illusion muscles can be one of two things. one: illusion muscles are when a weak person looking person who looks like he doesnt have any muscles tenses his muscles and his muscles noticeably buldge from his skin, and this person is also quite strong.
the other form of illusion muscles is when, a strong looking person, with big muscles tenses and there is no visible change to his muscle size, this person is also fairly weak.
person 1: dude check out that guys arms he must be strong
person 2: hes not that strong, he just has illusion muscles