Can You Trust Heart Rate Readings from Wearable Fitness Bands?

Pulse Monitor Fitness Watch
Pulse Monitor Fitness Watch. © alexey_boldin /

For 2015, more wearable fitness trackers and smartwatches are detecting your heart rate to measure moderate intensity and vigorous intensity exercise, or to measure your resting heart rate, but can you trust the numbers they are showing? Can you really use them instead of a chest strap heart rate monitor?

"The wrist is a terrible location for biometric monitoring," said Steven LeBoeuf, president and co-founder of Valencell.

"Wearable devices are less accurate at the wrist than at the head or hip." His company licenses biometric data sensor technology used in devices,including Bluetooth headsets, earbuds, sports armbands and wristbands. We discussed how these new devices compare with the standard chest strap heart rate monitor.

Apple Watch Heart Rate Accuracy Fails Interval Test

The Apple Watch's heart rate readings were a miserable 47% accurate compared with a chest strap heart rate monitor in an interval training treadmill test conducted for Valencell. Their PerformTek biometric technology on a wrist sensor was 82% accurate in the same test.

Fitness Trackers Move from the Hip to the Wrist

Fitbit debuted in 2008 with a hip-worn fitness tracker that improved on old-school pedometers by being accurate, easy to use and view its data on an online dashboard and phone app. Fitness bands such as the Fitbit Flex and Jawbone UP moved the data detection to the wrist and gained widespread popularity by 2014.

Now these devices are going beyond telling you how many steps per day you've walked and integrating biometrics to tell you both how much you are moving and how that movement is affecting you.

Heart rate is a measure of exercise intensity. Those who aim to exercise in higher heart rate zones want to know when they are in the zone and how long they have spent in the zone.

That requires accurate readings. Dieters want to know how many calories they have burned through exercise, and that requires heart rate for the best accuracy. Many of us want to track whether we are achieving not just 10,000 steps but also 30 minutes per day in moderate-to-vigorous exercise recommended for fitness and health by health authorities.

Heart Rate Readings from Wrist Sensors May Lack Accuracy

But are heart rate reading from wrist devices tracking what they say they are tracking? LeBoeuf says it is harder to register blood flow in the wrist than elsewhere on the body. Bone and muscle movement make interpreting readings complex. The heart rate monitor may really be tracking steps instead of heart beat.

LeBoeuf concludes that wrist-worn heart rate devices are not yet accurate for most uses. He lists two such devices which are the most trustworthy:

  • The Mio Alpha is worn on the wrist and he says is accurate for steady state runs.
  • The Scosche Rhythm+ worn on the forearm is very accurate and good even for interval training with rapid changes in heart rate. The Rhythm+ uses both green and yellow LED light. The yellow light helps penetrate tattoos and darker skin, which the green light used by the Apple Watch and other wearables can't penetrate. The Valencell technology used in the Scosche was validated as being 82 percent accurate in an 8-minute variable variable-intensity interval test on a treadmill. This compares with the Apple Watch being only 47% accurate in the same test.

    The inaccuracy of other devices means that they can't really tell whether exercise is helping you or hurting you. LeBoeuf says today's wrist heart rate measuring devices are accurate 50-70% of the time for 50% of the people, far short of a decent standard of working 90% of the time for 90% of the people. His company, Valencell, is looking for better technology and also for devices to signal when they are getting accurate readings and when they are not.

    Experience with Wearable Heart Rate Readings
    I have seen the effects of this with several devices. The Jabra Sport Pulse earbuds are made to detect your heart rate as well as play music.

    But I could not get a good fit consistently to get steady heart rate readings. When I got a fit that was better than usual, it was uncomfortable. Ultimately, I wanted something I could simply put on in a few seconds that would work accurately rather than having to adjust it for several minutes before each workout.

    Wrist heart rate detection has been hit-or-miss. I was able to get good, stable readings with both the Fitbit Charge HR wristband and the Fitbit Surge super watch, by placing it on the wrist the distance from my wristbone as recommended. The original Basis Health Tracker measures resting heart rate, and I saw times when it wasn't getting a stable reading. Fitbit and Wellograph also track resting heart rate as a measure of fitness. LeBoeuf notes that there is no industry standard as to how resting heart rate is measured or reported. The device may take a reading at some point during your sleep cycle, but is that the most accurate point?

    Why Not Just Wear a Chest Strap Heart Rate Monitor?
    For simplicity, it is easier to use a device you are already wearing most of the day rather than having to put on a chest strap for exercise sessions. All-in-one devices such as the Fitbit Surge, which includes GPS for speed and distance, are appealing if they are accurate enough for fitness use.

    Smartphones Can't Do It
    But another all-in-one device doesn't work at all for biometric monitoring, which requires continuous contact with the skin. You can take your pulse with a smartphone app, but you'll need a sensor somewhere on your body for continuous data. While a smartphone can act as a hub for the information and provide GPS data, we are still in need of a comfortable, convenient, accurate wearable biometric sensor.

    Bottom Line: How Good is Good Enough?
    For the average person who wants to be more active and to have an objective measure of whether their brisk walking is moderate-intensity exercise, the heart rate on the current crop of wristband monitors may be accurate enough. But they still fall short of medical monitoring standards and the readings should not be trusted if you your health depends on a higher accuracy. A chest strap heart rate monitor is still a better choice if you need accuracy.

    Next: Where are Fitness Wearables Going?

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