Simple LoRA to help with adjusting a subjects muscle mass. You can swing it both ways pretty far out from -3 to +5 without much distortion. Positive gives them more muscles. Negative gives them less muscles.
One issue you will find is that the more muscles they get, the more tan they get, and vise versa. Bodybuilders grease themselves up with shoe polish, and muscular guys probably get more sun... Probably as simple as that. I will try to find a way to regularize it so it is less pronounced. If you use my Skin Tone Slider you can counteract that effect.