It’s easy to attribute omnipotence to one’s devices. I’d hate to know what my iPhone knows about me. That said, there are still aspects of existence that Silicon Valley has yet to quantify (though not, I’m sure, for lack of trying). Calorie-burn might be one of these. While countless apps/devices profess to track it with rigor, it’s not at all clear how accurate their efforts actually are. For this week’s Giz Asks, we sought clarity on this issue with a number of relevant experts.
Chair of the Department of Genetics and Director of the Center for Genomics and Personalized Medicine at Stanford University
In some cases yes, in others no.
These things work reasonably well, but not that well. And they’re not going to measure the effects of activities that don’t involve accelerometers. They might pick up on calories burned while running on a treadmill, but less so on calories burned while lifting weights. But, in my experience, even the best wearables aren’t perfect on this front, and so much of calorie burn has to do with your intrinsic metabolism, which a wearable wouldn’t be factoring in.
If you’re doing something that’s elevating your heart rate, these devices will probably pick up on it. What they won’t do is distinguish between calories burned the way you’d want to to burn them (an elevated heart rate through yoga, say) and calories burned the way you wouldn’t want to burn them (an elevated heart rate through stress).
For these devices to provide detailed calorie-burn information, they’d need to calibrate for each individual wearer—they’d have to know how much energy you burn when you do this or that specific exercise. With more sophisticated equipment, you can sort of standardize this to yourself.
We’ll get better at measuring these sorts of things in the future, because the devices are getting more and more sophisticated around things like respiration, which can help to signal different kinds of activity. But it’s still pretty early, in my opinion.
Professor and Chair, Biomedical Engineering, University at Buffalo
Our bodies expend energy continuously. The term “energy expenditure” can also be thought of as “burning calories” in this context. The amount of calorie burn, or how much energy we expend doing tasks like walking, running, swimming, or just sitting, is important as we look at overall health. This information can indicate how efficiently the body is using nutrients (food), and to know if you’re using more calories each day than you’re taking in.
Among other things, wearables have the ability to provide information on how many calories a wearer burns.
Determining calories burned is not a direct measurement; it must be calculated based on a number of parameters, many of which vary from person to person. Studies indicate that the energy expenditure reported by all wearables deviates from “gold standard” measures of energy expenditure by significant amounts. And generally, whether wearables under-estimate or over-estimate the amount of calorie burn depends on the wearable, the activity, and rate of that activity, which further confounds things.
So, wearables are not at a level of sophistication that allow them to be used for critical measurements, such as precisely monitoring energy expenditure for medical reasons or for significant athletic training purposes. If someone is interested in a general gauge of their calorie burn for trends over time or for daily comparisons, then a wearable is an acceptable tool. And one should not overlook wearables as motivators to getting people moving.
However, one should not rely on exact numbers reported.
We expect wearable devices to get better, and indeed, newer versions of wearables allow you to input even more types of activities in an attempt to improve the accuracy of the built-in algorithms. And the more customizable the wearable is to the individual user, the better the calculation should be. As newer wearables come onto the market, more studies are needed to determine if they are better at measuring calorie burn than the wearables we’ve studied in the past.
Professor, Electrical and Computer Engineering, University of Alabama, whose research interests span wireless, ambient and wearable devices, and methods of biomedical signal processing and pattern recognition
The accuracy of wearable devices that measure energy expenditure (“calorie burn”) varies greatly.
The key defining factors are the sensors and algorithm used. The most prevalent sensor to date is an accelerometer: a device that measures motion, most frequently the whole body’s movement. The accelerometer is used, for example, to recognize when someone is walking or running, and to count steps.
The problem is that not all types of physical activity register well on the accelerometer. Strength training often targets isolated muscle groups, and even rigorous exercise does not register on the accelerometer if it remains stationary during the workout. Imagine doing push-ups or pull-ups with a wrist-worn activity tracker. Hence, the second most frequently used sensor is a heart rate sensor. By monitoring heart rate, it is possible to obtain a more accurate picture of the exercise intensity and energy expenditure.
Research-grade energy expenditure monitors may include additional sensors, such as body temperature, air temperature, temperature flow, perspiration, galvanic response, barometric, and other sensors. These sensors paint a more complete picture of the activity being performed and potentially improve the accuracy of the monitor.
The next key contributor to accuracy is the algorithm used to convert sensor measurements to the energy expenditure. These algorithms may be extremely simple (more steps you do, the more calories burned) or extremely complicated. For example, the algorithms that we develop and test in my lab may include recognition of the physical activity as the first step (such as sitting, standing, walking, cycling, driving, etc.) and then the estimation of the energy expenditure using a model specific to that physical activity. For consumer-grade wearables, these algorithms are often a black box owned and carefully guarded by the company. Over the past years, several research studies have shown that the accuracy of wearables varies dramatically in comparison to a highly accurate reference measurement. And, of course, one should be keenly aware of no-name monitors that frequently fake both the sensors and energy expenditure calculation algorithms. In my lab, we tested many devices that show completely unrealistic readings, such as a banana demonstrating a heart rate.
In summary, there is no universal answer on accuracy claims. The accuracy depends on the device, the type of activity being measured and the algorithms used. That said, the accuracy is gradually improving through the inclusion of new sensors and the development of more advanced algorithms. In comparison, the accuracy of physical activity measurement by a modern wearable is much higher than the accuracy of measuring when, what, and how much we eat (measuring energy intake). In my lab, we develop new solutions to tackle both problems and perform an accurate measurement on both sides of the energy balance equation.
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