accuracy, while FitBit Charge HR and Basis Peak were around 80 percent—but for cardiac patients or athletes who are closely monitoring their heart rates, that’s not good enough.

“For those groups, I'd recommend medical-quality chest-strap monitors," Gillinov said, though he added the devices aren’t dangerous, in his opinion, just not all that useful.

For more on why these wearable devices encounter the same accuracy problems as hospital equipment, click on the link below: