A group of running rebels are shedding their shoes and reporting years of injury-free miles. Some ultramarathoners, biomechanics experts and doctors think that's probably a good thing. Others go so far as to say running shoes are in fact causing injuries. But what about the glass, nails,and concrete of modern courses? Meanwhile, running shoe companies continue to precisely measure runners, and pound and flex shoes in their high-tech labs. Could shoes—and shoe companies—be covering hundreds of thousands of perfectly able bare feet?