In his new book, The Sports Gene, DAVID EPSTEIN dives deep into the gene pool to study what sets world-class athletes apart.

The arctic circle in the dead of ­winter is hardly a dream destination for most people. But there was David Epstein, a senior writer for Sports Illustrated, elated to be standing face to face with the Finnish reindeer herder he’d spent months tracking down for his new book, The Sports Gene: Inside the Science of Extraordinary Athletic Performance (Current Hardcover, $27).

Eero Mäntyranta wasn’t extraordinary for reindeer games but for his performance at the 1964 Winter Olympics, when he glided to gold in the 15K cross-country skiing competition 40 seconds ahead of the pack. What enabled ­Mäntyranta to set this remarkable (and still unbroken) record? Do exceptional athletes like him have exceptional genes, or are they instead examples of the 10,000-hour rule, which posits that repeated practice trumps genetic influence?

Those questions propelled Epstein to expand upon his award-winning 2010 Sports Illustrated story on the nature vs. nurture debate and embark on a 109,000-mile globe-trotting quest for answers. “At the time, that idea [of the 10,000-hour rule] was all the rage in both sports-science writing­ and among a segment of sports psychologists,” Epstein tells American Way. “So I wanted to find out if genes do, in fact, matter. It turns out that they absolutely do, but not always in the ways that I would’ve guessed.”

Epstein is skillful at taking such genetic jargon as microsatellites and myostatin and putting it into clear and captivating context. He does this in part by taking readers not only into the lab with scientists but onto the track, field and race courses with the amazing athletes they study: Kenyan marathoners, Jamaican sprinters, Major League Baseball players and even Alaskan sled dogs. “We tend to think of natural talent as something that exists before you start training,” Epstein says. “But a large proportion of talent is actually the body’s ability to respond to and adapt to training.”

All this genetic know-how may some day benefit the less fit among us with exercise plans ­customized to our specific genome. In the meantime, Epstein gives his readers a front-row seat to the fascinating “parade of biological diversity” displayed by top-tier athletes near and far — or in the case of Eero Mäntyranta, very, very far.