Unit/value error for recorded activity?

7115741 25 days ago

I observe the following recording(s) for cycling activites with power meter.

140 W average power
2h 45min
thus total enegy = 140 * 2,75 * 3600 = 1.386 kJ

so far so good, it shows 1.389 kJ work in the power section

But what I wonder: it shows exactly the same value under "Active Calories" under section "Nutrition & Hydration", but with other unit "calories" (which technical actually means kilo caloreis or kCal). The factor between Joule and Calories is 4,184.

I checked several activities, and it's always the same ...

While I understand that only a fraction of eaten energy goes into mechanical energy to the pedals (and much energy gets lost e.g. as heat), I'm a bit surprised that the factor is always the same (4,184), and also cannot believe the "coincidence" that the factor actually is exactly the conversion factor between Joule and Calories.

0 FlowState 24 days ago

Here's a couple of threads about the same topic.

https://forums.garmin.com/apps-software/mobile-apps-web/f/garmin-connect-web/267497/calories-burned-vs-work-kj

https://forums.garmin.com/sports-fitness/sports-fitness/f/edge-1030/268613/calories#

A couple of relevant comments:

The human machine, at least when pedalling, requires to burn about 4-5 times as much stored energy as the mechanical work it produces. In other words, the human mechanical efficiency is around 20-25 %. Therefore since the invention of power meters, people have been using the 1 (burned) kcal = 1 kJ of mechanical work.

Have a look for example at the paper Thieme E-Journals - International Journal of Sports Medicine / Abstract (thieme-connect.de). If you do not have access to scientific papers, there is a quote from it in another paper that is freely accessible at Perspective: Is the Response of Human Energy Expenditure to Increased Physical Activity Additive or Constrained? - ScienceDirect. It tells:

... people on a high carbohydrate diet display a gross efficiency during cycling of ∼20.4% compared with 19.6% on a lower carbohydrate diet [61]

However, those are values coming from lab tests. In reality the efficiency range is much, much wider, because there are many more factors playing a role - the bike type & weight, size & pressure of the tires, terrain, wind, moisture, etc, etc. Already the differences between individual bike types (i.e. road bike vs. MTB, fixie, city bike, or a fat bike) are enormous. My uneducated guess is that the efficiency will be rather around 10% at people riding sub-optimal bikes.

If you google the topics, you'll get many more papers. I just took the first or the second hit that Google showed me, when searching for the expression "scientific paper on human body energy expenditure efficiency at cycling", so it all took around 20 secs to find it.

I don't have any personal knowledge on the topic, but it seems that other people believe this calculation is correct.