I started a new game with a new Sim, who had the traits listed below:
Athletic, Handy, Friendly, Natural Cook, and another 5th trait I don't remember.
I decided to make a 4 story house for my Sim, and finished the first 2 floors. I got the Power Burn Station, and when I commanded him to work out on it, the bar that tells you when he's going to finish didn't move at all! I mean, he worked out for 45 hours and 55 minutes straight and I didn't even see green on the bar, even when he had the Quick Burst work tone! I decided to go along with it and his Athletic level was maxed out after the marathon workout. He had no Skill Challenges completed, as he was working out for the first time.
Example: I have a Sim that hasn't grasped the basics of the Athletic skill yet. His needs are static, and he decides to work out. He should be fatigued at 1:30 PM or so, but at 1:30 PM, he doesn't get off, and the action bar shows that it is empty. This continues until I command him to stop working out. Of course, I command him to stop after I complete all of the *Work Out for X Hours Straight* and the mastering of the Athletic skill.