Athletic is a skill in The Sims 3. Sims with the Athletic trait gain the skill faster. Athletic Sims enjoy working out, and get a cool vibe when they are at the gym. Most Athletic Sims will aspire to work out, or to become a professional athlete. Athletic Sims can also train others, and enjoy talking about Athletics with other Sims.
Since Generations or patch 1.22, children will no longer gain Athletic skill. They will instead learn the hidden Child Athletic skill up to level 3, which will transfer into Athletic skill once they grow up to teenagers.
Athletic skill also determines the success rate of physical fights when two Sims are in poor social standings with each other. In general, the Sim with the higher Athletic skill is more likely to win the fight. Completing the Body Builder skill challenge significantly increases the chance of the Sim coming out on top at the end of a fight.