The critical power field test: 12 minutes then 3 minutes all-out, with 30 easy minutes between. Your two averages give your CP, the power you can hold without blowing up, and your W′, the reserve above it. 70 minutes, for experienced riders.
| Target | Zone | Time | FTP 200 W | FTP 250 W | FTP 300 W |
|---|---|---|---|---|---|
| 125–145% | 6Anaerobic | 3 min | 250–290 W | 313–363 W | 375–435 W |
| 100–110% | 5VO2 Max | 12 min | 200–220 W | 250–275 W | 300–330 W |
Put this session on your bike computer or watch
Do the test from your bike computer, guided block by block. Then enter your new FTP and every session follows.
This test doesn't make you fitter: it measures the two parameters of the critical power model. Your CP is the line between efforts you can keep going for a long time and efforts that will run you into the ground: above it, fatigue builds until you stop. Your W′ is the amount of work you can do above that line, a battery drained by attacks, surges and long sprints, which recharges when you drop back below.
The model rests on a straight line: the work you do flat out rises in a straight line with the length of the effort. Two points are enough to draw it. Twelve and three minutes bracket the range where the line holds, roughly 2 to 15 minutes. Unlike FTP, which scales a single effort to an hour with an average factor, you come away with two numbers, and W′ tells you something FTP can't.
Experienced riders with a power meter who already know how to pace a long effort: a 20-minute FTP test or a 10-mile time trial, say. It speaks most to riders whose races are decided above threshold: crits, cyclo-cross, rolling road races full of surges. For a first test, or if you only want to update your FTP, the ramp test is simpler and shorter.
At the start of a training block, or every 4 to 6 weeks: a common guideline. With the long recovery the session runs to 70 minutes, so save it for a day with time to spare, after one or two easy days, never the day after a hard session. Ride both efforts on the same stretch, ideally a steady climb, or on the turbo with ERG mode off. Repeat it in the same conditions to track how your CP and W′ change.
5′ at 45–55% FTP + 5′ at 56–65% FTP + 5′ at 66–75% FTP
15 minutes building from 45 to 75% of FTP, in 5-minute steps. Zero your power meter before you set off. If you're on a smart trainer, turn ERG mode off now: it would set the power for you and spoil both efforts.
12′ at 100–110% FTP
12 minutes all-out, but even. Start at a power you think you can hold, settle in, and save the push for the last two minutes. If your current FTP is right, your average will land around 100–110% of it: 250 to 275 W for an FTP of 250 W. Treat those numbers as guides only: ride by feel, and note your average power.
30′ at 40–55% FTP
30 genuinely easy minutes at 40–55% of FTP. Long, on purpose: it's the recovery Karsten and colleagues tested to get a valid CP. Keep pedalling, drink, eat a little if you need to, and roll back to the foot of your climb if you're outside.
3′ at 125–145% FTP
3 minutes all-out, paced the same way: this is not a sprint. An explosive start would cost you the final minute. If your FTP is right, your average will land around 125–145% of it: 313 to 363 W for an FTP of 250 W. Note your average power.
10′ at 40–55% FTP
10 very easy minutes at 40–55% of FTP. Then the maths: CP = (P12 × 720 − P3 × 180) ÷ 540, and W′ = (P3 − CP) × 180, in joules. With 270 W over 12 minutes and 360 W over 3 minutes: CP = 240 W, and W′ = 21,600 J, or 21.6 kJ.
The 12 minutes feel like a short time trial: hard but manageable for eight minutes, then very hard. The recovery flies by, far too quickly by most accounts once it's time to go again. The 3 minutes are maximal from start to finish. Guide: 9 to 10 out of 10 on each effort. If you finish either one with something left, your result is off.
With only two points, a poor effort doesn't show up: it just moves the line. If your 3 minutes come out at 340 W rather than 360 W, CP climbs to 247 W and W′ shrinks to 16.8 kJ. A cautious short effort inflates your CP and shrinks your W′; a cautious 12-minute effort does the opposite.
On both efforts, going off too hard costs you later, and your average drops. The first pedal stroke should be firm, not explosive. Over the 12 minutes, the first minute should even feel a touch easy.
Thirty minutes is a long time with cooling legs. But a shorter recovery leaves fatigue in your 3 minutes, and the line comes out wrong. If you're short of time, ride the two efforts on separate days rather than trimming the rest.
CP and FTP are close but come from different calculations, and they aren't interchangeable. Sessions set as a % of FTP stay anchored to your FTP; your CP and W′ are for understanding what you can do above threshold.
The critical power model comes from Monod and Scherrer (Ergonomics 1965), and was applied to cycling by Moritani and colleagues (Ergonomics 1981). Karsten, Jobson, Hopker, Stevens and Beedie (Eur J Appl Physiol 2015) compared field efforts of 12, 7 and 3 minutes, with 30 minutes of recovery between them, against a laboratory protocol: CP estimation error of 4.5 to 5.8%. Karsten and colleagues (J Sports Sci 2017) then showed that with 30 minutes of recovery, CP remains valid but W′ does not (error of about 33%). The two-effort version is the model's mathematical minimum; it was not validated as such in these studies. The % FTP ranges, the warm-up and the cool-down are Wattson choices. Bottom line: good evidence for CP, weak for W′.
Multiply each average by its duration in seconds: that's the work done. CP is the slope of the line joining the two points: CP = (P12 × 720 − P3 × 180) ÷ 540. Then W′ = (P3 − CP) × 180, in joules. 270 W over 12 minutes and 360 W over 3 minutes give a CP of 240 W and a W′ of 21.6 kJ.
It's your energy reserve above CP. The further above it you ride, the faster it empties: with a CP of 240 W and a W′ of 21.6 kJ, you drain it in 3 minutes at 360 W, or in 6 minutes at 300 W. It refills when you drop back below CP. Two riders with the same CP can have very different W′ values, and race in very different ways.
Both describe the power you can hold for a long time, without being the same thing. If your sessions are written as a % of FTP, keep FTP as your reference and use CP and W′ to understand what happens above threshold: how many attacks you can absorb, how long you can hold a surge. Don't swap one in for the other without thinking it through.
Because that's the recovery Karsten and colleagues studied: with 30 minutes, the calculated CP stays valid. W′, on the other hand, becomes much less reliable. A shorter break would leave fatigue in the second effort and skew the line. For a more accurate W′, ride the two efforts on separate days.
Yes, that's the whole point of a field test. Pick a steady climb long enough for the 12 minutes, with no junctions or descents, and do both efforts in the same place. Wind and heat affect the result, so note the conditions to compare your tests. On a turbo, turn ERG mode off.
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