Fitness, fatigue, and form: the three numbers behind a good taper
Three numbers, one decision. How hard should this week be?
The framework comes from the cycling world but applies cleanly to running: every workout you do produces a training stress score (some combination of duration and intensity). Stack the scores up and you get three derived numbers that tell you everything you need to know about where you are.
Fitness (CTL) is your rolling six-week average of daily stress. It moves slowly. It represents the work your body has actually adapted to. You don't gain fitness in a week, and you don't lose it in a week.
Fatigue (ATL) is your rolling one-week average. It moves fast. It represents the cost of recent training — the muscle damage, the depleted glycogen, the sleep debt.
Form (TSB) is just fitness minus fatigue. It's the answer to "how fresh am I right now relative to what I've been doing?" A positive number means you're rested relative to your training. A negative number means you're carrying fatigue.
The useful values worth memorizing:
- TSB between -10 and -20: productive overload. You're tired, but your fitness is growing. Most of a training block should live here.
- TSB below -25 or -30: too deep. Your body is no longer absorbing the work; the next week needs to step back.
- TSB between +5 and +15: race-ready. Fitness has held while fatigue has cleared.
- TSB above +25: too rested. You've lost some sharpness — common when a taper goes a week too long.
The biggest mistake intermediate runners make is reading those numbers in isolation. A TSB of -15 during a build week means something different than a TSB of -15 two days before a marathon. The number is a tool, not a verdict.
Source: TrainingPeaks — Applying the Numbers Part 3: Training Stress Balance
The coach maintains a rolling 42-day fitness load (CTL) and a 7-day fatigue load (ATL) per user, and reports their difference — form, or TSB — alongside the weekly summary. During build blocks, the coach expects and welcomes TSB in the -10 to -20 range; that's productive overload. If TSB sinks below -25 for more than a few days, the next week's plan trims intensity automatically. In the final two weeks before a goal race, the coach steers volume down so TSB lands in the +5 to +15 window on race morning — fresh, but not detrained.