Normalized Power® (NP) is a valuable training metric used to account for the variability of power output during different types of rides and races. It is also used in calculations for other common training metrics, including Training Stress Score, Intensity Factor, and Variability Index. Here’s a closer look at NP to understand its importance, how to use it, and how it relates to Average Power.
What is Normalized Power?
Cycling is a stochastic sport, meaning that during a free-ride situation, power output fluctuates widely and unpredictably. Second-by-second power data is so variable that most head units default to a 3-second average power to make the data more useable. Across the duration of a whole ride or race, the variability of power output makes it difficult to determine how hard the ride really was.
Andrew Coggan introduced Normalized Power, and it is a trademarked term by TrainingPeaks, to provide a more accurate picture of the metabolic cost of a workout, ride, or segment of a power file. It does so by deemphasizing periods of lower power output – and hence lower metabolic cost. It does not, as cyclists erroneously believe, just remove all times when power equals 0 watts.
Calculating Normalized Power
The only reason it is useful to explain the formula for calculating Normalized Power is because it helps cyclists understand what NP is and is not. You don’t need to calculate NP on your own. TrainingPeaks does it for you, as do several power meter head units. Behind the scenes, here’s the calculation:
- Determine the 30-second rolling average power for the data segment (whole ride or race, specific climb, interval session, etc.).
- Raise all these values to the 4th power (i.e. x4). This is how those higher power efforts are emphasized. Thirty seconds at 5 watts goes to 625 watts but 30 seconds at 300 watts goes to 8.1 billion watts!
- Calculate the average of raised values.
- Take the 4th root of that value. This brings the numbers back to reality.
Normalized Power by Other Names
“Normalized Power” is trademarked by TrainingPeaks. This means other apps and software companies use a slightly different calculation and a different name to present a similar concept. Strava, for instance, uses “Weighted Average Power”, and Today’s Plan calls theirs “Adjusted Power”. This explains why you see small variations in your “normalized” power data if you use more than one app or training software.
Normalized Power vs. Average Power
You can see the value of Normalized Power when you compare it to Average Power (AP). When you calculate AP, all power values have the same weight. As a result, periods of low power output or coasting can dramatically reduce the average. And it’s easy for zero power moments to add up, particularly in group rides, criteriums, hilly rides, mountain bike rides, etc.
Variability Index (VI)
You can use the comparison of NP to AP to characterize a ride and get a better picture of its variability. This is where another metric you can see in TrainingPeaks – Variability Index (VI) – comes from. Credited to Charles Howe and described in Training and Racing with a Power Meter, 3rd Ed., VI is the ratio of NP to AP, or NP/AP for a workout, race, or segment of data. A VI of 1.0 means NP=AP, which describes a perfectly steady effort. There were no surges in power substantial enough to raise NP.
As VI increases from 1.0 it indicates increased variability. A flat time trial or hill climb might be just slightly above 1.0, like 1.0-1.06, representing a very steady effort. In contrast, criteriums or mountain bike races, which feature repeated high-power efforts but also lots of coasting, could produce VI values of 1.15-1.50.
Using Normalized Power in Cycling Training
So, how do we use NP in the real world? One way is by calculating Training Stress Score (read this post for more about TSS) for each workout. In turn, daily Training Stress Scores are to create metrics on fatigue, training load, and readiness to perform.
TrainingPeaks also uses NP to calculate Intensity Factor, the ratio of NP to Functional Threshold Power (FTP) for given effort or workout. Essentially, IF gauges the intensity of a ride or workout based on NP as a percentage of FTP. An easy recovery ride might have an IF of .65-.7. Endurance rides might range from .7-.85. Steady Zone 2 aerobic rides would be closer to .7. Group rides and rides that include climbing efforts would come in closer to .85. Interval workouts typically will push you to .85-.95, depending on the intensity and duration of the efforts and recovery times. And IF between .95 – 1.05 would represent a difficult ride or race, like criteriums and short road races.
All these metrics – TSS, IF, VI – are best used after your rides and races to gain insights on your performance. Even Normalized Power is better as a post-ride metric. And because it relies on a rolling 30-second average power, NP is best used for efforts lasting at least 10 minutes (20 minutes or longer is even better). During rides, monitor your 3-second Average Power for pacing purposes or to stay in a training zone for an interval.
By Chris Carmichael,
Founder and Head Coach of CTS