Strava said I burned 799 kCal on a recent ride.
Then Strava says I output 456 kjoules.
But if I remember my conversions, 1 kCal ~ 4.2 kjoules; I calculate ~3345 kjoules. Which would be an output ~ 670 watts.
As far as I remember, I did not just win the Tour de France, so I kind of doubt that. (Maybe I should enter next year???)

What am I missing (other than my yellow jersey)?

Don't even bother to do the math when it comes to Strava Estimated Power. It's a Blind Guessing Machine.

Also, 1kcal ~ 1kJ, because the human body is inefficient-- 20-22% metabolic efficiency, on average. The rest we turn into heat.

Oh yeah, heat. Doh. Thanks.
I know the numbers are just WAGs, but I've spent to much time solving equations to not be bothered by the discrepency. Now that I have the explanation, I'll be able to sleep tonight.
Although I am sad at having to give up the jersey.

I sincerely hope you divided by a time duration to conclude that...

One thing to consider would be efficiency. It's a basic rule of thumb that a person just sitting there generates 100 watts of heat (or maybe a little less). Even before trying to do anything useful. And obviously quite a bit more when exerting themselves, otherwise they wouldn't get all "hot and sweaty". That heat is the power wasted in inefficiency. Add to that whatever power you actually put into spinning the cranks.

Wikipedia seems to suggest that cycling muscles are 18-26% efficient. So assuming your earlier conversion was meaningful, take your 670 watts, let's subtract just 70 for overhead of staying alive. Then let's multiple the remaining 600 by 25% efficiency.

150 watts into the cranks.

Again, I have no idea if where you started from is accurate. But there will indeed be a huge difference between the food energy "burned" the and the useful mechanical work done at the cranks.

Yeah, Thermodynamics:

1. There's no free lunch.
2. If you pay for lunch you will spill soup in your lap.
3. Then the girl of your dreams will point at you and laugh hysterically.

If the soup is lemon chicken, can I now consider my jersey yellow?

Yes, this seems to be broken on Strava now. EDIT--it only started doubling the calories in mid July 2018.
I have a power meter that does give me consistent results.

For a recent ride recording:
Strava shows:
1164 kj
2815 calories!! uh, no.
Golden Cheetah shows 1159 kj.


