I have no mods neither CC in my game (except from the Store).
Now, you have 2 muscles sliders right!? One is for the volume of the muscles and the other one (Late Night) for the muscle definition.
So, I've made an experience: I put my sim exercising during all sim day (he didn’t get tired because he already reached 60H of exercising) and yes, he’s muscles, in terms of volume got a little bigger but muscle definition (Late Night slider) hasn't change at all.
Of course that this is not a "bug" but I believe that in the next patch the slider should move according to exercise.
