NEW YORK (Reuters) - Fitness is often a combination of personal choice and environmental support, experts say, and a ranking of the 50 healthiest U.S. cities seems to reinforce the theory.

High rates of physical activity helped to propel Minneapolis-St. Paul to the top of the list of the American College of Sports Medicine's 2012 American Fitness Index (AFI) for the second year in a row, while raised obesity levels and smoking pushed Oklahoma City to the bottom.

"When I say Minneapolis ranked No. 1, people give me an 'are you kidding me' kind of look," said Walter Thompson, the chairman of the AFI Advisory Board. "Between November 1 and April 1 they have cold and snow, but they've addressed that."

Thompson said the solution was a proliferation of exercise studios that dot main streets, and a local government that has invested resources in park lands.

When people in Minneapolis were asked if they had any physical activity in last 30 days, he said 82.9 percent said they had.

The index, which considered factors ranging from the number of tennis courts to the percentage of smokers, relied on information from federal government data, such as Centers for Disease Control and U.S. Census reports, as well as information from the 50 cities.

Washington, D.C., Boston, San Francisco, and Hartford in Connecticut were the five fittest, healthiest metro areas, while Birmingham, Dallas, Texas, Louisville, Detroit, Michigan and Oklahoma City fared the poorest.

"A couple of cities have made significant improvements," said Thompson about the rankings, which began in 2008. "A policy decision can dramatically impact environmental indicators, like smoking bans or bicycle lane ordinances."

New York City, ranked 22, climbed eight spots since 2011, while Nashville, at 27, gained 10 and Las Vegas climbed four spots to 39.

Shirley Archer, a spokeswoman for the American Council on Exercise, said studies show that many environmental factors affect how physically active people are in their communities.

"The more available green spaces, the more likely kids and adults are to enjoy that outdoor space, so simply having more parks and playgrounds makes a huge difference," said Archer. "If you add walking, jogging or biking paths, people will use them."

Connecticut-based exercise physiologist and running coach Tom Holland also believes that if you build it, they will come.

He said last year his home town of New Canaan, in Connecticut, filled holes and covered up tree roots and rocks on a popular public park trail to make it safer for running.

"Stimulus money that went to fixing/repaving roads has made running and biking on them much safer and more fun," he said.

With an obesity rate of over 31 percent and almost 23 percent of residents still smoking, according to the index, Louisville's 48th place ranking does not surprise Kathy Harrison, communications director for Louisville Metro Public Health & Wellness.

But she's optimistic about the future.

"I really feel like we mirror a lot of the country. We have quite a few problems when it comes to health," Harrison said. "But since 2004 we've been making quite few changes."

Initiatives include bringing farmers markets into so-called food deserts, which Harrison defines as low-income areas saturated with fast-food options but short on public transportation choices.

"We're also working with convenience stores to become healthier," she said. "We'll assist them if they meet certain criteria."

Projects are also under way to dramatically increase the biking/hiking trails around Louisville.

"We're a working on a ‘road diet,' taking highways from four lanes to three," Harrison said about the project to accommodate and encourage more foot and bicycle traffic.

"We've created some fun events to promote fitness. We have natural amenities," Harrison said. "Promoting a sense of fun brings people to them."