The exercises would be aimed at strengthening the upper portion of the femur, the leg bone that fits into the hip, said doctors at the University of Cambridge.

Dr. Paul M. Mayhew and his colleagues collected the femurs of 81 adults who died suddenly; the researchers then performed computerized tomography to create three-dimensional images of the middle of the neck of the bones.

The investigators found that the cortex, the outer layer of the femur, became progressively thinner with age. The thickness of the cortex in women decreased 6.4 percent every decade, and the reduction was only slightly less for men.

That thinning reduces the amount of energy the femur can absorb before buckling to cause a hip fracture, the researchers said. And the reason the thinning occurs is that older people tend to limit their exercise to walking, which does not strengthen the cortex of the femur, they suggested.

Making Stronger Hips

The same thinning does not occur in tree-dwelling apes -- close relatives of humans -- because climbing trees is an exercise that strengthens the femur, the researchers said.

"Several popular forms of exercise also involve extension of the flexed femur under load (cycling, sculling, gymnastics, weights)," they wrote. "Some should be investigated for their potential to protect, as part of our society's drive to increase physical activity for health."

The findings appear in the July 9 issue of the journal The Lancet.

But Charles H. Turner, a professor of biomedical engineering at Indiana University School of Medicine, said the solution may not be that simple.

"This new paper does do great service to the field," said Turner, who wrote an accompanying editorial in the journal. "But we should be doing more work to learn how best to strengthen that part of the hip."

The problem, he said, is that "I don't think anybody who prescribes exercises knows how to make one that strengthens that part of the hip preferentially."

Researchers are "now giving thought about the best approach to make stronger hips," Turner said. "That might come in the future, as we focus our efforts on it."

Muscle and Bone Strength

But even identifying such exercises would only be a partial solution to the problem of hip fractures in the elderly, said Dr. Kenneth A. Egol, chief of the fracture service at New York University-Hospital for Joint Diseases.

"This looks at only one kind of hip fracture," Egol said. "Breakage of the femoral neck accounts for only half of all hip fractures."

Other risk factors -- notably osteoporosis, the progressive thinning of all bones that occurs with aging -- also are important, he said.

Still, Egol recommends exercises, specifically those "that counteract tension forces across the femoral neck, the side-lift kind of exercise that is best at improving muscle strength and secondarily improving the strength of the bone."

More information: The National Center for Injury Prevention and Control has more on hip fractures and the elderly.