Just one in four U.S. adults exercised enough in the 1990s, says the Centers for Disease Control.

Almost 30 percent reported no physical activity at all.

Lack of fitness could contribute to workplace safety risks as baby boomer work population ages.

CDC recommends a half-hour of moderate exercise (such as walking, five times a week) or 20 minutes of vigorous exercise (such as running) three times a week.

Findings are based on random telephone surveys conducted by health officials in 43 states and the District of Columbia between 1990 and 1998.

CDC warned that its estimates could be off. Some people will exaggerate their physical activity; others won’t track a month’s worth of exercise.

What’s behind the lack of exercise? Blame it on high-stress jobs that consume time and make Americans more interested in convenience (fast food, hopping in the car instead of walking) and chilling out (watching TV or using the Internet).