Critical power workouts: what CP is and how to test it

What critical power actually is

Critical power (CP) is the power output an athlete could theoretically sustain for a very long time without fatiguing — the boundary between "sustainable" and "burning into a finite reserve." That reserve above CP is called W′ (pronounced "W prime"): a fixed amount of work, measured in kilojoules, that gets spent whenever the athlete rides above CP and slowly refills when they ride below it. Ride hard above CP for too long, and W′ runs out — which is exactly the feeling of a breakaway or a hard attack finally catching up with you.

CP vs. FTP — not the same number

FTP is a single, practical estimate of one-hour power, usually derived from a 20-minute or ramp test. CP comes from a different model entirely — it's fitted from several maximal efforts of different durations, plotted against how long each one lasted, to find where the power-duration curve flattens out. CP and FTP are usually close but rarely identical. The reason intervals.icu tracks CP separately is that, unlike a single FTP number, the CP/W′ model can also tell you how much high-intensity capacity an athlete has left right now, mid-ride — that's what its W′ balance chart is showing.

How a CP test actually works

A CP test isn't one effort — it's a small set of maximal, all-out efforts at different durations, spaced far enough apart to recover between them. A common structure is two efforts in the same session:

• A longer maximal effort — commonly 12 minutes.
• Full recovery (20+ minutes, or a separate day).
• A shorter maximal effort — commonly 3 minutes.

From those two data points, intervals.icu fits the CP and W′ values automatically once the efforts are logged. Some coaches use three efforts (for example 12, 7, and 3 minutes) for a more reliable fit — the structure matters more than hitting an exact duration.

Describe the test out loud

Building a two-effort max-test session by hand means getting the warm-up, both efforts, and the recovery between them exactly right. Describe it instead:

"Twenty minute warm up. Then a twelve minute all-out effort. Twenty five minutes easy recovery. Then a three minute all-out effort. Ten minute cool down."

Wattscribe structures the warm-up, both maximal blocks at their correct durations, the recovery between them, and the cool-down — reviewed and adjustable before it goes anywhere.

Send it, then let intervals.icu do the modeling

Push the finished test to the athlete's connected intervals.icu account. Once they complete it and their power data syncs, intervals.icu fits CP and W′ from the efforts automatically — no separate calculation. If they're testing on Zwift or a head unit instead, export the same session as a .ZWO or .FIT file.