By Robert Jimison

CNN

(CNN) -- The heart rate monitor inside your fitness tracker may not be as precise as the equipment used in doctors offices and hospitals, but researchers say the smartwatches and wristbands are accurate enough for most consumers' needs.

When tested alongside electrocardiograph (ECG) technology, devices from Fitbit and Mio performed reasonably well at measuring resting and active heart rates, according to a study published Monday in the Annals of Internal Medicine.

"It's very exciting because we've had so much advance in technology during such a short period," lead researcher Lisa Cadmus-Bertram said. "These trackers are such an enormous improvement over what we used to have."

For the small study, Cadmus-Bertram and her team at the University of Wisconsin looked at how the trackers worked for 40 healthy middle-age adults, compared with an ECG and against each other. The participants wore four devices during the test: a Fitbit Surge, a Fitbit Charge, a Mio Fuse and a Basis Peak smartwatch.

Compared with the ECG reading, wearable products varied in their accuracy. The Fitbit Charge performed the best at rest, measuring within 5 beats per minute of the ECG reading 95% of the time. The Basis Peak activity tracker was shown to be within 22.6 bpm of the ECG reading during the 10-minute resting test.

Accuracy lessened in all of the tested devices during increased activity. The monitors were off by a range of 20 to 40 beats per minute compared with ECG measurements. The findings that devices were more accurate during rest are similar to what previous research has found.

Although data were not provided to show readings at each measured time, Cadmus-Bertram suggested that outliers -- numbers well outside normal readings -- may have caused a wider range.

Critics question heart rate monitors

Critics, however, say that the devices are not performing up to their advertised promises and could supply users with dangerously inaccurate information.

Questions about the devices and their accuracy sparked a class-action lawsuit in 2016 over the technology in Fitbit trackers. The lawsuit was filed on behalf of people who bought Fitbits especially to help track their heart rates, whether for health reasons or to make sure they are getting the most out of their workouts. The lawsuit is ongoing.

"However accurate they may be at rest, the Fitbits are wildly inaccurate as heart rate monitors when worn during moderate- and high-intensity exercise, which is precisely the purpose for which Fitbit (in particular) markets them to consumers," said Jonathan Selbin, one of the attorneys who filed the lawsuit.

Physicians and researchers note that the trackers aren't medical devices. Selbin and others point out that Fitbit, in particular, "claims to be a 'Digital Healthcare Company' and is actively trying to get corporations and insurers to make health care decisions based upon data they collect."

Researchers and physicians said the technology has a long way to go. Dr. Nisha Jhalani, a cardiologist at NewYork-Presbyterian/Columbia University Medical Center, who was not involved in the new study, noted that the technology in ECG machines and LED-based activity trackers is entirely different.

"Electrocardiography, or ECG, involves placing electrodes on the skin surface to measure electrical impulses generated by the heart muscle itself. The LED technology used in fitness trackers is an indirect measurement looking at the changes in light reflection through the skin during each heartbeat," she said.

If you've ever worn a wrist heart monitor, you may have noticed a time or two during exercise when your heart rate read inaccurately low for a few ticks before getting it right. These hiccups are one reason, researchers say, why the range of accuracy of these devices is so wide.

"A tracker will give you the occasional crazy reading, but most of the readings will be quite a bit closer than that," Cadmus-Bertram said.

Tracking your health

Overall, researchers and experts are encouraged by the improving technology and continue to promote the use of fitness trackers to provide more information about a user's health.

"Fitness trackers are a great way to make people aware of their activity level throughout the day," Jhalani said. "Oftentimes, our perceived activity is much more than actual time spent we spend moving, especially people whose jobs involve sitting at a desk for hours at a time."

Heart rate data, she says, can be an empowering tool that can allow users to be more aware of their heart health and overall fitness.

"It is good for people to see their heart rate, both at rest and with exercise. Low resting heart rates, in the 60 to 70s bpm range, are considered generally healthy. High resting heart rates, especially when close to 100 bpm or higher, can be a sign of high stress levels or other medical conditions," Jhalani said.

"How quickly the heart rate goes back to normal after exercise also holds a lot of information. Quicker indicates better cardiopulmonary fitness level."

The compact, convenient and more stylish design achieved by using LED technology versus ECG comes with a few tradeoffs.

"LED technology can be affected by variables such as how loosely or tightly the tracker is worn, the user's skin tone or any other interference between the sensor and the skin. It can also become inaccurate with motion, which is why it doesn't fare as well as ECG with exercise," Jhalani said.

In response to the new study, Fitbit said in a statement that its 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."

Representatives from Mio also emphasized that the product is a consumer device. "We need to make understanding heart rate easier for the average consumer," the company said in a statement. "This opportunity defocuses the importance of needing to know the exact heart rate at any one time, and rather put emphasis on understanding what getting your heart rate up does for your health over time."

Representatives from Basis could not be reached; the fitness tracker used in this study was recalled in September. According to the company's website, the recall is "because the watch can overheat, which could result in burns or blisters on the skin surface."

As a commercial product, Jhalani said, the overall impact is a positive one. "I think it keeps people from becoming complacent and empowers them to work towards a healthier lifestyle."

"While you wouldn't want to rely on a commercial tracker if you need absolute precision during exercise," Cadmus-Bertram said, "there's also no reason for the general public not to use it for feedback and motivation."