4th Power (sic) for Normalized Power?

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Can anyone point me in the direction of an explanation for the 4th power used in the calculation of normalized power please? i'm assuming it's not squared (as it would be in 'RMS' values) because the 4th power better fits with the physiological impact of riding at higher powers? but has this been shown in a derived way or is it empirically based?

Cheers
Dan

Can anyone point me in the direction of an explanation for the 4th power used in the calculation of normalized power please? i'm assuming it's not squared (as it would be in 'RMS' values) because the 4th power better fits with the physiological impact of riding at higher powers? but has this been shown in a derived way or is it empirically based?

Cheers
Dan

http://www.amazon.com/Training-Racing-Power-Meter-Hunter/dp/1931382794


http://cyclingpeakssoftware.com/power411/PowerTrainingChapter.pdf

Thanks very much.

Dan