How JustGains calculates muscle fatigue

The JustGains muscle fatigue view is not reading your nervous system or measuring soreness. It is a practical estimate built from your logged sets. The goal is to answer a simple question: which muscle groups look recently trained, which are still recovering, and which look fresh enough to train again?

The shared recovery utility looks at muscle group metrics and separates work into recent sets and older sets. When explicit buckets are available, it uses them. When they are not, it falls back to completed set count and the last worked date.

Recent vs older sets

The fallback recovery window is about 2.5 days. If a muscle group was last worked inside that window, its completed sets are treated as recent. If the work is older than that, those sets move into the older bucket. If there is no recent work and no older work, the group is treated as recovered.

The utility also creates a weighted set count. If recent sets exist, older sets add only one-third of their count on top of the recent count. If no recent sets exist, older sets count at full weight for the recovering state. That keeps fresh fatigue visually stronger than work from farther back.

The diagram intensity is relative. JustGains totals the weighted work across muscle groups, finds the strongest share, and scales opacity from a low active value to a higher active value. So the color tells you how much that muscle group stands out compared with the rest of your recent logged work.

This is an estimate, not a medical recovery score. It does not know your sleep, nutrition, stress, injury history, or whether a set was technically easy unless that data is reflected in your logs. It is best used as a planning nudge, not a command.

If your chest is marked fatigued, that does not mean you are forbidden from pressing. It means your recent log history has enough chest work that JustGains is warning you to think before piling on more. Combine that with performance, soreness, and your plan.