W′bal Drop: the hidden cost of hard surges above FTP

Some rides do not look huge by TSS. They are not long. Average power is not special. But one repeated set of attacks can leave your legs empty. W′bal Drop is the metric that tries to capture that hidden cost.

W′, pronounced W prime, is your finite anaerobic work capacity above threshold. Think of it as the short-burst battery you spend when you attack, sprint, bridge a gap, or ride deep above FTP. W′bal Drop estimates the largest depletion of that battery during a ride.

The concept comes from exercise physiology research by Philip Skiba and colleagues. The math is simple in principle: above FTP, W′ drains; below FTP, W′ refills. But the implications for pacing, tactics, and recovery are profound.

See it live

Drag the sliders to change FTP and W′ capacity. Watch how the red W′ balance line responds. A higher FTP means more power stays aerobic. A bigger W′ means deeper reserves before the tank is empty.

What W′bal Drop means

Power below FTP is mostly sustainable aerobic work. Power above FTP is not free. Every second above FTP spends part of W′. When you back off below FTP, the model lets W′ recover. W′bal is the modeled balance remaining at each moment.

W′bal Drop = full W′ capacity minus the lowest modeled W′ balance reached in the ride.

That means W′bal Drop is not total work and not total calories. It is the deepest hole you dug into your anaerobic reserve. A ride can have several hard efforts, but the reported drop is the largest modeled depletion at any point.

This is important because the depth of depletion determines how quickly you can respond to the next attack. At 80% W′ remaining, you have options. At 20% remaining, one more surge might break you.

How the recovery curve actually works

W′ recovery is exponential, not linear. The closer you are to empty, the slower each kilojoule comes back. This is the key reason why deep depletion is disproportionately costly. Going from 90% to 80% recovers in seconds. Going from 20% to 30% can take minutes.

Try it below. Set a deep depletion and see how long it takes to get back to even 50%. Then try a shallow depletion and notice the recovery is nearly instant.

This is the mathematical reason behind the race tactic everyone knows intuitively: if you can force your opponent to go deep, they cannot answer the next move even if they are resting. The recovery is asymptotic. Sitting on a wheel at 50W below FTP after burning 85% of W′ still takes several minutes to get back to half capacity.

Same rider, very different rides

TSS and W′bal Drop often tell opposite stories. A long endurance ride has high TSS and almost no drop. A short criterium has low TSS but a massive drop. Click through these scenarios to see why coaches need both numbers.

How Atomic Metrix estimates it

Atomic Metrix uses your power stream and FTP reference. When power is above FTP, the model spends W′. When power is below FTP, W′ recovers exponentially with a time constant (tau) that depends on how far below FTP you are riding. The further below, the faster the recovery.

Until you have a personalized W′ model, Atomic uses a default W′ capacity of 20 kJ. So if your metric guide says W′bal Drop 2.8 kJ, it means the ride is modeled as having depleted about 2.8 kJ from a 20 kJ anaerobic reserve at its deepest point. That is a small controlled drop, not a near-empty sprint battery.

Read your own numbers

Enter your ride's W′bal Drop and TSS below. The combination tells you what kind of stress you encountered.

How to read the ranges

• 0–3 kJ: mostly controlled aerobic or tempo riding. You probably did not spend much snap.
• 3–8 kJ: meaningful anaerobic use. Common in VO₂ work, short climbs, or group ride surges.
• 8–15 kJ: hard repeatability demand. The ride likely stressed your ability to attack again after partial recovery.
• 15+ kJ: near-empty reserve in the current default model. This usually points to a decisive surge, sprint, or very hard interval sequence.

Why it is different from TSS

TSS asks how much total training stress the workout created. W′bal Drop asks how deep the hardest anaerobic moment went. A long Zone 2 ride can produce high TSS with almost no W′bal drop. A short race finish can produce modest TSS with a large W′bal drop.

That makes the metric useful for race analysis. If you cracked after the fourth attack, look at whether W′bal was still recovering or already near empty. If your final sprint was flat, check whether earlier surges had already spent too much reserve.

Practical applications

W′bal Drop is especially useful when the goal is repeatability: criteriums, punchy climbs, short Zwift races, gravel surges, and any session where the question is not just how high you can go once, but how many times you can go hard and still respond.

• Low drop, high TSS: aerobic load, not much anaerobic cost. Good base training.
• High drop, low TSS: sharp but costly work, often needs more recovery than duration suggests.
• High drop late in the ride: good race-specific fatigue signal. You were deep when it mattered.
• Repeated drops with incomplete recovery: useful marker for anaerobic repeatability. This is what crits test.
• Pacing guide: if your W′bal drop was 90%+ in a race, you may have started too hard. Saving 5% can be the difference between responding to the winning move and watching it go.

When not to over-trust it

W′bal is a model. It depends on clean power data and a valid FTP. If FTP is too low, the model will think you spent too much W′. If FTP is too high, it may understate the cost of hard work. A missing or noisy power stream also weakens the estimate.

The largest limitation is personal W′. Some riders have a small anaerobic reserve (12 kJ) and recover quickly. Others have a large reserve (30+ kJ) and recover slowly. Time triallists tend to have a large W′ but slow recovery; sprinters often have a large W′ with fast recovery. Atomic currently starts with a default 20 kJ capacity, then becomes more accurate as more personal power history is modeled.

The model also assumes steady-state recovery kinetics. In reality, factors like temperature, hydration, muscle glycogen, and accumulated fatigue all affect how fast W′ reconstitutes. A fresh rider recovers faster than one 5 hours into a race, even at the same below-FTP power.

The practical takeaway

Use W′bal Drop to answer one question: how much of my above-threshold reserve did I burn at the hardest point? Pair it with TSS, IF, normalized power, heart rate, and how the effort felt. The number is strongest when it explains a specific moment in the ride, not when it is treated as a standalone fitness score.

References

• Skiba et al., Modeling the Expenditure and Reconstitution of Work Capacity above Critical Power, 2012.
• cycleRtools W′ balance documentation, a concise implementation reference.
• Critical power and W′ review for cycling prescription.
• Vanhatalo, A., Jones, A. M., & Burnley, M. (2011). Application of critical power in sport. International Journal of Sports Physiology and Performance, 6(1), 128–136.