How difficult is ballet? An answer from Slate.
That kind of damage comes from spending hours a day training. They move as if they don't weigh anything at all, but that's a carefully crafted illusion: They move that way because they are intensely strong. They combine that strength with a grace that comes from practicing the same moves over and over and over until it looks as if it's weightless.
It breaks a body. Most ballet dancers are completely shot by their mid-20s, and many will suffer lifelong disabilities from the effort.