Open the fitness chart in Intervals.icu and there are two lines with a coloured band under them. Fitness 52, Fatigue 64, Form minus 12, and the band says you are in the optimal training zone. None of those numbers were measured. All of them were calculated from a single number per run, and that number was calculated from a threshold which, for pace, is one you typed in yourself, possibly two years ago.
Everything starts with load
Every activity gets a training load. Intervals.icu's definition is built on power, where an hour at your threshold power gives 100, but it can also compute the load from heart rate or grade adjusted pace. Which one wins when several are available is a setting per sport. Two runners' load numbers are therefore not always computed the same way, and your own may change if you buy a running power pod.
For heart rate, Intervals.icu's help text recommends the model called HRSS for running, and it needs your resting heart rate to work. For pace, the load is measured against the threshold pace in your run settings. That is a number you enter yourself, and the help text suggests the fastest pace you have been able to hold for 30 minutes.
Everything else in the chart is built on the threshold. If your load comes from heart rate, it is the threshold heart rate in the same settings that counts, and if it comes from pace, the threshold pace. If your threshold is from a spring when you were in better shape, every run counts as easier than it was and fitness grows more slowly than your training did. If it is too slow, the reverse. Intervals.icu also computes critical speed from your best times between 1 and 5 km. If that pace moves a lot while your setting stays the same, the setting probably needs updating.
The other thing load doesn't know is what kind of run it was. Seventy points from an hour and a half of easy running and seventy from a hard interval session are the same seventy in the chart. Your tendons do not agree, which the post on injuries goes into.
Fitness and fatigue are averages
Intervals.icu defines fitness as a 42 day exponentially weighted moving average of your training load, and fatigue as the same over 7 days. Recent days count most. Both time constants can be changed in the settings.
The idea goes back to a model Eric Banister and colleagues published in 1975, where performance is fitness minus fatigue. The 42 and 7 days, and the names CTL, ATL and TSB that many apps still use, come from the Performance Manager Chart that Andrew Coggan developed for TrainingPeaks.
A fitness of 52 means you have averaged around 52 load points a day, weighted towards the last few weeks. It doesn't mean you can run a certain time. Two runners with the same fitness, one who only runs easy and one who does two hard sessions a week, can be fit for very different races, and the number can't tell them apart.
So compare the line with itself. Fitness 52 this October against 45 last October, with a similar mix of sessions, says something. Fitness 52 against a friend's 70 says almost nothing.
Form is the difference
Form is fitness minus fatigue. When it is negative, the last week has been heavier than the weeks before it, which is what training looks like. When it is positive, you have been doing less than usual, which is what a taper is for, and also what a holiday does.
Intervals.icu puts form into named zones: transition, fresh, grey zone, optimal and high risk. According to the help text you gain fitness in the optimal zone and are ready to race when you are fresh and fit at the same time. High risk is not a zone to stay in for long. The zones are rules of thumb built on guidance from coaches like Joe Friel, whom the help text links to.
Form can also be shown as a percentage of fitness instead of an absolute number. Intervals.icu warns about that itself: if your fitness is below 60 the percentage exaggerates form and can put you in the high risk zone, so it is only worth using if you train a lot. Six hours of easy running a week gives a fitness of roughly 50.
What form is good for is seeing a taper work, and spotting a block that has been deep in the negative for weeks without a lighter week. What it can't do is decide today's session on its own. A form of minus 20 says nothing about whether you slept badly or have a cold coming.
Ramp rate and the ratio next to it
Ramp rate is how much fitness has changed from one week to the next. Positive means you are building, negative is normal in a recovery week. Intervals.icu warns that too high a ramp rate risks overtraining, but gives no number that applies to everyone.
Intervals.icu also shows an acute to chronic ratio, fatigue divided by fitness, with a band that calls 0.8 to 1.3 the safe zone. The help text links to an article asking whether the ratio is science or religion, which is a fair summary of where the research stands. Franco Impellizzeri and colleagues went through it in 2020 and found no evidence that the ratio should be used to steer training to avoid injury. It shows how fast your load is rising, but not whether your body can take it.
Decoupling says more about a single run
Aerobic decoupling compares the first half of an activity with the second: how much power you produced per heartbeat. If heart rate climbs while power stays the same, the number is positive. Intervals.icu's help text describes it as power against heart rate and says the measure is meant for long endurance sessions. The rule of thumb many coaches use, originally from Joe Friel, is that under five percent on a long easy session means your aerobic base holds for that duration.
