Woman running in a track field

Fitness Trackers - Do They Work?

Millions of people use fitness trackers in their day-to-day activities as a way to measure their physical activity, keep tabs on calorie expenditure, track heart rate and more.

The wearable market is currently worth billions and is forecasted to generate $25 billion by 2019, according to CCS Insight. That alone is spurring competition amongst tech giants Apple, Fitbit, Samsung and etc. to release products and upgrades regularly.

Millions of people rely on the data collected by these fitness trackers for medical reference or as a way to track their own fitness progress.

However, new research from the Stanford University School of Medicine published in the Journal of Personalized Medicine revealed that the data these fitness trackers show may not be accurate.

Stanford University evaluated seven devices - Fitbit Surge, Microsoft Band, Apple Watch, Samsung Gear S2, Basis Peak, Mio Alpha 2, and the PulseOn on 60 volunteers performing 80 tests. What’s interesting is that the researchers found that not only were some devices more accurate than others, but that some devices were more accurate for certain tasks.

They found that the lowest percentage of error for heart rate was observed for the cycle ergometer task and the highest error percentage for heart rate was shown in the walking task. For the cycle ergometer task, they found that six of the devices achieved an acceptable “median error of below 5%” for heart rate while the Samsung Gear S2 achieved a “median error of 5.1%” for heart rate. For the walking task, only three of the devices achieved a median error percentage below 5% (Apple Watch, PulseOn and Microsoft Band). When compared across all devices and tests, the Apple Watch achieved the lowest percentage of error at 2.0% and the Samsung Gear S2 had the highest at 6.8%.

While majority of the devices where fairly accurate in measuring heart rate, they were missing the mark for energy expenditure (i.e. calories burned) entirely. The median error rate for energy expenditure was higher on all devices when compared with the results from the heart rate tests. These ranged from 27.4% to 92.6% and beyond what the researchers classified as “acceptable”.

Another surprising result from that study is that darker skin tone, larger wrist circumference and higher BMI affected measurements. The team noted that these factors “correlate positively with increased HR [heart rate] error rates across multiple devices”.

It’s worth noting that all devices tested in the Stanford study were all wrist-worn devices. But you could make a case for a shoe-based device. Researchers at the Colorado State University found that shoe-based fitness trackers provided a valid estimate for energy expenditure when compared to measured energy expenditure.

With an ever-burgeoning market for these devices already in the billions and companies releasing updates and upgrades regularly, it makes sense to think that further research is needed to verify their accuracy at an on-going basis.

Lastly, the main takeaway we can get from these results is that maybe we need to rethink some of the claims made by these companies. Looking for an effective way to lose weight? Start the Simply Slender 48 hour Detox Diet today!


“Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort.” - Journal of Personalized Medicine, May 24, 2017
“Wearables Market to Be Worth $25 Billion by 2019.” - CCS Insight 
“A comparison of energy expenditure estimation of several physical activity monitors.” - Medicine and science in sports and exercise, November, 2013