What the power-duration curve describes
The critical power model describes the relationship between the intensity an athlete can sustain and how long they can hold it. Plotted as power against time, the curve is hyperbolic: critical power (CP) is the asymptote, an intensity theoretically sustainable for a long time in a metabolic steady state, while efforts above it draw on a finite work capacity.
That finite capacity above CP is termed W-prime, often visualized as a fixed reservoir of energy that depletes during hard efforts above critical power and partially recovers when intensity drops back below it.
Why coaches use it as a modern endurance model
Critical power and W-prime give a framework for pacing and interval design: staying near CP targets sustainable steady efforts, while repeated surges spend and recover W-prime. It is considered a more physiologically grounded reference than older single-point markers because it captures both a sustainable threshold and a depletable reserve.
CP is a mathematical estimate from test efforts and approximates, but is not identical to, physiological thresholds. This is general educational information about endurance science, not individualized training advice.