My sister, who showed some decent athletic potential when she was young, isn't a big exerciser as an adult.  Back in our youth, it was she who caught on to horseback riding early, she who had no fear of the water, she who did all the team sports.  For me, these skills and inclinations were hard-won.

But now that we're both adults, with jobs and spouses and kids and the like, it's me who insists on getting my workouts in, no matter what else may be happening; me who endlessly obsesses over flexibility, strength, body fat, and the like, me who makes his living as a champion of good clean livin.'  Years ago, she said to me, in one of our many conversations in which I flail my arms and extol the virtues of working up a virtuous sweat, "I think you're just genetically predisposed to liking exercise."

I dismissed such nonsense, and was vaguely offended.  She had all the athletic genes; she did all the sports, I worked like a dog to endow myself with even the vaguest whiff of athleticism, and here she was claiming I had the genetic edge?  How could it be?  All she needed to do is find some activity she enjoyed, throw herself into it, suffer through the first few weeks of unfamiliarity and then, ecstatically, get on the fast-track to mastery of whatever exercise she chose, and the glowing health and vitality that came along with it. 

She once told me she liked being in a pool, and I launched into a spiel about sprint intervals and stroke form and working steadily up to 40 laps three times a week.  Her response?  "That just sounds too much like exercise." 

But now it appears she may have been right.  According to this,

 

...a growing body of evidence suggests that it's not merely motivation but also genetics that separate slouches from fitness fanatics, and at least some of these genes appear to act on the brain's pleasure and reward center.

Studies of twins suggest that some of the differences between these types of people come down to genetics. A 2006 Swedish investigation looked at leisure-time physical activity in 5,334 identical and 8,028 fraternal twins. The findings revealed that the exercise habits of identical twins were twice as closely matched as those of fraternal twins.


In an effort to find the genes involved, physiologist Theodore Garland at UC Riverside turned to rodents. He placed exercise wheels in the cages of ordinary mice and measured how often they scurried around in the wheels.

"This was voluntary exercise," Garland says. "It's sort of like how some people jog and others don't."

Researchers then selected the mice who ran the most and bred them with other so-called "high-runners" and repeated the experiment for more than 50 generations.  The result was a strain of high-runner mice that run as many as eight hours per night.

 

The image of a mouse on a wheel, running doggedly away for eight hours, is both hilarious and a little sad.

I've known a couple of truly fanatical exercisers--I'm on the edge, myself, I think--who are like these mice, running away on their hamster wheels far, far more than is good for them. 

The researchers tied this compulsive need to exercise to dopamine.  Sedate mice run more when plied with stimulants like cocaine and Ritalin; high-running mice don't:

Because cocaine and Ritalin alter levels of the brain chemical dopamine, a neurotransmitter involved in pleasure and reward, the drugs' different effects on the two breeds suggest high-runner and regular mice may process dopamine differently in the brain -- and that may dictate how much pleasure they get out of running.

I'm not altogether blown away by these studies.  Like most trainers, I've had my share (maybe more than my share) of clients who don't seem to take to exercise no matter how much arm-waving I do.  They just...hate it. 

The article points out that there's little correlation between genes and the tendency to exercise 60 minutes a week or less.  But for the few, the proud, who hit the recommended 150 minutes or more?  Apparently it's all in the genes.