Olympic-level, elite female gymnastics are spectacles of ethereal grace and poise. But they’re built on a foundation of brutality. To win a gold medal, girls have to retain a childlike slenderness, develop rubber for bones and accept a life in a veritable boot camp from as early as the age of 4. From the U.S. to China, it’s a regime that critics liken to child abuse — by the end of their adolescence, most female gymnasts are already past it, too ungainly for the feats required of them and fated to nurse chronic pains that will linger long after that last lithe vault into the air.