CTL, ATL and TSB explained: how to read Fitness, Fatigue and Form

Open Intervals.icu (or TrainingPeaks) and you're greeted by three curves labelled Fitness, Fatigue and Form — under the hood, CTL, ATL and TSB. Most athletes glance at them and move on. But once you understand what they measure, they become the single most useful dashboard for deciding whether to push, hold or back off today.

The three numbers in plain English

All three are built from your daily training load — a single score per session (Intervals.icu calls it Load; it can be power-based TSS, heart-rate based, or pace based). CTL, ATL and TSB are just different time-averages of that daily load.

• CTL (Chronic Training Load) = your Fitness. A 42-day weighted average of daily load — it rises slowly and falls slowly.
• ATL (Acute Training Load) = your Fatigue. A 7-day weighted average — it reacts fast to hard days and rest days.
• TSB (Training Stress Balance) = your Form. Simply yesterday's CTL minus yesterday's ATL — how fresh you are right now.

Fitness (CTL): your training bank account

Think of CTL as a bank balance you build through consistent deposits. Because it averages six weeks of training, one big weekend barely moves it, and a week off barely dents it. That's the point: fitness is slow to earn and slow to lose. Chasing a higher CTL number for its own sake is a trap, though — a CTL of 80 built on junk miles is worth less than a CTL of 65 built on well-targeted sessions.

Fatigue (ATL): the short-term cost

ATL is the same idea over just seven days, so it spikes after a hard block and drops quickly when you rest. On its own it's not very actionable — its value is in the gap between it and CTL, which is exactly what Form measures.

Form (TSB): are you fresh or buried?

Form is the number you'll actually check each morning. Positive Form means fatigue has dropped below your fitness — you're rested. Negative Form means you're carrying fatigue — normal, and even necessary, during a build. The widely used bands:

• Above +15 — Fresh and race-ready (but sit here too long and you're detraining).
• +5 to +15 — Tapered, good for key races and breakthrough sessions.
• -10 to +5 — The grey zone: rested enough to train well, not so fresh you're losing fitness.
• -30 to -10 — Productive fatigue. This is where most of your build weeks should live.
• Below -30 — High strain. Fine for a short overload block, risky if you stay there.

These are guidelines, not laws. Your personal race-ready Form might be -5 or +20 depending on your history — the goal is to learn your own numbers over a season, not to hit a textbook value.

Ramp rate: how fast can you build?

Ramp rate is how many CTL points you add per week. A rise of roughly 3–7 points per week is sustainable for most athletes; push much past 8–10 week after week and injury or illness tends to follow. If your CTL is climbing fast and your Form is deep in the red for weeks, that's the model waving a flag.

How to actually use these numbers

• Plan backwards from your race: aim to arrive with Form in the +5 to +15 range by shedding fatigue in the final 1–2 weeks.
• During build blocks, expect and accept negative Form — that's where adaptation comes from.
• Watch the trend, not the daily wobble: a single number matters far less than the direction over 10–14 days.
• If Form stays deeply negative and your session quality is dropping, that's your cue to insert a recovery day — not to grind harder.

Where the model breaks down

CTL/ATL/TSB is a model, and every model simplifies. It knows nothing about a bad night's sleep, work stress, a cold coming on, or a session that felt far harder than its load score suggests. Two athletes at the same Form can be in completely different states. That's why the smartest approach is to read these curves alongside how you actually feel and metrics like resting heart rate or HRV — and to adjust the plan when they disagree.

That last point is the whole idea behind Upform: it reads your real Intervals.icu Fitness, Fatigue and Form every day and adapts your upcoming sessions around them, so you don't have to reverse-engineer the curves yourself. If you already live in Intervals.icu, it's the natural next layer on top.