W' (Anaerobic Work Capacity)

W' (pronounced “W prime”) represents your Anaerobic Work Capacity. It is an energy quantity, typically expressed in joules or kilojoules, describing the finite amount of work available above Critical Power (CP).

Core Concept

Think of your performance as two parts:

1. Critical Power (CP): the model’s high sustainable threshold.
2. W': the finite work reserve used when power is above CP.

W' depletion and recovery

• Depletion: when power > CP, depletion rate = current power - CP. The higher above CP, the faster W' is spent.
• Recovery: when power < CP, W' begins to recover. Recovery rate depends on how far below CP you are and on your aerobic recovery ability.

What does “empty” mean?

When W' reaches 0, it doesn’t mean you can’t move. It means you can’t sustain any power above CP. You’re forced to drop to CP or below.

Practical uses

• Race pacing: in short time trials or climbs, manage W' so you finish near “empty” at the line.
• Interval training: track W' balance (W' bal) to see whether your intervals truly deplete anaerobic reserves.

How Trainingload.ai uses W'

Trainingload.ai uses W' to interpret work above CP. A workout can have a similar total TSS but very different above-threshold demands depending on how much W' it uses.

W' is useful for reviewing:

• whether short attacks, hill repeats, or VO2 intervals were demanding enough;
• whether recovery intervals were easy enough to restore repeatability;
• whether an athlete has a larger short-power reserve or a stronger sustained-power profile.

For a model-based estimate, use the fit Critical Power and W' for training load planning, which fits CP and W' together.

Limitations

W' is model-dependent. It is only as good as the maximal efforts used to fit the power-duration curve. If your recent data lacks hard efforts in the 1–5 minute and 10–20 minute ranges, the estimate may be unstable.