How to read Fitness, Fatigue and Form in Intervals.icu

First Load: what was on the input

Load — assessment of the load of a specific activity. Intervals.icu supports several calculation methods, including power, heart rate and tempo; model and priority settings are important. Features are listed on Fitness Graphics page.

The same 60 minutes can give a different load. And the same trip with different FTP or calculation sources means different numbers. Therefore, first check the specific activity: duration, schedule, thresholds and method of assessment.

In heart rate calculation, there's also a model, not a direct measurement of workload. The developer describes it in explanation of the assessment of the heart rate load. Heart rate and power should not be considered interchangeable rows without checking the settings.

Three lines — three different meanings

Fitness

A model of the accumulated load over a longer period. The default settings use a time constant of 42 days. The increase in number does not equal the proven increase in power or speed.

Fatigue

A more rapidly changing estimate of the recent load; the standard time constant is 7 days. This is not a measurement of physiological fatigue.

Form

The ratio of Fitness and Fatigue. It helps to see how the recent load differs from the accumulated one.

The model uses history weighting, not just the sum of the last 42 and 7 calendar days. The assignment of indicators and settings are described in Fitness page Guide.

Check: number or percentage?

The Form can be shown as the Fitness − Fatigue difference or as a percentage of Fitness. These scales cannot be compared directly.

Before comparing with a friend's screenshot, check the format in the chart settings. Details — in parsing the absolute and percentage Form. With low Fitness, percentages are especially sensitive to a small load.

How to disassemble a week

1. Choose a week with a clear travel history.
2. Check if all activities are in place and if there are any duplicates.
3. Find the days with the highest Load and match them with the Fatigue change.
4. Look at the scheduled classes separately: the forecast is not equal to the work already done.
5. Match the graph with notes about sleep, well-being and subjective severity.

In Intervals.icu, the indicators of the day can take into account the completed and still planned load. This is clarified in explanation of daily values. Before comparing the morning and evening screenshot, check the composition of the day.

Why did the line suddenly change

• The story has loaded up. The model received data that did not exist before.
• Thresholds or calculation method have changed. Check the settings and see if the changes have been applied to past activities.
• The record contains outliers or an incorrect duration. First, open the workout itself.
• The plan counted differently than the fact. Check the planned and completed classes.

Do not change the time constants for the sake of the desired graph color. First, find the reason for the change. To start with an empty history, you need time to accumulate data — the first values especially depend on the initial conditions.

How to use a graph without chasing a number

A useful question: "What has changed in my workload and does it fit the plan?" Not useful: "How to make Fitness higher at any cost?" Compare yourself to yourself with the same settings, not the rating of numbers from different people.

The color of the zone does not confirm the health, recovery or safety of the load. If the numbers differ from how you feel, do not use them as an excuse to ignore the symptoms. It is better to disassemble training solutions with coach; medical questions — with a doctor.

Back to setting up Intervals.icu · How to choose a coach.