先講結論：
真的滿準的，自己的功率值 CP10、 CP20、CP40 在類似的心率下，剛好差約 5%呀！
(※懶的寫，從昇陽網站上 copy 下來的)
5%原則 (5% Rule)：運動時間增加一倍，那你的功率輸出會衰退5%，例如你在20分鐘的平均功率為200瓦，那在40分鐘的平均功率很可能落於200 X 0.95＝190瓦。
原文內容：
Power and Time
You’re probably already starting to get some thoughts about how to train with power. From the last section, you should now understand that the power data on your handlebars is closely related to your effort and expended energy while riding a bike. Power is also closely related to the duration— time—of the workout or race or a segment of one of those.
As time increases, power decreases if you are working at or near maximal effort. This should be obvious by now if you’ve gotten in a few rides with your power meter. You’ve probably done a short sprint of a few seconds at some point in a workout or race and seen the spike in power on your head unit and in the software chart after downloading the session. Do you think you could hold that same sprint power output for an hour? Absolutely not. Would you be able to hold that sprint power for a minute? Again, absolutely not if the sprint was an all-out effort of only a few seconds.
Your personal power levels are specific to the duration of the output. As the time of the workout increases, the normalized and average powers will decrease if you are riding workout increases, the normalized and average powers will decrease if you are riding with a high effort. This should be obvious in racing. It is also true of intervals, which we’ll examine in much greater detail in later chapters. Power and time are inversely related—when one changes, the other changes in the opposite direction. The “5 Percent Rule” explains this.
The 5 Percent Rule says that when the duration of a session (or a segment) doubles, the power you generate to ride at a maximal effort for the longer duration decreases by about 5 percent. For example, if you do a short time trial race that takes 20 minutes and you will soon do another that is expected to take 40 minutes, you can estimate that the power of the longer one will be about 5 percent less than that of the shorter race. So if your average power was 240 watts in the 20-minute race, the estimated average power for the 40-minute race would be 228 watts (240 × 0.05 = 12; 240 – 12 = 228). The 5 Percent Rule is helpful whenever you try to calculate from a known duration to a new duration so that you can estimate the required power for a maximal effort. (There is an interesting exception to this rule that I’ll explain in Chapter 4 in the section that describes how to determine your Functional Threshold Power.)
