Power File Terms Part I

Monday evening I led a workshop on power file analysis at SBR Coaching.  After spending years geeking out and obsessing over power data, it was really fun to get out and show others how I look at power files and maybe give them some new ideas on how they can better utilize their power meter.  

I decided I would share part of the talk here on my blog.  There are a lot of terms to know, but I focused on 6 main terms/variables you need to understand if you're training with power.   And if you don't train with power but are considering it, knowing these terms may give you a better understanding of the benefits of training with power.

Before I get into those, you need to know what FTP is (yes, a 7th term).  FTP stands for Functional Threshold Power (aka Threshold) and is the most power you can hold for 60 minutes.

If you're training with power, you absolutely must have a good estimate of your FTP.  A few variables are calculated off your FTP so if you don't know your FTP some of the data in your power files will be wrong.  And that means you must continue to retest your FTP as your fitness changes, or your data is wrong.  You paid a lot for a power meter so it only makes sense to make sure you have good data.

Plus, there are the training zones which are calculated off your FTP, but I'm not going to get into those right now.

Here are the terms/variables I focused on:

1. Average Power (AP) - This is an easy one. It's your average power over the duration of the ride.  I think most people understand this one.

2. Normalized Power (NP) - This one is a little more confusing than Average Power.  Normalized Power is what you could have averaged for the same physiological cost had you ridden steady on flat ground for the entire ride.  It gives you a better gauge of intensity than Average Power.  A ride with some hard efforts and easy efforts might give you an AP of 150 watts, but a NP of 200 watts.  The NP tells you that ride had the same physiological stress as riding steady at 200 watts.  The formula is complicated, but I think the main takeaway is that it's a better measure of intensity than Average Power.

3. Intensity Factor (IF) - This is the ratio of Normalized Power to your FTP.  IF = NP/FTP.  An example:  A ride with a NP of 200w for a rider with an FTP of 250w would be .8.


I know what you're thinking.....wait a minute.  You just said Normalized Power was a better gauge of intensity than average power, but now you're defining Intensity Factor.  If we have Intensity Factor, why do we need Normalized Power?  NP is a great way to compare rides, but it doesn't take fitness into account.  Notice that the formula for Intensity Factor has your FTP in it.  Intensity Factor takes your current FTP into account, meaning it takes fitness into account.  A ride that gives you a NP of 200w won't always give you the same IF.  Intensity Factor allows you to compare your fitness throughout the season or from year to year.  It also lets you compare fitness among different riders.  200w NP will not be the same intensity for everyone.


4. Variability Index (VI) - This shows you how much variability there was in your power.  It's the ratio of Normalized Power to Average Power.  The bigger the gap between those two numbers, the more variability and the higher the VI.  A time trial typically has almost no variability and will give you a VI of 1.0.

5. Kilojoules (KJ) - This is the total work you did over the course of the ride.  1 KJ = 1 Calorie burned.  You can use KJ to estimate your caloric needs (I went into detail on this at the workshop and may post some of that here - this can do a lot to help you nail down your nutrition and discover mistakes).

6.  Training Stress Score (TSS) - This is a number that uses intensity and duration to measure the training load or physiological stress placed on your body by that training session.  It's similar to KJ, except TSS is calculated off your FTP so it takes fitness into account.  KJ does not.

So you have NP and IF, both measures of intensity.  The difference is that Intensity Factor takes fitness into account.  

And you have KJ and TSS, both measures of workload.  The difference is that TSS takes fitness into account.   


In part II of this post I'll compare 3 rides to show you how you can use these variables to compare rides and fitness.