Critical Power

Critical Power can be thought of as a threshold between aerobic and anerobic exercise. While that's an oversimplification, it reflects the subjective feelings of the athlete quite well. In some ways it's similar to the idea of Lactate Threshold, but far easier to determine. The idea of 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 that can be sustained without using the anerobic reserve and is often considered the workload that can be sustained "indefinitely." In practice, "indefinitely" here means until glycogen reserves start to get depleted, or other limiting factors set in, so typically 30-60 minutes can be maintained at Critical Power. Above the Critical Power, there's an amount of work that can be performed anaerobically. This anaerobic work capacity is typically called W' (W-prime), and it can either be used at high intensity for a short period or a lower intensity for a longer period. This creates a curve of how long a given high intensity power level can be sustained.

Critical power, from [1].

1 Why should you care?

I see the greatest benefit from Critical Power for HIIT. A HIIT workout can be structured based on CP and W', using those metrics to understand how stressful (and how practical) a given workout will be. I use Golden Cheetah which will show the W' balance as a curve for a given workout. The CP and W' can be used for pacing shorter races (30-60 minutes) as an ideal pace will be one in which W' is depleted 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. However, CP can be used as a metric to evaluate for running potential (see below.)

2 Finding Your Critical Power

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

2.1 The 3AOT Approach

There's a simpler "3 minute all out test" (3AOT) that can estimate CP. This is an "un-paced" test where the athlete goes 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 W' is depleted. The idea is that by the end of the 3 minutes, all of the W' will be depleted and only the CP can be maintained. The limitation of the test is that any pacing by the athlete, even subconsciously will overestimate CP and underestimate W'. There are several studies that have found the 3AOT gives a good estimate of CP/W', and they found that it can detect relatively small changes in CP with training (230-255w). One validation technique that was used was to exercise at 15w above and then below the critical power estimate. The found the 15w above could not be maintained past 10-15 min, where 15w below is stable from ~10 min to 30 min. However, one study found the 3AOT did not provide valid results in elite cyclists, with the 3AOT estimating an average of 351w compared with 402w with multiple tests. This study found that the W' was underestimated, with an average of 15.5 kJ rather than 24.3 on average.

3 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 corelated with 75-85% of both values (r=0.75-0.85).