Boku no Hero Academia

[DarthEnderX]

If it is unrelated to One for All, you're basically saying a quirkless person who has atrophied like him can suddenly become buff for short periods "like holding a flexed muscle."

That's exactly what I'm saying. If the show was realistic, I might think that was ridiculous. But it isn't. So I don't.

As it is, I think it's literally just Master Roshi's ability to make himself buff again for a short time even though he normally looks frail and old.