HOW TRAINING WORKS · 12 MIN

Fitness, fatigue and form

Read fitness, fatigue and form, and use a taper to arrive fresh without losing fitness.

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Interactive · rTSS

Fitness, fatigue and form

Pick a plan and see how sixteen weeks of running add up. Fitness is the 42-day average of your daily load, fatigue the 7-day average, and form is yesterday’s fitness minus yesterday’s fatigue.

Plan

The same week, sixteen times.

Hours a week8 h

Form (TSB)

Scrub the block

Day: Race day
Load: 0 rTSS
Fitness: 68
Fatigue: 68
Form: +1 grey zone

Fitness, fatigue and form are read going into the day; load is that day’s training.

Race day: fitness 68, fatigue 68, form +1 — grey zone. Without a taper, you arrive carrying a normal week’s fatigue.

Why fitness is slow. Each day only moves the 42-day average by 1/42 of the difference, so a hard week barely shifts it and a missed week barely costs it. It takes months to build and weeks to lose.

Why fatigue is quick. The 7-day average moves by 1/7 each day. It jumps after a big week and drops within days of easing off — which is what a taper uses.

Running. Run load is scored from pace against your threshold pace (rTSS). The same score costs the legs more on foot than on the bike: every landing loads muscles and tendons while they lengthen (eccentric loading). That is why run ramps are usually kept more conservative than ride ramps.

Load here is modelled at about 60 rTSS an hour of mostly easy training, with a rest day, two harder days and a long weekend session. The form bands follow the spirit of Intervals.icu’s chart; they describe load, not how you feel.

STEP 1 OF 5

Fitness is a slow average

Each session gets a training load score from its duration and intensity. Fitness, often called CTL, is a rolling average of that load over about six weeks.

Because the window is long, fitness rises slowly and falls slowly. One big week barely moves it; months of consistency do.

THE TAKEAWAY

Fitness reflects the last six weeks, not the last session.

Read the whole lesson

01 · Fitness is a slow average

Each session gets a training load score from its duration and intensity. Fitness, often called CTL, is a rolling average of that load over about six weeks.

Because the window is long, fitness rises slowly and falls slowly. One big week barely moves it; months of consistency do.

The takeaway: Fitness reflects the last six weeks, not the last session.

02 · Fatigue is a fast one

Fatigue, or ATL, is the same idea over about a week. It jumps after a hard block and drops quickly with rest.

Choose build and recover. Fatigue climbs through each building week and falls in the easier one, while fitness keeps its upward line.

The takeaway: Fatigue responds within days.

03 · Form is the difference

Form, or TSB, is yesterday’s fitness minus yesterday’s fatigue. Negative form is normal in a training block: you are carrying fatigue on purpose.

Very negative form for long stretches is where training stops being productive and starts being risky.

The takeaway: Negative form is normal while building; it has limits.

04 · The taper

Before a race, cut volume for about ten days and keep some intensity. Fatigue falls much faster than fitness, so form turns positive and you arrive fresh with most of your fitness intact.

Turn the taper on and compare race-day form.

The takeaway: A taper trades a little fitness for a lot of freshness.

05 · Big jumps show up in fatigue first

Choose sudden spike. Fatigue shoots up, form goes deep, and fitness barely notices. For runners, the lesson before this one applies too: the same load number costs your legs more.

Form reads these numbers from your Intervals.icu data, and can tell you when your block is building well and when it is building risk.

The takeaway: Sudden jumps cost more than they build.

Sources for this lesson

Teaching models of typical patterns in healthy, trained adults. Individual responses vary. Not medical advice.

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