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If you think your fitness tracker on your wrist is giving you a good excuse to eat that donut, you may want to think again. According to a recent study from the Stanford University School of Medicine, fitness trackers are not accurate at tracking calories, though they are accurate at measuring heart rate.
Researchers at Stanford wanted to determine whether fitness trackers were accurately measuring heart rate and energy expended when people were at rest and when they exercised. “People are basing life decisions on the data provided by these devices,” said Euan Ashley, one of the authors of the study.
The study looked at seven fitness trackers worn on the wrist:
- Apple Watch
- Basis Peak (version 1)
- Fitbit Surge
- Microsoft Band (version 1)
- Mio Alpha 2
- PulseOn
- Samsung Gear S2
60 participants (29 males and 31 females) wore up to four trackers on their wrists as they sat and as they exercised on a treadmill and a stationary bike. The study included volunteers of diverse height, weight, age, body mass index (BMI), wrist circumference, and fitness levels.
The trackers measured participants’ heart rates and calorie/energy expenditure in activities from sitting to a leisurely walk to a run, and with cycling at different speeds on the stationary bike.
Their heart rates and other vital signs were also measured using FDA-approved medical instruments. The measurements of the fitness trackers were compared to the “gold standard” measurements of the FDA-approved medical instruments to determine the accuracy of the trackers.
Energy Expended (EE)
The researchers studied whether the fitness trackers accurately measure how many calories were burned, in other words, how much energy was expended by participants, both at rest and while exercising. In designing the study, the researchers determined that an acceptable error rate for a tracker measuring energy expended was ≥ 5%, but none of the fitness trackers met that level.
The final report on the study, published last week, stated,”No device met our prespecified error criterion for energy expenditure.”
The most accurate tracker for measuring energy expended was the Fitbit Surge with a 27.4% average error rate. The least accurate tracker was the PulseOn with a 92.6% error rate.
Interestingly, the error rate was higher for males than for females with all fitness trackers tested. The average error rate for all tested trackers with all forms of exercise tested was 4% higher for males than for females.
Heart Rate (HR)
The good news is that the study found that the fitness trackers studied were accurate at measuring heart rate. All but one of the trackers measured heart rate within the 5% accuracy range deemed acceptable by the researchers.
Of the trackers studied, the Apple Watch was the most accurate at measuring heart rate, with an average error rate of 2.0%, while the Samsung Gear S2 was the least accurate with an average error rate of 6.8%.
Study Findings
Overall, for both the measurement of energy expended and heart rate the study found, “of the devices tested, the Apple Watch had the most favorable error profile while the Samsung Gear S2 had the least favorable error profile.”
While the trackers’ measurements of heart rate were within the acceptable range, their measurements of calories/energy expended were not. “The heart rate measurements performed far better than we expected,” Ashley said, “but the energy expenditure measures were way off the mark. The magnitude of just how bad they were surprised me.”
The authors suggest that the heart rate was reliably measured by the trackers because that measurement can be monitored directly, while the measurement of energy expended depends on analyzing many different factors and varies between individuals.
More Information
The researchers acknowledged this study was constrained by the participants exercising within the confines of a laboratory. The next phase of their study will examine participants wearing fitness trackers throughout the day and while exercising outside a laboratory setting.
“In phase two,” said Anna Shcherbina, another author of the study, “we actually want a fully portable study. So volunteers’ ECG will be portable and their energy calculation will also be done with a portable machine.”
Another interesting study would be to determine whether the fitness tracker is relatively accurate for each individual over time. For example, if a tracker showed that the individual expended 500 calories for a run, then expended 1000 calories for a different run, would the difference in calories expended between the two runs be accurate? If so, maybe individuals would be able to gauge their relative fitness by these trackers.
The full study is available at The Journal of Personalized Medicine: Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort.
You can also read a summary of the study at Stanford University News Center, Fitness trackers accurately measure heart rate but not calories burned.
Your Thoughts
Do you wear a fitness tracker on your wrist? Do you think fitness trackers accurately measure heart rate and energy expended? Do you make decisions based on your fitness tracker?
Share your thoughts in the Comments section below!
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*Runner image courtesy of Eddie Codel via Flickr and Creative Commons
*Chart and graph images courtesy of Stanford University School of Medicine via Creative Commons
I am guessing that calorie tracking is hard, especially if you workout indoor. When speed is not a factor, only heart rate.
Hi Frane, Excellent point. These devices have technology that measures movement, but movement is limited on a treadmill and stationary bike. In fact, it is difficult to understand how a fitness tracker worn on the wrist could accurately determine the energy expended in riding a stationery bike.
I look forward to the next phase of the study that measures participants as they exercise outside the laboratory.
Not only stationary bike but any indoor fitness, especially if it’s mixed exercises.
Good point, Frane. Exercising outside where there is freedom of movement can make it easier for fitness trackers to get an accurate measurement of your activity. I do look forward to Phase 2 of this study.
This is good to know. So many of us are using FitBit’s to track calories. At least we can keep know that this might not be the best way. Especially helpful for those of us trying to lose weight.
Hi Jessie, Yes, getting in shape can be challenging and getting a fitness tracker can help motivate people to be more active. But it’s important to know that they may not be accurate so we shouldn’t use them as an excuse to eat that sweet treat!