Forget FTP. Critical Power Already Replaced it Without Anyone Noticing

The first thing you might get familiar with once starting with cycling is the concept of functional threshold power (FTP). I mean even if you don’t care, someone at some point will ask you, “What’s your FTP?” – probably a guy at a party who’s still wearing his race kit at 2AM.

On the other hand, the last thing you hear about is critical power. And yet, it’s what actual exercise physiologists and WorldTour teams rely on. Research backs it. The test is more reliable. And what’s more, it’s an actual physiological parameter.

So why does everyone still obsess over FTP? I think most cyclists just haven’t noticed, or simply don’t understand it, even though critical power has already replaced FTP.

But don’t worry. It’s easy to use critical power in your own training. Here’s what it is, why it matters, and how to test it.

What Critical Power Can and Can’t Do

The critical power concept is based on the idea that the power-duration relationship follows a hyperbolic function above the critical power as seen in the graph below.

The function, accordingly, is defined by two parameters: On the one hand critical power or CP. On the other hand W’ (pronounced W prime). Being a mathematical construct, the critical power model is great at predicting exactly two things: Number one itself and number two its own downfall. To understand why, it’s necessary to understand its compounds…

W’

W’ is a fixed amount of work measured in kJ that can be utilized above the critical power. For those unaware, work is a function of time x power. In the case of W’ its a power output relative to critical power that can be maintained for a given amount of time. In other words, you can empty W’ quite rapidly with a high power output like sprints, or more slowly with a HIT effort. Use it fast or use it slow, the amount of kJ will be the same.

What makes W’ so interesting is the fact that it’s a function of your VO2max, your VO2 slow component and your anaerobic capacity. Therefore, W’ can vary widely among cyclists. Female cyclists typically show lower values of 10-14 kJ, while males usually show higher values of 15-25 kJ. With values of pro riders exceeding a 30 kJ W.’

Now, by completing several all-out efforts lasting between 2-15 minutes, we can assess the relationship of W’ and CP to determine their values. The function being tlim = W’/(P-CP).

Which brings us to the first limitation of the CP model. Power calculations made from W’ are only accurate for efforts lasting between 2-15 minutes, where the relative contribution of VO2max and anaerobic capacity is sufficient and an athlete is certainly above his or her CP. In other words, don’t care too much about W’ prime and see it more as a way to determine a more precise FTP or CP in this case.

Critical Power

Critical power is the asymptote, where the hyperbolic curve is shaped toward. However, mathematically it never actually reaches it. Otherwise P-CP would be become zero and every mathematician would tell you that dividing something by zero means bollocks. So instead CP is the highest power output that a cyclist can maintain without progressively feeding on W.’ Sounds familiar, doesn’t it? Yep, it also reminds me of the original FTP definition.

But how long can you maintain your CP then? The model tells you CP can be maintained indefinitely. But we all know that doesn’t really sound right. Instead, time-to-exhaustion depends on your training status. Research has shown that it’s 30-40 minutes on average but can be as high as 75 minutes or as low as 20 minutes. I think, however, that with the testing I’m about to show you TTE at CP can be as high as 90 minutes. This is because CP is close to your second lactate threshold (LT2), and TTE at LT2 is realistic to be 90 minutes in pro cyclists. But yet again similar to the FTP or MLSS concept.

Interestingly, a study comparing FTP20 and FTP 60 found that FTP determined from a 20 minute test can be maintained on average for 50 minutes. It seems like FTP and CP are not completely unrelated.

Instead, it’s the fact that CP in contrast to FTP has more physiological significance. And according to exercise physiologist Dr. Mark Burnley, predicting its on downfall on the one hand and predicting itself on the other, is what makes critical power a strong model.

“I would argue that the weakest models in science purport to explain absolutely everything, but predict absolutely nothing.”
Dr. Mark Burnley

If you want to step-up your training, my plans are available at TrainingPeaks:

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Making Sense of Critical Power

Despite being a mathematical construct, critical power has collected a decent amount of studies that all proved its physiological relevance in a variety of ways.

For example, a current review by Anni Vanhatalo and colleagues stated that critical power and maximal lactate steady state are essentially the same thing. Differences of the two parameters only occurred in past studies due to inaccurate measurements and neglection of the delayed steady state effect in blood lactate concentration.

Furthermore, another study has proven a VO2 steady-state occurring at 15 watts below critical power but not at 15 watts above critical power. Based on this, it has been shown that critical power is a fatigue threshold and that fatigue at 80-90% of critical power occurs 4-5 times slower than above the critical power.

And ultimately, several studies found a high correlation of the critical power and the relative amount of type 1 muscle fibers and an even greater correlation with the capillary contacts of the type 1 fibers and critical power.

So by adding both the oxidative machinery, or type 1 fibers, and their supporting structure, the capillary content, you get an even stronger correlation. And if you ever came across an article or podcast from Inigo San Millan, you already know that it’s the type 1 fibers that drive lactate clearance capacity.

Redefining The Gold Standard With Critical Power Testing

As I’ve promised in the beginning, a critical power test is everything else than sorcery. All you need is a power meter, a solid climb, and good legs – with good legs probably being the hardest thing to coordinate.

To get reliable results that exceed those of an FTP test I recommend completing a 3 and 12 minutes all-out test in one session with an optional 5 minute test on a separate day. Keep in mind, though, that your results might be impacted by pacing in the first 2 or 3 attempts. Additionally, it’s crucial that you choose a road or climb, where you can complete the efforts uninterrupted and with constant pedalling, as this will otherwise falsify your test results.

The CP Test’s greatest strengths are also its greatest weaknesses. As your CP is determined through the relationship of the 3 and 12 minute power, poor pacing can give you a falsely high CP if your 3 minute power is lower, for example. On the other hand, this is also a great way to track improvements in your CP. If, for example, you could bring your 12 minute power closer to your 3 minute power, you obviously have a higher CP. But as I’ve already said, you will get used to it quickly.

You might even find to have a lower CP than your FTP from your 20 minute test result. This is because of your lactate kinetics. You might be anaerobically strong allowing you to go deep into acidosis and high lactate levels. So your FTP might be higher than your actual Lactate Threshold. The CP test, however, takes this into account.

Once done with the test, you can either use the equation below or search for an online calculator. I suggest the one from High North Performance.

Pain is Inevitable, But Suffering is Optional

No matter if FTP or CP-Test, we shouldn’t forget that all-out field testing will help us get good at tolerating pain, especially mentally. Now, one more thing I want to make clear is that you should replace the FTP value in a training platform like TrainingPeaks with your exact CP and not a correspondingly lower value. Despite studies finding CP to be higher than FTP, it’s again a problem of test protocol comparison.

I found in me and my athletes that if you compare a fresh 20 minute FTP test with just an easy warm up beforehand to the CP 3+12 minute test, you will get similar results. Therefore, simply replace FTP with CP.

Ultimately, you won’t only get used to CP testing quickly but it will also be more accurate, more fun and above all, it includes 5 minutes fewer suffering compared to the 20 minute test. With the fewer suffering being broken down into 3 and 12 minute chunks on top.

Happy testing.

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