CTL, ATL, TSB Explained — Fitness, Fatigue and Form
If you use a training app like TrainingPeaks, Strava or Yama, you've certainly run into these three acronyms: CTL, ATL and TSB. They sit at the heart of nearly every serious training system — and yet hardly anyone really understands what they mean.
The good news: the concept behind them is surprisingly simple. It comes down to three questions every athlete knows: How fit am I? How tired am I? And am I in form right now? In this article I'll explain all three metrics from the ground up — no sports science degree required, but with the real math behind them.
CTL = your fitness (long-term, accumulated capacity). ATL = your fatigue (short-term tiredness from the last few days). TSB = your form (CTL minus ATL — fresh or tired?).
The foundation: what is TSS?
Before we get to the three metrics, we need a unit for training load. That unit is called TSS (Training Stress Score). It captures how demanding a single session was — from duration and intensity.
The idea: one hour at your threshold (FTP on the bike, threshold pace running) is defined as 100 TSS. An easy one-hour run might give 50 TSS. A hard two-hour interval session can produce 150 TSS or more.
Don't worry, you don't need to compute this by hand — every training app does it automatically. What matters is the principle: TSS is the currency of training load. Everything else builds on it. If you want to go deeper into how intensity is measured, our article on the FTP test helps.
CTL — your fitness
CTL stands for Chronic Training Load. In plain terms it simply means: your fitness.
CTL is a rolling average of your daily TSS over the last 42 days (6 weeks). The reasoning: fitness builds slowly and fades slowly. A single hard week doesn't make you permanently fit — but six weeks of consistent training do.
This is an exponentially weighted moving average. In practice the number 42 means: training from six weeks ago still counts, but far less than what you did yesterday. The higher your CTL, the more training load you can handle.
A well-trained recreational endurance athlete often has a CTL between 50 and 90. Pros reach over 150. More important than the absolute value, though, is the trend: if your CTL rises over weeks, you're getting fitter.
ATL — your fatigue
ATL stands for Acute Training Load — that is, your fatigue. It works exactly like CTL, just over a much shorter window: 7 days.
Because the window is so short, ATL reacts quickly. A hard training week drives your ATL up immediately. Two rest days bring it back down. So ATL reflects how much fatigue is currently sitting in your legs.
On its own, ATL says little. It only becomes meaningful in combination with CTL — and that's exactly what TSB is.
TSB — your form
TSB stands for Training Stress Balance and is the most important of the three numbers for race planning. It's defined as simply as can be:
Fitness minus fatigue = form
TSB answers the question: Am I fresh or tired? If your fatigue (ATL) is higher than your fitness (CTL), your TSB is negative — you're tired. If the fatigue has cleared while the fitness remains, TSB turns positive — you're fresh and in form.
What the TSB numbers mean
| TSB range | State | Meaning |
|---|---|---|
| +25 and above | Very fresh | Fitness may be decaying — rested too long |
| +5 to +25 | Race form | Ideal for racing — fresh and fit |
| −10 to +5 | Neutral | Normal training day-to-day |
| −30 to −10 | Productively tired | Training block — fitness being built |
| −30 and below | Overreached | Elevated injury and overtraining risk |
Good form doesn't come from rest alone — otherwise every couch potato would be in peak shape. It comes from high fitness (CTL) plus cleared fatigue (ATL). You build fitness over weeks, then reduce load just before the race so the fatigue disappears. That's called tapering.
A real-world example over 12 weeks
Let's say you're preparing for a half marathon. Here's how the three values might develop over a typical training cycle:
| Phase | CTL | ATL | TSB | Feeling |
|---|---|---|---|---|
| Week 1 (start) | 45 | 45 | 0 | Neutral |
| Week 4 (build) | 58 | 70 | −12 | Tired, productive |
| Week 8 (peak block) | 72 | 88 | −16 | Really cooked |
| Week 11 (taper) | 70 | 55 | +15 | Coming fresh |
| Week 12 (race day) | 68 | 48 | +20 | Peak form |
Note race week: CTL has only dropped slightly (from 72 to 68 — you've barely lost fitness), but ATL has fallen sharply (from 88 to 48 — the fatigue is gone). Exactly this combination produces the positive TSB of +20 and therefore race form.
The most common misconceptions
"High TSB is always good"
Wrong. A permanently high TSB means you're training too little — your fitness (CTL) is falling. TSB should only be high around races. In normal training a slightly negative value is completely right and even desirable.
"I need to keep raising my CTL"
Also risky. Too rapid a CTL increase (more than about 5–7 points per week) is considered a warning sign of overload. Pushing CTL up too fast earns you an injury sooner than a fitness gain — the body needs time to adapt to higher load.
"The numbers are directly comparable"
No. A CTL of 70 means something different for a runner than for a cyclist, because TSS accumulates differently per sport. Always compare your values only with yourself over time, never with other athletes.
CTL, ATL and TSB are only as good as your TSS — and TSS is only as good as your threshold values (FTP, threshold pace, threshold heart rate). If your FTP is outdated, all three metrics are skewed. Keep your thresholds current: you can calculate your FTP here, and how to determine the lactate threshold without a lab is covered in our article on the lactate threshold.
Why these metrics aren't everything
CTL/ATL/TSB are powerful, but they have one weakness: they're distance-agnostic. A positive TSB tells you you're fresh — but not whether you have the specific endurance for your target distance. For a marathon, what counts isn't just freshness but also whether you have enough long runs in your legs.
That's exactly the gap Race Shape fills — a metric that additionally accounts for volume and distance specificity. How that works is explained in our article on Race Shape.
Yama shows all three live
Connect Strava with Yama — CTL, ATL and TSB are computed automatically from your activities and displayed as a performance management chart. See at a glance when you're coming into form and when you need rest.
Try YamaConclusion
Three acronyms, one simple principle: CTL is your fitness (slow, 42 days), ATL is your fatigue (fast, 7 days), and TSB is the difference — your form.
Anyone who understands these three can steer their training deliberately: ramp up fitness during build, shed fatigue before the race, and spot overload in time. The concept is simple — the discipline to stick to the plan even in the tired phase of a training block is the harder part.
One last bit of perspective: CTL, ATL and TSB are a model, not a law of nature. They reflect how much you've trained — not how well you've slept, eaten or recovered. Take the numbers seriously, but not as the whole truth.