Calculating Calories is Hard!

I’ve been using both MapMyRide and Strava to track my bicycle rides recently. In addition, I’ve been using the activity app on my slick new Apple Watch. Each estimates how many calories I burned on my ride, but the numbers are very different. For example, on my ride to work yesterday morning:

MapMyRide: 814 Calories
Strava: 643 Calories
Watch: 757 Calories

Dang – those are quite different numbers, especially when you consider that MapMyRide and Strava are using pretty much the same data and coming to very different conclusions. What gives? CAN I EAT THAT DONUT OR NOT?

Strava and MapMyRide use speed and (maybe) elevation change in a formula with the rider’s weight to come out with an estimate of how many calories the rider burned. Strava lets me set the weight of my bike; I don’t know what MapMyRide assumes. I’m pretty confident that neither really uses elevation changes well. And headwinds? Forget it.

Both services can come up with a better wild-ass guess if you use a special crank or pedals that directly measure how hard you are working. They directly measure the output of your muscles, so the only remaining guesswork is how many calories you burned to do that work (some people are more efficient than others). There’s a Garmin setup that will tell you if one leg is doing more work than the other. I have no such device.

The most accurate way available to measure calories burned is to measure how much carbon dioxide one exhales. Rather than measure the work you did, you’re measuring how much exhaust you produced. This is impractical on a bike ride, however.

Which brings me to the gizmo strapped to my wrist. It estimates calories based on my heart rate. I have no idea what formula it uses, but hopefully it incorporates my resting heart rate (which it measures throughout the day) and my weight (which I have to remember to tell it), and maybe even my age. The cool thing is that heart rate is directly related to carbon dioxide production. When I’m riding fifteen mph with a tail wind, I’m barely working at all. When I’m pushing against gale-force breezes at the same speed, I’m huffing and my heart is thumping. To Strava and MapMyRide, the rides look the same. The watch knows the truth.

When WatchOS 2 comes out (the “features we couldn’t get perfect in time for WatchOS 1” release), Strava will be able to access my heart data. I’m interested to see what that does to the numbers.

In the meantime, I’m listening to my watch.