Originally Posted by
Bacciagalupe
Uh... I'm sorry to say that you do not appear to understand how power meters and the corresponding mathematical models work.
Power meters measure the
exact power applied to the bicycle, i.e. they're measuring the actual torque generated by the rider, not "how many calories the rider is burning" or "how many watts the rider's body needs to generate." They are usually installed in the crank, bottom bracket or as the rear hub. So, when we talk about a rider applying an average of 200 watts over 20 minutes, that means that the pedals are receiving 200 watts of power from the rider.
That's why a power meter is a more objective measure. It doesn't matter who the rider is, how good or bad their aerobic system happens to be working that day, or the temperature, or whether you're climbing or descending, or whether the rider has more or less mechanical losses for some reason, or if you're sore (or if your blood glucose is low) from the previous day's efforts, or if that yogurt you had that morning was too close to the expiration date, and so forth. The power meter doesn't care; it's just measuring power that is actually applied to the bike. Thus if the rider is applying 200 watts to the pedals, they're applying 200 watts to the pedals and that's exactly what the power meter records.
The Kreuzotter and other mathematical models are also using this as their measure. Thousands of riders are comparing their power meter results to these calculators as well. As I may have said before, obviously it will be simplified compared to the real world, but it's generally good enough for these types of questions and would be corrected if they were off by orders of magnitude.
By the way, the actual measure is of "watt-hours." Going from an average of 200 watts over 2 hours to 201 watts over 2 hours really isn't terribly significant -- unless you're in a race or, per staephj1 and invisiblehand, an extremely competitive club ride.
I understand quite well how it works and I also understand that the power meter measurement is typically used to try to factor out the influence of variations in power applied to the pedals. That's how you were proposing to use it earlier in this thread and that's exactly the opposite of how it should be used. Filtering out "whether the rider has more or less mechanical losses for some reason" is not an objective way to consider the influence of bike weight when that reason
is bike weight. On the contrary what it is is a very objective way to rig the numbers to get the answer you want.
Moreover, there is a bit of a difference in the importance of power at the pedals between racers and nonracers. For a racer the only thing that matters is how much power he can apply and how far it will take him. So a racer doesn't care if it takes him 1000 watts or 2000 watts to apply 200 watts to the pedals as long as he gets 200 watts at the pedals. However, for the rest of us the 1000 or 2000 watts being exerted is the most important thing and not the 200 watts leftover at the pedals and
that 2000 watts a power meter does not measure. So considering how the users of such calculators and power meters are self selected to be mostly racers (and of those mostly amateurs who only want to flatter themselves on rides where they can experience their moments of glory, unhindered by the realities of everyday acceleration) it wouldn't be at all surprising to find that the results of the Kreuzotter calculator going uncorrected by orders of magnitude (especially considering the strong psychological tendency for most people to believe anything masquerading under the name of "science" or "math" regardless of actual scientific or mathematical validity).
Going from an average of 200 watts over two hours to 201 watts over two hours could be massive if it means a difference of 1000 watts in actual energy expended by the body. This is typical of muscles. For example, although bench pressing a 10 pound weight takes no more power "at the bar" than bench pressing a 500 pound weight (in both cases it's zero), the difference in power exerted ranges from nearly insignificant to more than your body can generate.
So
if a difference of 0 watts at the body machine interface can be the difference between easy and impossible (and it can) then it hardly makes sense to conclude that a difference of 1 watt isn't terribly significant. It can be massive and it's not just important for competitive folks, but for everyone. The fact is that less than 1 watt is the difference between a world class athlete and your average joe. I said it before and I'll say it again because you don't seem to understand it yet.
200 watts is not 200 watts is not 200 watts. At least not measured at the pedals it isn't.
P.S. By the way the actual measure is watts. Watt-hours would be energy, but we were talking about power.