Power Data Analysis: Understanding Training Stress Score (TSS) and Chronic Training Load (CTL)

Introduction

“I rode for 3 hours today and feel really tired, but I don’t know if it’s just the right amount of fatigue or too much.” This is a common confusion among many Taiwanese cyclists. A power meter not only tells you how many watts you’re producing at any given moment, but also allows you to understand the accumulation of your training load through data analysis tools. Among these, Training Stress Score (TSS) and Chronic Training Load (CTL) are the two most essential metrics for understanding your personal fitness status.

TSS: Quantifying the Stress of a Single Workout

TSS was developed by renowned exercise scientist Andrew Coggan, and the calculation formula is as follows:

TSS = (Training Seconds × Normalized Power × Intensity Factor) / (FTP × 3600) × 100

A more intuitive way to understand it:

• Riding at FTP for 1 hour = 100 TSS
• The higher the intensity, the faster TSS accumulates
• In the same amount of time, a Zone 4 threshold ride produces far more TSS than an easy Zone 2 ride

Reference TSS Values for Different Training Types

CTL: Long-Term Tracking of Fitness

CTL (Chronic Training Load) is the exponentially weighted moving average of TSS over the past 42 days, representing your current aerobic fitness base level. The higher the value, the greater your training volume and the better your fitness.

• CTL < 50: Low training volume, suitable for beginners or those in recovery
• CTL 50–80: Intermediate amateur level, with consistent training habits
• CTL 80–110: High-volume amateur / semi-professional level
• CTL > 120: Professional or pro-level cyclist standard

ATL and TSB: Fatigue and Race Readiness

To fully understand your training status, you also need to know two other metrics:

• ATL (Acute Training Load): The exponentially weighted average of TSS over the past 7 days, representing your current fatigue level
• TSB (Training Stress Balance) = CTL - ATL, representing your current race readiness

Interpreting TSB

• TSB = +10 to +25: Peak race readiness, ideal for racing
• TSB = -10 to 0: Training in progress, slightly fatigued but fitness is rising
• TSB < -20: Fatigue has accumulated too deeply, risk of overtraining increases
• TSB > +30: Training volume is too low, fitness may decline

Practical Application for Taiwanese Cyclists

Sample CTL Plan for Preparing for the Wuling Mountain Climb Challenge:

Assume the target event is 12 weeks away, and you want to arrive at the start line in peak condition:

1. Weeks 1–8 (Build Phase): Steadily increase CTL each week, aiming to raise it from 50 to 80
2. Weeks 9–11 (Overload Phase): Maintain or slightly increase CTL, allow ATL to rise relatively higher, and let TSB turn negative
3. Week 12 (Taper Phase): Dramatically reduce ATL to bring TSB back up to +10 to +20, while CTL only drops slightly
4. Event Day: High CTL maintained + positive TSB = the optimal race combination

Practical Recommendations

1. Use TrainingPeaks or Intervals.icu (free) to track the PMC chart (Performance Management Chart): Visualize CTL, ATL, and TSB trends
2. Do not let CTL rise more than 5 TSS/day per week: Rapidly stacking training volume is a leading cause of injury
3. Start tapering 7–10 days before a key event: Keep 1–2 sessions to maintain stimulus, allowing TSB to naturally recover
4. Compare the relationship between FTP and TSS: If TSS is the same for each workout but your power output keeps increasing, it means your fitness is genuinely improving
5. Winter training advice for Taiwan: During the northeast monsoon season, use indoor workouts to maintain CTL so you don’t lose your fitness base during the rainy period

Conclusion

TSS and CTL elevate training from subjective feelings to the level of objective data management. For Taiwanese cyclists, learning to read these numbers isn’t just about making training more scientific—it’s about knowing when to push forward and when to rest. Data is the map of training, FTP is the milestone, and your fitness progress is the most beautiful journey on that map.

Related Reading

• The Complete Guide to TSS, CTL, ATL, and TSB: The Calculation Principles and Applications of Training Stress Metrics
• TSS, CTL, ATL: A Complete Breakdown of the Three Key Training Load Metrics
• Training Load and Fatigue Management: Applying CTL, ATL, and TSB
• The Relationship Between Chronic Training Load (CTL) Tracked by Power Meters and Fatigue: A TrainingPeaks Data Study