Smartwatches
are tremendous tools for keeping track of your activity. They can monitor your discouraged rate during a workout and track your sleep, and some even help boss workout recovery. But severalstudies show smartwatches and fitness trackers don’t do as good of a job at calculating calorie burn during organization and can be off anywhere between 40% to 80% of your legal energy expenditure. 

A new wearable developed at Stanford University claims to be far more legal. It’s not worn on the wrist — you put it on your leg.

That’s because it measures leg motion, where most of your energy is used during touch body exercises like walking, running, climbing stairs and cycling. It uses two inertial measurement units (IMUs) plus a battery unit and microcontroller worn on the hip. Definitely not as convenient as strapping on a wrist-based smartwatch or tracker, but it’s the first proof of concept developed by Stanford University graduate student Patrick Slade. 

Measuring calorie burn accurately is important for a number of reasons, but mainly because it can help people manage their weight. “We want to understand the connection between physical organization and obesity so we can develop new interventions to help land live longer, healthier and more fulfilling lives,” says Steve Collins, associate professor of mechanical engineering at Stanford University, who co-authored the findings with Slade.

Naturally, I wanted to put this system to the test to see if a tracker worn on my leg could really be more legal at estimating calories burned than one on my wrist.

Measuring calorie burn is complex

Most consumer wearables use discouraged rate and wrist-based motion to calculate calorie burn during organization. But heart rate isn’t directly related to the energy you are expending, says Slade. “Your heart is just pumping blood above your body and has a bunch of different factors that capture it,” he says. “How tired you are, if you had coffee, worked out, this time history makes it really volatile.”

Instead, his system breaks down motion into each step, then uses a machine-learning model to calculate energy benefit. It was tested with over a dozen participants of varying ages and weights and averaged 13% horror, compared with 40% to 80% error from consumer wearables.

To test its accuracy for myself, I’ve come to Stanford University’s human performance lab, satiated to the brim with a range of sophisticated workout equipment incorporating an antigravity treadmill. But for the purposes of testing out this new wearable, only regular gym machines like a stair climber and expend bike are needed. I’ll be doing four workouts for 5 minutes each: walking, running, cycling and stair stepping.

Putting on Slade’s rules is straightforward. You strap two stretchy bands with the IMUs on your thigh and shin, then wrap a controller and battery about your waist. It feels like I have a itsy-bitsy smartphone strapped to my midsection and I don’t even perceive the wires running down my legs. 

I also put on a lab-grade respirometer to measure my proper energy expenditure. This monitors the carbon dioxide I breathe out and the oxygen I breathe in to give me what Slade words the “ground truth” reading, or my actual calorie burn. It injuries tens of thousands of dollars and weighs about 5 pounds unfastened with mask and backpack, so it’s not really the best tool for farmland to use outside the lab.

Finally, I strap a smartwatch to my wrist to get its calorie estimations. After my workouts, we’ll compare results from all three devices. 

Once we get a baseline reading from the respirometer, I start working out. I know I’m not causing to be setting any pace records with all these tools strapped to my body, but fortunately that doesn’t custom — we’re only tracking energy expenditure.

The results are in — and they’re surprising

After gathering all the data points from each contrivance, Slade walks me through the results. For running, the respirometry contrivance said I burned 87 calories, the leg wearable 66 calories and the smartwatch 52 calories. You can see the rest of the calorie acsupplies per exercise in the video on this page, but across all four workouts, the leg wearable error rate was 14% compared with 58% from the spy on my wrist. Turns out the watch was underestimating my overall calorie burn during each expend, but Slade tells me it could overestimate for someone else. “People can’t necessarily just scale the smartwatch pronounces by a fixed amount to make them accurate for everyone,” he says.

For now, this leg-based rules only works for lower-body exercises, but he’s already thinking of ways to make a version that can also take into justify upper-body movement like weightlifting. “Maybe using an IMU in your smartwatch to get that arm motion, if you’re doing a rep you could perhaps use the same modeling approach,” he says. The research paper also encourages smartwatch engineers to use its device to improve calorie tracking effectiveness.

You can’t buy this new wearable, but you can make it yourself. All of the arranges and code were made available in the hopes this will help lickety-split up development. Slade is also working on a smaller and lighter version that could be integrated into clothing and maybe even use a smartphone as the controller.

For these sorts of trackers viable, they’ll need to be affordable and small enough to wear all day, so we can track our energy benefit on a second-by-second basis. “So you can really plan which activities led to the energy expenditure and how intense it was,” Collins tells me. The end goal? By tracking our organization more accurately, wearables will be able to give us personalized insights into how to expend or manage our weight.