So apparently some people can have a ton of muscle and still be weaker compared to others because they cant command those muscle fibres to contract only activating others so a cool quirk could be something that gives them absolute control of every single fiber this could also be a way to limit muscle output. By only using those muscle strands to the max slot they will break and reconnect again so he could now exactly what to train and can exercise those muscles. Also if I give him absolute control over his anatomy (stollen from that one magic knights anime with robots)he could braid the muscles which should pull more from that animes logic.