Sit ups, should i keep my back straight or bend it?

I do a lot of sit ups, but i keep my back straight when i lift myself and bend from the hips (i row and thats what ive been taught to do when i'm in the boat). I've bever thought about it before so should i bend my back to lift myself when i do sit ups (it feels unnatural and much harder for me).
Thanks for the help.

Anonymous2012-04-02T15:56:30Z

Do crunches.
http://www.youtube.com/watch?v=Xyd_fa5zoEU