Board Thread:Manga Discussion/@comment-27082821-20171220152544

I find it interesting that out of all the users of the One For All Quirk, All Might has a "Muscle" form that we know of but not the other users of it. At the point of time, the Quirk seems to be just a simple base multiplier that the user can use to amplify their human abilities for a short or prolonged amounts of time. As shown by the fact that All Might can revert to muscle form back to his real form and vice versa but with a time limit due to his injuries. Instead of what Toshinori Yagi/All Might stated what One For All is.

That said, if even the best known user of it can't even understand how this Quirk really works. So it's safe to think that whatever said by the story should be deemed as speculation of what One For All actually grants the user. 