Edit: (Credit: FNSUITE GHOST) Unlike the first Escapists game, your stats don't decrease over time.
For this achievement you will have to max out (100) the following statistics:
- Intellect: Libary / Bookcase around prison
- Strength: Gym "Workout: Strentgh"
- Fitness: Gym: "Workout Strentgh"
Tip: To get energy back fast you can consume food items, lay on a bed, get a shower or sit in a chair.
Once you have got your last statistic to 100 the achievement will pop.