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polar calories to average power calculation.

Old 09-06-07, 11:49 AM
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radshark
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polar calories to average power calculation.

I hoping a power/energy/VO2 calculating wizard can help me out.

So I've been biking most of the summer to and from work. I'm getting a little faster and, after doing a bit of reading, started wondering about my power output. Well, I thought, I have a Polar F6 therefore I have a measure of energy (calories) and time - I should be able to calculate my average wattage. And so the story begins...

My typical journey to work is 18.6 km. It takes me 36-37 min and my polar F6 indicates that I typically expend 500-510 calories (actually kcal.). So here is my power calculation:

Given: 1 cal. = 4.185 J

Power(W) = Joules/sec = (calories x 4.185) / (min x 60)

= (500 000 x 4.185) / (37 x 60)

= 942 Watts.

I'm getting better .. but not that better. Not sure where I went wrong on this calculation.

So I set out this morning to try another approach. I have a relatively smooth 6 km stretch of road on my daily commute. So I reset my Polar F6 and recorded the cal. and time and my average speed on my cateye.

This is what I got: 154 calories , 10 min and 58 seconds with an average speed 32.5 km/h, HRavg=149 (81%), HRmax=160 (87%).

Next I use www.kreuzotter.de and plug in my data (178 cm, 91.6 kg, 9.5 kg (bike), 20C, 350m, 0 head wind at a cadence of 85 and speed of 32.5 km/h). The bike is a "performance" hybrid - 32C, flat bar - I have an aero bar but didn't use it. So I choose a road bike with robust touring tires - it seemed the most appropriate.

My power (average over the distance) was calculated by the site to be 313 Watts. I was rather pleasantly surprized - I think that's not bad for relatively new rec cyclist - certainly higher than I expected.

Then I noticed the calorie calculation at the bottom after I entered the time and distance - it was calculated to be 197 calories - but my polar indicated 154 calories. Hmmmm.

I've read posts where the kruezotter site is praised for providing fairly accurate estimates of power and calories. At the moment I would like to believe my F6 is not properly recording calories - or perhaps I haven't properly entered a VO2 and/or max heart rate parameter (?). (age = 41, hrmax = 184, hrsit = 50, and VO2 = 44)

Q1: Can someone point out where my power calculation became derailed? The fact that I cannot resolve calories/min into J/s is driving me a little bonkers. As an electrical eng I must confess this is a little humilitating .

Q2: Is anyone else witnessing similiar discrepancies in polar calorie calculations vs. alternative methods such as kruezotter?

Q3: Given the above data with respect to time speed and distance ... is there a method of estimating one's VO2?

Cheers,

R.
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Old 09-06-07, 12:39 PM
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If I'm at 300W of effort (i.e. work) my HR is at least in the 170's, not high 140's. But maybe that's just me.

36, 106 kg 187 cm 193 Max, hrsit 62, VO2 unknown

SU
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Old 09-06-07, 12:51 PM
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Originally Posted by radshark View Post
I hoping a power/energy/VO2 calculating wizard can help me out.

My typical journey to work is 18.6 km. It takes me 36-37 min and my polar F6 indicates that I typically expend 500-510 calories (actually kcal.). So here is my power calculation:

Given: 1 cal. = 4.185 J

Power(W) = Joules/sec = (calories x 4.185) / (min x 60)

= (500 000 x 4.185) / (37 x 60)

= 942 Watts.

I'm getting better .. but not that better. Not sure where I went wrong on this calculation.

So I set out this morning to try another approach. I have a relatively smooth 6 km stretch of road on my daily commute. So I reset my Polar F6 and recorded the cal. and time and my average speed on my cateye.

This is what I got: 154 calories , 10 min and 58 seconds with an average speed 32.5 km/h, HRavg=149 (81%), HRmax=160 (87%).

Next I use www.kreuzotter.de and plug in my data (178 cm, 91.6 kg, 9.5 kg (bike), 20C, 350m, 0 head wind at a cadence of 85 and speed of 32.5 km/h). The bike is a "performance" hybrid - 32C, flat bar - I have an aero bar but didn't use it. So I choose a road bike with robust touring tires - it seemed the most appropriate.

My power (average over the distance) was calculated by the site to be 313 Watts. I was rather pleasantly surprized - I think that's not bad for relatively new rec cyclist - certainly higher than I expected.

Then I noticed the calorie calculation at the bottom after I entered the time and distance - it was calculated to be 197 calories - but my polar indicated 154 calories. Hmmmm.

Q1: Can someone point out where my power calculation became derailed? The fact that I cannot resolve calories/min into J/s is driving me a little bonkers. As an electrical eng I must confess this is a little humilitating .

Q2: Is anyone else witnessing similiar discrepancies in polar calorie calculations vs. alternative methods such as kruezotter?

Q3: Given the above data with respect to time speed and distance ... is there a method of estimating one's VO2?