From a ride in Oct 2017:
Strava:
1003 kj
1118 calories
Golden Cheetah shows 997 kj.

~~~~~~~~~~~~
From one of the many explanations via google:
Kilojoules for a ride are quite close to calories for the the ride. (Calories might be slightly higher than the kj number).

Essentially:
about 4 kj is one calorie.
but people are about 25% efficient in converting food into useful work. (The other 75% is wasted heat!)
so 4 kj *.25 is effectively 1 calorie.

Surely those are different things. The kCal is what you burned for the whole ride whereas the kJ is average energy output, like the watts figure?

From my post in another thread, excerpted for this discussion:

Put in your effort, and your weight, get the burn rate:

https://www.bicycling.com/health-nut...ed-calculator/

Or if you don't trust it, we look here for the METS:

https://sites.google.com/site/compen...010.00104010.0

01040 10.0 bicycling, 14-15.9 mph, racing or leisure, fast, vigorous effort

So, 10.0 METS.

And use this math:
METS x 3.5 x body weight in kilograms) / 200
to get by minute, and then x 60 for calories per hour

10.0 x 3.5 x 104kg / 200 x60 = 1092

It is a guess, because you don't account for going up hills or going down. If you want Strava to be accurate, you need a power meter, and then you need to know how efficient you really are on the 20 to 25% scale.

So, either $2k in gear and metabolic testing or... guess.

But because after accounting for human inefficiency, for cycling purposes we can work from 1kJ = 1Kcal.

Strava uses a 1.1Kcal = 1kJ calculation if you have a PM, and I use that one because I have power2max meters which report notoriously low.

I have never gotten any ridiculously high calorie burn reports from Strava, even pre-PM. I seem to recall some ~30% high (by my post-PM math) but nothing like the people seeing double or triple Kcal to kJ.

Edit: That calorie calculator on the Bicycling Magazine linked above is just bonkers. I put in my numbers from my 64 miler yesterday, and it gave me a result of 4,182Kcal. Man, in my dreams. That ride was 2,135kJ via my PM, or 2,380Kcal according to Strava.

Bottom line: 900-1,000Kcal/hr takes a pretty much wide-open effort. My FTP tests are around 1,000kCal/hr. During rides, I ballpark-guess based on 35Kcal/mi. 64 miles X 35Kcal = 2,240... which is pretty much spot between the PM and Strava.

Well, unless the METS are wrong, you know how many calories you burned. METS is Multiples of Basal Metabolic Rate, so it does include the calories you would burn sitting on a couch for the same amount of time.

I don't think Strava includes BMR in the calorie burned count.

FWIW, Strava is far more conservative than other tools I have used. Samsung Health and Wahoo Fitness come up with much higher numbers. Strava does not appear to have a linear slope when it comes to body weight, increasing body weight does not have the 1:1 effect that a METS calculation would have. Double the mass, double the calories required.

If you really wanna know, it takes a properly calibrated Power Meter. Can't cheat with those!

If you go off of ANY of those guesstimator websites to plan caloric intake based off of exercise output, you're gonna gain weight. I'm over 200lbs, and my BMR is ~82Kcal/hr. So on a 2-hour ride at my typical pace, I will burn ~165Kcal being a person, and ~1,300Kcal from moving the bike.

All three of my rides this week have been metric centuries over different routes, at different speeds. But roughly the same lengths.

63.0 miles / 1,572ft = 2,014kJ
63.1 miles / 2,994ft = 2,145kJ
64.7 miles / 2,000ft = 2,135kJ

Notice a pattern there?

I have done 4,000Kcal on rides before, though. Back in April I did 108 miles @ 19mph. 3,984Kcal.

METS is generally accepted as good science, and the calcs are built off of that... but yeah, real life experience says don't replace all the calories. I shoot for 25 to 30% replacement on the bike and in recovery.

Yeah, seems like a bug in Strava. The ratio, even if it isn't exactly 1:1 should be fairly consistent in your rides. They're essentially converting between two units of energy and multiplying by an estimate for human efficiency. Perhaps different conditions cause it to estimate different levels of efficiency, but you shouldn't see huge differences like this at all.

Don't bother arguing with rgconner. He has no desire to learn.

Stalking me now?
Nice!
I love you too man!

But it seems I can't follow my own advice... cycling doesn't follow this kind of linear relationship. The main resistance is air, so body weight means very little. The only time when body weight is a big factor in terms of energy expenditure is when your are going up a hill. Double the mass does not mean double the calories required.

What can I say. I get triggered by ignorance. You should see how worked up I get when someone posts a simple order of operations question on facebook and people get it wrong and then insist that they are correct.

Oh, and for the record, I posted on this thread before reading your posts here.

Body size (not weight per se) can be a factor in determining drag, however.

Weight is a factor in acceleration, but that probably has really small calorie implications unless you're hitting a stop sign every 2 blocks.

I am sure you did post before reading.

How else could you glibly get what I say so wrong?

Body weight does matter.

If you read the links to METS and how you calculate workload, you would know it is exactly a 1 to 1 ratio:
1 METs = 3.5 x weight in kg ÷ 200



But I understand you take your opinion over the actual science.

You keep doing you!

Indeed both of these are true. But, as you no doubt know, the effect is pretty small overall. The idea that calories burned should scale linearly with weight isn't close to being true (as you no doubt also know).

Tell
Ainsworth, Barbara E.; Haskell, William L.; Herrmann, Stephen D.; Meckes, Nathanael; Bassett, David R.; Tudor-Locke, Catrine; Greer, Jennifer L.; Vezina, Jesse; Whitt-Glover, Melicia C.; Leon, Arthur S. (2011). "2011 Compendium of Physical Activities". Medicine & Science in Sports & Exercise.

they are wrong.

Those idiots, they should have just asked you guys!

My "opinion" is the science. You are in the wrong here. No one is agreeing with you. Think about why that is.

Or maybe... just maybe your conversion of speed to METS or your application of METS is incorrect? That couldn't possibly be true. Those power meters must be lying.

Absolutely as to weight--we're talking low double digit differences if that over the course of an hour.

The drag actually goes up much faster than linearly, however, so body size can make a much bigger difference depending on how it's distributed. It will not make much difference between 13 and 14 mph, but will account for a much larger difference in drag going from 19 to 20 mph, for example. Drag, obviously, translates much more directly into calories as it will need to be overcome on level surfaces as well as hills, unlike weight which is pretty much a non-factor on level due to inertia..