It is hard to remember another first lady who appeared as comfortable in her own body as does Mrs. Obama, who studied dance as a girl and later served on the board of an African dance company. She has shimmied, skipped, hopscotched, hula-hooped, jumping-jacked, and potato-sack-raced her way through her tenure as first lady, using not just her position but her body to push for more exercise and better nutrition for children.