It is one of the few numbers that says something about how you held up rather than how much you did. It is also the simplest way to follow durability, what happens after 90 minutes, outside a lab, which the post on the five performance factors explains. But heat, hills, a skipped gel and a pace that crept up all move it. One run says little. The same long run on the same route in similar weather, a month apart, says more.
HRV and resting heart rate
If your watch measures them overnight, Intervals.icu stores HRV as rMSSD, resting heart rate and sleep, one value per day, among other wellness data. It can also show HRV normalised by resting heart rate, with the warning that both have to come from the same source for that to mean anything.
The value on a given date describes the night before, not the day. A single low HRV after a late dinner means nothing, while four mornings in a row below your usual range mean something. Compare with your own history, never with someone else's number, because HRV varies a lot between people. The post on recovery goes further.
Where the numbers stop
When Philippe Hellard and colleagues fitted Banister's model to a season of training for nine elite swimmers in 2006, it tracked their performances well. But the best estimate of the fitness time constant was somewhere between 17 and 59 days, and the fatigue one between 6 and 32. Fixed values of 42 and 7 are a convention.
The numbers also know nothing about the rest of your life. A week with a sick child and four hours of sleep a night has the same load as a week on holiday, if you ran the same runs. HRV catches some of that, the load chart catches none of it.
We wrote the same thing in the info sheet of our own app's load view: the numbers describe how much you have trained, they don't measure your fitness or how tired you feel. That applies to Intervals.icu too, and to every app that draws these lines.
None of this is criticism of Intervals.icu. It is free to use and explains its own model openly in the help texts quoted above.
What Coach Fartlek does with the same data
Coach Fartlek is our own app and reads your training through Intervals.icu, so expect bias, both towards our app and towards Intervals.icu. It fetches your runs with their laps and full data series, plus HRV, resting heart rate and sleep, and computes its own load from them. It does not use Intervals.icu's fitness numbers, which means the numbers in the app won't match the ones in Intervals.icu exactly.
The biggest difference is the threshold. The app sets it itself, from your fastest runs and your races over the last three months with pace adjusted for hills, and updates it after races and threshold sessions, so you don't have to remember to change a setting. The load formula is the simplified one, intensity squared times hours times 100, and fitness and fatigue use the same 42 and 7 days.
Form affects where the hard sessions go and how the coach phrases things, never how much you run in a week. The morning check only reacts to HRV when last night's value is clearly below your own normal range from the last three months, and to resting heart rate when it is more than three beats above the top of that range. The app computes decoupling from pace as well as power, and uses it in its durability estimate, which it calls an estimate.
One practical thing if you connect: runs that reach Intervals.icu through Strava can't be read by other apps, because Strava doesn't allow them to be passed on. Sync your watch directly to Intervals.icu, which the guide shows. The app covers 5K to marathon. The tiers are on the pricing page, and you can try it free for a week.
Sources
- Intervals.icu's help texts in the app, for the definitions of load, fitness, fatigue, form, ramp rate and decoupling
- Friel: Managing training using TSB
- Coggan: The science of the TrainingPeaks Performance Manager
- Hellard et al. (2006): Assessing the limitations of the Banister model in monitoring training
- Impellizzeri et al. (2020): Acute:chronic workload ratio: conceptual issues and fundamental pitfalls
Questions and answers
What is Form in Intervals.icu?
Form is Fitness minus Fatigue. Negative form means the last week was heavier than the weeks before it, which is normal while you are training. Positive form means you have done less than usual, as in a taper. Intervals.icu names the zones from high risk to transition, and they are rules of thumb rather than measured limits.
How does Intervals.icu calculate training load for running?
From power, heart rate or grade adjusted pace, in the order you set per sport in the settings. The scale is set so that an hour at threshold power gives 100. For pace the load is measured against the threshold pace you enter in your run settings, so an outdated threshold makes every number in the fitness chart wrong.
What is a good Fitness number in Intervals.icu?
There is no number that fits everyone. Fitness is a weighted average of your training load over 42 days and describes how much you have trained, not how fast you can run. Compare it with your own earlier training blocks with a similar mix of sessions, not with other runners.
What does aerobic decoupling mean in Intervals.icu?
It compares how much power you produce per heartbeat in the first and second half of a session, and Intervals.icu says it is meant for long endurance sessions. A common rule of thumb from Joe Friel is that under five percent on a long easy session means your aerobic base holds for that duration. Heat, hills and fuelling move it too.