Fitness, Fatigue, Form Explained for Runners
Your training status turns weeks of training into three numbers: how fit you are, how tired you are, and whether you're ready to race. For runners building toward a marathon, half marathon or any goal race, this is the most useful planning view you can have.
The three numbers, in plain English
Fitness — the 42-day load
Fitness is the exponentially-weighted average of your daily training load over 42 days. It moves slowly. Adding one big long run barely shifts it — months of consistent work shift it a lot. Fitness is your aerobic engine in numerical form.
Fatigue — the 7-day load
Fatigue is the same average, but over 7 days. It reacts fast. A hard interval session spikes the value the same day; two rest days drop it sharply. Fatigue captures how loaded you are right now.
Form — Fitness minus Fatigue
Form is simply Fitness minus Fatigue. Negative Form = your fatigue exceeds your fitness — you're in a loading phase. Positive Form = you've recovered more than you've trained — you're rested and primed.
What good values look like
| Phase | Fitness | Fatigue | Form | What it feels like |
|---|---|---|---|---|
| Maintenance | 30–50 | 30–50 | −5 to +5 | Steady, no big build |
| Build week | 50–80 | 60–100 | −10 to −25 | Heavy legs, productive |
| Recovery week | 50–80 | 40–60 | +5 to +15 | Springy, fast |
| Marathon peak | 80–110 | 90–120 | −10 to −20 | Tired but strong |
| Race day | 75–105 | 60–85 | +5 to +20 | Fresh, slightly bored |
The core principle: build slowly
Fitness can only safely rise about 5 points per week without breaking down. Faster ramp rates correlate strongly with injury — overuse injuries appear when Fatigue spikes 1.5× faster than Fitness has built.
Reading the curves over time
Plotted over time, Fitness, Fatigue and Form form a clear pattern:
- Build phase — Fitness rising, Fatigue above Fitness, Form negative
- Unload week — Fatigue drops below Fitness, Form swings positive
- Taper — both lines converge: Fatigue falls fast, Fitness falls slowly, Form lifts
- Race day — Form peaks at +5 to +15, Fitness still high
Race-day Form targets
| Race | Form target | Notes |
|---|---|---|
| 5K / 10K | +5 to +10 | Short taper (5–7 days), Form rises gently |
| Half marathon | +10 to +15 | 10–14 day taper |
| Marathon | +15 to +25 | 3-week taper, Form swings sharply positive |
| Ultra (50K+) | +5 to +15 | Long taper risks losing aerobic edge — keep Fitness high |
Common mistakes
- Chasing maximum Fitness. Higher Fitness is better only up to your recovery capacity. A Fitness of 90 you can recover from beats a Fitness of 110 that injures you.
- Tapering too aggressively. A 3-week aggressive taper drops Fitness fast and you arrive at the line under-fit. Hold easy volume; only cut intensity and total volume in week 3.
- Ignoring Form after races. Racing produces 100–340 load points in a day — your Fatigue spikes hard. Don't resume hard training until Form is back near zero.
- Comparing absolute Fitness across runners. Fitness 60 in a high-volume runner is easy; in a low-volume runner it's a heavy block. Always compare Fitness to your own history.
How these three numbers improve your training
Without this view, runners plan from feel. Hard week, easy week, race week — but how heavy was the hard week? How recovered are you really? Fitness, Fatigue and Form turn that into numbers:
- You can pre-plan a 16-week marathon block with a target peak Fitness and work backward to weekly load targets.
- You can detect overreaching early — when Fatigue stays above Fitness for 3+ weeks straight, recovery is overdue.
- You can taper with precision — dial Form to a known target instead of guessing.
- You can compare seasons objectively. Same Fitness, faster paces? You got fitter.
See your Fitness, Fatigue and Form live with WattRun
Connect Strava — WattRun computes training load from every run and updates your curves daily. Plus race-day Form targets and AI-built training plans that hit your Fitness goals.
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