Fitness, fatigue, and form, explained

The short answer

Fitness is your training load averaged over about 42 days, and fatigue is the same load averaged over about 7 days. Form is fitness minus fatigue, so it rises when you rest and falls when you train hard. A high form number means you are fresh, and a low one means recent training is still weighing on you.

The 3 numbers

Every session you do gets a training load, a score for how hard it was. Rides, runs, and lifts all land on the same scale, where 100 is 1 hour at your threshold. The method page shows how each kind of session is scored.

Fitness is your load averaged over about the last 6 weeks. Recent days count most, and older days fade smoothly. Each day keeps about 97.6% of yesterday's fitness, so it moves slowly. It reflects what weeks of training have built.

Fatigue is the same load averaged over about the last 7 days. Each day keeps about 86.7% of yesterday's fatigue, so it rises quickly after a hard day and falls quickly when you rest.

Form is fitness minus fatigue, the gap between the 2. If you train the same amount every day for weeks, fitness and fatigue settle at the same level and form sits near 0. When you train more than usual, fatigue climbs faster than fitness and form goes negative. Rest days count as a load of 0, so form rises on the days you rest. Each session counts on your local calendar day, so an evening ride stays on today.

All 3 numbers are free to see in the app.

Where the idea comes from

These numbers follow the Banister impulse-response model, which describes how your capacity to perform changes over time as a function of the training you do (Clarke and Skiba 2013). In Peak Hybrid's version, each session adds to a slow-fading fitness and a fast-fading fatigue.

Peak Hybrid uses the same windows of 42 and 7 days for everyone, rather than windows fitted to your own response. Even fitted to each swimmer, these models described training and performance well but did not predict future performance closely enough to plan an individual's training (Busso and Chalencon 2023). Form is a guide to how fresh you are, and how your legs feel on the day still counts.

The form bands

The app gives your form a simple label from 5 bands:

• Fresh: form 5 and above.
• Steady: form from -10 to 5.
• Productive: form from -20 to -10.
• Overreaching: form from -30 to -20.
• Red zone: form below -30.

The bands are a coaching read of training state, set from practice rather than a study cutoff. In a normal building block, Steady or Productive is where you would expect to be most of the time. A dip into Overreaching after a hard week is fine as long as form lifts again with easier days, and a long stretch in the Red zone is a sign to ease off.

Why a heavy leg day shows up in form

Lifting counts toward your load, so a hard gym session moves your form just as a ride does. A 60 minute leg session at RPE 8 scores 38. On that day it adds about 5 to fatigue and about 1 to fitness, so your form drops by about 4.

That drop matches what the research shows about leg days. A single heavy leg session can lower endurance performance for hours to days afterward (Doma 2017). Because fatigue fades quickly, most of the drop is gone within a few days. 3 days later, that leg session holds your form down by about 2.5.

If lifting were left out, form would read fresher than you are on the day after leg day. Counting it keeps form accurate for people who both lift and ride or run. The article on how Peak Hybrid counts a lift explains the lift score in detail.

Using form around key sessions and races

Before a key session, as a rough coaching guide, a form near 0 or above usually means your legs are ready for hard work. A heavy leg day the day before shows up as a dip. Before a key run, Peak Hybrid's strength plans make that day upper body only, and before a hard or long ride the leg day keeps its place while split squats and other lowering-heavy lifts come off.

During a building block, expect form to sit below 0. Training that pushes fitness up also adds fatigue, and in the coaching read of the bands, a Productive reading is a normal part of a building block.

Before a race, a taper cuts volume while keeping intensity, and fatigue falls faster than fitness. In the taper research, cutting volume by about half while keeping intensity and the number of sessions gave the best results (Bosquet 2007). In the numbers, say you have averaged a load of 60 a day. After 7 days at half that, fitness would be about 55, fatigue about 41, and form about 14, in the Fresh band. The taper article covers what to cut and what to keep.

Try it yourself

The load calculator runs the same math the app does. Set a lift and a ride or run, repeat that day for 6 weeks, and watch the 3 numbers move. The formulas are on the method page.