What are CTL, ATL and TSB?
CTL is a 42-day average of training load standing in for fitness, ATL a 7-day average standing in for fatigue, and TSB is CTL minus ATL — a proxy for how fresh you are.
These three numbers come from the fitness-fatigue model, and they are all exponentially weighted averages of the same input: daily training load. Chronic training load uses a 42-day time constant, so it moves slowly and represents accumulated fitness. Acute training load uses 7 days, so it moves fast and represents recent fatigue.
Training stress balance is simply CTL minus ATL. When you train hard, ATL rises faster than CTL and TSB goes negative — you are fitter than you were, but too tired to show it. When you ease off, ATL decays faster than CTL and TSB climbs into positive territory as fatigue clears while fitness lingers. That gap is what a taper exploits.
The practical use is planning rather than prediction. Watching CTL climb steadily through a build tells you the training is accumulating. Watching TSB during a taper tells you whether you are shedding fatigue fast enough to race. Race-day readiness is usually described as slightly positive TSB — fresh, but not so detrained that fitness has started falling.
What it does not do is measure you. These are convenient summaries of what you logged, and they are only as good as the load model behind them. Two runners with identical CTL can be in entirely different states depending on how that load was accumulated.
Key numbers
| CTL window | 42 days | fitness proxy |
|---|---|---|
| ATL window | 7 days | fatigue proxy |
| Race-ready TSB | slightly positive | fresh without detraining |
Work it out for yourself
Marathon periodization planner — Track CTL, ATL and TSB across a full marathon build and taper.
Frequently asked questions
What TSB should I race at?
Slightly positive is the usual target — enough that fatigue has cleared, not so much that fitness has begun to decay. The exact number depends on how you compute load, so what matters is the shape of your own history rather than hitting a specific value.
Is CTL the same as fitness?
No. CTL is a rolling average of logged training load, which correlates with fitness but is not a measurement of it. A runner can raise CTL with volume that produces very little adaptation, and race results remain the honest test.
Sources
- Banister et al. (1975). A systems model of training for athletic performance. Australian Journal of Sports Medicine.
- Coggan & Allen (2010). Training and Racing with a Power Meter.
Related explainers
Glossary: CTL (Chronic Training Load), ATL (Acute Training Load), TSB (Training Stress Balance)