The Power Meter Handbook: A User’s Guide for Cyclists and Triathletes

same for both rides. They obviously weren’t, and NP revealed this.
    So what NP is actually telling us is what the workout felt like, which is a much more revealing training component than a simple measurement of average power level for the ride. In my example, the hill repeats felt much harder than the steady, moderate-effort ride, and NP reflected a difference that average power would not. Normalized Power also gives us a much better idea of the energy cost of a ride. Doing surges on the hill burned a lot more calories than riding steadily. That’s why we will use NP for much of our riding and analysis. (If you are still unsure about what Normalized Power means, please take a few minutes to reread this section.)
POWER’S RELATIONSHIPS WITH TIME AND HEART RATE
    Earlier I suggested that in setting up your head unit, you should place duration and heart rate in prominent fields near current or instantaneous power since they are the next most important data fields. After riding with your new power meter for a week or so, you may notice some interesting things happening in the relationships between power and time and power and heart rate. These are critical relationships for training and racing, which we’ll explore in much greater detail later in the book. For now, let’s look at how they interact.
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 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.)
Power and Heart Rate
    You’ve probably been training with a heart rate monitor for a long time now. They’ve been around since the late 1970s and can be found on nearly all riders at the start lines of races. To make the best use of your heart rate monitor, you’ve set up zones. In the next chapter, I’ll show you how to set power zones to use in much the same way as you’ve done with your heart rate zones. But before doing that, I want to make sure you understand the relationship between these two sets of zones as it is confusing for many athletes, especially when they start comparing heart rate zones and power zones during rides.
    In your first year or so of serious training, your heart rate zones stabilize as you become more aerobically fit. Once your zones have stabilized, there will be only