I check my iPhone. Time has slowed to a crawl and, according to all of these gadgets, so have I. My heart rate is a mere beats per minute. I adjust one of the three monitors strapped to my chest and pick up the pace. I want to get a better idea of how they track calories. All of these gadgets promise to help you get in shape by tracking how much you move and how many calories you expend during a wide range of activities.
This is especially true of wearables, as these gadgets are known. Every device takes a different approach to calculating the mechanics of moving your body a certain distance at a certain speed. For instance, fitness trackers feed step counts and even sweat rate into their equations, while GPS watches lean heavily on heart rate. To establish a level playing field in benchmarking these gadgets, I need to do each test with every device strapped to my body. I look like I just escaped from a lab.
Before hitting the bleachers I ran 5 kilometers. This followed the minute plyometric workout and 2-mile walk I did the previous day. I endured the same routine three times in two weeks.
For each activity, the calorie counts reported by these exercise monitors were all over the map. These algorithms attempt to discern your rate of caloric expenditure using measureable or self-reported metrics like heart rate, distance, weight and age.
Each device I tested crunches the numbers differently. Wahoo Fitness, for instance, calculates burn rate in its iPhone app using the same two algorithms for every activity.
The formula for women is [ For men it is Everyone else declined to reveal their algorithms, citing the usual pending patents and trade secrets. But some shared the metrics they used in the equation, which helps explain the results of my testing. My running test had by far the most consistent numbers. I mistakenly thought their approach of counting steps and overall movement was geared toward less-intensive activities like walking and yoga.
Oddly enough, when it came to the least intense of my four routines, walking, the Fitbit delivered some unusual results. It claimed that I burned almost as many calories walking for 35 minutes as I did running for I thought this might be due to the fact I wore the Fitbit on my waist and the other fitness trackers on my right arm, since each swing is often interpreted as an extra step.
The Fitbit automatically calculates separate values for running and walking stride lengths based solely on your height. In my case these default values were off by about three inches for my running stride length, and this significantly threw off the caloric calculations. I asked Heil why it is so hard to build a good algorithm based solely on movement. He cited two reasons.
Correctly identifying the activity means knowing how much muscle mass you use — more mass equals more calories. They often compared fitness trackers to proven methods of monitoring energy expenditure that are typically used only for research, like locking people in a room to assess every calorie consumed and burned or asking people at home to drink specially treated water that makes it possible to detect energy output with a urine test.
Taken as a group, the consumer devices tended to slightly underestimate energy expenditure, the researchers found. Devices tended to be more accurate at measuring energy expenditure when they also monitored heart rate or body heat, but this was not consistent across all types of activities. Accuracy also depended on the activity. Fitness trackers often did a poor job of measuring less vigorous movement like walking, climbing stairs, and doing household tasks.
Participants in the studies were 35 years old on average and typically not overweight. Results might be different for older people or obese individuals, the study authors note.
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