This is an introduction and overview of Critical Power and W' (pronounced W Prime).
If we made a graph of power and time where power is on the left and time is on the bottom, and then we asked you to do a bunch of different tests, where you complete 3-4 maximal efforts, each lasting between 2-minutes and 20-minutes. What you get is a bunch of points that looks like the chart below.
Now we can draw a line through all of them.
Now the interesting thing is that when you draw this line, it looks like it levels out somewhere. But the important thing is that that's critical power, or threshold power.
And whilst it’s been shown that Critical Power occurs close to the maximal lactate steady state (Poole et al., 2016), in most cases Critical Power should not be used as a direct replacement for FTP (Karsten et al., 2021), which admittedly limits it’s direct application given almost all commercially available training platforms use FTP as their central power-based value to calculate intensity factor, training stress etc.
But Critical Power is the effort level you feel when you say, "Okay, I can do this hard for a while, but if I go a little bit harder, I'm going to blow up right away."
In practice, people can typically only sustain power outputs at CP for around 30-minutes (Vanhatalo et al., 2011).
The other interesting thing is that we can draw a little box. So if we make a box that connects the one axis over to one of these points and down to the critical power line, that area is called the W prime.
W' is a measure of energy (measured in kilojoules). And as you can see, we can draw boxes around any of these points.
And the interesting thing is that they're always the same size. Sometimes they're tall and skinny. Sometimes they're long and skinny. But no matter what, there's the same amount of energy in each of these boxes.
Now, what that means is that once you get above your Critical Power, you have a very limited amount of energy that you can expend, and you can expend it really quickly by going really hard. Or you can expend it over a longer period of time by going easier. But either way, you've only got the same amount. And if you use it all up, you're going to blow up or have to slow down.
One of the ways you can think of this is like a battery.
At the start of your workout, you've got 100%. And if you do intervals until you explode, you end up with 0% left in your battery. That's obviously something you want to avoid, especially if you're in a race.
When we measure W’ we can compare it to some typical levels for endurance athletes.
For example, this level is around 9–15 kJ for men and around 6–10 kJ for women, although these values can be bigger for athletes with a very high VO2max. For more punchy endurance disciplines, slightly higher W’ are better suited, such as between 15-18kJ for men or 11-13kJ for women. At the other end of the spectrum, sprinters can have W’ values in excess of 25-30kJ (Vanhatalo et al., 2011).
