Actress Jaimie Alexander, who plays Lady Sif in Marvel’s Thor films, was seriously injured on the set of Thor: The Dark World on Monday. "Today I sustained a pretty horrible injury," she Tweeted. "I’m lucky I’m not paralyzed." Alexander was hospitalized but said she was having a "speedy recovery." One scene that involved some risk was a major battle scene in which Alexander’s Sif was wielding a sword from horseback.