How the Banister impulse-response model predicts cycling performance
WattsUP calculates Training Stress Balance as CTL minus ATL, using a 42-day exponentially weighted average for chronic load and a 7-day average for acute load. A negative TSB means you are carrying fatigue, while a positive TSB signals freshness and recovery.
In 1975, exercise physiologist Dr. Eric Banister published his foundational mathematical model showing that physical performance is the net outcome of two opposing physiological responses to exertion: a slow-decaying fitness impulse and a fast-decaying fatigue impulse.
Every ride generates a Training Stress Score (TSS), a metric formalized by TrainingPeaks. TSS weights both workout duration and intensity relative to your Functional Threshold Power (FTP). One hour ridden steadily at 100% of FTP equals exactly 100 TSS.
How WattsUP calculates CTL, ATL and TSB
WattsUP models your daily fitness, fatigue, and form using exponential smoothing algorithms applied across every indoor smart trainer session and outdoor GPS file:
Chronic Training Load (CTL) — Your Long-Term Fitness
CTL is a 42-day exponentially weighted moving average (EWMA) of daily TSS. It represents your accumulated physiological engine size. A weekend recreational cyclist often maintains a CTL of 40–50, while competitive amateur racers reach 85–110 during peak season.
CTL(today) = CTL(yesterday) × e^(−1/42) + TSS(today) × (1 − e^(−1/42))
Acute Training Load (ATL) — Your Short-Term Fatigue
ATL uses the exact same exponential weighting equation but with a 7-day time constant. It responds rapidly to recent hard training blocks. A demanding weekend stage race will cause ATL to jump immediately, reflecting acute systemic fatigue.
ATL(today) = ATL(yesterday) × e^(−1/7) + TSS(today) × (1 − e^(−1/7))
Training Stress Balance (TSB) — Your Form and Freshness
TSB is simply yesterday’s CTL minus yesterday’s ATL. It indicates whether your body is primed to express its full aerobic fitness on race day:
TSB = CTL(yesterday) − ATL(yesterday)
Interpreting TSB: What do negative and positive training balance numbers mean?
A TSB between +5 and +25 indicates optimal race form where acute fatigue has cleared while aerobic fitness remains elevated. A TSB below -20 flags excessive fatigue and heightened risk of overtraining.
In my third season riding with a power meter, I thought dragging TSB below −35 during spring base miles was proof of toughness. By April, I was struggling up mild hills and caught two colds in three weeks. TSB isn’t a bravery contest — staying between −10 and −20 gives you 90% of the adaptation with none of the illness risk.
| TSB Range | Status | Recommended Action |
|---|---|---|
| > +25 | Detraining / Super-Fresh | Fitness is decaying; time to start the next training block |
| +5 to +25 | Optimal Race Form | Prime window for peak race day, Gran Fondo, or FTP breakthrough attempt |
| -10 to +5 | Neutral / Maintenance | Normal training zone for steady mid-season maintenance |
| -30 to -10 | Productive Overload | Typical range during hard 3-week build phases; high adaptation stimulus |
| < -30 | High Overtraining Risk | Immune suppression and injury danger; schedule an immediate rest week |
What is a safe weekly CTL ramp rate?
A safe weekly CTL ramp rate is 5 to 7 TSS points per week for trained cyclists. Exceeding 8 to 10 points per week drastically increases the incidence of illness and tendon injuries. Read our dedicated breakdown on safe CTL ramp rates to structure your macrocycles.
How to taper for race day peak form
A successful cycling taper reduces training volume by 40–60% over 10 to 14 days while maintaining high workout intensity. This allows ATL to drop sharply while CTL remains virtually intact, driving TSB into the positive sweet spot (+10 to +20).
Model your own taper curves in the WattsUP Training Load Modeler to eliminate guesswork before target events.
Frequently Asked Questions
What is Chronic Training Load (CTL)?
CTL is a 42-day exponentially weighted moving average of your daily TSS. It represents your aerobic engine size and physiological tolerance for sustained cycling volume.
What is a safe CTL ramp rate per week?
Most coaches recommend increasing CTL by 5–7 TSS points per week during a build phase. Ramp rates above 8–10 per week significantly elevate overtraining risk.
What TSB number indicates peak race form?
Cyclists typically perform best when TSB sits between +5 and +25 on race morning, reflecting fully recovered glycogen stores and cleared muscular inflammation.
Related Tools and Guides
12-Week Training Load Modeler
Simulate your future CTL, ATL, and TSB across build, taper, and race weeks interactively.
Open Modeler →CTL Ramp Rate Guide
Discover safe weekly build targets and how to avoid overtraining during intensive training blocks.
Read Ramp Rate Guide →FTP & Watts/kg Calculator
Calculate your benchmark threshold wattage and power-to-weight category in seconds.
Open Calculator →