The Banister Impulse-Response Model
In 1975, physiologist Eric Banister proposed a mathematical framework for modeling athletic performance as the balance between two opposing responses to training: a positive fitness effect and a negative fatigue effect. This became the impulse-response model, and it's the theoretical foundation under CTL, ATL, and TSB.
Every time you complete a training ride, you produce a Training Stress Score (TSS). TSS quantifies the dose — duration × intensity — of that session relative to your FTP. A 1-hour ride exactly at your FTP = 100 TSS. An easy 90-minute Z2 ride = ~55 TSS.
The Three Core Metrics
Chronic Training Load (CTL) — Your Fitness
CTL is a 42-day exponentially weighted moving average (EWMA)of your daily TSS. It's the slow, smoothed signal that represents your accumulated aerobic fitness. Think of it as your engine size. A recreational rider might have a CTL of 40–50. An elite amateur preparing for a Gran Fondo might sit at 80–100. Pro road racers often peak at 120–160+.
CTL(today) = CTL(yesterday) × e^(−1/42) + TSS(today) × (1 − e^(−1/42))
CTL(today) = CTL(yesterday) × e^(−1/42) + TSS(today) × (1 − e^(−1/42))
Acute Training Load (ATL) — Your Fatigue
ATL uses the same formula but with a 7-day time constant instead of 42. It responds much faster — a big training block in the last 7–10 days will spike your ATL sharply. ATL represents how much stress your body is currently carrying.
Training Stress Balance (TSB) — Your Form
TSB is the simple difference: TSB = CTL − ATL. When TSB is negative (your recent fatigue exceeds your fitness score), you're in a training hole — fatigued but building fitness. When TSB is positive, you're rested and fresh — ready to perform.
Interpreting TSB: What Do the Numbers Mean?
| TSB Range | Interpretation | Training Phase |
|---|---|---|
| +25 to +5 | 🟢 Optimal race form | Taper / Race week |
| +5 to -10 | 🟡 Transition / neutral | Recovery week |
| -10 to -25 | 🟠 Training load, manageable fatigue | Build phase |
| -25 to -40 | 🔴 Heavy training block | Peak overload block — short duration only |
| < -40 | ⚠️ Overtraining risk | Reduce load immediately |
CTL Ramp Rate: How Fast Is Too Fast?
The most common mistake in structured training is building CTL too aggressively. Most coaches recommend a safe ramp rate of 5–7 TSS/week during a build phase. At that rate, building from CTL 50 → 80 takes approximately 6 weeks.
Pushing above 8–10 TSS/week is possible for short overload blocks (2–3 weeks) followed by a recovery week, but sustained aggressive ramp rates lead to injury, illness, and form plateau.
Race Day Taper Strategy
The ideal taper preserves CTL (fitness) while eliminating fatigue (ATL), pushing TSB into the optimal +5 to +20 zone on race day.
- Start your taper 7–14 days before an A-priority event.
- Reduce volume by 30–50%, but maintain intensity (keep some Z4/Z5 efforts).
- Cutting both volume and intensity will let CTL drop too fast — TSB goes positive but fitness erodes.
- Target race-day TSB of +10 to +20 for most athletes.
Frequently Asked Questions
// 01No. 01What is a good CTL for a competitive amateur cyclist?
A recreational fit amateur targeting century rides typically builds CTL of 50–70. Competitive Cat 3–4 racers preparing for road races often aim for 80–110. Well-trained Cat 1–2 riders may peak at 120–140+ before a target event.
// 02No. 02Does TSB work without a power meter?
You can approximate TSS using heart rate-based Training Impulse (TRIMP) or perceived effort scores, but accuracy is lower. Power-based TSS from a meter or smart trainer is significantly more reliable for training load management.
// 03No. 03How does WattsUP display CTL/ATL/TSB?
WattsUP includes a live 12-week training load modeler with interactive CTL, ATL, and TSB curves, scenario selectors (Build / Taper / Overreaching), and the Sun Log 84-day activity heatmap. Try it free without an account.
Related Tools & Guides
12-Week Training Load Modeler
Visualize your CTL/ATL/TSB curves, test taper scenarios, and explore the WattsUP Sun Log heatmap — all in your browser for free.
Open the Modeler →CTL Ramp Rate: How Much Fitness?
Deep dive into safe ramp rates, overtraining warning signs, and how to structure 3:1 build-to-recovery week ratios.
Read the Guide →FTP & Power Zones Calculator
Without an accurate FTP, your TSS calculations are off, and so is your entire training load model. Start here.
Calculate My FTP →