CTL, ATL and TSB Explained for Cyclists
Three numbers tell you almost everything about how ready you are to train or race: CTL (fitness), ATL (fatigue) and TSB (form). Here's what they actually mean, how they're calculated, and how to use them to make smarter training decisions.
Where these numbers come from
CTL, ATL and TSB were introduced by Dr. Andrew Coggan and Hunter Allen in their book Training and Racing with a Power Meter. The model is based on a simple insight: training creates both a fitness adaptation (which builds slowly) and a fatigue response (which hits quickly). The difference between the two tells you how ready you are to perform.
The model runs off TSS — Training Stress Score. TSS is the fundamental unit of training load. A 60-minute ride at exactly your FTP scores 100 TSS. An easy 2-hour ride might score 80. A hard 3-hour ride might score 250.
The CTL formula
CTL is a 42-day exponentially-weighted moving average of your daily TSS. In plain language: it's a rolling measure of how much training you've done over the last six weeks, with recent sessions weighted more heavily than older ones.
This means it takes roughly 42 days (the "time constant") for a training stimulus to fully affect your CTL. If you do nothing, your CTL will decay by about 1/42 each day.
The ATL formula
ATL works the same way, but with a 7-day time constant. It responds much faster to training stimulus — a big week of training will spike your ATL quickly. A week of rest will drop it quickly.
TSB: the number that drives decisions
TSB is simply CTL minus ATL. When you're training hard, ATL rises faster than CTL, making TSB negative — you're fatigued. When you rest and taper, ATL drops faster than CTL, making TSB positive — you're fresh.
You need a period of negative TSB to build fitness — that's training stress. But you need positive TSB to race well — that's recovery. The art of training is knowing how much stress to apply, and when to pull back and let fitness emerge.
What your TSB number means
TSB is the most actionable number in the model. Here's a rough guide to what different ranges mean for most cyclists:
Very fresh. You've been resting more than training. Good for race day or a key test. Not sustainable for fitness building.
Race-ready. Ideal TSB range for peak performance. A 7–10 day taper typically lands you here.
Normal training. Slightly fatigued but functional. Most of your training weeks should sit here.
Fatigued. Deep in a training block. Performance will feel hard. Back off when you hit −20 or below.
Overreached. Risk of illness, injury and performance regression. Rest is not optional — it's urgent.
How to use CTL, ATL and TSB to train smarter
Build CTL gradually
A good rule of thumb: don't raise your CTL by more than 5–8 points per week. Building too fast causes ATL to spike unsustainably, pushing your TSB dangerously negative. The cyclists who sustain the highest CTLs are the ones who build it slowly over years, not months.
Use TSB to decide daily intensity
Before you head out for a session, check your TSB. If it's below −15, consider replacing your planned quality session with Zone 2. If it's above +10, you're fresh — you can push harder than planned. TSB is your daily permission slip.
Plan your taper around TSB, not a calendar
Most cyclists taper for a fixed number of days before an event. A smarter approach is to taper until your TSB reaches your target range (+5 to +15 for most events). If your CTL is high and ATL is very elevated, you may need 10–14 days to reach race form. If CTL is lower and ATL is already modest, 7 days is enough.
Watch the ratio, not just the number
TSB of −15 means different things at CTL 40 vs CTL 90. At low CTL, −15 is relatively stressful and suggests slowing down. At high CTL with significant fitness built up, the same TSB can be within normal training range. Context matters.
The limits of the model
CTL, ATL and TSB are powerful but they're a model — not reality. They don't capture:
- Sleep quality — eight hours of broken sleep is not the same as eight hours of recovery sleep
- Life stress — work pressure, illness, travel all affect recovery in ways TSS doesn't see
- Muscle damage — eccentric loading (running, descending) causes more damage than the power meter measures
- Individual variation — some athletes handle high TSB poorly and perform better at −5 to −10
Use the numbers as a framework, not a prescription. When your body disagrees with your TSB, trust your body.
How VeloCoach AI uses these numbers
VeloCoach reads your CTL, ATL and TSB from Strava, Wahoo and Intervals.icu, then interprets them in plain language each morning. Rather than staring at a dashboard and trying to work out what the numbers mean, you get a coaching decision: what to do today, and why.
If your TSB is at −18 after a hard week, VeloCoach says: "Fatigue is outpacing fitness. Back off today." If your TSB is back to −6 and your race is in 12 days, it says: "Time to start easing into your taper — here's the plan."
The numbers are already in your Strava. VeloCoach is the layer that tells you what to do with them.
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