CTL, ATL and TSB: Understanding the Performance Curve

Fitness alone wins no races — what matters is how freshly you can access that fitness at the start line. That is exactly what the fitness-fatigue model captures with its three metrics CTL, ATL and TSB. Once you understand the performance curve, you stop training by feel — and start timing your form precisely for race day.

The model behind it: fitness minus fatigue

The model goes back to physiologist Eric Banister and rests on a simple observation: every training session has two effects. It makes you fitter — and it makes you tired. Both effects decay at different speeds: fatigue fades within days, fitness gains last for weeks. Your performance on day X is therefore not your fitness, but your fitness minus your current fatigue.

This yields the three metrics found in almost every modern training platform — from Peakora to the well-known analysis tools:

• CTL (Chronic Training Load) — long-term load, commonly called "fitness". An exponentially weighted average of your training load over the last 42 days.
• ATL (Acute Training Load) — short-term load, commonly called "fatigue". The same average, but over just 7 days.
• TSB (Training Stress Balance) — the difference between CTL and ATL, commonly called "freshness" or "form".

The basis for all three values is your daily training load — ideally measured as Training Stress Score (TSS), which condenses the duration and intensity of every session into a single number.

CTL: your fitness — the 42-day memory

CTL answers the question: "How much training is in my legs?" It is an exponentially weighted average of your daily load over 42 days — recent weeks count more than training from a month ago. A CTL of 80 means you have recently averaged 80 TSS per day, roughly one hour of moderate training daily.

Three properties of CTL matter in practice:

• It rises slowly. A single monster weekend barely moves it. Sustainable build means 3 to 5 CTL points per month, not per week.
• It falls slowly. The half-life is about 42 days. After two weeks without training you lose roughly 20 percent — reassuring during forced breaks, but a warning against months of inconsistency.
• It says nothing about freshness. A high CTL on race day is useless if fatigue completely masks it. That is what TSB is for.

ATL: your fatigue — the 7-day memory

ATL mirrors the load of the last 7 days and reacts much faster: three hard sessions in a row and ATL shoots up; two rest days and it drops noticeably. Exactly this quick response makes it the steering instrument for race week — during tapering you deliberately lower ATL while CTL stays nearly stable. The gap widens, freshness returns.

Read it correctly: a high ATL is not a badge of honour. Anyone who proudly trains at constant high fatigue is mostly collecting exhaustion — adaptation only happens during recovery; the keyword is supercompensation.

TSB: the difference that decides your form

TSB = CTL minus ATL. An example: CTL 80, ATL 110 gives TSB minus 30 — you are fit but tired. After ten days of tapering: CTL 76, ATL 62, TSB plus 14 — almost the same fitness, but fully recovered. That is form: the coincidence of high fitness and low fatigue at the right moment.

TSB zones in practice

• Above plus 25: Very fresh — often too fresh. If you sit here, you have usually rested too long; fitness starts to crumble. Typical after longer injury breaks.
• Plus 5 to plus 15: The target corridor for your A-race day. Fresh with maximum available fitness.
• Minus 10 to plus 5: The neutral zone — normal training weeks where building and recovery are in balance.
• Minus 10 to minus 30: Productive loading block. This is where the training stimulus lives — but nobody should race here.
• Below minus 30: Red zone. Tolerable for a few days at a training camp; over weeks it is a direct route towards overtraining.

Worked example: one training week on the curve

Starting point: CTL 75, ATL 85 — TSB minus 10, a solid building week. On Saturday a long run worth 180 TSS is scheduled, on Sunday 60 easy minutes worth 50 TSS. Over the weekend ATL climbs to about 100 and TSB drops towards minus 25: heavy legs on Monday are no coincidence but the model at work. Follow it with a recovery week at 40 percent less volume, and ATL sinks below CTL within five days — the next weekend you line up for the next quality session at TSB plus 5, fresh and stronger. This wave pattern of loading and unloading is exactly the logic that the ACWR monitors from a different angle: it checks whether the short-term load sits in a healthy ratio to the long-term one.

Common mistakes when reading the curve

• Chasing CTL. Maximising fitness at all costs leads to monotonous overload. Your best performance does not come at the highest CTL but at the best ratio of CTL and TSB on the right day.
• Reading negative values as an alarm. A TSB of minus 20 mid-build is plan fulfilment, not a problem. Minus 20 on race day would be the alarm.
• Scoring sessions inconsistently. Sometimes by power, sometimes by feel, sometimes not logged at all — then the curve mostly measures bookkeeping gaps. Consistency beats precision.
• Putting the curve above the body. Sleep, resting heart rate and motivation remain the final authority. A "green" TSB with poor sleep and low mood is a measurement error, not clearance.

How Peakora works with the performance curve

Peakora's recovery monitor computes exactly on this basis: your form (TSB) is derived from daily training load — about 65 percent — and combined with your sleep, which accounts for roughly 35 percent of the recovery picture. So you see not just a number but the context: is form dropping because the loading block is working, or because recovery is lagging? The engine also times your taper automatically — volume drops as planned over the final two weeks, so you stand at the start line with a positive TSB instead of tired legs and fitness you cannot access.

Frequently asked questions about CTL, ATL and TSB

What do CTL, ATL and TSB actually mean?

CTL (Chronic Training Load) is your fitness — the average training load of the last 42 days. ATL (Acute Training Load) is your fatigue from the last 7 days. TSB (Training Stress Balance) is the difference between the two and describes your freshness: positive means recovered, negative means loaded.

What is an ideal TSB value on race day?

For most athletes the optimal range is plus 5 to plus 15. Much higher and you would be fresh but might have lost fitness already. A slightly negative value down to minus 5 is fine for shorter races.

Is a negative TSB dangerous?

No. During loading blocks, values between minus 10 and minus 30 are normal and intended — that is where the training stimulus happens. It only becomes critical if TSB stays below minus 30 for weeks or coincides with poor sleep and falling motivation.

How fast does form decay without training?

CTL roughly halves every 42 days without training — after two weeks off you lose about 20 percent of your fitness. Fatigue (ATL), on the other hand, dissipates within days; that is why even a short break brings noticeable freshness.

Do I need a power meter for the performance curve?

No. The model works with any load metric: TSS from power, heart rate data, pace, or even RPE times duration. What matters is that you score every session the same way — the curve lives on consistency, not on the sensor.