# Calculate calorie expenditure from power output

Discussion in 'Power Training' started by VS1, Oct 1, 2007.

1. ### VS1 New Member

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Hello,

As Polar uses a model that estimates the energy expenditure (using age weight HR VO2max..) I tried to double check the calories burned using this http://austinimage.com/bp/velocityMetric/velocity.html .

For any value od wattage, say for example 250W I picked a distance so that the riding time will be 60.00 minutes and thus I thought I’m getting (at the bottom of the table) the calories burned for this average power level:

Do you think this is correct ? reservations ?

Thanks..!

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2. ### daveryanwyoming Well-Known Member

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Calories burned for an average power level is actually pretty easy, it's roughly 4*AP*hours

That comes from hours*APwatts*3600seconds per hour*1.1(~metabolic efficiency)/1000(kcal to Cal) or very close to 4*AP*hours. That's consistent with the way gym machines like Lifecyles estimate calories and agrees with the way PowerTap estimates calories.

-Dave

3. ### VS1 New Member

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Thanks Dave.

Good thing that I asked - - now I see how inaccurate the polar calculation can be (considering the parameters one asign to it).

Taking as an example the 1:32 hours ride I have just finished - according to the S725 it's 1120 Kcal and acording to 4*233(AP)*1.5333=1429 Kcal... (!) and that's only a 1.5 hrs ride..

Thanks again.

4. ### VS1 New Member

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Thanks Dave.

mm...Good thing that I asked - - now I see how inaccurate the polar calculation can be (considering the parameters one asign to it).

Taking as an example the 1:32 hours ride I have just finished - according to the S725 it's 1120 Kcal and acording to 4*233(AP)*1.5333=1429 Kcal... (!) and that's only a 1.5 hrs ride..

Thanks again.

5. ### sidewind New Member

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I suppose more widely used way for estimating the calory consumption is just to take amount of work done in kJ and use it as the kcal amount. For your ride, this would be 233 W * 3600 * 1.533 = 1285 kJ work done, and ~1285 kcal energy consumed by your body.

6. ### VS1 New Member

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Dave, Sidewind,
See what ric said about it in the past - according to the calculation he suggests it's: AvgPower / 1000 * 3600 / 4.18 * 4 = ... AP * 3.44 (3.444976)

Take a 2 hours ride at AP=200W:
According to Ric's suggestion 200*3.44*2 =1,376 Kcal
while Dave's suggests 200*2*4= 1,600 Kcal
and sidewind's: 200*3600*1.53 is only 1,103 Kcal...

you would agree that differences are too big..

Thanks.

ric_stern/RST
Power (watts) and calories
To work out energy expenditure in joules, you multiply average power (/1000) by time in seconds. Thus, the work done in 1-hr at 190 W avg is 684 kj (0.19 x 3600). To convert this to kcal you divide by 4.18, i.e., 164 kcal.

However, the human body when cycling is only around 25% efficient (normal range ~ 20 - 26%), thus, this needs to be calculated, to work out the human energy consumption, i.e., 164 / 0.25 = 656 kcal.

To work out the exact amount of energy expended you'd need to go to a sports science lab and have your expired air analysed. As a *good* ballpark figure, it's best to estimate energy expenditure simply by quoting the work done in kj as kcal, i.e., in the above example just say you 684 kcal.

Efficiency changes with both cadence, and absolute power output, and also with fatigue. At higher cadences, efficiency *decreases*, while at higher powers it *increases*. As you fatigue efficiency decreases.

Ric
bobkny

7. ### sidewind New Member

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Nope; for two hour ride the last formula should be 200*3600*2.0 = 1440 kcal.

As Rick refers, the human efficiency range is quite large, 20-26 %, so can't say which one(s) is(are) right or wrong.

8. ### sugaken New Member

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All those are meant to give you ballpark figures, and you can't go much more precise than that with only power.

As Ric mentions, you need to go to a sports science lab to get your metabolic efficiency pinned, but even that changes as you go (cadence/power/fatigue).

Hope it helps,

Ken

9. ### NomadVW New Member

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I'll say that all math and calculations aside, I use straight kjoules from the powertap as my calories burned, and that's been pretty effective for me - both for on the bike nutrition requirements for training and for overall weight management.

10. ### VS1 New Member

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Right. my mistake.

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Thanks

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