What is CTL in running?

CTL (chronic training load) is a 42-day weighted average of your daily training stress. It is usually called fitness: it rises slowly when you train consistently and falls slowly when you stop.

ATL (acute training load) is the same average over about 7 days. It is usually called fatigue: it jumps after a hard week and drops within days of easy running or rest.

What is TSB and what is a good number?

TSB (training stress balance) is yesterday's CTL minus yesterday's ATL, usually called form. It is negative while you build and positive after a taper. Commonly cited ranges are about -10 to -30 in a heavy block and about +5 to +25 on race day, but the right number is individual.

How do I raise TSB before a race?

Cut training load for one to three weeks before the race (a taper). Fatigue falls faster than fitness, so form rises while most of your fitness stays.

Training load

CTL, ATL and TSB Explained: Fitness, Fatigue and Form

By EverRun Team · October 18, 2026 · 6 min read

Three numbers sit behind most training-load charts: CTL, ATL and TSB. They answer one question runners ask all the time: am I getting fitter, or am I just tired? Here is what each one means, how it is worked out, and what a taper does to them.

The short version

Number	Usually called	What it tracks
CTL	Fitness	Your training load averaged over about 6 weeks (42 days). Rises slowly, falls slowly.
ATL	Fatigue	The same average over about 1 week (7 days). Moves fast in both directions.
TSB	Form	Fitness minus fatigue. Negative while you build, positive when you are fresh.

Where the numbers come from

Every workout gets a training stress score (TSS): how hard it was, multiplied by how long it lasted. By convention, an hour at your threshold effort scores 100. An easy 45-minute run might score around 40, and a long run with a fast finish well over 100. Running versions of the score use pace (rTSS) or heart rate (hrTSS) instead of power.

CTL and ATL are then exponentially weighted averages of those daily scores, the model first described by Eric Banister. Each day moves the average a fraction of the way towards that day's load:

CTL today = CTL yesterday + (TSS today − CTL yesterday) ÷ 42
ATL today = ATL yesterday + (TSS today − ATL yesterday) ÷ 7
TSB today = CTL yesterday − ATL yesterday

Because ATL uses 7 days and CTL uses 42, a hard week lifts fatigue about six times faster than it lifts fitness. That gap is the whole idea: training makes you tired first and fitter later.

What a good TSB looks like

There is no single right number, but these ranges are commonly used as a starting point:

About −10 to −30: a normal training block. You are carrying fatigue on purpose.
Below about −30: a heavy overload. Fine for a short camp, risky as a habit.
Around 0: balanced. Typical of easy or recovery weeks.
About +5 to +25: fresh. Where many runners aim to be on race day.
Well above +25 for weeks: rested, but fitness is starting to slip.

Read your own history rather than someone else's ranges. If your best races came at +10, that is your target.

How fast can fitness rise?

The rate CTL climbs each week is called the ramp rate. Many coaches keep it to a few points a week for most of a block, with easier weeks in between. A sudden jump in ATL with no matching rise in CTL usually means a week that was much bigger than what came before. That is worth a second look, especially after a break.

What a taper does to your form

A taper cuts training for one to three weeks before a race. Fatigue drains in days, fitness in weeks, so form climbs while most of your fitness stays. Try it with your own numbers:

Average daily TSS in your build Taper length (days) Load cut during taper (%)

On race day	Value
Fitness (CTL)	–
Fatigue (ATL)	–
Form (TSB)	–
Fitness kept	–

The calculator assumes a long, steady build where fitness and fatigue have settled at your daily load, then the same cut every day of the taper. Real weeks are lumpier, so treat it as a sketch of the shape, not a prediction.

What the numbers can't tell you

They only know the training you record. Sleep, stress, illness and heat all add fatigue the model never sees.
A score built on heart rate or pace is only as good as your zones. Update your threshold when it changes.
Form is a guide to freshness, not a race-time guarantee.

Seeing your own CTL, ATL and TSB

You don't need a paid tool to see these numbers (here is how EverRun compares with TrainingPeaks). EverRun reads the workouts already on your phone (Apple Health on iPhone, Health Connect on Android) and charts fitness, fatigue and form for you, next to a daily readiness read. For race planning, our race pages count back from your race date to when the taper starts, and the race time predictor turns a recent result into a goal time.

This guide is general training education, not medical advice.