It seems, not all fitness trackers are created equal when it comes to accuracy. A new study published in JAMA Cardiology pits four popular wearable devices against each other to determine which is best at measuring your heart rate.

Researchers at Cleveland Clinic compared heart rates obtained by Fitbit Charge HR, Apple Watch, Mio Alpha and Basis Peak with a medical-grade ECG machine, the gold-standard test for measuring heart activity. They also assessed a chest strap, the Polar H7.

The study monitored 50 healthy adults while running on a treadmill, then walking and at rest. As you would expect, the chest strap showed the most accurate results, just 1% off EKG readings. Both solutions obtain information directly from the heart.

Essential reading: Best heart rate training chest straps

The research clearly shows, fitness wearables do a fine job of monitoring your heart rate while you’re resting or briskly walking but can struggle as you increase pace. Of the four wrist-worn monitors, the Apple Watch and the Mio Fuse performed the best with a success rate slightly over 90%.

Other trackers were in the low 80s. The Charge tended to underestimate heart rate, while the Peak overestimated heart rate. The Basis Peak was recalled a couple of months ago due to overheating issues.

Fitbit commented on the study in a statement.

“Fitbit trackers are not intended to be medical devices. Unlike chest straps, wrist-based trackers fit 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 much more informative picture of overall health and fitness trends.”

Fitbit also added that their internal tests showed an accuracy rate of 94%.

There are currently two different types of heart rate monitors on the market. The chest strap style and the wrist band style. A wireless sensor on a chest strap detects your pulse electronically and sends that data to a receiver which displays your heart rate. This is similar to what you can see in hospitals with patients who have sensors strapped to their chest.

Sensors that are embedded into a wristband operate by shining a light into your wrist, which is then reflected by blood vessels passing through your veins. When your heart pumps, the blood moves through your veins at a quicker rate, causing less light to be reflected back. The tracker will then calculate your heart rate using an algorithm.

The current trend may be to move heart-rate monitoring away from the chest and over to the wrist but accuracy at these new locations is clearly questionable.

“What we really noticed was all of the devices did not do a bad job at rest for being accurate for their heart rate, but as the activity intensity went up, we saw more and more variability,” says Dr. Gordon Blackburn, one of the study’s authors and director of cardiac rehabilitation at Cleveland Clinic. “At the higher levels of activity, some of the wrist technology was not accurate at all.”

Blackburn added that you need to have good contact between the photosensing cells. As a person is exercising more vigorously, there’s more bounce, so you may lose some of that contact.

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Apple has recently come out with the Apple Watch Series 2. While the Cupertino company didn’t deliver a new design – waterproofing, an integrated GPS and a faster processor make it better, while software upgrades makes it smarter and more useful. Buyers should no longer be considered early adopters.

The study makes it clear though. If you are really serious about heart rate training, chest straps are still the way to go.

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