NYTimes article on Michael Phelps

He knew that prepubescent children can, through training, increase the size of their hearts and lungs in ways that are no longer possible later on. "The larger the heart and lungs," he has noted, "the bigger the aerobic engine." Beginning when Phelps was 12, he worked the swimmer seven days a week, guided by the assumption that competitors who rested on Sundays were at least one-seventh less conditioned. "Michael has a pretty easy life," he would joke, "if you don't count the five hours a day of torture I put him through."