Weird calorie stats

neilus

Morning all!
Its my first post here in this forum so...hello! Im a mtb'er, i posted this in the mtb section but im guessing you guys might be a bit more clued up about this subject:

I kinda enjoy keeping track of my bpm and how many calories i burn (so i know know much fudge I can eat!)...but ive recently just started going for an easy spin around town in the evening and it seems im using the same energy as i do on reasonably strenuous mtb routes and im a bit miffed! Eg:
Easy evening ride around town:
8.66km/00:40:35/410Kal/Av. bpm 130/87m height gain: 47.3 Cal/km
Hardish mtb loop (with 4 climbs):
36.92 km/02:58:42/1736Kal/Av. bpm 156/1105m height gain: 47.04 Cal/km
According to this im actually working harder on the first ride !?! It makes no sense whatsoever. The 36km mtb loop isnt brutal by any stretch but it still feels far more strenuous than my leisurely cruise around town...
:?:
Which ever way you look at it, the first ride is clearly easier. The average bpm confirms that. I fear im probably not crunching the numbers in the right way, if so, how can i make an accurate comparison of two rides which feel at least to be completely different?
Im just curious
Cheers

VamP

You don't say how you arrive at your calorie estimates, but even you can see I'm sure that it's garbage.

Measuring calories burned is impossible without a power meter, the best you can hope for is an estimate. Most devices that claim to measure based on HR rates are really not accurate at all. You might as well just work of an average per hour ridden - say 500 per hour moderate intensity and 800 per hour high intensity. You'll be more accurate that way in the long run than using most HR based devices.

Don't despair though, measuring calories in is equally error prone so it doesn't really matter too much. If you want to lose weight, you need to make sure your daily calorie deficit is large enough to not be obliterated by the error on both sided of the equation.

And finally, weird is spelled like this, not like this (wierd), which reads more like ''wired'' to my brain, and is the only reason I clicked on your thread in the first place

BrandonA

I don't mind being proven wrong but I believe that even with a power meter calories can not be computed accurately. With a power people you can compute the amount of work you have performed in terms of KJs.

Different people will burn a different number of calories per KJ though.

Looking at the interval section I did last night. I did 1,396KJ of work. Checking the estimated calories burnt, Garmin Connect states 1,385, Strata states 1,557. Both were computed from the same power data. Who is to say which is correct.

Also, power meters are not 100% accurate. If you bought two different ones you might get a slight difference in power figures. I think they claim to be accurate to 2-3%. If my power meter records power at +2.5% and yours at -2.5% then we be credited with a different amount of KJs even if we put in the same effort.

neilus

Cheers, some good info there. Yes its a HR monitor run through a Runtastic app. I'll take the calorie readings with a pinch of salt from now on...
Cheers!

cruff

Yep - impossible to count calories burned without a calorimeter. A properly-calibrated power meter at a steady state effort on an indoor trainer has a decent chance of being reasonably accurate - but even then it's only an educated guess. Throw intervals in (with associated heart rate variation, hydration, individual physiological differences, ambient temperatures etc) and that becomes far less accurate - and doing it outside (with wind and other environmental factors taken into account) means it's impossible to get an accurate figure.

All calorie counting should be taken with a pinch of salt (not literally!) and factor in an over-estimate of 20% for most tools - they seem to measure on the high side, presumably because giving you an inflated sense of achievement is more likely to keep you coming back for more

VamP

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19779356#p19779356]BrandonA[/url] wrote:

I don't mind being proven wrong but I believe that even with a power meter calories can not be computed accurately. With a power people you can compute the amount of work you have performed in terms of KJs.

Different people will burn a different number of calories per KJ though.

Looking at the interval section I did last night. I did 1,396KJ of work. Checking the estimated calories burnt, Garmin Connect states 1,385, Strata states 1,557. Both were computed from the same power data. Who is to say which is correct.

Also, power meters are not 100% accurate. If you bought two different ones you might get a slight difference in power figures. I think they claim to be accurate to 2-3%. If my power meter records power at +2.5% and yours at -2.5% then we be credited with a different amount of KJs even if we put in the same effort.

The error bounds are two orders of magnitude less than with HR based systems. And a lot less than what you will introduce on the intake side.

Standard advice is not to use computed calories but to take the kJ figure as being equivalent to calories burned.

And finally, the purpose of measuring calories is to balance your intake/output in a way that delivers the results you want. It's not to compare with another user who might have a different power meter. It is fairly easy to adjust your deficit/surplus depending on your past experience, e.g. if you thought you had a 300 kcal deficit but are not losing weight. The important thing is it's consistent. Unlike the OP's system.

stretchy

The problem is that the app has no idea if you are riding on rough terrain or not. I'd guess the calorie calculation is based on an assumed rolling resistance and drag coefficient, then it will factor in distance, speed and elevation to come up with an approximation of calories burnt. So if you're riding around town (low rolling resistance) and comparing to off road (high rolling resistance) and you are not changing any parameters in the app you will have one figure that is miles off and one figure that is potentially a bit closer but not to be assumed as accurate.

VamP

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19779395#p19779395]zebulebu[/url] wrote:

Yep - impossible to count calories burned without a calorimeter. A properly-calibrated power meter at a steady state effort on an indoor trainer has a decent chance of being reasonably accurate - but even then it's only an educated guess. Throw intervals in (with associated heart rate variation, hydration, individual physiological differences, ambient temperatures etc) and that becomes far less accurate - and doing it outside (with wind and other environmental factors taken into account) means it's impossible to get an accurate figure.

All calorie counting should be taken with a pinch of salt (not literally!) and factor in an over-estimate of 20% for most tools - they seem to measure on the high side, presumably because giving you an inflated sense of achievement is more likely to keep you coming back for more

Accuracy of around 5% given by the 1kJ = 1kcal assumption is widely accepted for power meters by cycling coaches. Regardless of type of workout or whether it's outdoors or indoor.

bobmcstuff

It doesn't sound like it is taking HR data into account? Since that would/should reflect you working harder on the rougher terrain. Sounds more like a simple speed/distance correlation to me (given the faster ride had a higher calorie burn per km).

Personally while I would love to have PMs on all my bikes I'm not in a position to afford that so I use HR via my Garmin, which uses the FirstBeat algorithm and is supposedly about as accurate as you can get for a HR based calc without lab testing (see DCR's excellent review). I wouldn't like to venture an opinion on whether it is as accurate as advertised (I assume not), however it is certainly pretty consistent for me - e.g., if I spend a ride struggling into a headwind it will report a higher cal burn compared to the same ride at the same speed soft pedalling with a tail wind. As mentioned above consistency is key here, if you find you're eating 2k cal and supposedly burning 2.5k cal and still not losing weight (or vice versa) then you can compensate for that, provided the methods are reasonably consistent. I think it is good practice to factor in a big margin for error anyway.

I try not to pay too much attention to calorie burn figures anyway, except when I am trying to lose weight (currently trying to sneak back in under 70kg after Christmas...).