Do you cringe when a friend tells you about the awesome burn she got at a spin class? Does the idea of a pre-dawn run give you nightmares? You may have been born that way.
A study in The Journal of Physiology claims that the motivation to exercise may be at least partially due to genetic factors. Researchers from the University of Missouri selectively bred two groups of rats: one group came from breeding rats that had run voluntarily on a wheel, while the other group contained the descendents of rats who preferred not to run.
Researchers examined the groups after several days of not running, and after several days of voluntary running, and found that mice with low voluntary running behavior had fewer mature neurons that are affected by running-induced dopamine plasticity — in other words, mice who don't want to run don't have as many neurons that benefit from one of the feel-good hormones released by exercise. The New York Times has a good explanation of this portion of the study:
[W]hen the scientists closely examined the brains of the two types of rats, they found that by young adulthood the animals bred to run had more mature neurons in the nucleus accumbus than did the non-runners, even if neither group had actually done much running. In practical terms, that finding would seem to indicate that the brains of pups born to the running line are innately primed to find running rewarding; all those mature neurons in the reward center of the brain could be expected to fire robustly in response to exercise.
Conversely, the rats from the reluctant-running line, with their skimpier complement of mature neurons, would presumably have a weaker innate motivation to move.
This could explain why some rats (and potentially humans) don't like exercise: it just doesn't feel as good as it does for others. The reward isn't high enough.
The problem, of course, is that we all know exercise is good for us; what to do if you happen to be like those rats who don't run much on their own?
There's a light at the end of the tunnel. The researchers also found that placing the reluctant-running rats on a wheel increased the density of neurons related to their dopamine response, which suggests that you can train yourself to get more pleasure from exercise, regardless of genetics.
So if you always hated getting off the couch to go for a jog, that old adage may be especially applicable: no pain, no gain.
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