A recent scientific study shows evidence to suggest that the motivation to exercise may be genetic.

In rats.

While more studies are needed to draw the same conclusion in humans, it’s quite common for a person’s inspiration to exercise to be very personal. This motivation often comes from one’s life experiences.

This is true for Lisa Carney, who exemplifies the changing demographics of marathon runners. More and more of the half-million runners who annually finish the grueling 26.2 miles are women.

Running USA reports that the proportion of marathon runners who are women has increased from 11 percent in 1980 to 42 percent in 2012.

Marathon runners are getting older, too. Since 1980, the percentage aged 40 and older has gone from 26 percent to 46 percent.

The gender and aging trends hold true in non-marathon distance races, too, and are reflected in Tucson area local races.

“These days we are seeing a lot more women. It’s definitely gotten a lot older,” says John Sabatine, a board member of the Southern Arizona Roadrunners. “I think the difference these days is that the emphasis is really just about finishing.”