CTL is the slow number
Chronic training load moves over weeks. One big weekend does not buy fitness; consistent load does.
Load model
CTL, ATL, and TSB explained
Three numbers. Longer-term fitness, recent fatigue, and whether you are fresh or flat today. That is how Runapt reads load.
Included on this page
CTL ≈ longer-term fitness
ATL ≈ recent fatigue
TSB ≈ freshness (CTL − ATL)
Free resource
How the three numbers show up in a week
A simple week view of what rising ATL, stable CTL, and a dipping TSB feel like in practice.
- CTL ≈ longer-term fitness
- ATL ≈ recent fatigue
- TSB ≈ freshness (CTL − ATL)
WhenSessionHow to run it
CTL rising slowlyFitness buildingConsistent weeks, not one heroic spike
ATL spikesHard block or travel stressExpect heavier legs for a few days
TSB negativeFatigued / in loadProtect easy days; do not stack quality
TSB climbing into race weekFreshness returningTaper doing its job
Training notes
Make the plan fit the runner.
ATL is the loud number
Acute load jumps when you pile hard sessions or long runs. It is why you feel cooked after a surge week.
TSB is the freshness read
Training stress balance is roughly CTL minus ATL. Deeply negative means fatigued; rising into race week is the taper you wanted. Details on /science.
What CTL, ATL, and TSB mean in plain language
CTL (chronic training load) estimates the fitness you have been building. ATL (acute training load) estimates how much you have done lately. TSB (training stress balance) is the gap between them — a shorthand for form or freshness.
Runapt uses this model with readiness and your actual sessions so the week can get shorter when load is high, instead of pretending every Tuesday is identical. It is not a medical diagnosis; it is a training load read.
How this changes the plan
When ATL jumps and TSB dives, quality sessions get protected or delayed and easy days stay honest. When you taper, you want TSB climbing so race day is not another fatigue spike. For the product-level story of adaptation, see the science page.
FAQ
What do CTL, ATL, and TSB mean?
CTL is longer-term training load (fitness), ATL is recent load (fatigue), and TSB is roughly CTL minus ATL (freshness or form).
What is training stress balance?
Training stress balance (TSB) estimates how fresh you are relative to recent hard work. Negative usually means fatigued; rising TSB into a race week is typical of a taper.
Does Runapt use CTL/ATL/TSB?
Yes. Together with readiness and missed runs, those load signals help reshape the week so you are not forced into quality work when you are already flat.
Is a negative TSB always bad?
No. Training blocks often run with some fatigue on purpose. It becomes a problem when hard sessions keep stacking with no recovery or taper.
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