What W′bal is, and why your pacing falls apart on the climbs

W′balJuly 12, 20266 min readBorja Alfaraz

Physiology

≈ 20 kJ

Anaerobic work reserve (W′) above critical power in a trained cyclist.

Depleting it before the finish is the most frequent cause of collapse on the climbs.

Abstract. This article introduces the critical-power model and the W′ reserve, and explains through W′ balance (W′bal) why pacing that ignores the depletion and recovery dynamics of that reserve systematically fails on climbing segments.

Almost everyone has lived this scene: the plan’s numbers added up, the average power was right… and you still blew up halfway up the final climb. The problem was not your fitness. It was that you were distributing the effort with the wrong metric.

CP and W′: two tanks, not one

The critical power model describes your engine with two numbers:

CP (Critical Power): the power you can hold for a long time in a ‘near’ steady state. Below it, you last.
W′ (‘W prime’): a finite reserve of energy — measured in kilojoules — that you spend only when you pedal above CP. It is your anaerobic tank.

The key is that W′ is limited. Every time you push above CP, you drain that tank. When it hits zero, there is no more: power collapses whether you like it or not.

W′bal: the tank in real time

W′bal (W′ balance) is how much of the tank you have left at any instant. It rises and falls throughout the race:

Above CP, W′bal falls. Below CP, it recovers — but more slowly than you think.

That ‘more slowly’ is what ruins naive plans. W′ recovery is neither instant nor linear: it follows a time constant that depends on how depleted the tank is (Skiba’s model). After a hard effort you need to ride easy for a good while to truly refill.

360 W (above CP)315 W (sustainable)

Figure 1. Remaining W′ during a climb (CP = 300 W, W′ = 20 kJ). At 360 W the reserve empties at 6 minutes and power collapses to CP; at 315 W the climb is sustainable.

Why climbs fool you

On a climb, gravity forces you to raise power just to keep moving. If the climb comes late and you have been nibbling your W′ on every earlier ramp, you enter it with the tank half empty… and that is where you blow up. The stage’s average power could be perfect; the problem is when you spent the reserve.

Pacing by average power is like planning a budget looking only at the monthly total and ignoring that halfway through the month the account is already empty.

Pacing with W′bal: spend to arrive, not to explode

A good plan does not minimise average power: it distributes W′ so you reach the line with the tank almost empty, but never at zero too early. In practice:

Push a little above CP on climbs, where time is won.
Ease below CP on the flat and descents, where W′ recovers.
Keep a buffer for the decisive stretch: nobody wins cooked 3 km from the line.

AthletePro Super Record does exactly this: it optimises the watt-by-watt split per segment via dynamic programming over your 3-parameter CP/W′ model, with Skiba’s realistic recovery, and shows you the W′bal trace so you can see that you finish with reserve, not empty. You can also tune your recovery factor if you are a highly trained rider who recovers more slowly.

Understand your W′ and you will stop blowing up in the wrong place.