Estimating calorie counts is an imperfect science, especially when it comes to tracking how many you’ve burned during a given activity. Wrist-based fitness trackers and smartwatches’ estimates for calorie burn can be off by as much as 40-80%. However, engineers from Stanford claim they’ve created an inexpensive, accurate device that you can actually make for yourself. But instead of wearing it on your wrist, you strap it to your leg.
The device, which was published in Nature Communications, uses two “inertial measurement” sensors worn on a person’s shank and thigh. The components are run by a microcontroller on the hip and measure how fast the leg rotates and accelerates while in motion. To measure its accuracy, the engineers had test subjects wear their device along with two smartwatches and a heart rate monitor during activities such as walking, running, biking, climbing stairs, and transitioning from walking to running. The results were then compared to calorie burn measurements collected from a laboratory-grade system. Study participants were also of varying ages and weights.
Long story short, the leg-based device had an error rate of 13% across common activities, compared to 42% for smartwatches and 44% for activity-specific smartwatches.
According to Stanford engineers, the problem with wrist-based devices is they generally calculate calorie burn based on a combination of wrist movement, trunk movement, and heart rate. However, heart rate isn’t always related to how many calories you’re burning. A cup of coffee, for instance, may increase your heart rate even though you’re not metabolizing at a faster rate. Likewise, exercises like outdoor cycling don’t have as much wrist or trunk-based motion. Another benefit is the device is capable of delivering data in a faster, more accessible way. Currently, laboratory-grade methods of measuring calorie burn are expensive, require a mask, and aren’t exactly portable.
“A lot of the steps that you take every day happen in short bouts of 20 seconds or less,” Patrick Slade, lead author of the study and Stanford graduate student in mechanical engineering, said in a press release. “Being able to capture these brief activities or dynamic changes between activities is really challenging and no other system can currently do that.”
“An issue with traditional smartwatches is they only get information from the movement of your wrist and heart rate,” Mykel Kochenderfer, associate professor of aeronautics and astronautics at Stanford and the paper’s co-author, said in a statement. “The fact that Patrick’s device has a lower error rate makes sense because it detects motion of your legs and most of your energy is being expended by your legs.”
This doesn’t mean that fitness buffs or anyone looking to lose weight will be able to start strapping devices to their legs, however. This particular system is a proof-of-concept device. However, a neat thing is the engineers have made the code and component list publicly available for anyone who wants to try making their own rig for about $100. In a video, Slade noted that in a commercial setting, the device could potentially be even more affordable.
The calorie burn problem is one that’s plagued wearables for a long time. For tech reviewers, there’s no way to test or gauge calorie burn accuracy, even if it’s a metric that many consumers want to know for managing weight. Plus, different devices use proprietary algorithms that may be based on different sensor data. However, it’s important to note that this doesn’t mean smartwatches or fitness trackers are completely useless for health tracking. They can still provide valuable insight into your baselines, as well as help track your progress in various sports. It’s just a friendly reminder that you shouldn’t put much stock into how many calories they say you’re burning.