Well, in NY own simpler opinion, I don't think it's tthe major cause to flat chest of women participating in sport. From a particular research I stumple upon; I read from the research that, there are different types of female breast or chest pattern as designed bby God and due to individual body structure. Some chest are naturally flat even from the beginning and doesn't need any other thing to further make it flat.
So are slim and pointed and doesnt go flat no matter the situation. While some are big and gets flat if not properly maintained. Some women started spoiling their chest them self right from tender age by allowing rough play with it.
However, I trust ladies who participate in sport usually wear some smart wears to keep all part of their body sound and active. Trust me, they don't wear what will make it flat. Sport makes the body to form properly at young age .