Any Sim who appreciates a slim body should focus on the Fitness skill. The Sims 4 has several ways to develop this skill. Read page to learn how to improve your Sim's Fitness skill as quickly as possible.
The best way to develop this skill is to train with one of these devices: a treadmill, a punching bag or a weight machine. Remember that during exercises your Sim will lose his hygiene and soon he will be exhausted (if he will keep exercising he will be in pain later). Leveling up this skill allows you to do some new exercises such as endurance running on a treadmill, lifting on a weight machine, box on a punching bag and so on.