Compared to mothers with children 5 to 18 at home in 1965, their contemporary counterparts spend on average 11 fewer hours per week in physical activity - including housework, meal preparation, child care, laundry, and exercise. But they are logging seven more hours per week in sedentary activity, such as watching TV, surfing the Web, or driving.

Mothers of children under 5 have shed about 14 hours of physical activity time weekly, but have increased their weekly log of sedentary time by six hours, on average.

The new research, in the Mayo Clinic Proceedings, looked at data from the American Heritage Time Use Study, which has detailed the time Americans engaged in paid and unpaid work from 1965 to 2010.

The dramatic reallocation of mothers' time has consequences for the women: The resulting reduction in energy burned up in physical activity averages 177 calories more per week for women with older children at home, and 225 calories per week for those with preschoolers.

And because mothers' patterns of activity, consumption, and weight gain influence those of their children, this 45-year shift from exertion to inertia has likely played an outsize role in the nation's epidemic of obesity. - L.A. Times