You get to use these toggle bars when you first create your sim, however, if you go to modify your sim in create a sim after you have started playing the game these bars won’t be an option any more.
One will change how much muscle max your sim has and another will change how much body fat your sim has. When you first create your sim you have two bars that show up on the left of your sim.
How to get weight gain and weight loss potions in Sims 4? Toggle Sims’ weight in create a sim
