If you rely on fitness trackers to see how hard you work out, you may want to rethink this approach. According to a small study from the University of Wisconsin in Madison, these popular devices may be more accurate at reading heart rate when you are at rest rather than during exercise.
The study tested four popular wristbands, each of which has a light-emitting diode (LED) that measures heart rate from tiny changes in skin blood volumes by using light reflected from the skin.
Participants in the study, 40 healthy adults, wore two trackers on each wrist and compared resting and exercise heart rate readings on the devices to the gold standard used by doctors: an electrocardiogram (ECG or EKG) test.
At rest, the Fitbit Surge got heart rate measurements that most closely matched the ECG results, and the Basis Peak was furthest off.
In tests that also included the Fitbit Charge and Mio Fuse, none of the trackers got exercise heart rate readings that came close to the ECG.
These results suggest that while the trackers may help monitor daily activity, it’s not clear the heart rate readouts would be accurate enough to help patients with certain health problems make medical decisions, the authors noted.
At any moment, the tracker could be off by a fair bit, but at most moments, it won’t be, said lead study author.
That’s why the paper does not suggest that the commercial trackers tested are sufficient for medical applications where high precision is needed during exercise. For the typical recreational user, though, they may still provide feedback that’s useful and motivational.
At rest, Fitbit Surge most accurate, Basic Peak the least
To assess the accuracy of the trackers, researchers examined heart rate data for participants who were 49 years old on average and slightly overweight.
First, they looked at the amount of agreement between the readings from the trackers and the ECG tests.
When participants were seated, researchers took readings for the trackers and the ECG tests at one-minute intervals for 10 minutes.
The narrowest range of differences between the trackers and the ECG, indicating the most accuracy, was for the Fitbit Surge. The range for this tracker ranged from an underestimation of 5.1 beats per minute to an overestimation of 4.5 beats per minute.
The widest range of difference at rest was for the Basis Peak, which ranged from an underestimation of 17.1 beats per minute to an overestimation of 22.6 beats per minute.
During exercise, all trackers gave wide range of readings
When participants exercised on a treadmill, the ranges were even wider. The Mio Fuse ranged from an underestimation of 22.5 beats per minute to an overestimation of 26 beats per minute, for example, while the Fitbit Charge range from an underestimation of 41 beats per minute to an overestimation of 36 beats per minute.
The study is small, and researchers found only limited repeatability with results for the same participant under the same conditions.
Still, the findings are an important first step in understanding the clinical validity of wrist trackers many patients already use. In particular, people with the most common heart rhythm disorder, atrial fibrillation, shouldn’t rely on the trackers to detect abnormal rhythms.
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