Some fitness trackers may fall short in measuring heart rate accurately, study finds

— -- Your favorite wristband fitness-tracker may be good at measuring your steps, but not as accurate at monitoring your heart rate when you are exercising, according to a new study published today.

The study, published in the Annals of Internal Medicine, found that four popular wrist-worn fitness trackers fell short in measuring heart rate during moderate exercise.

A co-author said the study aimed to help inform people who use the popular devices.

“We wanted to help provide people with some guidance and feedback and learn about how accurate this [heart-monitoring] feature was,” said co-author Lisa Cadmus-Bertram, an assistant professor of kinesiology at the University of Wisconsin.

Researchers examined four wrist-worn fitness trackers on the market in early 2016, each of which depends on a light-emitting diode (LED) to measure heart rate from tiny changes in skin blood volume as reflected by light on the wearer's skin.

Such wrist-worn devices have the advantages of being unobtrusive, useful for long-term wear and, as previous research has shown, generally accurate in measuring the number of steps a person takes, the study says.

But the researchers found that the four devices tested were less accurate in measuring heart rate during exercise in comparison with an electrocardiograph, the most accurate way to track heart rate.

For the study, a group of 40 healthy adults aged 30 to 65 wore the fitness trackers when they were at rest and during 10 minutes of treadmill exercise at 65 percent of the maximum heart rate.

All of the devices were more accurate in measuring heart rate when the wearers were at rest than during exercise, the study said.

When participants were at rest, the Fitbit Surge's measurement of heart rates was the closest of the four devices to the electrocardiograph (EKG) reading. Among the three other wrist-worn trackers, the Basis Peak was furthest off in its heart rate measurement during rest, and the Fitbit Charge and Mio Fuse were in the middle.

During the 10-minute exercise test, none of the four trackers was in close agreement with the EKG heart-rate readings.

In addition to discrepancies between the activity trackers’ heart-rate measurements and EKG readings, the four wrist-worn devices also showed inconsistencies in measuring the same heart rate in the same person under the same conditions, the study said.

Researcher Cadmus-Bertram said people undergo various physical changes during exercise that can affect heart-rate measurement.

“Heart rate is easiest to measure during rest, but once you start exercising, more variables come into play including sweat, which may have an effect”, said Cadmus-Bertram.

The study concluded that more research is needed on the heart-rate monitoring feature on wrist-worn fitness trackers before they can be used to help clinicians advise patients on heart issues.

Some physicians say the activity trackers on the whole are helpful motivational tools.

"Awareness is a separate issue from accuracy," Dr. Suzanne Steinbaum, director of women's heart health of Lenox Hill Hospital in New York City, told ABC News. Awareness of activity levels makes people more motivated, and these devices are useful for that."

She said, however, that people should not mistake a fitness tracker for a medical diagnostic tool.

"All of these monitors help motivate people to exercise, but you can’t use these monitors as diagnostic tools or rely on them to give accurate answers to what’s going on from a cardiac perspective," she told ABC News.

Cadmus-Bertram agreed that even an imperfect fitness tracker could help many people better understand their health and wellness needs.

“We didn’t find (trackers) to be perfectly accurate,” she said. “But they don’t need to be perfectly accurate for most people wanting to live a more active lifestyle.”.

In a statement to ABC News, Fitbit officials said the trackers are not meant to be medical devices but give other valuable wellness measurements not measured by an EKG, such as estimated maximum oxygen consumption during exercise, which can indicate cardiovascular fitness.

“Fitbit is committed to making the best activity trackers on the market for consumers who want information to make informed decisions about their health and fitness, and we stand behind our heart-tracking technology,” Fitbit said in its statement. “Fitbit trackers are not intended to be medical devices and, unlike chest straps, wrist-based trackers fit conveniently and comfortably into everyday life, providing continuous heart rate for up to several days without recharging (vs. a couple hours at a time) to give a more informed picture of your overall health. “

We conducted extensive internal studies which show that Fitbit’s PurePulse technology performs to industry standard expectations for optical heart rate on the wrist. There also is independent scientific validation of the accuracy of Fitbit's sleeping heart rate, and Consumer Reports independently tested the heart rate accuracy of Fitbit Charge HR and Surge, and gave both products an “excellent” rating.”

Mark Gorelick, chief science officer for Mio Global, said in a statement that there is a need for devices that help people understand their own heart rate changes. He emphasized that the company's Personal Activity Intelligence (PAI) could help people target their heart rate zones more easily.

"Mio has an extraordinary reputation for optical heart rate accuracy, and the introduction of our Personal Activity Intelligence (PAI) technology makes heart rate zone training easy because people only need to know one number to understand the impact their exercise is having on their health,” Gorelick said.

“It is generally understood that health benefits are amplified during moderate- to high-intensity exercise, but most consumers don't know how to interpret their heart rate data. PAI is a simple, scientifically validated activity metric that helps consumers understand the intensity of their exercise, based on their personal profile and heart rate data, and empowers them to proactively manage their health and reduce risk of lifestyle-related diseases," Gorelick said.

The Basis Peak activity tracker was voluntarily recalled last December after some of the devices caused burns on users.

Dr. Chris Gu contributed to this report. He is a radiology resident at the Mayo Clinic and a resident in the ABC News Medical Unit.