Wisdom Wednesday: Don’t Bank on Heart-Rate Accuracy from Your Activity Tracker

Wrist-worn activity trackers such as Fitbit don’t reliably assess heart rate, a new study finds.

While the devices may have some legitimate benefits, they shouldn’t be used for medical purposes, researchers suggest.

Evaluating four wearable activity trackers for Fitbit, Basis, and Mio, the investigators compared results to those from an electrocardiograph (EKG). The researchers found results varied among the different models, and were much less accurate during exercise than at rest.

“These devices are probably good enough to inform consumers of general trends in their heart rate – high or low – [but] it’s important to have more accurate information when physicians are relying on this data to make decisions on medications or other tests and treatments,” said Dr. Mitesh Patel.

Patel is an assistant professor of medicine and health care management at the University of Pennsylvania. He wasn’t involved in the study.

However, the study’s lead author cautions against making too much of the discrepancies. “At any moment, the tracker could be off by a fair bit. But at most moments, it won’t be,” said Lisa Cadmus-Bertram, an assistant professor of kinesiology at the University of Wisconsin at Madison. “The heart-rate feature performed better at rest,” she said. “They’re not as precise during exercise.”

A 2014 survey by PricewaterhouseCoopers found that 20% of American adults owned a wearable activity tracker.



For the new study, 40 healthy adults, aged 30 to 65, were recruited to test the Fitbit Surge, Fitbit Charge, Basis Peak and Mio Fuse. Generally, when compared with the EKG results, the activity trackers were near the correct mark, Cadmus-Bertram said. But occasionally, their estimates of heartrate could swing too high or too low.

At rest, the Fitbit Surge was most accurate; Basis Peak was least accurate, the study authors said. The monitors could overestimate heart rate by as much as 39 beats per minute (Fitbit Surge), or underestimate it by as much as 41 beats per minute (Fitbit Charge), the study found.

In general, Cadmus-Bertram remains a fan. “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,” she said.

The study findings were published online April 11 in the Annals of Internal Medicine.

My Take:
I have the Fitbit Surge and came to the same conclusions. My “study” wasn’t as well designed. I began to suspect early on that at peak levels of exercise my Surge was reading too high. So, I started wearing both my heart rate monitor and Fitbit during spinning class. The Fitbit is very accurate until I go above my aerobic zone (>130 bpm). However, it will often shoot up over my Vmax (155 bpm) when the heart rate monitor shows I’m really at about 140 bpm.

Once you understand the limitations of these devices they still have great value. Anyone who exercises regularly knows their approximate heart rate by how they feel and how hard they are breathing. I love the tracking feature for running and biking. The device automatically downloads all the data on my activity. I can even view my route on a map showing heart rate, calorie expenditure, pace, even elevation.

The Bottom Line:
These devices are an excellent tool to monitor and improve your exercise patterns. Even daily wear to count your steps, millage and calorie consumption can motivate you to exercise more.

Source: April 10, 2017 National Institutes of Health