Are Activity Monitoring Devices Accurate?

A new study by researchers at Stanford University evaluated the accuracy of seven commercially available wristband activity monitoring devices. These provide the user with their heart rate data and calculated energy expenditure based on heart rate. They evaluated different brands such as Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn, and Samsung Gear S2. Participants wore devices while being simultaneously assessed with continuous telemetry and indirect calorimetry while sitting, walking, running, and cycling. Sixty volunteers (29 male, 31 females, age 38 ± 11 years) of diverse age, height, weight, skin tone, and fitness level were selected.

Although heart rate is closely related to energy expenditure, it has long been known by exercise physiologists that heart rate does not provide an accurate measure of energy expenditure (calories expended). This study clearly showed that these devices measure heart rate reasonably well, but their estimation of energy expenditure is very inaccurate.


Why the Apple Watch is Different:

Personally, I use an Apple Watch to monitor my daily steps and active time and I love it. Even though it may not accurately measure my energy expenditure, it does a great job in reminding me to stand up after sitting for a while and just being mindful of my daily activity. It is different because instead of counting steps or calories, the Apple Watch focuses more on your overall health and well-being.

September iwatch

According to my weekly average, I do about 30 minutes of exercise, which proves that running after my toddler is great cardio! Video below is not even on fast forward, this is his usual stride.

Apple defines exercise as any activity you perform that is the equivalent of a brisk walk or more. It determines exercise by monitoring your heart rate and movement data. That means that things you do on a regular basis like getting up and walking around your office or taking your dog for a walk probably won’t raise your heart rate enough for the Apple Watch to deem it as exercise. Like I tell my patients who opt for walking as their cardio “if you are walking and talking comfortably…you are not walking fast enough”.


Ideally you should use your exercise monitoring device to have an estimate of your overall activity. However, do not use it to justify binge eating or simply that those are some “extra” calories to treat yourself. It is more important to count NUTRIENTS than calories, because it is not the same thing to eat 100 calories of bagged chips than 100 calories of fresh apples.



Shcherbina et al. Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort. J. Pers. Med. 2017:7(2);3. To download the original, click HERE.

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