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This phenomenon has also been demonstrated repeatedly with lab rats. When rats who can run farther than average before giving in to exhaustion are bred with one another, each generation has successively greater endurance.
Family lineage is critical for humans as well. Since 1992 the HERITAGE Family Study has analyzed hundreds of families to find out how "heritable" exercise capacity is, or how much variation in a particular fitness trait is due to genetic inheritance. In one segment of the study, 98 families that had not been exercising were put on identical 20-week training programs on stationary bikes. Their aerobic capacity—a gauge of how much oxygen the body can use during exercise—was measured before the program and at the end. In terms of improvement, family members tended to be similar, while improvement varied widely among families. Even after five months of training, some people in families that benefited little on average did not improve their aerobic capacity one iota, while others in families that generally showed marked improvement increased it up to 50%. Statistical analysis showed that about half of a person's ability to improve with training was determined by his or her parents. The amount any person improved in the study had nothing to do with how good he was to begin with—his "baseline aerobic capacity"—but about half of that baseline, too, was attributable to family inheritance.
In work from the HERITAGE study that has not yet been published, the researchers have identified 20 genetic markers that can predict entirely the genetic component of an individual subject's aerobic capacity improvement after months of bicycle training. The point of the work is to be able to give people individualized advice regarding whether exercise is the key to improving their health, or whether they are among the low responders who might be better off resorting to medication. There are also implications for athletes: These 20 genetic markers can help predict whose strong work ethic will pay off in aerobic spades and who might be pedaling largely in vain.
The HERITAGE Family Study is controlled science, with all subjects on identical training plans. In reality, though, one of the most hallowed notions in sports is that we can each carve our athletic destiny by waking up earlier and working out harder than the guy or girl at the next locker—that is, by adopting different training plans. But researchers have found that even motivation to work out has an important genetic component. A 2006 Swedish study of more than 13,000 sets of fraternal and identical twins—fraternal twins share half their genes on average, while identical twins share all of them—found that the exercise tendencies of identical twins were twice as likely to be similar as those of fraternal twins.
Another 2006 study, of more than 37,000 pairs of adult fraternal and identical twins from six European countries and Australia, concluded that about half to three quarters of the variation in the amount of exercise people engaged in could be accounted for by their genetic makeup, while environmental factors, such as access to a gym, often had less influence. And studies of laboratory mice suggest that the difference could be in genes that regulate dopamine, a brain chemical involved in sensations of pleasure and reward.
For more than a decade Theodore Garland, a physiologist at UC Riverside, has been giving mice a wheel and allowing them to hop on or spurn it. Normal mice voluntarily run three to four miles each day, but Garland and his colleagues separated the mice that chose to run the farthest and bred them with each other. By generation 16 the high runners were pounding out more than seven miles a day. "The normal mice are out for a leisurely stroll," Garland says. "They putz around on the wheel, while the high runners are really running."
But one interesting difference was that the high runners' brains were larger. "Presumably, the centers of the brain that deal with motivation and reward have gotten larger," Garland says. His team plied mice with Ritalin, a stimulant that alters dopamine levels. Once doped, the normal mice, apparently gleaning a greater sensation of pleasure from activity, suddenly ran more. But the doped long runners did not. Whatever Ritalin does in the brains of normal mice is already happening in the brains of the high runners. They are running junkies.
Other studies have begun to identify the specific genes related to dopamine processing that differ between the marathon mice and their average counterparts. "Who says motivation isn't genetic?" Garland asks rhetorically. "In these mice it's absolutely the case that motivation has evolved." A 2003 study found an association in women between physical activity and a particular dopamine receptor gene, DRD2.
Wayne Gretzky famously said, "Maybe it wasn't talent the Lord gave me, maybe it was the passion." But what if the two are inextricable? What if passion is a talent? Might not, say, Floyd Mayweather Jr.—who has been known to jolt awake in the middle of the night salivating to get to the gym—get an innately outsized sense of reward from working out? This gets to the questions that genetics has brought us no closer to answering: Where does genetic motivation bleed into free will? How can we affect our athletic fate? Who's in charge here—is your body running you, or are you running it?
After Mayweather's win over Oscar De La Hoya in 2007, he told SI about an unhappy period in the past when he constantly worried about money. "But I'm happy now," he said giddily, mentioning the $25 million he had made from the fight. Certainly, rewards more obvious than dopamine—such as money—are huge motivators. And, as it does with so many human endeavors, money can counter athletic heritage. "How many of the top Kenyan runners have sons or daughters who are excelling at running?" asks Pitsiladis. "Almost none. Why? Because their father or mother becomes a world champion, has incredible resources, and the child never has to run to school again."