Talk:One For All/@comment-116.88.10.75-20160406120440/@comment-31.205.110.42-20160709151856

Pretty sure we see that if you're not used to using it yet, it does hurt you when you use it. I think when he's still learning how it works, we see Izuku use it to make his punches super strong, but he doesn't know how to make it also increase toughness in his bones and muscles yet, so the force of the quirk shatters the bones in his arms.