Using the treadmill doesn't bring the muscle slider down but it is really the only option to keep sims slim without them bulking up. The thing is you have to get the muscle slider to max out to complete the requirement for the bodybuilder aspiration. After it is checked though you can stop using weights etc as it stays checked.
I always think that female sims look so gross with muscles, it's the way the shading is used to represent that on their skin, almost as if they are permanently dirty. Eww.