CTL, ATL, and TSB are three metrics that describe where you are in the fitness-fatigue cycle at any given point in your training. Originally developed for cycling by Andrew Coggan, they've become essential tools in performance running — and they're the reason adaptive training plans can respond to your actual state rather than following a static schedule.
If you've ever seen these abbreviations in TrainingPeaks or wondered what "form" means in a training app, this is the complete explanation.
The core idea: fitness is built slowly, fatigue accumulates fast
Training stress is not uniform. A hard interval session creates acute fatigue that fades within 48-72 hours. But the fitness gained from that session takes weeks to build and months to decay. These two dynamics operate on different timescales — and CTL, ATL, and TSB measure each of them.
What is CTL (Chronic Training Load)?
Chronic Training Load (CTL) is a rolling average of your training stress over approximately 42 days. It represents your fitness — the accumulated aerobic capacity you've built through consistent training.
CTL is calculated as an exponentially weighted moving average of your daily Training Stress Score (TSS). Recent training days count more than older ones, but all 42 days contribute.
What CTL tells you:
- A rising CTL means you're building fitness
- A stable CTL means you're maintaining current fitness
- A falling CTL means fitness is declining (you're detraining or tapering)
Typical ranges for runners:
| Training Level | Typical CTL |
|---|---|
| Recreational (40-50km/week) | 40–60 |
| Intermediate (60-80km/week) | 60–80 |
| Advanced (80-110km/week) | 80–110 |
| Elite marathon runners | 110–140+ |
CTL is not a competition. The goal is to build your CTL progressively without creating excess fatigue — not to hit a specific number.
What is ATL (Acute Training Load)?
Acute Training Load (ATL) is a rolling average of your training stress over approximately 7 days. It represents your fatigue — how much stress you've accumulated recently.
Because ATL uses a shorter window than CTL, it reacts quickly to changes in training volume and intensity. A hard week sends ATL up fast. A rest day brings it down within 24 hours.
What ATL tells you:
- A high ATL means you're currently under high training stress (fatigued)
- A falling ATL means you're recovering
- A very low ATL means you're fresh — possibly too fresh if it stays low for weeks
ATL naturally peaks during your build phase and drops sharply during taper as you reduce volume before your race.
What is TSB (Training Stress Balance)?
Training Stress Balance (TSB) is the difference between fitness and fatigue:
TSB = CTL − ATL
TSB is also called "form" in many training platforms. It tells you how ready you are to perform right now.
Reading your TSB
| TSB Range | Interpretation |
|---|---|
| −30 or below | Overreaching. High injury risk. Performance will decline. |
| −10 to −30 | Heavy training load. Normal during build phase. |
| −5 to +5 | Moderate freshness. Day-to-day training. |
| +5 to +20 | Positive form. Tapering or peaking. Good race window. |
| +20 or above | Very fresh. Detraining risk if sustained too long. |
The goal on race day is a TSB of approximately +10 to +20 — meaning you've built fitness (high CTL) and shed recent fatigue (ATL has dropped during taper).
Training Stress Score (TSS): the input to all three metrics
CTL, ATL, and TSB are all derived from Training Stress Score (TSS) — a per-workout number that quantifies how much physiological stress a session created.
TSS was originally defined using power output for cycling. For running, it's calculated using a combination of duration, intensity (pace relative to threshold), and heart rate:
TSS ≈ (duration in hours) × (intensity factor)² × 100
Where intensity factor is how hard you worked relative to your lactate threshold pace. An easy 1-hour run might score TSS 50. A hard 90-minute threshold session might score TSS 140.
Key points about TSS:
- Easy long runs contribute high TSS due to duration, moderate intensity
- Interval sessions contribute high TSS due to high intensity, shorter duration
- Rest days score TSS 0 — and this is where recovery happens
How these metrics work together in a training plan
The relationship between CTL, ATL, and TSB over a training block follows a predictable shape:
Base phase (weeks 1-8): ATL rises as training volume increases. CTL lags behind but rises steadily. TSB is often negative (−10 to −20). This is normal — you're building fitness through productive fatigue.
