Why are Midoriya's arms weaker than his legs and why do they get broken more easily?


In episode 14 season 3 of My Hero Academia we've seen that

> Midoriya develops an attack with his legs because his arms get broken more
> easily when he uses One for All

Why are Midoriya's arms weaker than his legs and get broken more easily?




From the start of season 1, Midoriya always used his arms, but since he is not
able to control power output that caused his bones to shatter all the time.
With the help of Gran Torino he was able to develop Full Cowl, but that was
not enough. His fight with Muscular was a prime example of that. Thus he again
shattered his bones multiple times in that fight, and that was the "wait stop"
point for him.

The human body can fix fractures like that to an extent, but he was using this
again and again. So much that even Recovery Girl said "I can't fix you next
time."

Therefore, if he continues to use his arms with uncontrollable power output,
they will break to the point of him not being able to use his arms. With the
help of Full Cowl, Midoriya can use 20% of his power now, but that's a recent
power up he got. To not risk his arms, he decided to find another solution or
fighting style. Using his legs was the answer.

In conclusion his arms weaker than his legs because he shattered his arms so
many times.




Human legs are naturally stronger than arms. This is something I already knew,
but to back up this claim I checked
[wikipedia](https://en.wikipedia.org/wiki/Kick):

> As the human leg is longer and stronger than the arm...

While I was watching the show, I was given the impression this was the reason
he switched to Shoot Style, not because of his arms being repeatedly broken
and thus weakened. The [_My Hero Academia_
Wiki](https://bokunoheroacademia.fandom.com/wiki/One_For_All%3A_Full_Cowl_-
_Shoot_Style) backs up my memory (emphasis mine):

> To use Shoot Style, Izuku concentrates Full Cowl's effects in his legs
> rather than his arms for his smash attacks, enabling him to use their
> **naturally** greater power to improve his striking strength.

