What is the Banister fitness-fatigue model?
A model that treats every training session as producing two effects at once — a long-lasting fitness gain and a short-lasting fatigue cost — with performance being the difference between them.
Banister's insight was that training does not have a single effect. Each session adds to a fitness term that decays slowly and to a fatigue term that decays quickly. Because fatigue is larger but shorter-lived, performance immediately after hard training is worse than before it, and better some days later once the fatigue has drained away and the fitness has not.
That single structure explains a surprising number of training phenomena: why you feel worst in the middle of a hard block, why performance often peaks a week or two after the heaviest training stops, why a taper works at all, and why removing training entirely for too long eventually costs more fitness than the freshness is worth.
The implementation uses two exponentially weighted averages of daily load, conventionally 42 days for fitness and 7 for fatigue, giving CTL and ATL. Their difference — training stress balance — is the model's performance proxy.
It is a deliberately simple model of a complicated system. It has no concept of what kind of training you did, no injury term, and no individual calibration unless you fit the constants to your own history. It is best used to reason about the shape of a plan, not to predict a race time.
Key numbers
| Fitness time constant | 42 days | slow decay |
|---|---|---|
| Fatigue time constant | 7 days | fast decay |
Work it out for yourself
Marathon periodization planner — Watch the fitness-fatigue model play out across a whole training block.
Frequently asked questions
How accurate is the Banister model?
Directionally useful, quantitatively loose. It reproduces the shape of adaptation — the dip, the rebound, the taper effect — reliably, but the time constants vary between individuals and the model ignores everything about training other than a scalar load.
Why does performance improve after you stop training hard?
Because fatigue decays much faster than fitness. Stop the heavy load and the fatigue term drains within days while the fitness term persists for weeks, so the gap between them widens. That gap is what a taper is designed to open.
Sources
- Banister et al. (1975). A systems model of training for athletic performance. Australian Journal of Sports Medicine.
- Busso (2003). Variable dose-response relationship between exercise training and performance. Medicine & Science in Sports & Exercise.
Related explainers
Glossary: Banister Model, TSB (Training Stress Balance)