Sunday, September 18, 2022

How Accurate Is Your Smart Watch?

I have noticed almost all my patients wear a smart watch now. Mostly an Apple watch. Actually, Aized recently has a patient who wears a Rolex on his left hand and an Apple watch on his right. He says he is a watch connoisseur (hence the Rolex), but the Apple watch is more practical since it monitors his health, allows him to read his whatsapp messages, email and also to make payments easily.

Are the readings for heart rate, calories used and energy expenditure accurate? Definitely more so if you have updated personalized metrics like your weight, height, age and gender to make the calorie estimates more accurate. That was what I thought until I looked it up.

I found a study that studied the heart rate and energy expenditure accuracy of the Apple watch, Fitbit Charge HR and Garmin Forerunner 225. All the watches overestimated energy expenditure. 

Another study looked at the Apple watch, Samsung Gear 2, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2 and the PulseOn. That study found that even though heart rate was adequately measured, energy expenditure was not.

A third study concluded that 2 fitness watches (Polar V800, Garmin Forerunner 920XT) and 3 activity trackers (Garmin vivosmart@HR, TomTom Touch and Withings pulse Ox) were not accurate enough to be used for sports or heathcare applications. Ouch! And I thought Polar and Garmin are the market leaders for sports watches. No prizes for guessing when measuring distances and altitude gain then.

The quality of heart rate data reading can affect accuracy. Compared to using a chest strap to measuring your heart rate with having an Apple watch on your wrist, the chest strap will be more accurate.

According to Apple, how snugly the watch fits against your skin, the surrounding temperature and tattoos can affect readings. The ink, pattern and saturation of some tattoos can block light from the sensor, affecting the reliability of readings. 

Yes, you read correctly, tattoos can affect your sporting performance as we have written before

Note that all the 3 studies had less than 100 participants each and technology for such watches and activity trackers are consistenly improving so the latest versions can be more accurate. For now, bear in mind that the readings are just an estimate.


Dooley EE, Golaszewski NM and Bartholomew JB (2017). Estimating Accuracy At Exercise Intensities: A Comparative Study Of Self-monitoring Heart Rate And Physical Activity Wearable Devices. MMIR Mhealth Uhealth. 5(3): e34. DOI: 10.2196/mhealth.7043

Passhler S, Bohrer J, Blochinger L et al (2019). Validity Of Wrist-worn Activity Trackers For Estimating VO2 Max and Energy Expenditure. Int J Environ Res Public Health 16(17), 3037. DOI: 10.3390/ijerph16173037

Shcherbina A, Mattsson CM, Waggott D et al (2017). Accuracy in Wrist-worn, Sensor-based Measurements Of Heart Rate And Energy Expenditure In A Diverse Cohort. J pers Med. 7(2): 3. DOI: 10.3390/jpm7020003

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