Fitness (CTL), Fatigue (ATL), and Form (TSB): What Your Training Numbers Are Actually Telling You

You're staring at a chart with three lines and a pile of acronyms — CTL, ATL, TSB — and all you really know is that up is good and negative is bad. That's where most age-group athletes leave it, which is a shame, because those three numbers answer the two questions that decide your season: Am I getting fitter? and Am I fresh enough to race? Let's decode them properly, because a metric you don't understand is a metric quietly making your decisions for you.

It Starts With One Number: TSS

Before the three lines mean anything, you need the input that feeds them: Training Stress Score (TSS). Developed by Dr. Andrew Coggan and Hunter Allen as part of the framework for power-based training, TSS captures how much stress a session imposed by combining its intensity and duration into one number. An hour at threshold and three easy hours can produce a similar TSS by very different routes. Every day you train you generate a TSS; every day you rest, it's zero. That daily stream is the raw material for everything else.

The model itself traces to physiologist Eric Banister, whose impulse-response model in the 1970s proposed something elegant: your performance on any given day is the result of two competing effects from training — a slow-building positive effect (fitness) and a faster-building negative effect (fatigue). Modern platforms simply operationalized that idea with TSS.

Athlete Takeaway: TSS = intensity × duration in one number — the daily "dose" of training stress. Everything downstream is just clever averaging of your daily TSS.

CTL = Fitness (The Slow Line)

Chronic Training Load (CTL) is your Fitness: an exponentially weighted average of your daily TSS over roughly the last 42 days. Because the window is six weeks long, CTL moves slowly — you don't build it in a weekend and you don't lose it in one either, which is exactly why it's a good proxy for genuine fitness. It rewards consistency over heroics. A single monster ride barely nudges it; six weeks of steady, progressive work moves it meaningfully.

This is the number to watch across a build. You generally want CTL rising gradually — emphasis on gradually. Ramp it too fast and you're not getting fitter, you're getting injured; a controlled rate of rise is a feature, not a limitation.

ATL = Fatigue (The Fast Line)

Acute Training Load (ATL) is your Fatigue — the same idea over a much shorter window, roughly the last 7 days. Because it's short, ATL reacts fast: a heavy week spikes it, a few rest days crash it back down. ATL isn't good or bad on its own. It's the price you're currently paying to build fitness. The point of a hard block is to drive fatigue up temporarily; the point of recovery is to let it fall. What matters is the relationship between the two — which is the third number.

Athlete Takeaway: CTL (Fitness) is the slow 6-week line — build it gradually. ATL (Fatigue) is the fast 7-day line — expect it to spike in hard weeks and fall in recovery. Neither means much alone.

TSB = Form (Fitness Minus Fatigue)

Training Stress Balance (TSB) is your Form, and it's the simplest calculation of all: TSB = CTL − ATL. Yesterday's fitness minus yesterday's fatigue. It's your freshness — how ready you are to perform right now.

TSB (Form)	What it means	When you want it
Negative	Fatigue > fitness: loaded, building, not fresh	Normal during a training block
Around neutral	Fatigue and fitness balanced	Transitions, easy weeks
Positive	Rested: freshness on top of fitness	Race day / A-race taper

Negative TSB: You're Loaded

When fatigue (ATL) sits above fitness (CTL), TSB goes negative. This is normal and necessary during a block — it means you're doing the work. Deeply negative TSB means you're carrying heavy fatigue and shouldn't expect to race well on it. You're not fresh; you're building.

Positive TSB: You're Fresh

Back off, fatigue falls below fitness, and TSB climbs positive — rested and race-ready. This is the entire physiological point of a taper: shed fatigue faster than you shed fitness so you arrive fresh but still fit. Most athletes target a positive TSB for an A-race; the exact figure is individual, which is where coaching judgment beats a rule of thumb.

The Trap: Too Fresh for Too Long

Here's the nuance that separates athletes who use these numbers from those who obey them. A TSB that stays high for weeks doesn't mean you're perfectly peaked — it means you've stopped training and your fitness (CTL) is now falling. Freshness only counts when it sits on top of fitness you actually built. A high TSB on a collapsing CTL is just detraining with good vibes.

Athlete Takeaway: TSB (Form) = Fitness − Fatigue. Negative = building (normal). Positive = race-ready (the goal of a taper). But high Form only matters if Fitness is still high underneath it.

A Quick Worked Example

Say you finish a big build with a CTL of 90, deep in negative TSB — loaded, fit, and flat. You start a two-week taper: training drops, ATL falls fast, TSB climbs positive while CTL slips only slightly, from 90 to maybe 85. That's a textbook peak — you shed a lot of fatigue and kept almost all your fitness. Now imagine you got nervous and stopped training for three weeks instead. TSB would look gorgeous, but CTL would slide toward 75, and you'd toe the line "fresh" but measurably less fit. Same beautiful Form number, completely different race. Reading the two lines together is the whole skill.

Using the Numbers Without Being Used by Them

Two cautions. First, the model is only as honest as its inputs. If your FTP or threshold zones are out of date, every TSS is wrong and so is every line built from it — garbage in, garbage out. Recalibrate your zones regularly. Second, this is a model, not a measurement of your body. It knows nothing about your sleep, your life stress, your illness, or the fact that today's "fresh" TSB is sitting on a head cold. The chart is a powerful lens for spotting trends and planning a taper — not a substitute for how you actually feel, or for a coach who reads the context the numbers can't.

Athlete Takeaway: Keep your zones current or the whole chart lies to you. And treat TSB as a planning lens, not a verdict — it can't see your sleep, stress, or health.

Bottom Line — What This Means For You

Three lines, two questions. CTL tells you if you're getting fitter — keep it rising, gradually. ATL tells you what fatigue you're carrying — let it spike and recover with intent. TSB tells you if you're fresh — negative while you build, positive when you race. Understand that and you stop staring at a confusing chart and start using it: to time a taper, justify a rest week, and tell the difference between fitness you built and freshness you're faking.

Want a season built around your fitness, fatigue, and form — and peaked on the right day?

NVDM Coaching turns the chart into a plan: periodized load, a real taper, and a peak that lands on your A-race. Book a free coaching consultation.