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"Calories burned" vs Work (kJ)


for many years the calories and kJ (mechanical work) count for me were almost identical for bike rides with power meter readings. In other words, Garmin used the traditional 1 kcal = 1 kJ. 

Now since a few days I get completely different readings, with significantly more calories burned than mechanical work (kJ). 

Does Garmin Connect add basic metabolic rate to the active burned calories, or does it estimate a lower mechanical efficiency?

I have a Garmin 1030 and pedal based powermeter, but I see the same for my friends too. 

  • This 1 kcal = 1 kJ formula is widely accepted as the most precise estimation of calorie consumption you can have without laboratory testing and is generally used by most websites and computer apps and textbooks, not only by Garmin. 

  • Yesterday ChatGPT explained to me why I was wrong: 20-25% applies to running, with vertical losses and bodyily heating losses, whereas for cycling the efficiency effectively amounts to 90%, ChatGPT said.

    From which I deduce that 1 burned kcal(cycling) corresponds to 0,9*4.18=3,8 kJ(cycling). And since nothing seems to be extremely exact in the first place, using a conversion factor of 4 would still seem pretty appropriate, leading to about a 1/4 of Garmin's provided active Calories based on its registered active workout energy. 

  • No, ChatGPT is wrong. You can check whatever scientific paper or applied physiology textbook out there and they all say the same thing: cycling efficiency is around 20-25 %, therefore 1 kcal (burned) is more or less equal to 1 kJ of mechanical work on the pedals. 

  • Right. If you, please, could refer to (a) basic research article(s) underlying the textbooks, that would be great.

  • If you, please, could refer to (a) basic research article(s) underlying the textbooks, that would be great.

    Have a look for example at the paper Thieme E-Journals - International Journal of Sports Medicine / Abstract ( 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.