W' Balance

W′ (W prime) represents the finite amount of work you can perform above your Critical Power (CP). W′ Balance (W′bal) shows you how much of that anaerobic "battery" remains, and how it depletes and replenishes throughout a session.

Every interval session is a back-and-forth between effort and recovery, moments where you spend anaerobic energy and moments where you recharge it.

• When you ride above CP, you draw from your W′ reserve.

• When you ride below CP, that reserve begins to replenish.

• Once it's depleted, you're forced to back off or stop.

W′ Balance tracks this process second by second, helping you visualize your high-intensity energy budget in real time.

In Vekta’s Critical Power (CP) modeling, W′ is individualized based on your physiology. Then, during every session, Vekta continuously calculates your W′ Balance:

• Above CP → W′ is depleted

• Below CP → W′ is partially restored (the lower the power, the faster the recovery)

This rolling balance updates second-by-second, providing a reliable estimate of how much anaerobic capacity you have available at any moment.

It helps answer key questions like:

• Can I go now, or do I need more recovery?

• How many more intervals can I handle?

• Is it too soon to attack in a race?

This makes W′ Balance a pacing and recovery tool, not just a post-ride metric.

Not technically, but it may appear to.

In theory, W′ Balance represents your finite anaerobic energy reserve. You can't spend more than you have, just like you can’t pour more water from an empty glass.

But in practice, W′ is an estimated value, derived from testing and modeling under controlled conditions. And that means:

• If your true W′ is lower than estimated (due to fatigue, poor recovery, illness, etc.), you might hit exhaustion before W′ Balance reaches zero.

• If your true W′ is higher or you recover faster than modeled, you may be able to perform more work above CP, temporarily driving W′ Balance below zero in the model.

This doesn't mean you’ve broken the laws of physiology, it means the model is imperfectly predicting a complex system.

💡 Pacing and Energy Management

• Time your efforts: Avoid attacking or pushing when W′ is nearly depleted.

• Know when to back off: Monitor recovery and avoid hitting empty.

📊 Interval Design

• Build smarter intervals: Use W′ depletion and recovery patterns to set rest periods.

• Target adaptation: Design sessions that challenge your anaerobic capacity without full recovery.

🎯 Race Execution

• Plan critical surges: Track W′ availability during races to know when to commit.

• Avoid overcommitting: Prevent burning all your anaerobic capacity too early.

W′ Balance turns complex anaerobic energy systems into something visible and actionable. By modeling how your anaerobic reserve is spent and restored throughout a session it helps you train and race with better precision.

It doesn’t just explain why you blew up, it helps you avoid it in the first place.

• Skiba, P. F., & Clarke, D. C. (2021). The W' Balance Model: Mathematical and Methodological Considerations. International journal of sports physiology and performance, 16(11), 1561–1572. https://doi.org/10.1123/ijspp.2021-0205

• Bartram, J. C., Thewlis, D., Martin, D. T., & Norton, K. I. (2018). Accuracy of W' Recovery Kinetics in High Performance Cyclists-Modeling Intermittent Work Capacity. International journal of sports physiology and performance, 13(6), 724–728. https://doi.org/10.1123/ijspp.2017-0034

• Bartram, J. C., Thewlis, D., Martin, D. T., & Norton, K. I. (2022). Validating an Adjustment to the Intermittent Critical Power Model for Elite Cyclists-Modeling W' Balance During World Cup Team Pursuit Performances. International journal of sports physiology and performance, 17(2), 170–175. https://doi.org/10.1123/ijspp.2020-0444

• Welburn, A.J., Pugh, C.F., Bailey, S.J. et al. W′ reconstitution modelling during intermittent exercise performed to task failure. Eur J Appl Physiol (2025). https://doi.org/10.1007/s00421-025-05912-0