The typical American spends just two hours a week exercising, according to researchers at Penn State and the University of Maryland, even though the U.S. Centers for Disease Control and Prevention recommends adults age 18 to 64 get four hours of exercise per week. That could help explain why a separate study by Duke University finds 42 percent of the population could be obese by 2030, adding nearly $550 billion to the nation’s healthcare tab.

Ideally, the CDC says, we should get 2.5 hours of moderate exercise — brisk walking, riding a bike on level ground, that sort of thing — each week. We ought to spend another 75 minutes per week engaging in vigorous activity like running or shooting hoops.

 The researchers, who analyzed American Time Use Study data the U.S. Census Bureau gathered from more than 100,000 people nationwide, found teens are the most active, spending about 41 minutes per day exercising. Adults spend about 17 minutes a day, while those 65 and older get roughly 13 minutes of exercise per day.

Walking is the most common activity, with about 5 percent of Americans spending about 53 minutes a week on foot. Among those people regularly breaking a sweat playing a team sport, basketball is the most popular activity. The results are reported in the 2011 edition of Time Use in Australia and United States/Canada Bulletin.