The Complete Guide to TSS, CTL, ATL, and TSB: How Training Stress Scores Are Calculated and Applied

Introduction

Many cyclists who buy a power meter are first confused not by “how to train,” but by “what do all these numbers mean?” The four metrics—TSS (Training Stress Score), CTL (Chronic Training Load), ATL (Acute Training Load), and TSB (Training Stress Balance)—form the “fitness management dashboard” of power-based training and are the core analytical tools of platforms like TrainingPeaks.

This article starts with the calculation principles to help Taiwanese cyclists truly understand the meaning of these numbers, rather than just staring at the line charts on the platform and guessing their training status.

TSS: The Stress Score of a Single Workout

The TSS calculation formula is:

TSS = (seconds × Normalized Power × Intensity Factor) ÷ (FTP × 3600) × 100

A more intuitive way to understand it: riding at FTP power for a full hour gives a TSS of 100. Riding in Z2 (about 65% FTP) for 2 hours gives a TSS of roughly 70–80. Doing 1 hour of Z5 interval training (including recovery) can yield a TSS of 100–120.

The greatest value of TSS is that the stress from different workout types can be quantified on a unified scale, allowing you to directly compare whether “2 hours of Z2 today” or “1 hour of threshold yesterday” places a heavier load on your body.

CTL: Your “Fitness Accumulation Index”

CTL (Chronic Training Load) is the exponentially weighted average of TSS over the past 42 days. It reflects the cumulative level of long-term training adaptation—in plain terms, “your current aerobic fitness level.”

• CTL rising = fitness improving
• CTL steady = maintaining current fitness level
• CTL falling = fitness declining (e.g., pre-race taper period, injury layoff)

Amateur cyclists typically have a CTL between 50–80; professional riders can reach 120–180. CTL should not rise too quickly—the general recommendation is to increase it by no more than 3–7 TSS points per week, otherwise the risk of overtraining increases significantly.

ATL: Short-Term Fatigue Index

ATL (Acute Training Load) is the exponentially weighted average of TSS over the past 7 days, reflecting the level of fatigue accumulated from recent training. A rising ATL means you have been training hard lately and fatigue is building up; a falling ATL means you are resting and fatigue is subsiding.

TSB: The “Form Index” of Training Status

TSB (Training Stress Balance) = CTL - ATL, sometimes also called “Form.”

The pre-race taper strategy is to start reducing training volume 7–14 days before the race (lowering ATL), allowing TSB to recover from negative values into the +10 to +20 “optimal form zone,” while keeping a few high-intensity efforts to prevent CTL from dropping too fast.

Common Application Scenarios for Taiwanese Cyclists

• Tapering before Wuling: Start reducing volume 2 weeks before the East Route Wuling climb, aiming for a TSB close to +15 on race day.
• Post-race cycle management: After a major event, ATL spikes; spend the next 1–2 weeks riding in Z1/Z2 to bring TSB back to positive before starting the next training cycle.

Practical Advice

1. Do a weekly “dashboard review”: Check the CTL trend, whether ATL is too high, and whether TSB matches the expected values for that week’s training plan.
2. Don’t blindly trust the numbers: TSS is an estimate—if your FTP is inaccurate, all metrics will be skewed; how your body feels remains the most important fatigue indicator.
3. Higher CTL is not always better: For amateur cyclists, maintaining a sustainable CTL (around 60–80) has more long-term value than chasing an extremely high CTL.
4. Free tools for tracking these metrics include Strava’s Fitness & Freshness chart (requires premium subscription) or intervals.icu (free).

Conclusion

The four metrics—TSS/CTL/ATL/TSB—form a complete language for managing training stress. From understanding the TSS of a single workout, to grasping the long-term fitness accumulation of CTL, to using TSB to plan pre-race tapering—this toolkit turns training from a guessing game based on intuition into data-driven scientific decision-making.

Related Reading

• Power Data Analysis: Interpreting Training Stress Score (TSS) and Chronic Training Load (CTL)
• Training Load and Fatigue Management: Applications of CTL, ATL, and TSB
• Training Load Management: Daily Applications of TSS, ATL, CTL, and TSB
• Complete Analysis of the Three Major Training Load Metrics: TSS, CTL, and ATL