Understanding Critical Power and W′

Critical Power is one of the most useful concepts in cycling science, and Pedal Pulse tracks it for you automatically. Here's what it means, where the numbers come from, and how to use them.

What it measures

Your power-duration relationship has two parts, and Pedal Pulse models both:

• Critical Power (CP) - the highest power you can sustain in a metabolically steady state — think of it as the ceiling of what's aerobically sustainable, measured in watts (and W/kg)
• W′ (pronounced "W-prime") - your anaerobic battery, measured in kilojoules. It's the fixed amount of work you can do above CP before you have to back off

Together they describe your engine: CP is the size of your sustainable motor, W′ is the turbo you can call on for attacks, climbs, and sprints.

How it's worked out — the process

Trust matters with modelled numbers, so here's the full pipeline:

1. Your power curve is built from real riding. Pedal Pulse tracks your best mean power over standard durations from 5 seconds to an hour, using a rolling window of your recent months so the curve reflects current you (older efforts are gently discounted).
2. The classic two-parameter Critical Power model is fitted to your best efforts in the 2–20 minute range — the same model used in sports science research for decades. That fit yields your CP and W′.
3. The fit is quality-checked before you see it. The model must fit your data well statistically, the results must pass physiological sanity checks, and a guard prevents a single recalculation from swinging your numbers wildly. The app openly grades its own confidence — you'll see the model fit quality (excellent to poor) on the page.
4. Dedicated tests are recognised automatically. If you ride a CP, FTP, or ramp test, Pedal Pulse detects it and uses the structured intervals for extra precision — a test is the cleanest input you can give the model.

From the fitted curve the app derives predicted power at any duration and time-to-exhaustion at any intensity. Your power curve also feeds your rider label — see our guide to rider types for how that works.

Where to find it

1. From the home page, tap the Body grid
2. Tap the Critical Power card

You'll see your current CP (with W/kg and how it relates to your FTP), your W′ capacity and category, your full power-duration curve with the model fit quality, and trends over time. The deeper W′ analysis — anaerobic capacity trends, 30-day changes, and recovery insights — is a Pro feature.

Using it in your riding

CP tells you what's sustainable; W′ tells you how many matches you have to burn. A rider with a big W′ can attack repeatedly; a rider with high CP can simply ride everyone off their wheel. Knowing your own balance helps you pick tactics — and watching the trends tells you whether your training is moving the right lever.