Two after-effects of one workout
The fitness-fatigue model, associated with Eric Banister's work in the 1970s, proposes that every training session leaves two traces. It builds a positive fitness effect and simultaneously imposes a negative fatigue effect. Performance at any moment is modeled as the current fitness minus the current fatigue, so what you can do on a given day reflects the balance of the two.
The key insight is that the two traces decay at different rates: fatigue is larger but fades faster, while fitness is smaller per session but lingers.
Why timing matters
Because fatigue dissipates more quickly, a window opens after hard training where fatigue has receded but much of the fitness gain remains, leaving net performance elevated. This is the logic behind tapering before competition: reduce load enough to shed fatigue without losing the accumulated fitness.
A model, with caveats
It is a simplified mathematical abstraction. Real adaptation involves many systems, individual recovery varies, and the model's parameters are hard to measure precisely, so it guides thinking more than it predicts exact numbers. This is general educational information about a training theory, not medical advice.