Training load

Fitness, Fatigue and Form

Fitness is your long-term training load, Fatigue is your short-term training load, and Form is the gap between them: Fitness minus Fatigue.

Together they show whether you are building, overreaching or freshening up, which makes them the most widely used way to plan a block and time a taper.

By Nick Buckley-Mills, Team GBR age-group triathlete Reviewed 15 October 2026 8 min read

At a glance

Term	What it tracks	Timescale	Rises when
Fitnessalso CTL	Training your body is used to absorbing	About 6 weeks	You train consistently for weeks
Fatiguealso ATL	Recent training you are still carrying	About 1 week	You have a big few days
Formalso TSB	Fitness minus Fatigue	Daily	You back off and recover

How does it work?

Every session adds load, and the three numbers are running averages of that load over different time windows. A hard week lifts Fatigue quickly because it only looks back about a week. Fitness moves slowly because it looks back about six weeks, so one big week barely shifts it, but two months of consistent training does.

The idea comes from a model published by Eric Banister and colleagues in 1975: training produces two effects at once, a slow-building positive effect (fitness) and a fast-building negative one (fatigue), and performance on any given day is roughly the first minus the second. Andrew Coggan later adapted it for power-based cycling, and TrainingPeaks made it mainstream as the Performance Management Chart.

Load is the unit underneath it all: a single score per session combining how long you went and how hard. Each platform scores Load in its own way, but all of them weigh duration against intensity.

How are they calculated?

Each is an exponentially weighted average, updated once a day. Recent days count more than older ones, and the weight fades smoothly rather than dropping off a cliff.

Fitness today = Fitness yesterday + (Load today − Fitness yesterday) ÷ 42
Fatigue today = Fatigue yesterday + (Load today − Fatigue yesterday) ÷ 7
Form today    = Fitness today − Fatigue today

The 42 and 7 are time constants in days. They are conventions rather than laws of physiology, and some platforms let you change them. Some tools also calculate Form from yesterday's Fitness and Fatigue, so your Form number can differ by a few points between apps on the same data.

Worked example

You start the day with Fitness 60 and Fatigue 70, then complete a long ride scoring 100 Load.

Step	Calculation	Result
Fitness	60 + (100 − 60) ÷ 42	61.0
Fatigue	70 + (100 − 70) ÷ 7	74.3
Form	61.0 − 74.3	−13.3

One big day lifts Fatigue by more than four points but Fitness by only one. That asymmetry is why Form drops during a hard block and climbs when you rest.

What does a training block look like?

Fitness climbs in steps, Fatigue spikes and drops with each week, and Form dips negative through the build then rises through the taper.

A sample athlete over two 3:1 build cycles and a two-week taper. Form bottoms out around −25 in the hardest week and reaches about +23 on race morning, while Fitness eases from its peak of about 66 to 55. That trade is the point of a taper: shed Fatigue faster than you lose Fitness.

How do I read my Form number?

Read it as a direction, not a score. As a rough guide that many coaches and athletes use:

Below about −30: very heavy load. Fine for a short, planned overreach; a warning sign if it lasts.
−10 to −30: the usual range for a productive build.
Around 0 to +10: fresh enough to train well or race a B event.
About +5 to +25: where many athletes land for an A race after a taper.
Positive for weeks on end: rested, but Fitness will be sliding.

Your ranges are your own. Compare your Form on past race days with how those races went. That history is a better guide than any table, including this one.

What changes for triathletes?

One combined number hides which sport is actually building. Most platforms add swim, bike and run load together. That shows your total training stress, but the sports aren't interchangeable:

Running costs more recovery. Impact makes a run harder to bounce back from than a ride with the same Load score, so a run-heavy week can feel worse than the numbers suggest.
The bike can inflate Fitness. Long rides generate a lot of Load, so a strong combined Fitness can sit on top of a run that hasn't progressed.
Swim load is the least reliable. Without power, swim Load is estimated from pace or heart rate, both of which are less precise in the water.

Watch the combined number for fatigue and taper timing, and per-sport Fitness to check each discipline is moving the way your race needs.

