The least impactful way to measure critical power is to do the 6/3 lap test or 9/3-minute test on the road. Stryd uses the power and pace for each segment to estimate critical power. Critical power is the power you can sustain for 50 minutes, obviously much longer than the time of the test.
Since we extrapolate CP from the 2 test segments, the power ratio between the short and long segment of the test is important. It is possible to repeat the critical power test and run at higher power in each segment, yet get a lower critical power number. If that happens, the power ratio likely changed.
1. In the first test, the short section was not an all-out effort, causing CP to be overestimated.
2. In the second test, the long section was not an all-out effort, causing CP to be underestimated.
3. Your training was focused more on short distances, improving your speed but not your endurance.
You can always repeat the critical power test, and with each test, you will get more proficient and closer to the best possible CP estimation. If you have done a 10k race at full effort, it is the best way to estimate CP, but it has high impact.
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