Do fitness tracker wearables/apps measure power?

Do fitness tracker wearables/apps measure power?

Postby Velocomp » Sun May 17, 2015 10:58 am

Are you thinking "No way!"?

Well, wait a moment...nearly all wearables/apps/fitness trackers report calories expended during exercise...

Calories are work, and work is nothing more than power added up over time. So, YES, if you ride a bike with your fitness app or wearable, you can turn its calorie measurements into average power measurements.

As it turns out, on bike rides there is a simple relationship between calories and average power:

Average Power = 17.3 x (calories reported) / (minutes in motion)

Don't believe this simple formula? Try this with one of your ride files: take the total calories expended, multiply by 17.3, then divide by the time (in minutes) over which you exerted that effort. This will be VERY CLOSE to the average power for your ride.

So, now, have fun trying out your wearable/app as a power meter...

1) do a bike ride with your wearable/app

2) get its calorie measurement and ride time

3) use the simple formula

4) compare your wearable/app average power to your PM average power

Today I rode with the Endomodo app and my Newton. My Newton measured 170W average power for the ride.

The Endomondo app, of course, has no sensors at all. Still, it reported authoritatively that I burned 626 calories on my bike ride. I was in motion for 39.5 minutes (I did not pause the app clock when I stopped briefly), so that's equivalent to 274W average power.


Endomondo average watts were 61% high...

Yesterday I rode with an Apple Watch, which has a heart rate sensor. Since HR changes are somewhat related to energy expended, it seems like the Apple Watch should do better. Well, it did, sort of...the Apple Watch says I expended 1246 calories over a period of 87 minutes of actual riding. That's 247W average.


Apple Watch average watts were "only" 50% high...

Now we all know, intuitively, that calorie numbers from apps and wearables are bogus. That is a big reason people give them up after a few weeks of using them.

When expressed as calories we don't realize how inaccurate these devices are...but their inaccuracies are laughably obvious when their measurements are re-expressed as average power.

Even worse: the magnitude of these gross errors aren't consistent; the inaccuracies will very, depending on ride conditions and terrain.

Would you pay $400 for a wearable that gives cycling power measurements that are 50% high?

Would you use an app that reports cycling power that is 61% high?

Would you use any wearable or app whose measurement consistency is variable?

I didn't think so.

Tell your friends that if they want to measure calories and power correctly (and affordably), they need a PowerPod.
Endomondo051715.jpg (126.01 KiB) Viewed 13 times
AppleWatch051615.PNG (22.16 KiB) Viewed 15 times
John Hamann
Velocomp CEO
Posts: 4021
Joined: Fri Jan 11, 2008 9:43 am

Re: Do fitness tracker wearables/apps measure power?

Postby EHB » Wed Jan 06, 2016 3:52 pm

But what if we take into account muscular efficiency and basal metabolic rate?

Quick look on the web; people seem to use 20%....

This means Endomondo underestimated your calories by 93%!

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Joined: Sun Oct 26, 2014 5:52 pm

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