April 10, 2017
2 min read

Wrist-worn activity trackers yield unreliable heart rate data

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Wrist-worn activity trackers that measure heart rate do not offer consistent data, suggesting that more research is needed to evaluate their efficiency before they are used clinically, according to a recent study.

“Activity trackers may motivate persons to engage in healthy behaviors… and may help manage chronic conditions related to lifestyle,” Lisa Cadmus-Bertram, PhD, from the University of Wisconsin, and colleagues wrote in Annals of Internal Medicine. “Although previous studies have shown that they are generally accurate for measuring the number of steps a person takes, less is known about their accuracy when measuring heart rate.”

In their study, the researchers examined the accuracy of four LED-dependent, wrist-worn activity trackers, which measure heart rate from tiny changes in skin blood volume using light reflected from the skin.

They randomly placed two trackers on each wrist of 40 participating adults aged 30 to 65 years without cardiovascular conditions. They performed electrocardiography on seated participants and measured the heart rate for each of the four trackers at 1-minute intervals for 10 minutes. They also measured participants who exercised on a treadmill at 65% of the maximum heart rate, which they calculated at 220 beats/minute minus the participant’s age. Using Bland-Atman plots, researchers compared the heart rates measured by electrocardiography with those measured by each of the trackers.

The results showed that the limit of agreement was best for the Fitbit Surge (–5.1 to 4.5 beats/min) and worst for the Basis Peak (-17.1 to 22.6 beats/min) among participants at rest. For the Fitbit Charge and Mio Fuse, the agreement was intermediate (–10.5 to 9.2 beats/min and –7.8 to 9.9 beats/min). For participants measured during exercise, the limits of agreement were relatively poor for all the activity trackers.

The repeatability coefficient, which determined how close one measurement was to another when using the same device in the same participant under the same conditions, for electrocardiography was 5.4 beats/minute at rest and 9.1 beats/minute during exercise.

Comparatively, the investigators found that the coefficient at rest was 4.2 beats/minute for the Fitbit Surge, 19.3 beats/minute for the Basis Peak, 9.3 beats/minute for the Fitbit Charge and 10.9 beats/minute for the Mio Fuse. During exercise, it was 20.6 beats/minute for the Fitbit Surge, 20.2 beats/minute for Basis Peak, 21.6 beats/minute for the Fitbit Charge and 23.7 beats/minute for the Mio Fuse.

“Although wrist-worn trackers may help monitor daily activity, more research is needed before we can confidently conclude that the monitoring feature for heart rate is sufficient to help clinicians advise their patients about health issues and conduct clinical trials that require a high level of accuracy and reliability for heart rate measures,” Cadmus-Bertram and colleagues wrote.

“While our data did not find exceptional accuracy of consumer-based heart rate monitoring during exercise, on the whole, fitness trackers still provide a tremendous amount of useful information to the average user who just wants some feedback to help them to increase their exercise level,” Cadmus-Bertram told Healio Internal Medicine. “They are such an improvement over what was available to us less than a decade ago. Our findings need to be taken into careful consideration given that we tested only two conditions (rest and treadmill exercise) and the data were collected a year ago, whereas hardware and algorithms change frequently.” – by Savannah Demko

Disclosure: The researchers report no relevant financial disclosures.