Fitbit owners from several US states claim that despite the company’s products purporting to accurately measure heart rates, Fitbits “do not and cannot consistently and accurately record wearers’ heart rates during the intense physical activity for which Fitbit expressly markets them”.
One claimant in the class-action lawsuit says that while her personal trainer measured her heart rate at 160 beats per minute, her Fitbit Charge HR recorded a rate of 82 bpm. Another who said his doctor had told him not to exceed 160 bpm found that his Fitbit Surge device was as much as 25 bpm below what other trackers said.
It looks as though the device derives the Beats Per Minute from a different measurement. So although it might be correct in a high percentage of cases, given enough absolute cases, it can have a high number of failures.
It’s like the new saying goes, “Where there’s an algorithm, there’s an error.”