Critical Power

You can think of Critical Power as a threshold between aerobic and anaerobic exercise. While that's an oversimplification, it reflects the subjective feelings of the athlete well. In some ways it's like Lactate Threshold, but far easier to determine. Critical Power has been around since 1965, but the rise of power based cycling trainers, cycling power meters, and now the ability to estimate running power with Stryd, the concept has gained new traction. Critical Power is the highest workload you can sustain without using your anaerobic reserve, and research papers often consider this workload that an athlete can sustain "indefinitely." Or course, "indefinitely" here means until glycogen reserves get depleted, or other limiting factors set in. Typically, you can maintain Critical Power for 30-60 minutes . Above the Critical Power, there's an amount of work you can perform anaerobically. This anaerobic work capacity is name W' (W-prime). You can either use your W' at high intensity for a short period or a lower intensity for a longer period. This creates a curve of how long you can sustain a power level above Critical Power.

Critical power, from Golden Cheetah.

Contents

1 Why should you care?

There are several benefits to knowing your Critical Power as a runner. Improvements in your Critical Power show improvements in your fitness. This is a good way of evaluating the effectiveness of your training. Critical Power can help define your running paces. To use Critical Power for pacing, you can't use your Critical Power when cycling directly. One option is to perform a Critical Power test while running, using Stryd (the only running power estimation that works as of April 2020.) The other option is to convert your cycling power to your Stryd running power. You can use Critical Power for HIIT. You can structure an HIIT workout based on your CP and W'. You can use these values to understand how stressful (and how practical) a workout will be. I use Golden Cheetah which will show the W' balance as a curve for a workout. The CP and W' can allow you to pace shorter races (30-60 minutes). An ideal pace will be one in which you deplete W' evenly over the race, reaching near zero at the end. For longer races, other factors beyond W' depletion become the limiting factors, so it's less valuable there.

2 Finding Your Critical Power

The gold standard for measuring CP is to perform multiple tests to define points in the CP/W' curve. You need to perform each test needs while well recovered, so each needs to be on a separate day. After a warmup, you do a fixed power workout that should cause your exhaustion between 3 and 15 minutes (durations less than 3 minutes may give an overestimate of CP.) Given these results, several online calculators will give CP and W' numbers. (One study found that CP occurred at around 83% of Max HR ).

3 The 3AOT Approach

There's a simpler "3 minute all out test" (3AOT) that can estimate your CP. This test must be "un-paced" where you go all out for the duration of the test. This means that power output will be very high for the first few seconds, then drop off as you deplete your W'. Between 90 and 120 seconds, you should have depleted all your W' and you can only maintain your CP . The limitation of the test is that any pacing, even subconsciously, will overestimate CP and underestimate W'. There are several studies that have found the 3AOT gives a good estimate of CP/W'. These studies found that a 3AOT can detect relatively small changes in CP from training, such as increasing CP from 230 to 255w. One validation technique that a study used was to exercise at 15w above and then 15w below the critical power estimate. The study found that athletes could not maintain 15w above their CP for over 10-15 minutes, where 15w below their CP was stable up to 30 minutes. However, one study found the 3AOT did not provide valid results in elite cyclists,. That study found that the 3AOT estimated an average of 351w compared with 402w with multiple tests. This study found that the underestimated the athletes' W', measuring an average of 15.5 kJ rather than 24.3 Kj.

4 Critical Power and Running Performance

A study looked at the relationship between Critical Power and Running Performance over 40m, 1km, 10km, and 21.1km (half marathon distance.) The study found a reasonable correlation between CP and both running performance and V̇O₂max for distances of 1km and longer. They found that CP correlated with 75-85% of both values (r=0.75-0.85).

5 Critical Power and Power at V̇O2max

I find it interesting that Critical Power is halfway between Lactate Threshold and the maximum power attained during an incremental V̇O₂max test. This suggests that the power achieved at V̇O₂max is partly from anaerobic sources.