At our wedding, my mother-in-law jokingly expressed a wish that my "athletic genes" would be more prominent in her future grandchildren, since her sons never really had a strong desire to be active. I never really thought much about what genes had to do with it...I mean I just had a stronger desire to play team sports and be active than not. Then an article appearing in the New York Times this week caught my attention. It detailed a study published in The Journal of Physiology finding genetics may influence our motivation to exercise.
So yes, the study used rats...but how they conducted this study is even more fascinating to me. Researchers at the University of Missouri inter-breed normal rats that had voluntarily run on wheels in the lab the most and then bred those that ran the least together. They did this a couple times, got distinct groups and then studied their brains.
Those rats who were "born to run" had more mature neurons, which meant they would be primed to find running rewarding.
Again these are rats and you can't really make the leap from rats to humans...but its possible that there really are people who are pre-programmed to exercise and those who would rather not.