When the power went out Sunday at the New Orleans Superdome, coaches wanted their players to stay calm, focused and ready to spring back into action. So as the lights gradually came back, they illuminated lots of Ravens and 49ers on the field in a variety of stretches. That strategy would win approval at the Mayo Clinic, where staffers say stretching increases blood flow to the muscles and can help improve flexibility. They admit, though, that studies show mixed results on the benefits of stretching. Travelers have been known to ward off pain and stiffness by doing simple stretches before …