Yu-na, now heart broken and ashamed of her vampyric state, paid an extraordinary sum of money to a group of mystic experts so that they would quest into the Arirang mountain and hopefully procure an ancient remedy for her malady. It's said that only few extremely talented and lucky individuals are able to complete the quest every century. People are plucked out of childhood and trained in the arts their whole lives just to fulfill this purpose, but the task is nearly impossible. While living out her days as a vampire, uncertain if she would actually be cured, Yu-Na decided to also experiment with radical scientific procedures in order to cleanse herself.
One very diabolical scientist said he had a cure for Yu-Na, which had so far been successful on mice. She was willing to try anything and underwent the surgery. The scientist had lied to her, though. What he actually wanted was to experiment on her supernaturally altered body, trying genetic mutations that would quickly kill any normal human. His experiment was successful; he altered the DNA of the bones in Yu-Na's legs to make them nearly unbreakable, which was tested by trying to slice through part of her lower leg with a silver saber. However, a side effect was caused by this procedure: Yu-Na was in immense pain daily as the genetic mutation constantly created internal pressure within her body. Enraged by the scientist's treachery after discovering what he had really done, Yu-Na quickly consumed his blood and used his corpse as kindling for her fireplace.
Luck shined upon Yu-Na, though! One of the mystics had returned from the Arirang mountain and actually been able to obtain the cure for her vapirism! She immediately drank the potion and found herself returned to a warm-blooded human state the next morning. The pain in her legs was even gone. Upon returning to the ice rink to train for the glorious Golden Spin of Zagreb competition and get back on track with her life, she found her legs to feel a little heavier than before. The experiment performed on her had carried through back to her human state and she now had unbreakable leg bones that were a little denser then before. At first this extra weight threw her jumps off, but every time she fell her legs were never in danger. This quickly gave her the confidence to jump without fear and adapt to the additional weight gain.
In summation, it's going to take a lot of strong hits with a nice heavy crowbar to really hurt Yu-Na's legs. Her bones are super-reinforced now and one wimpy little Nancy Kerrigan whack won't be enough.