Who you are: You do not enjoy exercise. You hate to sweat. Endorphins don't work on you, either. (And, by the way, you are not fooled by the name "Zumba," and see it for what it really is: aerobics.)

What's going on: You may be genetically predisposed to couch-potato-ism.

The back story: Scientists at the University of Missouri wanted to find another explanation for why so few people exercise, despite being bombarded with messages about how wonderful it is for you. Sure, they're busy, but some of the busiest people in the world—surgeons, the president—still find time to squeeze in workouts. So, the researchers wondered, is there perhaps something else going on, something biological?

What they did: A team led by Frank Booth, PhD, a professor of biomedical sciences at the University of Missouri, noticed that when rats were placed near an exercise wheel and left alone, there were some who ran for nearly all of their waking hours, while others showed no interest in running—or in any type of movement that wasn't critical to their survival. The scientists then bred these two different groups to make more runners and more non-runners and closely examined their brains—specifically the nucleus accumbens, which is the hub of the reward system in animals as well as in humans.

What they found: The scientists found distinct differences in the genes of the Forrest Gump mice, and they also found that the running enthusiasts had more mature neurons in the nucleus accumbens than the couch potato mice. This would seem to indicate that the runners found this activity to be innately rewarding, explains Booth. As for the couch potatoes: Running just didn't have the same effect on them —so why waste their energy doing it?

What else they found: In the final part of the experiment, the scientists encouraged the unmotivated mice to run by setting them on the wheels. After six days, this produced noticeable changes in their brains: Compared with sedentary family members, they had more mature neurons in the nucleus accumbus, making it seem like they were finding activity to be more rewarding. (Unfortunately, this didn't really translate into mileage: After six days, the unmotivated mice had only covered about 3.5 kilometers, compared with the whopping 34 kilometers covered by their jog-a-holic brethren.)

Why you should still try that free trial membership at the gym: Research has shown that just a few minutes a day of exercise can bring significant health benefits. At the same time, you'll also be subtly reconfiguring your brain to enjoy it more.


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