John Robinson of the University of Maryland and Geoffrey Godbey at Pennsylvania State University analyzed data from the U.S. Census Bureau's most recent national diary study of more than 100,000 respondents of all ages across the country.
The U.S. Centers for Disease Control and Prevention recommends adults ages 18-64 get about 4 hours of physical activity each week by exercising moderately for 2.5 hours per week and engaging in a vigorous activity -- running and muscle strengthening -- for an 1 hour and 15 minutes per week.
The study Americans spend, on average, only about 2 hours each week participating in sports and fitness activities.
"The United States is the fattest country in the world," Godbey said in a statement. "The amount of exercise Americans get has become a major concern."
The study found that walking is the most prevalent activity -- about 5 percent of Americans walker about 53 minutes. Basketball was the most popular active sport followed by football, soccer, baseball, volleyball and hockey.
"Baseball may be our national pastime and football our main spectator sport, but the daily time spent on basketball is higher than both of them combined," Robinson said in a statement.
The findings were published online in Time Use in Australia and United States/Canada Bulletin.