An Introduction to Power Metrics
Functional Threshold (FTP)
Most of the metrics in TrainingPeaks are based on Functional Threshold Power (FTP) so it is important that you have this right before jumping ahead to some of the other metrics. Functional Threshold Power represents the maximal effort you can maintain for around an hour (30-70 minutes) without fatiguing. For some athletes, this will be less than an hour and for others, it might be slightly longer, but the important thing is that it provides a useful benchmark by which to measure your training.
Normalized Power (NP)
In cycling, Normalized Power provides an estimate of the metabolic cost if the ride would have been paced steadily and places an emphasis on the surges during the ride.
Think of riding your bike like driving a car. You are going to use less gas while driving on the highway at a steady speed than you would driving in the city and constantly having to come to a stop and then hit the gas again to get back up to speed. Normalized Power provides a picture of how hard a ride felt and while the Average Power may be relatively low, the Normalized Power could be high due to surges in speed throughout the ride duration.
Intensity Factor (IF)
Now that we know how hard we can go for around an hour (FTP) and we have a way to estimate how metabolically taxing a workout was (NP/NGP) we can measure how intense that workout was relative to our FTP through the Intensity Factor (IF).
Intensity Factor is simply how intense a workout was relative to your FTP and can be read as sort of a percentage of 1 with 1.0 being your threshold. If the Intensity Factor for your workout was .80 then we could say that you performed that workout at 80 percent of your threshold.
General endurance work falls in the 60 to 70 percent range, while a harder tempo workout would be closer to 80 to 90 percent. Additionally, depending on the duration of your race you may see an IF of up to 105 percent or 1.05. Intensity Factor is calculated by dividing your NP or NGP by your functional threshold. So if your FTP is 250 watts and you did a workout with a Normalized Power of 200 watts then you would end up with an intensity factor of .8 or 80 percent of your threshold. In addition to providing information on the relative intensity of your workout, you can also use this number to get an idea of the relative intensity of different race durations.
Training Stress Score (TSS)
Finally, we have Training Stress Score, which is a score given to a workout to tell you how hard it was. Whereas IF told you the intensity of a workout relative to your threshold, it is only one side of the equation. If you did a workout at 90 percent of your threshold for two hours it would surely be much more stressful than doing 90 percent of your threshold for 45 minutes.
Training Stress Score takes both intensity and duration into account and provides you with a more complete view of how stressful that workout was in the overall picture of your training. To take this idea a step further, the Performance Management Chart uses this daily Training Stress Score to model your long-term training load (CTL), short-term training load (ATL), and the balance between the two (TSB) by using a weighted average of a set number of days.