Anaerobic reserve (W′ / D′)
Your anaerobic reserve is the finite amount of extra work you can produce above your endurance limit before you have to ease off. That endurance limit is called Critical Power (cycling) or Critical Speed (running); the reserve above it is called W′ (cycling, pronounced "W-prime", measured in kilojoules) or D′ (running, measured as distance).
What is your anaerobic reserve?
Below your endurance limit you can keep going for a long time: you supply energy mainly with oxygen, nicely in balance. Go above it — a sprint, a steep hill, an attack — and you tap into a separate, limited supply that can’t be refilled with oxygen. That is your anaerobic reserve. It is finite: once it’s empty, you have to drop back below your limit to recover.
Think of a battery alongside your "endurance engine". W′ measures that battery in cycling in energy (kilojoules), D′ in running in extra distance. The bigger your reserve, the longer or more often you can produce short, hard efforts above your limit — important for sprinting, attacking and surviving hills.
How does the reserve fill and empty?
Every effort above your Critical Power or Critical Speed draws on the reserve, and the further above, the faster it empties. Ride or run easy below your limit again, and it fills gradually — the lower the intensity, the faster that recovery. That explains why you hold interval blocks together better with enough easy recovery in between.
Your reserve isn’t a fixed number for life: it shifts with your training and form. Calculation models estimate it together with your Critical Power or Critical Speed from your best efforts over different durations — your duration curve.
How Trimetra personalises this
Trimetra continually estimates your Critical Power or Critical Speed and the matching reserve (W′/D′) from your duration curve, without a separate test. On the card of a finished session you also see, as a bar, how strongly that session drew on your anaerobic reserve — from barely to heavily — which helps in gauging your recovery need.
Frequently asked questions
What does W′ (W-prime) mean?
W′ is your anaerobic reservoir on the bike: the finite amount of extra work (in kilojoules) you can deliver above your Critical Power before you have to back off. Think of a battery next to your endurance engine. The running variant is called D′ and is measured as distance.
Why do you sustain intervals better with rest in between?
Because your reservoir refills below your endurance boundary — the easier the recovery, the faster. Every effort above the boundary drains it; with enough easy recovery between blocks you start each interval with a fuller battery.
Is your anaerobic reservoir a fixed number?
No, it shifts with your training and form. Models estimate it together with your Critical Power or Critical Speed from your best efforts over different durations — your power-duration curve. It is a model estimate, not a direct measurement.
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