Fatigue, Form, and the CTL/ATL/TSB Model Explained
Understanding why you feel flat on some days and sharp on others is one of the most valuable skills in endurance training. The CTL/ATL/TSB model gives that feeling a mathematical structure — one you can track, manipulate, and plan around.
The Foundation: Training Stress Score
Before CTL, ATL, or TSB make sense, you need one input: a consistent measure of how hard each ride is. Training Stress Score (TSS) serves that role. Every ride produces a TSS value that accounts for both intensity and duration. A gentle ninety-minute spin scores far lower than a hard sixty-minute threshold effort, even though the clock time difference is small.
TSS is calculated from your power output relative to FTP. If you don't train with power, heart-rate-derived TSS or pace-based equivalents can approximate it, but the model is most reliable with power data.
With a daily TSS figure in hand, you can feed the three derived metrics.
CTL: Chronic Training Load (Your Fitness)
Chronic Training Load is a rolling, exponentially weighted average of your daily TSS, calculated over a long window — typically equivalent to a six-week time constant. In plain terms: CTL is your fitness. It rises when you train consistently and falls when you rest or reduce volume.
CTL moves slowly by design. You cannot meaningfully raise it in a week. It represents the accumulated adaptation from weeks and months of work — aerobic capacity, muscular endurance, metabolic efficiency. When athletes talk about "building a base," they are talking about raising CTL.
What CTL tells you
- Your current aerobic fitness relative to your own history
- Whether a training block is actually accumulating load or just maintaining it
- How much fitness you stand to lose during a taper or illness
CTL does not tell you how you feel today. It is a trailing indicator.
ATL: Acute Training Load (Your Fatigue)
Acute Training Load uses the same exponential averaging logic but over a short window — typically equivalent to a one-week time constant. ATL tracks recent training stress. After a hard week, ATL spikes. After a few easy days, it drops quickly.
ATL is your fatigue signal. It responds to what you have done in the last several days, not the last several months. A single brutal block — a stage race, a training camp, a week of back-to-back long rides — will send ATL well above CTL.
What ATL tells you
- How fatigued you are right now from recent training
- Whether you are recovering between blocks or accumulating unresolved fatigue
- When you might be approaching overreaching territory
TSB: Training Stress Balance (Your Form)
Training Stress Balance is the simplest calculation in the model:
TSB = CTL − ATL
TSB is form. It is the difference between your long-term fitness and your current fatigue state. A positive TSB means fatigue is lower than fitness — you are rested and ready to express the fitness you have built. A negative TSB means fatigue is masking your fitness.
This is the number that explains why a well-trained athlete coming off a hard training block feels terrible in a race, while the same athlete a week later — after a structured taper — rides the best of their season. The fitness (CTL) barely changed. The fatigue (ATL) dropped. TSB turned positive, and form emerged.
Reading TSB values
| TSB range | What it typically means |
|---|---|
| Deeply negative | Heavy training load; fitness building but performance suppressed |
| Mildly negative | Moderate fatigue; sustainable training state |
| Near zero | Transition between loading and recovery |
| Mildly positive | Good form; ready to race or test |
| Highly positive | Well-rested but fitness may be declining if sustained too long |
The right TSB for a target event depends on the athlete and the event type. A short punchy race may call for a higher positive TSB than a long ultra-endurance event, where some residual fatigue is tolerable.
How the Three Metrics Interact
The model is most useful when you read all three numbers together, not in isolation.
Building phase: CTL rising, ATL elevated, TSB negative. This is what a productive training block looks like. You are accumulating fatigue in service of fitness.
Overreaching risk: ATL far exceeding CTL, TSB deeply negative for extended periods. Sustainable for short planned blocks; dangerous if unintentional or prolonged.
Taper: ATL falling faster than CTL. TSB moves toward positive. Fitness is largely preserved while fatigue dissipates. This is the mechanism behind why tapering works.
Detraining: Both CTL and ATL falling, TSB positive. You feel good but fitness is eroding. Extended periods here cost more than they appear to.
Practical Application
Planning a training block
Define your target event date and work backward. Identify how many weeks you have for loading, then how long your taper needs to be to bring TSB to the range that works for you. Track CTL week by week to confirm the block is actually raising fitness and not just maintaining it.
Recognizing accumulated fatigue
If TSB has been deeply negative for several weeks and performance in training is declining rather than improving, the load is likely too high. Planned recovery weeks — where ATL drops and TSB briefly recovers — are not laziness; they are the mechanism by which CTL gains become permanent rather than temporary stress responses.
Avoiding the freshness trap
A high positive TSB feels good. But if ATL has been very low for several weeks, CTL is declining alongside it. Athletes sometimes confuse feeling fresh with being fit. The model makes that distinction visible: check whether your CTL is holding steady or dropping during a period of good feelings.
Testing and calibration
The model is only as reliable as the TSS values feeding it. That means your FTP estimate must be current. An FTP that is set too low understates the stress of every hard ride, making your CTL appear lower than your actual fitness and distorting TSB. Retest FTP after significant training blocks and update it before interpreting the model's outputs.
Common Misunderstandings
TSB must be positive to train hard. False. Most productive training happens at negative TSB. Positive TSB is for racing and testing, not for generating adaptation.
Higher CTL is always better. CTL without adequate recovery translates to injury, illness, or burnout. The ceiling for sustainable CTL is highly individual and rises gradually over years, not weeks.
The model predicts performance. It does not. It quantifies training load and estimates readiness, but external factors — sleep, stress, nutrition, illness — affect how you actually perform on a given day. Use TSB as one input among several, not as a race-day guarantee.
Integrating the Model Into Your Training
The CTL/ATL/TSB model is covered in depth across the Reading Your Ride Data hub, including how to structure blocks around it and how to interpret your numbers after specific ride types. Start by establishing a baseline — two to three months of consistent TSS logging — before placing too much confidence in your CTL figure. The model rewards patience: the longer your consistent data history, the more meaningful the numbers become.