Supercompensation and adaptation
Evidence: moderate
Adaptation happens during recovery, not during the run. The principle is sound; the tidy textbook supercompensation curve is an oversimplified model.
Training does not make a runner fitter; recovering from training does. A hard session is a catabolic stress that transiently lowers capacity, and the adaptation that lifts fitness above baseline happens afterwards, during recovery. This overload-then-recovery logic is well accepted and underpins why rest, sleep, tapering and sensible spacing of hard sessions matter as much as the sessions themselves.
The familiar textbook “supercompensation curve”, showing fitness dipping and then rebounding above baseline on a tidy schedule, is a simplified one-factor model rather than a validated law. The two-factor fitness-fatigue model, in which a single session simultaneously raises fitness and fatigue that decay at different rates, is regarded as more accurate, and the literal curve and its exact timing are not strongly validated. So the principle that recovery drives adaptation is sound, while any precise prescription read off the curve is not.
The same logic explains much else: the value of the taper, whose 2 to 3% gain is the curve made visible; the case for rest days and deloads; the risk of stacking hard work without recovery, which is overtraining; and the reason cold water immersion can backfire by blunting the adaptive signal it interrupts.
Spacing hard work
Why hard and easy days alternate
The fitness-fatigue model has a direct practical consequence: a hard session needs enough recovery after it for the fatigue to fade while the fitness gain remains. For most runners that means leaving roughly 48 hours between hard efforts and filling the gaps with easy running or rest, rather than stacking quality sessions back to back. The longer or harder the session, the more recovery it earns. A week of relentless hard training produces worse results than a hard-easy mix, because without the recovery the adaptation never appears.
This is also why the same weekly sessions can build one runner and break another: the limiting factor is not the work but whether recovery keeps pace with it, which is individual and is the thing monitoring tries to track.