FTP versus Critical Power

Functional Threshold Power (FTP) and Critical Power (CP) both measure a cyclist’s endurance performance, but differ in how they are calculated and applied.  Dr. Andy Coggan, the godfather of FTP, defines FTP as the highest power output a cyclist can maintain in a quasi-steady state without fatiguing. Critical Power, meanwhile, represents the hyperbolic curve of different max efforts (e.g., 1 min, 5 min, 12 min, 60 min, etc) which can be used to predict what a rider can sustain for various durations, including 60 minutes. 

This is where the confusion and debate begin because technically FTP is the maximum power a rider can sustain for about an hour-ish, and it is often estimated using a 20-minute test,  both of which may be plotted with a critical power curve. 

When you have good data, CP and FTP are aligned and they are the same.

But most riders do not have true 60-minute max efforts or formal CP testing in their data set.  Any 40k time trialists out there? You’re in luck, as the 40k time trial is the gold standard power output for measuring FTP.  And the duration may be used in one’s CP curve, whatever duration that is.   In any case, because CP estimates 60-minute FTP, the methods and definition will continue to be debated. 

I/we use them both

FTP is easy to test for defining training zones and improving performance. Critical Power is more precise for specific power outputs but requires more complex testing and testing protocols.  Critical power is especially helpful for helping athletes understand how hard they can go for an 18 minute effort (for example) that they may have in a time trial, a hill climb or a Strava segment.

However, one can curate their critical power curve from their data, including their 20-minute tests. That’s the beauty of critical power curves: you can use any length power outputs* from race and group ride data.  It does not have to be exactly 20 minutes or exactly 60 minutes.   The major caveat is that those power outputs have to be max efforts. Otherwise, the curve is inaccurate.

As a coach, I am not a fan of critical power testing because it requires rest and time away from training, but there is a workaround: I pluck maximal power outputs from an athlete’s data set to populate their CP curve. For example, peak 1-minute power outputs or a maximal 12-minute effort from a short prologue TT or Strava segment.  Both curate the curve.  All-out Strava segments are incredible pieces of data for critical power curves because any duration works.  There are tow caveats with this workaround:

#1 Is the relationship between one’s fitness and the date of the power output.  You would not want to populate your power duration curve with your best 1 minute power output from last year and your 20-minute field test from last week.  In my opinion, a rolling 6-week average captures your fitness accurately for cherry picking your best power outputs for your CP curve. 

#2 is to populate your curve with max efforts from 30 second to 5 minutes and 8 - 30 minutes.  The point is to have one short and one long effort. 

In summary 

FTP and Critical Power are closely related measures of cycling endurance, with FTP being simpler and more practical for setting training zones, while CP provides greater precision for predicting specific performance durations. We like both. The key distinction is in their testing and data requirements: FTP tests are simpler (e.g., a 20-minute test or a 40k time trial), while Critical Power demands maximal efforts across multiple durations. One workaround is to use a rolling 6-week window of peak power outputs to accurately build a CP curve without disrupting regular training.

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