You can't touch this, Part III

NP Busters again today.

This is third part in a chat about Normalized Power and NP Busters.

In Part I, I reviewed the concepts of Average and Normalized Power and how and when they are useful tools for assessing your ride, the differences between them and how NP accounts for the highly variable nature of our power output as well as the non-linear relationship between the strain we experience and our power output, both things that are masked by inspection of Average Power alone.

In Part II, I expanded on how Normalized Power, well, normalizes rides by providing a reliable indicator (i.e. NP) of the metabolic strain experienced from rides of quite different types, and demonstrated this with an example by comparing a time trial with a criterium race by the same rider.

Part II also provided a definition for an "NP Buster", a term coined many years ago to indicate a ride with an NP somewhat higher than is typical or higher than what would normally be associated with the level of strain experienced. IOW, it significantly over estimates the rider's steady state power output capability. And by significantly, I don't mean double, but by more than 5%.

So, how common are NP busters, and what can we learn from them?

Just to quickly recap, an NP Buster is a ride (or part of a ride) of about an hour's duration where the NP is > 105% of a rider's well established Functional Threshold Power (FTP). There are a few caveats about accurate power measurement, correct application of the NP algorithm, and a valid FTP setting before we declare an NP Buster, but assuming those requirements are satisfied, then we can declare a buster.

Like cyclones, NP busters do happen, and there are places and times when they are more likely to happen. They are also relatively rare, especially when compared to the vast "weather system" of all rides performed. But unlike cyclones, they don't represent some vast destructive force to be feared, but rather provide an opportunity to learn something.

Over the years, of all the files from riders I've coached (and myself of course), I have seen maybe one or two true NP Busters. That represents less than ~0.01% of all rides. Of course I have seen multiple ride files with an NP > 1.05 but they don't qualify for one or more reasons as true busters.

Now it's entirely possible that I have a lower than normal representative sample of NP busters in all of my client's data, but even if NP Busters are under sampled by a factor of 10, that still means they only occur less than 0.1% of the time. For a large proportion of riders, they will just never happen. And that's because for a large proportion of riders, they are just not capable of generating a buster. More on that later.

Over the years the creator of the NP concept, Andy Coggan, did actively seek and collect files from those who had genuinely generated an NP Buster, and I have data on those 20 examples to share (thanks Andy). Yep, only 20 examples out of many tens, perhaps hundreds, of thousands of power meter files. OK, perhaps not a brilliant statistical indicator of their true frequency, but you get the idea that these are not exactly an every day occurrence.

As an exercise in testing his Normalized Power algorithm, Andy also issued a challenge in 2009 at the Google Groups wattage forum for riders to attempt to complete one of a series of suggested workouts, any of which would have resulted in an NP buster. There were few, if any, takers.

Below is a chart showing information about each of the NP Busters Andy had collected files for. Click on the chart to see a larger version.

Let me take you through what is shown.

Each horizontal green and blue bar on the chart represents an NP Buster. The green bars on the left show how far below FTP the Average Power of the NP Buster was. The blue bar on the right shows how far above FTP the Normalized Power of the NP Buster was. Hence, only rides with an NP more than 5% over FTP are shown.

On the right side of the chart I have also included the Intensity Factor (IF) and the Variability Index (VI) for each ride. IF is simply the ratio of NP to FTP (IF = NP/FTP). VI is the ratio of the NP to the AP (VI = NP/AP). So looking at the first bar at the top of the chart, the Average Power for the ride was 22% less than the rider's FTP, the Normalized Power was 5% higher than the rider's FTP, the IF was 1.05 and the VI was 1.34.

Now we can see something interesting when you look at all of the examples of true NP Busters. Even though the Normalized Power over estimates the rider's well defined capability of sustaining an equivalent steady state power output for about an hour, on most occasions NP is significantly closer to the rider's FTP than is Average Power, and by quite some margin. On only four occasions out of the twenty was the rider's AP closer to FTP than NP, and not by much. These four are highlighted by the red translucent boxes.

So even though all of these rides are NP busters (and hence by definition are extreme examples of stretching the NP algorithm) and are clearly nothing like steady state efforts (see the VIs), 80% of the time the NP was still somewhat closer to a rider's FTP than was AP.

So if you have a ride or part of a ride of about an hour with an NP > FTP (and the data and calculation of NP is valid) then it's very likely your FTP will be closer to NP than AP, even if it's an extreme case of an NP Buster.

Indeed, if you are still trying to work out your FTP, and you haven't really performed any testing or settled on a reliable means to establish it just yet, but do have some very hard one-hour ride/race data, then you can peg a reasonable initial estimate of your FTP to be somewhere in the region of 95-100% of the NP.

The other thing to note is if you are in the majority of people who can't or don't produce NP Busters, then charting your 60-minute mean maximal NP, for example with a periodic chart in WKO+ software, is a reliable means to track longer term aerobic fitness changes. Here's an example chart of quarterly progress in 60-minute mean maximal Normalized Power over two years:

Plotting progress with 60-minute mean maximal NP
is a handy way to track longer term fitness changes.

Can you generate an NP Buster?

Of course there are going to be people who see instances of NP Busters a little more frequently, and it comes down to two things:
- the physiological profile of the rider
- the type of rides/races they do

To generate an NP Buster you need to execute a ride which includes a lot of very hard efforts of 30+ seconds duration which are substantially higher than your FTP. Many riders simply do not have the physiological profile to do that, as it requires a rider to posses both high neuromuscular power and a high anaerobic work capacity, especially relative to their aerobic capabilities.

NP Busters often involve out of the saddle efforts that engage the upper body musculature to enable the high power outputs necessary to generate them. An example of the sort of ride where this is likely to occur is in a criterium, and in particular one where there is a 20+ second long hill and/or a U-turn to negotiate each lap. Of course you could design a training session with such efforts or try your hand at one of the challenge sessions posted by Andy Coggan.

The ride file shown below is an example of an NP buster candidate course - a U-turn at the bottom of a hill in a criterium. In this example the rider has an FTP of ~315W (shown by the horizontal dashed line) and was able to continually punch out of the corner hard enough and long enough to contest the race finale. Yellow line = power.

Races such a crits with U-Turns and hills are likely NP Buster candidates
for riders with the right physiological capabilities

To better illustrate the file, this is what it looks like when you apply a 30-second rolling average to the data. This makes it pretty clear how frequently and how hard the rider had to push themselves. And this was with a small break away group, not a large group of riders.

  Using a rolling 30-second average power trace
helps to see why a ride might have been an NP buster

In this case AP was 276W (13% less than FTP) NP was 354W (12% more than FTP).

Now whether we should include rides which involve out of the saddle efforts as examples of NP busters or not is a consideration, but since it's not an uncommon thing in bike racing (especially criteriums like the one above), then I figure we may as well since that's how bikes are raced.


OK, so if you are a rider that can generate an NP buster, then that tells you something about your unique capabilities as a rider, that is you likely posses both a good sprint and a high anaerobic work capacity. You are also in the minority that possess very potent race winning weaponry, provided your aerobic fitness is good enough to use it. But it also means that you'll need to take a little more care in how you choose to interpret the NP from such rides, and don't go immediately assigning yourself a high FTP on the basis of such rides.

For most everyone else, NP provides a robust and reliable means to assess the metabolic strain for a wide variety of rides, and if you see an NP > 105% of your FTP, then there is a very strong likelihood that you've under estimated your FTP and maybe it's time to validate though reliable test methods.