Even now, at fifty-one, Herschel Walker is one of the most impressive athletes in the world. It’s been three decades since he won the Heisman Trophy as a running back at Georgia, and 15 years since he retired from the NFL. He has trained in ballet, taekwondo (he’s a fifth-degree black belt), and, in 1992, was an Olympic bobsled pusher. Never one to rest on his muscled laurels, Walker is currently 2-0 as a professional mixed martial artist.

It was the workout regimen he began at age twelve—and has continued every day since—before he was involved in organized sports, that is most indicative of Walker’s drive. “I would start doing sit-ups and push-ups at seven p.m.,” he says, “and go until eleven. It was every night, on the floor. It was about five thousand sit-ups and push-ups.” These days, Walker says he “only” does 1,500 push-ups and 3,500 sit-ups a day—in sets of 50 to 75 push-ups and 300 to 500 sit-ups or crunches—plus his martial arts training.

Walker insists that the routine will remain, even after he stops competing. “It has nothing to do with my competitions,” he says. “It becomes a drug, or a medicine. Even if I’m sick, I do it. It’s like there’s something saying, ‘Herschel, you gotta get up. You gotta do it.’”

Perhaps that something resides in Walker’s dopamine system. Variations in he brain’s dopamine system make certain individuals more likely to feel reward when using particular drugs, and they are more likely to become addicted. Is it possible that—like sled dogs and lab mice that are bred to be junkies for running—some people are biologically predisposed to get an outsized sense of reward or pleasure from being constantly in motion? All sixteen human studies conducted as of this writing have found a large contribution of heredity to the amount of voluntary physical activity that people undertake.

In the largest study, of 37,051 pairs of twins from six European countries and Australia, concluded that about half to three quarters of the variation in the amount of exercise people undertook was attributable to their genetic inheritance, while unique environmental factors, like access to a health club, had a comparatively puny influence.

It is entirely clear that the dopamine system responds to physical activity. This is one reason that exercise can be used as part of treatment for depression and as a method to slow the progression of Parkinson’s disease, an illness that involves the destruction of brain cells that make dopamine. And there is evidence that the reverse is true as well, that physical activity levels respond to the dopamine system. Several lines of scientific evidence have begun to implicate genes that control dopamine.

Particular versions of dopamine receptor genes have been associated with higher physical activity and lower body mass index. Multiple studies—including a meta-analysis of all published studies—have also replicated the finding that one of those variants, the 7R version of the DRD4 gene, increases an individual’s risk for attention deficit hyperactivity disorder, or ADHD. Tim Lightfoot, director of the Sydney and J.L. Huffines Institute for Sports Medicine and Human Performance at Texas A&M has authored papers on voluntary physical activity in rodents and humans, and he sees a connection between ADHD, exercise, and dopamine genes. “The high active mice we bred in the lab,” Lightfoot says, “they mimic ADHD kids, at least as far as the dopamine system goes….They’re low on a [a particular kind of] dopamine receptors, and if you can drive the amount of dopamine up, their physical activity decreases.”

Ritalin drives dopamine up in hyperactive children, and their activity decreases. Obviously, this is a good thing for a child who is having difficulty sitting still in school. But, Lightfoot suggests, it might have unintended consequences. “These may be kids that have a very strong drive to be active, and maybe we’re blunting it with medications.”

“Our society is so scared right now of kids being fat,” Lightfoot continues. “Well, what if we’re putting some of these kids on drugs that actually may be contributing to this by driving their activity levels down?” In any case, that’s exactly what happened in mice Lightfoot bred in the lab for voluntary running.

A set of scientists have proposed the controversial idea that hyperactivity and impulsivity may have had advantages in the ancestral state of man in nature, leading to the preservation of genes that increase ADHD risk. Interestingly, the 7R variant of the DRD4 gene is more common in populations that have migrated long distances, as well as those that are nomadic, compared with settled populations.

In 2008, a team of anthropologists genetically tested Ariaal tribesmen in northern Kenya, some of whom are nomadic and some recently settled. In the nomadic group—and only in the nomadic group—those with the 7R version of the DRD4 gene were less likely to be undernourished. One of several hypotheses the researchers offered: “It might also be that higher activity levels in [the 7R] nomads are translated into increased food production.” In other words, it could that carriers of that version of the gene are harder workers when it comes to physical activities.

“One of the issues with our field is when we’ve looked at activity and what controls activity, we’ve forgotten that we know very clearly there are biological mechanisms that actually influence people to be active or not,” Lightfoot says. “You can have a predisposition to be a couch potato.”

Quite obviously, as is the case with Kenyan children—many of whom run to school barefoot long before becoming pro marathoners—the necessity of transportation by foot and the aspiration for a better life can have profound influences on physical activity levels. But those environmental factors do not exclude the significant contribution of genetics that has shown up in every study ever conducted on the heritability of voluntary physical activity.

Those consistent findings are reminiscent of a famous quote by Wayne Gretzky, the greatest hockey player in history: “Maybe it wasn’t talent the Lord gave me, maybe it was the passion.”

Or, maybe the two are inextricable.

Adapted from The Sports Gene: Inside the Science of Extraordinary Athletic Performance by David Epstein, in agreement with Current, an imprint of Penguin Random House. Copyright (c) David Epstein, 2013.