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Often it is argued that an 8 or a 20 minute effort can be sufficient to estimate Functional Threshhold Power (FTP), an indicator of how many watts a rider can put out over the course of an hour (or 40 minutes, depending on who you ask).

For a 20 minute effort, it is often argued that a 5% reduction over the 20 minute wattage gives an accurate estimate of the maximum wattage for the 40-60 minute effort.

For the 8 minute effort, an additional 5% (for a total of 10%) is taken off the total wattage over 8 minutes.

My question is as follows: does the reverse also hold? Suppose I do an 80 minute test, or a 160 minute test, will a respective 5 or 10% increase in the wattage over that period be my FTP?

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  • Probably pretty close, but FTP is an estimate, and you're estimating an estimate, which will have a margin of error. You'd have a hard time putting out your FTP for such long times - you want a quiet and non-stop, flat road for perhaps 40 km. That's hard to find, hence the shorter heuristics.
    – Criggie
    Jul 11 '17 at 0:02
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    I wouldn't put much faith in any of those numbers. Jul 11 '17 at 3:33
  • Worth noting that the 8min protocol isn't simply 90% of 8min wattage, but 90% of the average of 2 8min efforts in the same session. Additionally, you'll have a tough time convincing someone to do a 80 or 160min test to estimate the power they can hold for 40-60min. Better to just do a 60min test and forget the "estimation" portion.
    – Altom
    Jan 14 '20 at 20:50
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As a rough rule of thumb, those ratios are approximately right but there is quite a bit of individual variation and there will be more individual variation in the mean maximal power to FTP ratio at shorter durations than at longer ones.

If we consider a mean maximal power curve, a plot of the maximal power one is capable of sustaining plotted against duration, it is generally pretty flat from 8 minutes out to many hours. It's not of course totally flat and there are various individual factors that influence the shape of the curve. The slope is greater at the shorter duration, and it has more individual variability there.

This example curve taken from Jack Mott's website shows (note time is on a log axis):

enter image description here

Now while this is the general shape of a MMP curve, there is individual variability and I chose this image as it gives some clues as to why, namely the contribution of the various metabolic energy systems that supply the energy varies depending upon the duration of a maximal effort, and also upon the unique physiological characteristics of the individual.

So while long duration efforts are dominantly fuelled by aerobic metabolism, during shorter maximal efforts there is a greater contribution to the energy demand by anaerobic metabolism, and anaerobic capacity is highly individually variable.

As a result, the ratio of short duration power capability to FTP can vary quite a bit between individuals, while the ratio of long duration power capability to FTP will be less variable.

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