The same as "beer muscles", where when a person is drunk, they do and say things they wouldn't normally have to courage to do, or say. Internet muscles are where a person gets online, reinvents themselves, and says things they can't say in public, like using racial slurs for example.

When you eat dirt for the first time and leave your family to build a shack in the woods

When someone tries to act all big and cool on the internet by posting offensive and rude posts and comments on forums, youtube, etc. Often done to attempt to feel good about one's sad pathetic lonely life.