Anyone who remembers being a hard-working but middle-of-the-pack high school athlete knows that when it comes to being the best, genes matter. Being born with physical gifts simply provides an advantage, as sports pages attest day in and out. We think of the famed Kalenjin people of Kenya, whose lower body proportions and faster leg speed produce one after the other champion distance runners. Or this year's top-performing Los Angeles Dodgers roster, half of whom have uncorrected vision that is 20/10 or better.

Yet David Epstein, author of 'The Sports Gene,' says the relation of genetics to athletic performance isn't so black and white. "There are gifted athletes, but I don't think there's a totally natural athlete," Epstein says. "There doesn't seem to be anyone who is so rare that they don't still have to train, and train hard."

Epstein's book, which is founded on the results of scientific papers, interviews with a slew of experts, and reported stories, suggests there are far more complex factors that decide athletic success. "You may have the background to be a good athlete, but you also have to have the right kind of training," says Jim Skinner, an exercise physiologist out of Indiana University Bloomington. So those Kenyans don't wake up one day and become champions the next, Dodger team members have certainly put in thousands of hours of practice to be able to predict how curveballs and fastballs move, and even sled dogs have to be trained hard.

That's not to say there isn't a predictive aspect based on body types and success in sports, though. And at the highest performance levels, athletes are now being selected based on increasingly specialized traits. "Your genetic predispositions are actually becoming more and more important," says Epstein.

For instance, countries hosting the Olympics in particular have ramped up the process of rooting through their population to discover athletes with the greatest potential success based on genetic factors. Chinese officials found their medal-winning divers for the 2008 Beijing Olympics by asking children to stand and raise their arms above their heads. If their elbows didn't line up directly over their heads, they were cut from the team – based on the assumption they would create too much splash when entering the pool. And British officials plucked one female athlete from among thousands based on her height, limb length, and power output. Helen Glover had never touched an oar in her life, yet five years later she had a gold medal for rowing around her neck.

"The more competitive sports have gotten and the bigger the rewards have gotten, the more important it is to have the requisite genetics and the requisite body type," says Epstein. "And also the more important it is to have the right environment and massive amounts of practice."

Today, physical measurements still rule. Muscle biopsies offer a glimpse into how many fast or slow twitch muscle fibers an athlete might have, and whether they're more suited for sprint or endurance events. Testing of limb proportion, jumping ability, sprint time, hand-eye coordination, and reaction time all offer a look into what sport or position might best suit an individual. "The idea is that, in the future, you'll still want all those performance measures," says Stephen Roth, a professor of exercise physiology, aging, and genetics at the University of Maryland. "But you'll add the DNA onto them and maybe get some more subtle information. We're not at that stage right now – but that certainly may happen."