There is a rise in popularity of wearable devices that track steps and even heart rate, but research has shown many of these devices do not provide accurate information. Researchers tested four popular wearable heart rate devices for their study.

Research lead Dr. Marc Gillinov said, “All worked pretty well at rest. But as people exercised, the accuracy diminished.”

None of the devices were as accurate at monitoring heart rate as a chest strap monitor. The most reliable devices in the treadmill tests were the Apple Watch and Mio Fuse.

The Basic Peak device, which is no longer manufactured, overestimated heart rate during moderate exercise, and the Fitbit Charge HR underestimated heart rate during vigorous activity.

Basic Peak was pulled from shelves due to overheating accidents that caused skin burns. As for Fitbit, the company defended their technology, stating that the device meets industry standards.

Spokespersons for Fitbit said, “Fitbit devices were tested against properly calibrated industry devices like an EKG chest strap across the most popular activities performed worldwide, including walking, running, biking, elliptical, and more.”

Wearable activity trackers have been under much scrutiny as of late, as a number of studies have come to light revealing that these devices do not really boost activity levels. For cardiac patients, wearing a device to track heart rate is important for rehabilitation, so accuracy is key.

Dr. Mitesh Patel, assistant professor of medicine at the University of Pennsylvania’s Perelman School of Medicine in Philadelphia, explained, “For the general consumer, wearable devices may still be able to give them a general sense of their heart rate trends. However, further study is needed to determine which devices are more reliable for use in clinical care.”

The variations in heart rate readings ranged from minor to quite large, depending on the activity. The researchers advise that if you’re an athlete or a cardiac rehabilitation patient, you should use a chest strap for most accurate readings.