The Power Curve: Why CP and W’ Beat FTP

You do a 20-minute FTP test. It comes out at 250 W. All your zones flow into the app: Z2 up to 165 W, Z3 up to 205 W, Z4 up to 250 W. And you train that way for eight weeks. Meanwhile your anaerobic work, 30-second intervals, sprints on a descent, attacks on climbs, lives in a vacuum. FTP tells you nothing about it, because FTP is one number for one point on the curve.

The power curve changes that. Critical Power (CP) plus W’ give you two points instead of one. Suddenly you see the whole physiology of your legs, not just that one 20-minute segment.

What the power curve is

The power curve (the power-duration curve) is a graph showing the maximum power you can hold for a given length of time. Watts on the vertical axis, seconds or minutes on the horizontal.

5 seconds: a neuromuscular sprint. Trained age-grouper: 800-1100 W. Pro: 1500-1800 W.
1 minute: max VO2max effort. Age-grouper: 350-450 W. Pro: 600-700 W.
5 minutes: the classic VO2max test. Age-grouper: 280-340 W. Pro: 450-500 W.
20 minutes: the classic FTP test. Age-grouper: 220-280 W.
60 minutes: true threshold power (CP).

The longer the duration, the lower the power. The curve decays exponentially. Two numbers describe it: CP and W’.

Critical Power: your true aerobic limit

Critical Power (CP) is the highest power you can hold in physiological steady state. Below CP, your lactate, VO2 and pH stay stable. Above CP, all three climb exponentially and you move into non-steady state: fatigue accumulates and sooner or later you blow up.

CP is not the same as FTP, though the two sit close together. CP typically runs 4 to 5% higher than your true 60-minute maximum power. So if your real one-hour power is 240 W, CP comes out around 250-252 W. FTP usually lands somewhere in between, around 245 W.

The difference sounds marginal. But it is a meaningfully more precise metric, because CP is not “60 minutes that felt hard”. It is a value calculated mathematically from two or three tests, grounded in physiology rather than how you felt.

W’: your anaerobic tank

W’ (read “W prime”) is the work you can do above CP before you are exhausted. Measured in kilojoules (kJ) or joules.

It is a finite store of anaerobic energy. Picture a fuel tank: you do not burn it while idling below CP. But push the throttle above CP and the tank empties. When it is gone, you are at zero.

Typical values:

Age-group triathlete: W’ = 10-15 kJ
Road cyclist with a strong anaerobic engine: W’ = 18-22 kJ
Track sprinter: W’ = 25-30 kJ

With CP and W’, you can calculate exactly how many seconds you will hold a given power above CP. The formula is simple: time = W’ / (P – CP), where P is the power you are holding and CP is your critical power.

Example: CP = 250 W, W’ = 15 kJ. You hold 350 W (an attack on a climb). How long can you sustain it? 15,000 J / (350 – 250) = 150 s = 2.5 minutes. After that, you blow up.

How to test CP and W’

Two protocols are most common:

1. The 3 to 12-minute test (classic):

Day 1: 20-minute warm-up, then a 3-minute maximal all-out effort. Average power is P3min.
Day 2 (or after 48 hours of recovery): 20-minute warm-up, then a 12-minute maximal effort. Average power is P12min.
Calculation: CP = P12min – ((P3min – P12min) x 3) / 9 (an approximation). W’ = (P3min – CP) x 180 seconds.

In practice, apps such as TrainingPeaks, Golden Cheetah and WKO5 calculate this for you once you upload the files.

2. The 3-minute all-out test (Vanhatalo/Burnley, 2007):

20-minute warm-up.
3 minutes of maximal effort: maximum power from the first second, holding whatever you can to the end.
CP = average power over the last 30 seconds.
W’ = the work done above CP in the first 150 seconds.

A brutal test. But faster: you get both metrics in 25 minutes.

CP vs FTP: when to use which

Training for IRONMAN 70.3 or full-distance starts: FTP is enough, simpler and well proven over long training blocks.
Training for T100 or fast short-course racing: CP + W’, for precision in the anaerobic zones.
Age-group triathlon, building an aerobic base: FTP is enough.
Advanced triathlon, fighting for seconds in a 70.3 race: CP + W’, for better pacing analysis.
No power meter: neither, use heart rate and RPE.

For most age-groupers, FTP is still the practical standard. But for anyone already using WKO5 or Golden Cheetah, analysing every interval, moving to CP and W’ is the natural next step.

Training zones based on CP

The classic Coggan model has seven zones based on FTP. CP opens up an extra, seventh zone for anaerobic training:

Z1 (recovery): below 55% CP
Z2 (aerobic base): 56-75% CP
Z3 (tempo): 76-90% CP
Z4 (threshold): 91-105% CP
Z5 (VO2max): 106-120% CP
Z6 (anaerobic capacity): 121-150% CP, where you train W’
Z7 (neuromuscular): above 150% CP, sprints of 10-30 s

For a triathlete, Z2 and Z4 matter most. Most training, 64-74% of FTP or roughly 65-75% of CP, is aerobic base. Z4 is threshold intervals. Z6 and Z7 are seasonal, done in blocks, not daily.

Practical takeaways

If there is no power meter, the power curve does not matter. Go back to heart rate and pace.
If FTP alone drives the app, that still works well for 80% of sessions. No need to change.
If every session gets analysed in TrainingPeaks or WKO, test CP and W’. See how the zones shift and whether the intervals are actually hitting their targets.
If racing in virtual events, W’ is critical. Knowing there is, say, 15 kJ available for attacks means planning when to spend it and when to save something for the finish.

CP and W’ are not magic. They are mathematics, mathematics that shows real physiology, not the round marketing numbers from a 20-minute FTP test.