Fitness, Fatigue and Form Explained for Cyclists
Three numbers tell you almost everything about how ready you are to train or race: Fitness, Fatigue and Form (what other apps call CTL, ATL and TSB). Here is what they actually mean, how they are calculated, and how to use them to make smarter training decisions.
Where these numbers come from
Fitness, Fatigue and Form (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 Load (Training Stress Score). Load is the fundamental unit of training stress. A 60-minute ride at exactly your FTP scores 100 Load. An easy 2-hour ride might score 80. A hard 3-hour ride might score 250.
How Fitness is calculated
Fitness is a 42-day exponentially-weighted moving average of your daily Load. In plain language: it is a rolling measure of how much training you have 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 Fitness. If you do nothing, your Fitness will decay by about 1/42 each day.
How Fatigue is calculated
Fatigue 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 Fatigue quickly. A week of rest will drop it quickly.
Form: the number that drives decisions
Form is simply Fitness minus Fatigue. When you are training hard, Fatigue rises faster than Fitness, making Form negative. You are fatigued. When you rest and taper, Fatigue drops faster than Fitness, making Form positive. You are fresh.
You need a period of negative Form to build fitness. That is training stress. But you need positive Form to race well. That is recovery. The art of training is knowing how much stress to apply, and when to pull back and let fitness emerge.
What your Form number means
Form (TSB) is the most actionable number in the model. Here is a rough guide to what different ranges mean for most cyclists:
Very fresh. You have been resting more than training. Good for race day or a key test. Not sustainable for fitness building.
Race-ready. Ideal Form range for peak performance. A 7 to 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 is urgent.
How to use Fitness, Fatigue and Form to train smarter
Build Fitness gradually
A good rule of thumb: do not raise your Fitness by more than 5 to 8 points per week. Building too fast causes Fatigue to spike unsustainably, pushing your Form dangerously negative. The cyclists who sustain the highest Fitness are the ones who build it slowly over years, not months.
Use Form to decide daily intensity
Before you head out for a session, check your Form. If it is below −15, consider replacing your planned quality session with Zone 2. If it is above +10, you are fresh and can push harder than planned. Form is your daily permission slip.
Plan your taper around Form, not a calendar
Most cyclists taper for a fixed number of days before an event. A smarter approach is to taper until your Form reaches your target range (+5 to +15 for most events). If your Fitness is high and Fatigue is very elevated, you may need 10 to 14 days to reach race form. If Fitness is lower and Fatigue is already modest, 7 days is enough.
Watch the ratio, not just the number
Form of −15 means different things at Fitness 40 versus Fitness 90. At low Fitness, −15 is relatively stressful and suggests slowing down. At high Fitness with significant base built up, the same Form can be within normal training range. Context matters.
The limits of the model
Fitness, Fatigue and Form are powerful, but they are a model, not reality. They do not capture:
- Sleep quality. Eight hours of broken sleep is not the same as eight hours of recovery sleep.
- Life stress. Work pressure, illness and travel all affect recovery in ways Load does not see.
- Muscle damage. Eccentric loading (running, descending) causes more damage than the power meter measures.
- Individual variation. Some athletes handle high Form poorly and perform better at −5 to −10.
Use the numbers as a framework, not a prescription. When your body disagrees with your Form, trust your body.
How VeloCoach AI uses these numbers
VeloCoach reads your Fitness, Fatigue and Form 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 Form is at −18 after a hard week, VeloCoach says: "Fatigue is outpacing fitness. Back off today." If your Form is back to −6 and your race is in 12 days, it says: "Time to start easing into your taper. Here is 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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