Muscle Mode is the simplest kind of before-and-after: nothing about you changes except the one thing everyone notices. You hand it a normal photo, and five seconds later you are standing in the same room, in the same clothes, with shoulders that no longer fit them.
The reason it works is restraint. The background stays put, the lighting stays put, and the camera never moves — so there is no cut to hide behind and nothing to argue with. The growth plays out on the clothing, which is where a real physique change would show first: the sleeves go taut, the chest pulls at the seams, and the take lands on a held flex rather than drifting off. That single locked-off shot is what makes it read as a transformation instead of a new picture.
Use a photo with your shoulders and arms in frame, keep it vertical if it is going to a phone feed, and let the aspect ratio sit on Auto. The clip comes back silent at 720p, which makes it easy to drop under a beat.