Understanding Critical Power: What the CP curve reveals about your form
CP is more robust than FTP and more accessible than a step test. What the curve tells you and where its limits are.
Anyone who rides with a power meter eventually runs into the term Critical Power (CP). Some treat it like a myth, others as just another flavour of FTP. In reality CP is one of the most robust concepts in training science and the basis of almost every honest performance diagnosis.
What is Critical Power?
Critical Power is the output you could theoretically sustain forever, if your body stayed in a stable aerobic state. In practice, it's the threshold above which you start tapping into your anaerobic reserve (W') and below which you ride sustainably.
Mathematically CP comes from the hyperbolic model: Power = CP + W' / time. With two or more all-out efforts of different durations (e.g. 3 min and 12 min) the model can be fit. trace uses your best values from your entire activity history, so you don't have to run any manual tests.
CP vs. FTP: what's the difference?
FTP (Functional Threshold Power) is a pragmatic estimate: 95 % of your best 20-minute power. CP is mathematically cleaner and more reproducible because it uses multiple data points. For most athletes FTP and CP land within 5 % of each other. But CP is the physiologically more robust model.
W' – the anaerobic reserve
Alongside CP, the model gives you W' (pronounced „W-prime"), measured in kilojoules. That's the energy available above your CP, basically your „sprint battery". A high W' means you can absorb short, hard attacks. A high CP means you ride long climbs at a higher level.
How to read your CP curve
The power curve in trace shows your best value for each duration (5 s to 4 h). A few reading hints:
- Steep drop in the short range (5 to 60 s): you have sprint power, but the capacity falls off fast. Classic profile of a track or sprint rider.
- Flat run between 5 and 20 min: your VO2max range is well developed. Typical for climbers and time trialists.
- Strong values beyond 60 min: you have a deep aerobic base. Marathon and ultra profiles look like this.
- A „dip" in the middle range (3 to 10 min): often a sign you've never really pushed maximally in that range. A training gap, not necessarily a weakness.
What the curve doesn't tell you
The CP curve shows your best values, snapshots taken in a fresh state. What it doesn't show: how those values look after three hours of accumulated load. That's what the „Fatigued Overlay" is for (see durability), but that's a story of its own.
How often does CP change?
With structured training you see measurable shifts after 4 to 6 weeks. In a training camp CP can climb 5 to 10 W within 2 weeks. After illness or a forced break it drops faster than you'd like. trace recalculates CP automatically after every import as new activities come in, you don't have to do anything.
Bottom line
CP isn't a magic number, but it is the most honest indicator of your current endurance fitness: more robust than FTP, more versatile than VO2max estimates and accessible without a lab. If you ride regularly with a power meter, trace has everything you need to use it.