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...these devices most definitely DO report calorie expended during exercise...

Since calories are work, and work is nothing more than power added up over time, on bike rides there is a simple relationship between calories and average power:

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

This means that any device that reports bike ride calories expended is ALSO measuring your average watts!

Try this with one of your ride files: take the total calories expended, multiply by 17.3, then divide by the time over which you exerted that effort. This will be VERY CLOSE to the average power over that time.

So, have some fun with the wearable or fitness app of your choice. Do a bike ride with it and your Newton, get its calorie measurement, do the same math, and compare it to your Newton.

Today I rode with the Endomodo app. This 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 when stopped), 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. It did (in a manner of speaking)...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.

When expressed as calories we don't realize how bad they 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 error rate will change, depending on ride conditions and terrain.

Would you pay $400 to get power measurements that are 50% high?

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

Would you use a tracker or an app whose consistency is variable?

I didn't think so.

Tell your friends that if they want to measure calories and power correctly, they need a Newton.
AppleWatch051615.PNG (22.16 KiB) Viewed 2512 times
Endomondo051715.jpg (126.01 KiB) Viewed 2512 times
John Hamann
Velocomp CEO
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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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