Wrist-worn activity trackers measure heart rates better at rest than during exercise

An analysis of four, wrist-worn activity trackers found that the devices worked better at rest than during moderately active exercise and that they had mixed results when it came to measuring heart rates.

Lead researcher Lisa Cadmus-Bertram, PhD, of the University of Wisconsin, and colleagues published their results online in the Annals of Internal Medicine on April 10.

The researchers examined the following four trackers that use light-emitting diodes (LEDs) to measure heart rate: the Fitbit Surge, the Basis Peak, the Fitbit Charge and the Mio Fuse (Mio Global). They then compared the heart rates measured by electrocardiography with heart rates measured by each of the trackers.

“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 wrote in an email. “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.”

The study enrolled 40 healthy adults who were between 30 and 65 years old and did not have any cardiovascular conditions. Half of the participants were women, while the mean age was 49.3 years old and the mean body mass index was 25.1 kg/m².

The researchers placed two trackers on each wrist for each participant in random order. They then performed electrocardiography on the seated participant and measured the resting heart rate at one-minute intervals for 10 minutes for each of the four trackers. The participants then had their heart rate measured every minute for 10 minutes while they exercised on a treadmill at 65 percent of the maximum heart rate, which the researchers calculated as 220 beats per minutes minus the participant’s age in years.

When the participant were sitting and at rest, the limits of agreement was best for the Fitibit Surge and worst for the Basis Peak. When the participants exercised at 65 percent of their maximum heart rate, the limits of agreement were relatively poor for each activity tracker, according to the researchers.

The researchers also evaluated the repeatability coefficient, which they defined as the range of measurements when using the same device in the same study participant under the same conditions. They noted that smaller values were better.

At rest, the repeatability coefficients were 5.3 beats per minute for electrocardiography, 4.2 beats per minute for the FitBit Surge, 9.3 beats per minute for the FitBit Charge, 10.9 beats per minute for the Mio Fuse and 19.3 beats per minute for the Basis Peak. During exercise, the repeatability coefficients were 9.1 beats per minute for electrocardiography, 20.2 beats per minute for the Basis Peak, 20.6 beats per minute for the FitBit Surge, 21.6 beats per minute for the FitBit Charge and 23.7 beats per 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 measurement,” the researchers wrote.