Olympic gold medalist Michael Phelps says his rivalry with fellow Olympic gold medalist Ryan Lochte will motivate him to swim his best at the 2012 Summer Olympics in London.

But what motivates us non-Olympians to take part in athletic events?

The elite women start the 115th running of the Boston Marathon, in Hopkinton, Mass., April 18, 2011. (Stew Milne/Associated Press)

According to a Harris Interactive Survey commissioned by Eventbrite (an online event registration and ticketing service) and released on Tuesday, the top reason we opt to take part in marathons, 10K runs, bike races and other “endurance events” is — charity.

Of the 2,209 U.S. adults surveyed during the last week of June 2012, 34 percent said raising money for a good cause was their biggest motivation for taking part in such events. But while 71 percent of respondents said they’d like to participate in that kind of event, only 32 percent said they actually had done so.

Why not? Of those surveyed, 37 percent cited lack of time; 32 percent cited the monetary cost of participating, while 27 percent blamed a lack of motivation or just plain “laziness” as reasons for not taking part in endurance events.

Beyond the glow of supporting a worthy cause, motivators for participating included improved health and fitness (32 percent), the sheer challenge posed by the event (28 percent) and the desire to lose weight (27 percent).

Single people were more likely than married folk to report (43 percent vs. 34 percent) that they didn’t have time to participate in such events, while parents with kids under 18 were far more likely than people without children to cite family commitments as obstacles to taking part (37 percent vs.15 percent).

Well more than half (56 percent) of respondents thought going for a run would be more therapeutic than talking with a therapist or psychologist.

More than 40 percent of those surveyed said they figured training for a marathon would make them more attractive. Not only that, about a third of respondents (37 percent of the male respondents, 25 percent of the females) thought training for a marathon would make them better in bed.

How about you? What motivates you to sign up for a run, walk, bike ride or other “endurance event”? What stands in the way of your taking part?