I wanted to know how much carbs I would need for some vo2max intervals so decided to calculate the energy needed for the interval: power in Watt x time in seconds / 1000 = energy in kJ divide kJ/4.2 to get the calories. What I noticed is that the amount of calories strava gives a ride equals the average power x time in seconds/ 1000, so not the amount in kcals at all but in kJoules, almost a factor 4.2 too high! (1 cal = 4.1868J to be exact).
Did I do my calculations wrong or is Strava really off?
Edit: looking at trainerroad workouts, TR makes the same mistake and equates Joules to calories
The TrainerRoad software equals your calories to the work done in kilojoules during a ride. Power (watts) x duration (hours) x 3.6 equals work done in kJ’s. For example, 200 watts for 2 hours is 200x2x3.6 = 1,440 kJ, which is what TR suggests in calorie burn. HOWEVER, this assumes a gross metabolic efficiency of 24-25%, which most research seems to disagree with. Most people fall somewhere around 15% lower in terms of GME, which means they expend about 15% more calories than the work produced in kJ’s during a workout. For me, this has proven to be much more accurate. Thus, calculating calorie expenditure goes as follows. Power x Duration x 3.6 x 1.15 = calories burned. OR, to make it easier, 4.14 x Power (W) x Duration (hrs). GME rises as the intensity does, and at FTP power we may actually reach 24-25% efficiency, but at lower intensities most people are not that efficient. The only way to know precisely would be to perform a gas exchange test, but kJ’s + 15% seems to fall within the accurate range according to most articles I’ve read as well as my anecdotal experience.
As mentioned by others, this is because the human body is not 100% efficient. It turns outs that only 21-25% of the energy consumed is used to turn the pedals. If you actually were 100% efficient you wouldn’t produce any extra heat while exercising and you didn’t have the sweat.
Yes, it is “wrong” to assume that kcal is the energy burned by the body vs kJ is the work. The are both units of energy, but unfortunately that is how it is done in most apps. Actually, except for cycling where you have power meters I think the apps will typically only show the estimated energy burned by your body.
To make things more confusing some watches/apps will also include both active and inactive energy burn (what you consume at resting heart rate).
It’s not perfect, but it is as perfect as you’re gonna get for a big group i guess. Or at least, good enough for a general population.
For me it’s pretty accurate. Been tracking my weight, calorie intake and KJs burned for over two years now and it tracks pretty well for me. But with the margins of errors in KJ to Cal and nutritional values on food not being 100% accurate. You really have to test it for your self and perhaps adjust.