Training Metrics
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In almost every article related to power based cycling training, a reference to either Normalized Power (NP), Intensity Factor (IF) and Training Stress Score (TSS) is referred to at some point or another. To begin with, these terms are essentially ways of qualifying power data to help coaches and athletes better analyze training and racing sessions. Hunter Allen and Dr. Andrew Coggan developed the terms. The are best known for their use in the Cyclingpeaks WKO software, but were also first in use on the Ergomo Pro power meter. This means that any athlete who uses the Ergomo pro, can easily see NP, IF and TSS while still riding instead of waiting to get home to download the ride data. Now, what do these terms actually mean in regards to training and racing?
NP is essentially an attempt to show the amount real work being done by the rider by smoothing out the fluctuations. We have all been on rides where there are steep hills, descents, wind, rough roads and a variety of other factors that make it hard to keep a steady power output. There will be times when you are not pedaling but then others when you really are going to hard. Average power cannot begin to show this fluctuation. Say if you ride at 300 watts for 5 minutes and then only 100 watts for the next 5 minutes. Afterwards, your average power might be 100 for the ten minutes, but that is not really a reflection upon the actual work that took place. In this case NP would probably be in the 250 to 260 watt range for the ten minutes, which would be more evident of the energy output of the timeframe. Basically, NP shows the steady wattage that you could have ridden for the entire given time to have the same energy cost. This is valuable metric for analyzing race data, because for any race, most riders spend a large portion not pedaling which highly skews the average power data. However, NP usually gives an accurate picture of the actual demands of the race and can even be used to estimate Functional Threshold Power (FTP), the power that an individual rider can maintain for 60 minutes.
The next metric commonly referred to is IF or Intensity Factor. IF is simply NP divided by FTP for a workout or a race. For example, an individual racing a 40k time trial would want to stay as close to his or her FTP for the race and if accomplished, would likely have an IF of close to 1. For a tempo ride or road race the IF would be from 0.75 to 0.85. A recovery ride would fall below 0.75 on the IF scale. What this number can be used for is to compare the intensity of different rides between riders with different abilities. Obviously a rider with an FTP of 300 watts is going to have a lower IF for a hard group ride than a rider with an FTP of 200.
Finally, by combining all of the previous metrics stated, FTP, NP, IF and time, Coggan and Allen developed a formula to compute the TSS or Training Stress Score. This can also be used to compare rides done between different riders, but unlike IF, TSS is more indicative of the total work done. IF only shows how hard a ride was, as it does not differentiate between 30 minutes and 3 hours. Taking the 40k time trial example from before, if the same rider held his or her FTP for the duration of the event, then he or she would likely have a TSS close to 100. In their book, Training and Racing with a Power Meter, Coggan and Allen give examples for how one should interpret different training stress scores. A ride under 150 TSS is classified as low, as recovery will generally be complete by the following day. A ride from 150 to 300 TSS is medium with some residual fatigue, which is usually gone by the second day. 300 to 450 TSS is high as fatigue will probably be present after more than two days. Lastly, over 450 TSS is classified as very high, as fatigue will be present for several days, which usually requires the athlete to rest fully before attempting any more hard rides.
Hopefully this article has helped you understand more about Normalized Power, Intensity Factor, and Training Stress Score so that you can fully comprehend the data that your power meter is capturing. It also illustrates the benefit of training with an Ergomo Pro power meter, as these metrics are easily accessible on the non-glare screen so that you can see them during any training session or race.
