She was in training long before the Black Swan started filming. Entertainment Weekly had a long interview with her about her preparation which started a year before. This link seems to be the same article..
http://www.natalie-portman.org/?p=251
I have never met any dancers who used swimming as a part of their training. Dancers generally just dance most of the day-although I know a lot of male dancers who regularly do weight training in the gym. Swinning is a great exercise, but many times it tends to build the muscles in the upper body in a not-very-ballet-looking manner. But-I'm no authority on swimming!Maybe a pool was accessible when she was trying to drop some weight while filming the previous movie.



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It had me feeling tingly for about two hours after it ended. I'd definitely recommend it.
. It involves bodily harm. On a related, though different note, Jeremy tweeted that he walked out of Black Swan because he felt violated. Take that for what you will. 
