The Physical Activity Guidelines for Americans also recommends 75 minutes of vigorous-intensity activity per week, or an equivalent combination of moderate- and vigorous-intensity physical activity, the Centers for Disease Control and Prevention's Morbidity and Mortality Weekly Report said.

This report found that overall, 64.5 percent of respondents in 2007 reported meeting the guidelines -- 68.9 percent of men and 60.4 percent of women. Approximately one-third of U.S. adults didn't report meeting minimum levels of aerobic physical activity as defined by the 2008 guidelines.

Almost 49 percent of the same respondents reported meeting Healthy People 2010 objectives -- adults to engage in at least 30 minutes of moderate-intensity activity, five days per week, or 20 minutes of vigorous-intensity activity, three days per week.

Healthy People 2010 is a set of health objectives for the nation to achieve over the first decade of the new century, which is part of the U.S. Department of Health and Human Services.