Should you trust these numbers?

Trust them as a record of what you did. Be sceptical of them as a measure of what it did to you. Fitness, Fatigue and Form are useful, and they are also one of the most over-read charts in endurance sport.

What they get right

They make consistency visible. Weeks of steady work show up as a rising line, and gaps show up too. Most athletes underestimate how much one missed week costs and overestimate what one big week buys.
They flag spikes. A sudden jump in Fatigue is a prompt to ask whether you meant to do that.
They give a taper a shape. Watching Fatigue fall faster than Fitness is a reasonable way to see a taper working.

Where they fall short

They reduce a whole body to one line. Training changes your heart, muscles, tendons, metabolism and nervous system, each on its own timescale. Collapsing all of that into a single number, then into a single average, throws most of the information away.
Very different sessions score the same. On the most common scoring method, one hour at threshold and two hours of steady endurance riding both come out at about 100. They are not the same stimulus, they don't need the same recovery, and they don't prepare you for the same race.
The model assumes your body is simple. Fixed 42- and 7-day windows, the same for everyone, all year, with effects that add up in a straight line. Real adaptation is individual and non-linear: the same week lands differently when you are sleeping badly, travelling, or three years further into the sport.
It only sees what you record. Work stress, illness, heat, a new baby. None of it scores, yet all of it changes what training does to you. Studies fitting the model to individual athletes have found its settings imprecise and unstable, which limits how far it can be trusted for one person.
It can be gamed, mostly by yourself. Once Fitness becomes the goal, it is easy to raise: add easy volume, never take a down week, log everything. The line goes up and the athlete doesn't get faster. When a measure becomes a target, it stops being a good measure.

How to use them anyway

Treat the chart as evidence, not a verdict. Use it to ask better questions (why did Form crash this week, why is run Fitness flat while total Fitness climbs), then answer them with what the numbers can't see: how sessions felt, how you are sleeping, what your race actually demands. A number you understand the limits of is more useful than one you obey.

Common questions

Is Fitness the same as CTL?

Yes. Fitness is the plain-English name for Chronic Training Load (CTL), the 42-day weighted average of training load. Fatigue corresponds to Acute Training Load (ATL) and Form to Training Stress Balance (TSB), terms popularised by TrainingPeaks.

What is a good Form number for race day?

Many athletes race well with Form somewhere between about +5 and +25, but the right number is individual. Track your own Form against how your races actually went, and use that history rather than a fixed target.

Does a higher Fitness number mean I will race faster?

Not directly. Fitness measures how much training you have absorbed, not how fast you are. Two athletes with the same Fitness can race very differently, and the right kind of training for your event matters more than the size of the number.

Should triathletes track one Fitness number or one per sport?

Both. A combined number shows total training stress, but swim, bike and run load are not interchangeable: running usually costs more recovery than riding at the same load. Per-sport Fitness shows whether each discipline is actually building.

Can you game your Fitness score?

Yes, and most athletes who do it are fooling themselves rather than anyone else. Adding easy volume, skipping recovery weeks and logging every walk all raise Fitness without making you faster. Use the number to check consistency, not as a target to chase.

Sources

Banister EW, Calvert TW, Savage MV, Bach T. A systems model of training for athletic performance. Australian Journal of Sports Medicine. 1975;7:57–61.
Allen H, Coggan AR, McGregor S. Training and Racing with a Power Meter, 3rd edition. VeloPress, 2019.
Hellard P, Avalos M, Lacoste L, et al. Assessing the limitations of the Banister model in monitoring training. Journal of Sports Sciences. 2006;24(5):509–520.
Impellizzeri FM, Marcora SM, Coutts AJ. Internal and external training load: 15 years on. International Journal of Sports Physiology and Performance. 2019;14(2):270–273.
Goodhart CAE. Problems of monetary management: the UK experience. In Papers in Monetary Economics, Reserve Bank of Australia, 1975. The origin of the idea known as Goodhart's law.
Strathern M. "Improving ratings": audit in the British University system. European Review. 1997;5(3):305–321. The source of the phrasing "when a measure becomes a target".