The reasons behind your numbers, from the same sources the app uses. Where a detail is our own choice, we say so.
Each exercise has a rep range, like 6–10. Keep the same weight and add reps until every set reaches the top. Then the weight goes up by the smallest step and the reps drop back down.
Add 2–10% to the weight once you can do 1–2 reps more than your target. If you're new, 65+ or carrying extra weight, you reach the top of the range twice in a row first.
Bodyweight and band moves have no weight to add, so you add reps until every set reaches the top of the range, then move on to a harder version of the move.
Sessions are compared by an estimated one-rep max, so 60 kg × 8 and 57.5 kg × 10 can be compared fairly: weight × (1 + reps ÷ 30).
At least 10 hard sets per muscle a week grows more than fewer, with each muscle trained about twice a week.
Big compound lifts: 2:00 (3:00 when you do 5 reps or fewer). Smaller isolation moves: 1:30. Holds and balance work: 0:45. The exact times are partly our convention.
After a break we start lighter so the first session back goes well: 90% of your last weight after 14+ days off, 80% after 28+. These percentages are our convention, not a study result.
Bodies adapt. The old 7,700 kcal-per-kg rule overstates long-run weight loss, so the app models your weight week by week and you re-measure every 2–4 weeks.
Fat loss at 0.5–1% of your bodyweight a week. In one study the slower group kept more lean mass.
In a calorie deficit, lifting is there to keep the muscle you have. Resistance training helps hold on to lean mass while you lose weight.
Adults: 150–300 min a week of moderate aerobic activity, and muscle-strengthening on 2 or more days. For 65+: balance and strength on 3 or more days, to prevent falls.
The drop in mortality levels off around 8,000–10,000 steps a day for adults under 60, and 6,000–8,000 for adults 60 and over. Walking to work or the shops counts.
Muscle and strength hold on about a third of the building volume, if the sets stay hard.