FitnessOS / How it works
How FitnessOS estimates muscle recovery
FitnessOS does not guess from a feeling. After every logged set it calculates how much fatigue each muscle group has accumulated and how long it usually needs to clear. This page explains the idea in plain language.
What the engine counts
- Sets and reps. More hard sets mean more fatigue, with diminishing weight as sets pile up on one muscle in a day.
- Load compared to your best. A set near your best effort for that exercise counts more than a light one.
- Closeness to failure. If you log reps in reserve, it is used. If not, rep count and a drop in reps at the same weight act as a proxy.
- Helper muscles. A bench press also tires triceps and front delts, so they get part of the load.
Different muscles recover at different speeds
Each muscle group has its own typical recovery window. These are approximate starting values:
- Chest: about 56 hours
- Back: about 56 to 66 hours
- Legs: about 72 hours
- Shoulders: about 45 hours
- Arms (biceps and triceps): about 36 hours
- Core: about 24 hours
Age nudges these windows slightly, and a weekly “how did you feel” question calibrates them to you.
What the colors mean
- Green (85% and up): ready to train.
- Yellow (60 to 85%): still recovering, keep volume moderate.
- Red (under 60%): rest is the better call today.
These are advice, not locks. You always decide what to train.
What it is not
The percentages are an estimate calculated from your own training data, not a medical measurement of your body. FitnessOS is a training guide and does not give medical advice. If something hurts, stop, rest, and see a doctor or physiotherapist.