Jeff Bercovici, author of "Play On: The New Science of Elite Performance at any age." The book explores how scientific breakthroughs are helping elite athletes in a variety of sports sustain their excellence far beyond what once would have been possible. <This interview was recorded and initially broadcast back in 2019. Since then, two of these elite athletes mentioned in the interview, Roger Federer and Serena Williams, have endured physical setbacks that have taken them to the brink of retirement.)