Training Load Management: A Plain-Language Guide to CTL, ATL, and TSB

Introduction

When you open Garmin Connect or TrainingPeaks, you may have seen the “Training Status” chart, which displays three numbers: CTL (Fitness), ATL (Fatigue), and TSB (Form). Many runners find these abbreviations complicated and simply ignore them. But this system is actually very intuitive in its logic—once you understand the three core concepts, you can use it to manage training cycles and predict the optimal time for peak race performance.

A Plain-Language Explanation of the Three Core Concepts

CTL (Chronic Training Load) = Long-Term Fitness

CTL is the weighted average of your training load over the past 42 days. Think of it as “how thick your current fitness base is.” The higher your CTL, the more training you’ve accumulated over the long term, and the stronger your aerobic engine.

• CTL 0–50: Beginner runner, aerobic base still being built
• CTL 50–80: Intermediate runner, can steadily complete half-marathon preparation
• CTL 80–100+: Advanced runner, at the peak of full-marathon preparation

Characteristics of CTL: Rises slowly (requires weeks of consistent training), declines slowly (only drops noticeably after two weeks off).

ATL (Acute Training Load) = Short-Term Fatigue

ATL is the weighted average of your training load over the past 7 days. Think of it as “how tired you are right now.” The day after a long run, ATL rises noticeably; after several consecutive rest days, ATL drops quickly.

• High ATL = recent training volume is high, fatigue is accumulating
• Low ATL = recent training volume is low, body is relatively fresh

TSB (Training Stress Balance) = Current Status

TSB = CTL minus ATL, i.e., “fitness” minus “fatigue,” representing your actual current competitive state.

Training Periodization and Its Relationship with TSB

Build Phase

During the build phase, CTL rises steadily, ATL also rises, and TSB stays between -10 and -25. This is normal “functional fatigue”—the body is receiving stimuli and preparing for supercompensation.

Recovery Week

Every four weeks, schedule a deload week, reducing mileage by 30–40%. ATL drops rapidly, and TSB rebounds from negative values back toward 0. During this time, the body undergoes supercompensation, while CTL effectively remains at a high level.

Taper

In the 2–3 weeks before a race, deliberately reduce training volume significantly. ATL drops sharply, and TSB climbs toward positive values. The goal is to have TSB land between +5 and +20 on race day—enough fitness (high CTL) without fatigue (low ATL).

How to Calculate and Track?

Tool Options

• Garmin Connect: Automatically tracked with a Garmin watch; view it on the “Training Status” page
• TrainingPeaks (free version): Manually enter or sync from Strava to see full CTL/ATL/TSB charts
• Strava Premium: Provides a “Training Load” chart, using Relative Effort as a substitute for TSS

Calculating TSS (Training Stress Score)

TSS is the “stress score” for each workout and determines the values of CTL and ATL. For runners, it can be simplified as:

• Easy run, 1 hour ≈ TSS 50–55
• Tempo run, 40 minutes ≈ TSS 55–65
• Interval training, 50 minutes ≈ TSS 80–100
• Long run (3 hours easy) ≈ TSS 150–180

Practical Advice

• Don’t be a slave to the numbers: CTL/ATL/TSB are supporting tools; your perceived fatigue (subjective feeling) is the most important reference
• Two weeks before a race, aim for TSB +5 to +15: Too high (above +20) means you’ve tapered too much and fitness may be lost; too low (negative) means fatigue hasn’t been cleared
• The reality for amateur runners in Taiwan: Work stress and life fatigue are also part of “fatigue,” but they aren’t factored into ATL. When you feel especially tired, even if the numbers say you’re fine, respect your body

Conclusion

CTL, ATL, and TSB present abstract training status in numerical form, making the logic of periodized training clearer. Remember this formula: High CTL + Low ATL = High TSB = Peak Race Condition. Structuring your training cycle within this framework is about making race day the day your body is “most ready” across the entire preparation period.

Related Reading

• Training Load and Fatigue Management: Applications of CTL, ATL, and TSB
• The CTL, ATL, and TSB Fitness Status Model: Tracking Training Status with Data
• Training Load Management: Daily Applications of TSS, ATL, CTL, and TSB
• Training Load Metrics: A Complete Guide to TSS, IF, and CTL