(USA TODAY) -- Middle-age and older women can greatly reduce their risk of hip fractures by exercising regularly, according to a study out today.

Women who walk for four or more hours a week have a 40% reduced risk of hip fractures compared with women who do little or no activity.

Women cut their risk in half when they walk an hour a day or jog for three hours a week, a study in today's Journal of the American Medical Association shows.

These are the latest findings on the benefits of physical activity from the Nurses' Health Study, an observational study of thousands of women.

Hip fractures are a serious concern for older Americans because the injuries can result in serious health problems and even death. About 80% of the 300,000 hip fractures annually occur in women.

Sedentary women should start doing something, says the study's lead author, Diane Feskanich. "Doing anything is better than nothing; doing more is better than less.

"Walking alone will help, but if you do high-impact activities like jogging or running, you get even more benefit," says Feskanich, an assistant professor at Harvard Medical School and Brigham and Women's Hospital in Boston.

For the latest report, researchers reviewed questionnaires on the habits of 61,000 postmenopausal women, ages 40 to 77. Women were asked questions over time about walking time and pace as well as type and intensity of other forms of exercise. Researchers found that:

• For every hour of walking or an equivalent activity a woman adds to her weekly exercise plan, her risk of hip fracture is cut by 6%.
• Activity is protective for hip fractures in both lean and heavy women.
• Faster walking pace and amount of time spent standing is associated with lower risk of hip fractures.
• Those who regularly exercise, equal to about an hour of walking a day, reduce their risk of hip fractures about the same amount as women who take hormone therapy.

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