Cheers,

R.
I have an idea, although I should warn you I've never used a cycling computer or any of the calorimetric calculations. Nonetheless, as a fellow engineer I feel I ought to jump in whenever I see calculations, despite a lack of authoritative knowledge.

Here's my take: what if the CALORIC INPUT RATE and POWER OUTPUT are different because your body has some fractional efficiency between 0 and 1? For example, what if you "burned" energy at a rate of 942 W, but sent that energy to the pedals at a rate of 313 W, for an efficiency of about 33%? Given that each of those figures has a pretty large uncertainty, this would put the efficiency of the body *roughly* in the range suggested by a Wikipedia article on muscle.

As for the VO2, maybe you could do a rough calculation based on the stoichiometry of the chemical breakdown of food, based on an assumption that all food is glucose, etc. Or maybe there's a correlation that people use, i.e. the real approach.
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Old 09-06-07, 01:01 PM
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To get a rough idea of average power, divide your energy consumption in kcal/hour by 4.

I.e. if you've been out for two hours, and your HRM tells you you've spent 1100 kcal, you first find kcal/hour, which is 550 in this example, and then divide by four. ~140 W in this case.

In your case, given 500 kcal for 36 min (sounds fairly high), you get ~210 W.
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Old 09-06-07, 01:19 PM
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That of course assumes that the HRM provides an accurate estimate for energy expenditure. Not guaranteed...
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Old 09-06-07, 02:38 PM
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Good points on the efficiency. The assumed efficience on the kreuzotter site is 25% (in parenthesis on the bottom)

SU
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Old 09-06-07, 04:54 PM
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ah yes .. the efficiency factor - thanks njm ... that makes sense although my efficiency may be a little to high. The large difference in caloric calculations may be attributable to that as well.

You underscored the difference in efficiency (33% vs 27%). I wonder if the 6% difference is mechanical losses - oh wait .. only 27% is available. Is it possible cyclist may develop higher muscle efficiency?

I must read and tinker some more

Thanks for the help.
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Old 09-07-07, 12:07 AM
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Originally Posted by Sportute View Post
Good points on the efficiency. The assumed efficience on the kreuzotter site is 25% (in parenthesis on the bottom)

SU
I used 20%, since that's what I've seen reported in several places.
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Old 09-07-07, 08:52 PM
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Originally Posted by radshark View Post
I hoping a power/energy/VO2 calculating wizard can help me out.

So I've been biking most of the summer to and from work. I'm getting a little faster and, after doing a bit of reading, started wondering about my power output. Well, I thought, I have a Polar F6 therefore I have a measure of energy (calories) and time - I should be able to calculate my average wattage. And so the story begins...

My typical journey to work is 18.6 km. It takes me 36-37 min and my polar F6 indicates that I typically expend 500-510 calories (actually kcal.). So here is my power calculation:

Given: 1 cal. = 4.185 J

Power(W) = Joules/sec = (calories x 4.185) / (min x 60)

= (500 000 x 4.185) / (37 x 60)

= 942 Watts.

I'm getting better .. but not that better. Not sure where I went wrong on this calculation.

So I set out this morning to try another approach. I have a relatively smooth 6 km stretch of road on my daily commute. So I reset my Polar F6 and recorded the cal. and time and my average speed on my cateye.

This is what I got: 154 calories , 10 min and 58 seconds with an average speed 32.5 km/h, HRavg=149 (81%), HRmax=160 (87%).

Next I use www.kreuzotter.de and plug in my data (178 cm, 91.6 kg, 9.5 kg (bike), 20C, 350m, 0 head wind at a cadence of 85 and speed of 32.5 km/h). The bike is a "performance" hybrid - 32C, flat bar - I have an aero bar but didn't use it. So I choose a road bike with robust touring tires - it seemed the most appropriate.

My power (average over the distance) was calculated by the site to be 313 Watts. I was rather pleasantly surprized - I think that's not bad for relatively new rec cyclist - certainly higher than I expected.

Then I noticed the calorie calculation at the bottom after I entered the time and distance - it was calculated to be 197 calories - but my polar indicated 154 calories. Hmmmm.

I've read posts where the kruezotter site is praised for providing fairly accurate estimates of power and calories. At the moment I would like to believe my F6 is not properly recording calories - or perhaps I haven't properly entered a VO2 and/or max heart rate parameter (?). (age = 41, hrmax = 184, hrsit = 50, and VO2 = 44)

Q1: Can someone point out where my power calculation became derailed? The fact that I cannot resolve calories/min into J/s is driving me a little bonkers. As an electrical eng I must confess this is a little humilitating .

Q2: Is anyone else witnessing similiar discrepancies in polar calorie calculations vs. alternative methods such as kruezotter?

Q3: Given the above data with respect to time speed and distance ... is there a method of estimating one's VO2?

Cheers,

R.
Polar calorie measurements aren't very good. I would be quite surprised if you were burning 1000 cal/hour on your rides - that's a very very high fitness level.

I suggest you find a decent hill and compare what you get on Kurezotter vs the polar. I think you'll find the polar one is much higher.
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