the .package file from a trusted creator (like TSR, Patreon, or Tumblr).
This is the eternal war of Sims 4 modding. Your choice of muscle overlay depends on your game style. sims 4 muscle skin overlay
The built-in fitness sliders in The Sims 4 primarily change the body's volume. While they make a Sim larger, they don't always add the anatomical detail many players crave. sims 4 muscle skin overlay