CTL, ATL and TSB: reading your fitness, fatigue and form without going crazy
The first time you open a training load chart you meet three acronyms, three lines and a pile of numbers going up and down. It is easy to become obsessed with it, and just as easy to ignore it completely. The good news: the idea behind it is simple, and used with a cool head it is one of the best compasses for deciding when to push and when to rest.
The three numbers in one minute
CTL (chronic load, "fitness"): a weighted average of your daily load over the past weeks, with a time constant of about 42 days. It rises slowly and falls slowly. It is the best approximation of "how much training you are carrying".
ATL (acute load, "fatigue"): the same idea with a time constant of about 7 days. It reacts fast: one hard week sends it up and three easy days bring it down a lot.
TSB (balance): CTL minus ATL on the same day. Positive means you are fresher than your recent history would suggest; negative means you are carrying accumulated fatigue.
Think of a bank account: CTL is your long-term capital, ATL is your recent spending and TSB is what is left in your pocket today.
How Cyklot estimates the load of each ride
Everything starts with one number per activity, on a scale similar to TSS: one hour at your FTP is worth roughly 100 points. The formula is hours × IF² × 100, where IF is your intensity relative to threshold. Cyklot picks the most reliable method the data allows:
Power: IF = average power / FTP (capped at 1.5). Two hours at 70% of your FTP adds up to about 98 points.
Heart rate: without power, average heart rate is compared against your heart rate reserve (between a fixed resting rate of 60 bpm and your max heart rate), taking roughly 84% of that reserve as the threshold.
Duration and intensity estimate: the last resort. For cycling it starts from average speed and adds a bonus for climbing; for other sports without power or heart rate it assumes a fixed moderate intensity.
The references come from your profile (FTP and max heart rate). If you have not filled them in, Cyklot infers them from your last year of activities and, as a last option, uses 200 W and 190 bpm. The load page shows which values were used and where they came from, and how many rides were computed with each method.
How CTL, ATL and TSB are calculated
Each day the load of all your activities is added up and an exponential average is applied: CTL with a 42-day constant and ATL with a 7-day one. TSB is the difference between the two on that same day. So the chart does not depend on where the window starts, the calculation begins about 126 days before the range you are looking at, and you can view 6 weeks, 3 months or 6 months. The future is never filled in: the series ends today.
How Cyklot reads your TSB
Fresh: TSB above 5. Rested and ready for a hard effort.
In form: between −10 and 5. Fitness and fatigue are balanced; this is the sustainable training zone.
Loaded: between −30 and −10. You are accumulating fatigue, so plan some recovery.
Overtraining: below −30. High risk of overdoing it: time to rest.
These are indicative thresholds from the model, not a diagnosis. Some riders perform well at a TSB of −20 during a hard block, while others feel awful at −8.
Building load without overdoing it
CTL grows when you stack days of load above your average. The practical rule is to raise it gradually: many coaches work with increases of a few points per week, and less if you are coming back from a break or recover slowly. Some useful guidelines:
Alternate loading weeks with a lighter week every 3 or 4. ATL will drop and TSB will drift back towards positive.
Do not try to raise CTL with a single heroic block: the 42-day constant rewards consistency.
A negative TSB is normal while you build. Being permanently "Fresh" usually means you are not accumulating much stimulus.
If your plan ramps up the load, check how the chart responds instead of guessing.
Tapering before an event
When you cut volume in the final weeks, ATL falls fast and CTL only dips a little, so TSB rises. That is why many riders arrive at their goal with a positive balance. A taper of one to two weeks is typical: trim a good part of the volume but keep some short, sharp efforts so you do not lose your edge. There is no magic TSB number: the "Fresh" range is a good reference, but it is worth testing what works for you in lower-priority events.
Red flags
TSB below −30 for several days in a row, especially if ATL keeps climbing.
Workouts that used to go well now fall flat, worse sleep, irritability or an unusually high resting pulse. These signals matter more than any chart.
Sudden jumps in CTL over a few weeks: they rarely come for free.
If something genuinely worries you, talk to a health professional: this tool is no substitute for medical advice.
Common misreadings
Assuming a high TSB is always good. A very positive TSB for weeks usually means you have lost fitness, not that you are at your best.
Confusing CTL with performance. It measures accumulated load, not how fast you are. Two riders with the same CTL can perform very differently.
Forgetting it is an estimate. Cyklot uses average power, which tends to underestimate the load of very variable rides, and an FTP that may be approximate. Values will not match other platforms.
Not uploading every activity. Days without data count as zero load; if you did a trainer session and never logged it, the model does not know.
Comparing your CTL with someone else's. What is useful is your own trend, not the absolute value.
Next step
Open your training load page, pick the 3-month range and look at the shape of the three curves before your next hard week. If you do not have an FTP in your profile yet, add it to sharpen the estimates, and let your plans do the rest.