Testing fitness trackers with heart rate monitors – what we found

The heart rate monitor in your fitness tracker may not be giving you accurate results.

Flatlining monitors?

Fitness trackers make it possible to exercise smarter. These wearable devices gather information on sleeping patterns, eating habits and training sessions, before presenting them in graphs that are easy to understand. Owners can use this information to make decisions about what they eat, the kind of exercises they do and their workout intensity.

A feature emerging in the latest generation of fitness trackers is a heart rate monitor. Wearing a stand-alone heart rate monitor is difficult because of their large size, but by integrating them with an unobtrusive fitness tracker, it should be easier for people exercising to identify if they're in a heart rate zone ideal for burning fat, improving endurance or increasing performance.

While we discovered some of the built-in heart rate monitors may not be as accurate as you'd think in our mid-2016 test, our latest fitness tracker review revealed small improvements across the market. In this article we reveal what we found when we tested 36 popular fitness trackers with heart rate monitors from brands such as Fitbit, Sony, Garmin and Apple.

Wasting a workout

A heart rate monitor showing a reading that is even marginally off is enough to undermine the efforts of a workout, says Dr Kate Edwards, the course director of Exercise Sports Science at the University of Sydney.

People exercising should aim for a specific heart rate zone to reap the results they're after. The light zone, for instance, is the ideal zone for fat burning, while the moderate zone targets aerobic fitness.

But a difference of five percent of your maximum heart rate is all it'd take to diminish the results of a workout. "The zone is a ten percent range," says Edwards. If you were exercising at 75% of your maximum heart rate and your monitor said you were at 81%, you would be working in a different zone.

"As soon as those intensities aren't accurate, then the benefits aren't there."

Not getting the benefits could affect your motivation to exercise, says Edwards, as well as your ability to achieve your goal. It's easy to see how inaccurate readings can short-change the effort poured into dieting, sleeping and exercising.

How we test

We tested the heart rate monitors of 36 fitness trackers at the Faculty of Health Sciences at the University of Sydney. A 12-lead electrocardiography (ECG) monitor mapped the real-time heart rate of a professional athlete as he ran on a treadmill in the faculty's Sports Chamber. The readings from the ECG monitor – a device Dr Edwards described as the most accurate heart rate monitor, capable of identifying an immediate change in heart rate – were compared to the readings of each of the fitness trackers.

Readings were taken at one-minute intervals for five minutes to compare the accuracy of each tracker to that of the ECG. To compare how well the trackers respond to sudden changes in intensity – typical to interval training – we also took readings at 10-second intervals in the first minute of warming up, and then the sixth minute when cooling down.

Testing heart rate monitor accuracy at the University of Sydney in 2016.

Performing the same heart rate monitor test in 2017.

Of the 36 trackers equipped with a heart rate monitor, only ten had an acceptable margin of error when measuring active heart rate. Among the best performing were the monitors in bands from Garmin, Fitbit, Under Armour and Striiv, as these managed to produce readings within 95% accuracy – despite having to contend with motion and sweat. The Samsung Gear S3 Frontier smartwatch was the best performing with an overall accuracy of 100%, but it can only read and display data when your arm stops moving. However, the Apple Watch Series 2, Mio Alpha 2 and TomTom Touch return 99% accuracy even with movement, which makes them a much more effective training option.

Resting heart rate results fared better. We assessed these against industry-grade GE Healthcare Carescape V100, Dinamap V100 and Dinamap V150 heart rate monitors since first testing fitness bands in 2014. All models are calibrated and set to record data under the same conditions. The only difference is in input and export tools (the V150 is the first model to introduce a touch screen for example). 18 trackers fell within the acceptable margin of error, with the TomTom Spark Cardio + Music, Fitbit Alta HR and Garmin Vivofit Jr. all returning an overall accuracy of 99%.

Big electronic brands surprised. The Apple Watch Sport was accurate only 67% of the time, the Samsung Gear S2 53% of the time, and the Sony SmartBand 2 just 27% of the time.

See more test results in our fitness tracker reviews.

Testing resting heart rate accuracy against the GE Dinamap V150.

Pointless fitness trackers

Pausing in the middle of a workout to read a heart rate can undermine its intensity, but that's exactly what the fitness trackers from Apple and Samsung demand. These fitness trackers can't provide a reading in real time, so at key points in a run or a bike ride, you'll have to stop, hold your hand level to the ground, make sure it's quiet and wait between 20 to 30 seconds for your reading. But by then the reading is irrelevant, as a heart rate can drop by 20 beats during the period of inactivity.

Edwards believes the manufacturers have adopted this method because it may be more accurate. However, the need to stop moving and the time needed to take a reading meant we couldn't test how well these trackers respond to rapid changes in intensity, such as during the warming up and cooling down periods of a workout. For this reason, the active heart rate results for all trackers in our test are based on the recordings taken at minute intervals, and don't take into account the readings from the warm-up and cool down.

Waking heart rate readings

The heart rate monitors in most fitness trackers performed better in our resting heart rate test. Capturing readings every morning can help identify any changes in overall health; a spike in your waking heart rate can reveal injury, illness and even long-term health problems.

This part of the test took place in the CHOICE labs, using the GE Carescape V100 heart rate monitor to measure against. Most of the fitness trackers we tested captured resting heart rate readings within 95% accuracy, as sitting steadily, waiting for 30 seconds and not having to contend with sweat helped them hone in on an accurate reading.

But sitting and waiting never helped anyone lose weight or gain strength. Edwards recommends using a chest strap heart rate monitor when exercising, although there are other methods that can be used to deduce the intensity of a workout. The rate of perceived exertion, a scale judging the ease or difficulty of an exercise, is one.

"But there are simple ways, like the talk test," adds Edwards. "If you can't comfortably talk [while running], then it's a higher intensity."

In other words, take a running companion – of the human variety.

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