W′ and anaerobic capacity: the battery above threshold explained
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Critical Power tells you where the boundary is. It says nothing about what happens when you cross it — and crossing it is most of what makes cycling interesting. Attacks, bridges, the last kilometre of a climb, holding a wheel over a rise: all of that is life above threshold.
The Critical Power model has a second parameter for exactly this, written W′ and usually said aloud as "W prime". It is the finite amount of work you can perform above CP before you stop. Not a rate, not a power — a quantity of work, measured in kilojoules, that you spend and then have to earn back. If you have read Critical Power vs FTP, this is the other half of that model.
The battery, and why the metaphor is only half right
The useful mental image is a battery sitting on top of your aerobic engine. Ride below CP and the battery is untouched — you are running on the engine alone, indefinitely. Ride above CP and you start drawing down the battery, at a rate proportional to how far above you are. When it reaches empty, you are done: not slowing gracefully, but unable to continue at that power at all.
Stylised. The important feature is that each recovery repays less than the preceding effort withdrew, which is why a rider survives three repeats and comes apart on the fourth.
Where the battery metaphor breaks down is the recharging. A battery recharges at a rate you set. W′ recharges on its own schedule, and that schedule is slower than almost anyone expects.
How quickly does it come back?
In a study that deliberately emptied W′ and then re-tested after fixed recovery periods, the pattern was clear and slightly depressing.
Barely a third back. This is why a 30-second breather in the bunch feels like nothing.
Two-thirds. Enough to contribute again, not enough to repeat the same effort.
Close, but still short. Full reconstitution takes longer than most riders assume.
Group means from six participants. Individual recovery varies, and the study used passive-to-easy recovery — the rate depends strongly on how far below CP you sit while recovering.
That last point matters more than the headline numbers. Reconstitution is not a fixed clock: the further below CP your recovery power sits, the faster W′ comes back. Soft-pedalling at 40% of CP repays far quicker than sitting at 90% of it. This is the physiological justification for a piece of advice every experienced rider gives — after a hard effort, actually ease off. Recovering at tempo is barely recovering at all.
What it means on the road
A 25-minute climb is ridden almost entirely at or below CP. W′ barely matters to the outcome — and spending it in the first two minutes to hold a wheel is the classic way to ruin the other twenty-three.
A 40-second surge well above CP can cost several kilojoules of W′. You get a small number of those per race, and the number is genuinely finite.
Repeated short climbs with brief descents. Each effort withdraws; each descent repays only partially. Riders do not blow up on the first climb — they blow up on the fifth.
Whatever W′ you have left is the sprint you get. Arriving with a full tank after four hours is a pacing achievement, not a talent.
Typical values, and what a big or small W′ means
In trained cyclists, W′ commonly lands somewhere in the region of 15–25 kJ. In one validation of the three-minute all-out test, the work above end-test power averaged 15.0 kJ against an independently determined W′ of 16.0 kJ, in riders with a CP near 287 W. Treat any specific figure as indicative rather than a norm to be measured against — the estimate depends heavily on how the efforts were performed.
The interesting part is not the absolute value but its relationship to your CP. A rider with a high CP and a modest W′ is a diesel: strong on long climbs, dropped by repeated accelerations. A rider with a moderate CP and a large W′ survives punchy racing and loses time on a mountain pass. Neither is better; they are different jobs. Our post on power curve rider types goes further into what that shape says about you.
Can you train it? Somewhat — repeated supra-threshold work does appear to influence the parameter. But the larger and more reliable lever is CP itself. Raising CP shrinks the amount of the race that is spent above it, which is worth more than a slightly bigger battery, because it means you draw on the battery less often in the first place.
Where the model stops being trustworthy
W′ is a good idea being asked to do a lot of work. The limits are worth knowing before you build decisions on a live W′ readout.
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W′ is not a fuel tank
It is a modelling construct fitted to your power–duration curve, not a substance you can measure in a muscle. Phosphocreatine, hydrogen ions and potassium all move in ways that partly track it, but none of them is W′.
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Reconstitution is not one number
The widely used balance models assume a single exponential recharge. More recent work fitted a two-phase process — a fast component of about 11 seconds and a slower one of about 256 seconds — and found the single-exponential model underestimated recovery for every interval shorter than five minutes.
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CP itself is not perfectly stable
The model treats CP as fixed. Over long rides it drifts down, which means the ceiling W′ sits above is not where it was three hours ago.
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The fit depends on your efforts
W′ is estimated from the same curve as CP. If your short maximal efforts are stale or half-hearted, your W′ is wrong — and it will be wrong in a confident-looking way.
None of this makes the concept useless. It makes it a model — a compression of something complicated into two numbers that behaves well most of the time and misleads at the edges. Used as a way of thinking about pacing, it is excellent. Used as a fuel gauge you trust to three significant figures mid-race, it will let you down.
Practical takeaways
- Count your matches honestly. If a ride requires four maximal efforts, you cannot spend the battery on the first climb and expect it back by the third.
- Recover properly between intervals. Going genuinely easy between repeats is not softness — it is what makes the next repeat possible at the intended power.
- Pace climbs from CP, not from feel. Feel at the bottom of a climb is a poor guide, because you have a full battery and no sense of how long the climb will ask you to stay above the line.
- Train CP first. The battery matters, but the size of the engine underneath it determines how often you have to reach for it.
Sources 8
Where this article summarises a study, the study itself is linked — not a write-up of it.
- 1 Jones AM, Vanhatalo A The 'Critical Power' Concept: Applications to Sports Performance with a Focus on Intermittent High-Intensity Exercise · Sports Medicine · 2017
- 2 Ferguson C, Rossiter HB, Whipp BJ, Cathcart AJ, Murgatroyd SR, Ward SA Effect of recovery duration from prior exhaustive exercise on the parameters of the power-duration relationship · Journal of Applied Physiology · 2010
- 3 Skiba PF, Chidnok W, Vanhatalo A, Jones AM Modeling the Expenditure and Reconstitution of Work Capacity above Critical Power · Medicine & Science in Sports & Exercise · 2012
- 4 Caen K, Bourgois G, Dauwe C, Blancquaert L, Vermeire K, Lievens E, et al. W′ Recovery Kinetics after Exhaustion: A Two-Phase Exponential Process Influenced by Aerobic Fitness · Medicine & Science in Sports & Exercise · 2021
- 5 Skiba PF, Fulford J, Clarke DC, Vanhatalo A, Jones AM Intramuscular determinants of the ability to recover work capacity above critical power · European Journal of Applied Physiology · 2015
- 6 Vanhatalo A, Doust JH, Burnley M Determination of Critical Power Using a 3-min All-out Cycling Test · Medicine & Science in Sports & Exercise · 2007
- 7 Chorley A, Lamb KL The Application of Critical Power, the Work Capacity above Critical Power (W′), and its Reconstitution: A Narrative Review of Current Evidence and Implications for Cycling Training Prescription · Sports · 2020
- 8 Poole DC, Burnley M, Vanhatalo A, Rossiter HB, Jones AM Critical Power: An Important Fatigue Threshold in Exercise Physiology · Medicine & Science in Sports & Exercise · 2016
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