Build phase (weeks 9-14): CTL continues rising. Intensity increases, so individual TSS scores are higher. ATL spikes after hard weeks and recovers during easy weeks. TSB oscillates between negative and near-zero.
Peak phase (weeks 15-16): Maximum CTL. ATL is high. TSB is deeply negative. This is the hardest training of the cycle — and the point where most runners feel terrible during easy runs. That feeling is expected, not a sign of overtraining.
Taper phase (final 2-3 weeks): Volume drops sharply. ATL falls quickly. CTL declines slightly (unavoidable). TSB rises toward positive territory. Legs feel "springy" again. This is the adaptation window.
Race day target: High CTL (fitness built over months) + low ATL (recent fatigue shed during taper) = positive TSB = peak performance.
Why static 12-week plans get this wrong
A static training plan gives every runner the same taper, the same hard weeks, and the same rest days — regardless of where their CTL, ATL, and TSB actually sit.
A runner who missed two weeks due to illness will hit the final peak training week with a lower CTL than intended. Following the static plan anyway — with the same hard sessions — will produce a negative TSB that's too low to recover from by race day.
Adaptive plans read these metrics and respond. If you missed training, ATL is lower than expected, and the plan rebuilds volume more conservatively. If you had a particularly hard week, it may pull back the following session intensity automatically.
This is why Zarkus recalculates your plan every week based on your actual training data — not what the spreadsheet assumed you would do.
What to track
You don't need to obsess over these numbers daily. But checking them weekly gives you useful signal:
- CTL trend — Is fitness building at a sustainable rate? (Aim for +3-7 CTL/week during base phase)
- ATL vs CTL — Is ATL consistently above CTL? That's a red flag for overreaching.
- TSB before key sessions — For your most important workouts (long runs, tempo sessions), a TSB of −5 or better means you'll perform well. Below −20, consider adjusting intensity.
- TSB at race day — If you're within 2 weeks of your race and TSB is still deeply negative, your taper may be insufficient.
The limits of CTL, ATL, and TSB
These metrics are powerful tools, not oracle systems. They have real limitations:
They don't capture everything. Sleep quality, nutrition, life stress, and illness all affect your readiness in ways TSS can't measure. A runner with TSB +15 who slept 4 hours will not race well.
Accuracy depends on honest TSS logging. If you regularly under-log easy runs or skip uploading activities, your CTL will be artificially low and your plan will be calibrated incorrectly.
They're derived from threshold estimates. Your TSS calculation depends on knowing your lactate threshold pace accurately. An incorrect threshold estimate produces systematically wrong TSS values — and therefore wrong CTL, ATL, and TSB.
Use these metrics as one input in a broader picture, alongside how you feel, your sleep, and your RPE on key sessions.
CTL, ATL, and TSB take the guesswork out of training load management. They explain why you feel flat two weeks before a race (good — your CTL is high), why easy runs should feel easy even when you're fit (ATL management), and why taper tantrums are normal (ATL drops faster than CTL can respond).
Zarkus calculates all three metrics from your Garmin data and uses them to decide when to push, when to pull back, and when you're ready to race.
Frequently Asked Questions
What is CTL in running training?
CTL (Chronic Training Load) is a rolling 42-day average of your training stress that represents your current fitness level. A rising CTL means you're building aerobic capacity; a falling CTL means fitness is declining. Intermediate marathon runners typically have a CTL of 60–80, while elite marathon runners reach CTL values of 110–140.
What does TSB tell you about race readiness?
TSB (Training Stress Balance) equals CTL minus ATL — it measures the balance between your fitness and your current fatigue. A TSB of +10 to +20 on race day indicates peak readiness: you've built fitness through months of training and shed recent fatigue during the taper. A negative TSB on race day means you're still carrying fatigue and will likely underperform relative to your fitness.
What is a good CTL for a marathon runner?
For recreational marathon runners training 40–50 km/week, a CTL of 40–60 is typical. Intermediate runners at 60–80 km/week usually reach CTL 60–80. Advanced runners at 80–110 km/week reach CTL 80–110. The goal is to build your CTL progressively at approximately 3–7 points per week during base phase — not to hit a specific number.