As he learned to control his new quirk, One For All, Deku had a hard time keeping his body intact. But why does Deku break his bones?
Deku has to break his bones because he can’t control One For All. Deku puts all of his power into one body part at a time, willing to break it if it means using the quirk. It took time for Deku to regulate his quirk and evenly and safely spread it around his entire body.
He has now reached the point where he can use One For All freely and not hurt himself. It would take extreme amounts of power to cause self-damage at this point, as Deku’s body is well-acclimated to his quirk.
Plot Behind Why Deku Has to Break His Bones
In chapter #5, the plot begins the night after Deku received his acceptance letter from U.A. The previous day’s entrance exam was difficult. He was told by All Might to meet on a beach, and the two discussed Deku’s future. He was congratulated on passing and reassured that no one at the school has been told about his dynamic with All Might.
All Might explains that he took a job teaching at U.A. in order to find his successor. He had been searching for a long time before Deku came along. Looking down at his hands, Deku questions his ability to use One For All to its max potential.
“One For All,” Deku said. “My body is broken after a single punch or kick. I just can’t wield it.”
It’s natural for One For All’s power to be difficult to control at first, All Might explained. Deku is at the stage where he must figure out how to regulate the power on his own and only put out as much as his body can handle. The more Deku’s body is tempered, the better he can control the power.
So that’s the answer. Deku has to break his bones because he can’t regulate the power of All For One yet.