Exercise that targets a weak region of the hipbone may help prevent fractures, but simple walking will not, new research indicates.

As people age, the outer "cortical" layer of bone in a particular region the hipbone or upper femur become thinner, making the hip more prone to fracture, according to the report in The Lancet medical journal.

The findings are based on a CT scan analysis of 77 femurs from people between 20 and 95 years of age who died suddenly of causes not related to bone disease or fractures.

Substantially thinning of the cortical layer in the neck of bone that connects the hip "ball" to the rest of the thighbone was observed with aging, Dr. Jonathan Reeve, from Addenbrooke's Hospital in Cambridge, UK, and colleagues found.

After 60 years of age, bone thickness in this zone fell by 6.4 percent per decade, the report indicates.

Regardless of whether the bone-thinning disease osteoporosis was present or not, thinning in the cortical layer impaired the femur's ability to absorb energy, making it more likely to break, the researchers note.

"Because walking does not sufficiently (condition the upper femur), the fragile zones in healthy bones may need strengthening, for example with more well targeted exercise," the authors conclude.

They point out that while walking does not put a significant load the fragile area, other exercises that basically flex the hip joint do "involve extension of the upper femur under load." These include cycling, sculling, gymnastics and weights.

In a related editorial, Dr. Charles H. Turner, from Indiana University in Indianapolis, notes that the present report provides "a compelling argument for more diligent assessment of the regional patterns of bone loss in the (upper femur) and point to the need for targeted interventions that strengthen bone" in this area.

SOURCE: The Lancet, July 9, 2005.