TSS, CTL, ATL: A Complete Guide to the Three Key Training Load Metrics

Introduction: Why You Need to Understand Training Load

If you’ve ever looked at a bunch of numbers on TrainingPeaks, Strava, or any training platform and felt completely lost, this article is for you. TSS, CTL, and ATL are the cornerstones of modern scientific cycling training. Understanding them not only helps you quantify the stress of every ride but also empowers you to make informed decisions in your long-term training plan.

TSS: Training Stress Score

Definition and Calculation

TSS is a metric proposed by Dr. Andrew Coggan to quantify the physiological stress of a single training session. The calculation formula is:

TSS = (Training Time (seconds) × NP × IF) / (FTP × 3600) × 100

Where:

• NP (Normalized Power): Normalized Power
• IF (Intensity Factor): Intensity Factor = NP / FTP
• FTP (Functional Threshold Power): Functional Threshold Power

Intuitive Understanding

A simple reference point: riding steadily at FTP intensity for one hour yields a TSS of exactly 100. This means:

Practical Application

The greatest value of TSS lies in comparing across training types. Whether it’s short, high-intensity intervals or long, low-intensity endurance rides, TSS converts them all into the same yardstick for measurement. This allows you to allocate different types of training sensibly within your weekly plan.

Important Note: TSS has its limitations. It assumes your FTP setting is accurate—if your FTP is set too high, TSS will be underestimated; conversely, it will be overestimated. It is recommended to retest your FTP every 4-6 weeks.

CTL: Chronic Training Load

Definition and Calculation

CTL represents your long-term training volume, typically calculated using a 42-day exponentially weighted moving average:

CTL(today) = CTL(yesterday) + (TSS(today) - CTL(yesterday)) / 42

Intuitive Understanding

CTL can be viewed as your “fitness bank account.” It reflects the accumulated training adaptations from roughly the past six weeks. The higher your CTL, the more training volume your body has adapted to, and theoretically, the better your fitness level.

Typical Value Ranges

Key Concept: Rate of CTL Increase

A critical principle that is often overlooked: CTL should not increase by more than 5-7 points per week. A rapid CTL increase means a sharp spike in training volume, which significantly raises the risk of overtraining or injury.

ATL: Acute Training Load

Definition and Calculation

ATL represents your short-term training load, calculated using a 7-day exponentially weighted moving average:

ATL(today) = ATL(yesterday) + (TSS(today) - ATL(yesterday)) / 7

Intuitive Understanding

ATL represents your “fatigue level” from the most recent week. When you complete intense training, ATL rises quickly; when you rest, ATL drops rapidly.

TSB: Training Stress Balance

With CTL and ATL, we can calculate the most practically valuable metric:

TSB = CTL - ATL

Interpreting TSB

Real-World Example: Pre-Race Tapering

Suppose you are preparing for an important race. A typical tapering strategy is as follows:

1. 3-4 weeks before the race: Maintain high CTL, with TSB around -10 to -20
2. 2 weeks before the race: Begin tapering, reduce TSS by 30-40%
3. 1 week before the race: Continue tapering, reduce TSS by 50-60%
4. Race day: Target TSB between +10 and +20

This strategy ensures your fitness (CTL) stays at a high level while fatigue (ATL) has largely subsided, putting you in optimal racing condition.

Common Mistakes and Caveats

1. Over-Reliance on Numbers

TSS/CTL/ATL are excellent tools, but they cannot fully capture all physiological stress. Sleep quality, work stress, nutritional status, and other factors all affect your actual recovery.

2. Ignoring FTP Updates

If your FTP has improved but you haven’t updated the setting, all TSS-based metrics will be inaccurate.

3. Focusing on Numbers Rather Than Trends

More important than a single day’s TSB value is observing the trend of these metrics. A steadily rising CTL paired with periodic recovery in ATL is far more meaningful than chasing a specific number.

Conclusion

Mastering the three key metrics—TSS, CTL, and ATL—gives you the foundational language of scientific training. But remember, these numbers are tools that serve you, not masters you obey. By combining subjective feelings with objective data, you can forge the training path that best suits you.

Related Reading

• TSS, CTL, ATL, TSB Complete Guide: Calculation Principles and Applications of Training Stress Indices
• Power Data Analysis: Interpreting Training Stress Score (TSS) and Chronic Training Load (CTL)
• Training Load and Fatigue Management: Applying CTL, ATL, and TSB
• Training Load Management: Daily Applications of TSS, ATL, CTL, and TSB