Is it true that girls who do karate might lose their verginity during a strong hit ?

Hymen: The hymen is the lining that covers the internal area of the vagina. It can be broken with trauma. "virginity" refers to whether a person has had intercourse or not for most. Yes, the hymen can be broken with trauma of any kind in the vaginal area.

Not technically: It is possible for a small tear of the hymen, however this will usually heal as long as there is no further trauma. There are groin protectors that women can wear if this is a big concern. Groin strikes are illegal in tournaments for both sexes, but accidents can happen so it is "possible".