Velo-fit.com's monod.xls: Why use W/kg?



sugaken

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Mar 27, 2006
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I suspect many of you here use velo-fit.com's monod.xls ( http://velo-fit.com/articles/monod.xls ) when calculating Critical Power. I do, too, but it always seemed a bit odd to me that the file first calculates power-to-weight ratio (PWR, in W/kg), multiply the PWR by duration to get Work-to-weight ratio (WWR, in J/kg), then finds the best fit slope (=CP) for the duration-WWR pair points, and finally makes power predictions for various durations by multiplying the CP by "assumed weight for predictions" (I'm consciously omitting the part calculating AWC for simplicity).

So in essence, the file predicts that you will produce more power if you put on some weight? :confused:

Aren't one's body weight and power production ability largely independent from each other (barring sickness, weight loss from muscular atrophy, etc)? Otherwise it wouldn't make much sense to try to lose weight in an attempt to improve on one's PWR.

It seems to me a little, if not much, better to find the best fit slope from duration-work pair points (not duration-WWR pair points), then divide the slope by the "assumed weight for predictions" to get a power-to-weight ratio (=final CP value).

Does this make sense to you?

TIA,

Ken
 
sugaken said:
I suspect many of you here use velo-fit.com's monod.xls ( http://velo-fit.com/articles/monod.xls ) when calculating Critical Power. I do, too, but it always seemed a bit odd to me that the file first calculates power-to-weight ratio (PWR, in W/kg), multiply the PWR by duration to get Work-to-weight ratio (WWR, in J/kg), then finds the best fit slope (=CP) for the duration-WWR pair points, and finally makes power predictions for various durations by multiplying the CP by "assumed weight for predictions" (I'm consciously omitting the part calculating AWC for simplicity).

So in essence, the file predicts that you will produce more power if you put on some weight? :confused:

Aren't one's body weight and power production ability largely independent from each other (barring sickness, weight loss from muscular atrophy, etc)? Otherwise it wouldn't make much sense to try to lose weight in an attempt to improve on one's PWR.

It seems to me a little, if not much, better to find the best fit slope from duration-work pair points (not duration-WWR pair points), then divide the slope by the "assumed weight for predictions" to get a power-to-weight ratio (=final CP value).

Does this make sense to you?

TIA,

Ken
If I understand you correctly, you are asking why the spreadsheet takes the slope & intercept of (work/kilogram):time, then multiplies by mass to get absolute CP, AWC and predicted power values--as opposed to taking the slope of work:time, and then getting the absolute CP, AWC, and predicted power directly. The answer is that I don't know, but also that it doesn't matter. The two methods are 100% identical mathematically.
 
rr9876 said:
If I understand you correctly, you are asking why the spreadsheet takes the slope & intercept of (work/kilogram):time, then multiplies by mass to get absolute CP, AWC and predicted power values--as opposed to taking the slope of work:time, and then getting the absolute CP, AWC, and predicted power directly.
Thanks for rephrasing my question. I was having trouble expressing there.

The answer is that I don't know, but also that it doesn't matter. The two methods are 100% identical mathematically.
Well, if your body weight is absolutely constant, yes, but if you're alive, your body weight probably fluctuate from time to time. And if so, it's not mathematically identical anymore. And chances are, if you take samples from multiple rides across multiple days, your body weight for each duration would be slightly different, and the predictions would be different depending on the "assumed body weight" you input.

Ken
 
sugaken said:
Thanks for rephrasing my question. I was having trouble expressing there.

Well, if your body weight is absolutely constant, yes, but if you're alive, your body weight probably fluctuate from time to time. And if so, it's not mathematically identical anymore. And chances are, if you take samples from multiple rides across multiple days, your body weight for each duration would be slightly different, and the predictions would be different depending on the "assumed body weight" you input.

Ken
just keep the weight (mass) constant. I guess the spreadsheet was done like that to make it easier to compare multiple athletes etc and express everything in W/kg.
 
rmur17 said:
I guess the spreadsheet was done like that to make it easier to compare multiple athletes etc and express everything in W/kg.
Good point. Thanks.

For my own use, though, I've modified the file slightly so that it doesn't use the body weight values for CP/AWC calculations and predictions.

Ken
 
sugaken said:
Good point. Thanks.

For my own use, though, I've modified the file slightly so that it doesn't use the body weight values for CP/AWC calculations and predictions.

Ken

I've got no problem with displaying Watts/Kg since that's the units of CP and what I'm most interested in tracking in terms of performance gains. I was thinking how funny it is that the spreadsheet requires you to enter you weight for each test point and then again for the prediction curve. Are you really going to enter test points from days so far apart that your weight has measurably changed and as you point out it implies that losing weight always drops power and that gaining weight always gains power or that your W/Kg are constant across weight changes. They should have just coded a single weight entry cell and used it to populate all the test point and the prediction curve cells.

It's an easy enough spreadsheet to make and it's hard to complain about the price but it is funny how it was coded :)

-Dave
 
daveryanwyoming said:
It's an easy enough spreadsheet to make

Indeed, I don't even bother with a template - I just do the calculations "by hand" (by keystroke?) any time I happen to have the need.
 
daveryanwyoming said:
I was thinking how funny it is that the spreadsheet requires you to enter you weight for each test point and then again for the prediction curve.
Yeah, that's more in line with my thoughts about the spreadsheet.

daveryanwyoming said:
It's an easy enough spreadsheet to make and it's hard to complain about the price but it is funny how it was coded :)
I'm not complaining. Really. I was just wondering why. Thanks for your thoughts.:)

Ken
 
acoggan said:
Indeed, I don't even bother with a template - I just do the calculations "by hand" (by keystroke?) any time I happen to have the need.
I don't know... I'll probably have trouble remembering the names of those functions if I do that.:eek:
 
Quick question related to this.

My weight is absolutely not a constant! I have a "seefood" problem so over the holiday season, I tend to gain almost 15 lbs over my mid-season race weight - say 160ish to 175ish. I realize that's probably not too healthy but it is what it is.

My question...
Is it not correct to assume that (all other factors aside - like that's ever the case) as my weight comes back down, I'll benefit from a higher watts/kg threshhold? In other words, will losing weight (not muscle but what we Canadians call Molson muscle) cause any loss in one's ability to produce power?

I could understand if you were always out of the saddle standing on the pedals you might gain a small amount of power directly as a result of weight but otherwise, I wouldn't think that weight would be a factor.

Thanks,
Pad
 

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