What is CTL, ATL, and TSB? The Complete Guide to Training Load Analytics
If you have ever wondered how elite coaches decide when to push their athletes harder and when to back off, the answer often comes down to three numbers: CTL, ATL, and TSB. Together they form the fitness-fatigue model — one of the most powerful frameworks in endurance sports for managing training load and preventing overtraining.
The Fitness-Fatigue Model Explained
Every training session creates two competing effects. First, it builds fitness — your body adapts and becomes stronger, faster, or more efficient. Second, it generates fatigue — you are temporarily less capable until you recover. The fitness-fatigue model separates these two effects so coaches can manage them independently.
Dr. Eric Banister introduced this concept in the 1970s, and it has since become the foundation of modern periodization. Platforms like RaceLabs calculate these metrics automatically from your workout data, giving coaches and athletes real-time visibility into training readiness.
What is CTL (Chronic Training Load)?
CTL represents your long-term training load — think of it as your fitness bank account. It is an exponentially-weighted rolling average of your daily training stress over the past 42 days (approximately 6 weeks).
Key facts about CTL:
- A higher CTL means a larger fitness base and greater capacity for hard training
- CTL responds slowly to changes — it takes weeks of consistent training to move meaningfully
- Sudden spikes in training volume rarely increase CTL quickly but dramatically increase injury risk
- Elite marathon runners typically maintain CTL values between 100-150 TSS/day
- Recreational athletes usually sit between 40-70 TSS/day
How CTL is Calculated
CTL uses an exponential moving average with a time constant of 42 days. Each day's training stress score (TSS) is weighted, with more recent days having slightly more influence. The formula is:
CTL(today) = CTL(yesterday) + (TSS(today) - CTL(yesterday)) / 42
What is ATL (Acute Training Load)?
ATL represents your short-term training load — your recent fatigue. It is calculated identically to CTL but with a 7-day time constant instead of 42 days, making it far more responsive to recent training.
Key facts about ATL:
- ATL spikes after big training days or hard training blocks
- It drops quickly during rest days and recovery weeks
- A rising ATL means you are accumulating fatigue
- When ATL significantly exceeds CTL, you are in a fatigued state and risk overtraining
What is TSB (Training Stress Balance)?
TSB is simply CTL minus ATL. It represents the balance between your fitness and your fatigue — your form or readiness to perform.
- Positive TSB (+10 to +25): You are rested and ready to perform. This is the taper zone where athletes peak for races.
- Slightly negative TSB (-10 to 0): Normal training state. You are building fitness with manageable fatigue.
- Very negative TSB (-30 or below): Heavy fatigue. You are in a hard training block and need to plan recovery soon.
Practical Application: How Coaches Use These Numbers
Building a Base Phase
During base building, coaches aim for a gradual, steady increase in CTL — typically 3-7 TSS/day per week. ATL should stay within a manageable range, keeping TSB between -10 and -20. This ensures the athlete is training hard enough to build fitness without accumulating dangerous levels of fatigue.
Race Taper
Two to three weeks before a key race, coaches reduce volume to let ATL drop while CTL remains relatively stable. The goal is a positive TSB between +15 and +25 on race day — fresh legs with peak fitness.
Preventing Overtraining
If an athlete's TSB drops below -30 for more than a week, alarm bells should ring. This is where overtraining syndrome lurks. RaceLabs flags these danger zones automatically, alerting coaches before athletes cross the line.
Why Manual Tracking Falls Short
Calculating CTL, ATL, and TSB by hand requires logging every workout's training stress score and running exponential moving averages daily. For coaches managing multiple athletes, this is impractical. Platforms like RaceLabs automate this entirely — syncing workouts from Strava, Garmin, and WHOOP, calculating metrics in real time, and surfacing actionable insights without spreadsheets.
Getting Started with Training Load Analytics
Whether you are a coach managing a roster of athletes or a self-coached athlete looking to train smarter, understanding CTL, ATL, and TSB is a game-changer. The key principles are simple:
- Build CTL gradually — no more than 5-7 TSS/day per week
- Monitor TSB to prevent chronic fatigue (stay above -30)
- Taper for races by reducing ATL while maintaining CTL
- Use automated tools to track these metrics across all your athletes
Ready to see your training load data in action? RaceLabs calculates CTL, ATL, and TSB automatically for every athlete, with visual charts and smart alerts built in.