CAMBRIDGE, England, July 7-Exercise such as cycling, weight training, gymnastics, or rowing might help older people reduce their risk of a hip fracture.
That's according to researchers at the University of Cambridge here, who contend that the age-related thinning of the femur, particularly in women, puts older people at greater risk for fractures from a sideways fall.
Normal weight-bearing exercises such as walking, the most common type of exercise among older adults, is not sufficient for strengthening the femoral neck, write Paul M. Mayhew, Ph.D., in the July 9 Lancet.
Action Points
- Advise older patients that this study suggests that certain forms of exercise that straighten or extend the femur while it is under a load -- such as cycling, sculling/rowing, gymnastics or weight-training -- may help to reduce the risk of fracture.
- Inform patients that this information comes from a relatively small study of bones taken from cadavers, and that the proposed model for increased fracture risk with age is subject to debate and interpretation.
"Several popular forms of exercise also involve extension of the flexed femur under load (cycling, sculling, gymnastics, weights)," Dr. Mayhew and colleagues wrote. "Some should be investigated for their potential to protect, as part of our society's drive to increase physical activity for health."
They proposed that thinning of the superolateral cortex of the femoral neck, which may occur if the femur is not under sufficient mechanical load during walking, may lead to local buckling or elastic instability of the bone, putting people at increased risk for fracture from a sideways tumble.
In an accompanying editorial, which criticizes the idea, a biomedical engineer likened the concept to an aluminum can, the thin walls of which will buckle when it's stepped on.
"I am concerned that Mayhew and colleagues' shell-buckling theory might be overly simplistic," wrote Charles H. Turner, Ph.D., of the departments of biomedical engineering and orthopedic surgery at Indiana University-Purdue University in Indianapolis. The femoral neck is filled with trabecular bone and marrow, making it a far more complex structure than an empty shell.
Although the risk of falling increases only moderately with age, the risk of fracture rises much more dramatically -- 10-fold for every 20 years of aging, Dr. Mayhew and colleagues wrote.
In their search for a cause that could explain the disparity, they obtained the proximal third of femurs from 77 people age 20 to 95 who died suddenly. They used dual-energy X-ray absorptiometry (DEXA) scanning to determine bone mineral density, and CT scanning to measure the distribution of bone in the mid-femoral neck. They then plugged the numbers into a formula for figuring out the critical stresses that would occur in a sideways fall as a measure of elastic instability.
They found that the already thin cortical zone in the upper femoral neck became even thinner with age in the samples they studied. Among women, the relative cortical thickness at superior neck declined 6.4% per decade, and critical stress -- the amount each area of bone can tolerate before becoming critically unstable -- declined by 13.2% per decade in the superoposterior octant, the region compressed most during a sideways fall.
A smaller effect was seen in men, who had a 4.2% per decade decline in mean thickness (relative to age 60) in the posterior octant. The authors suggested that this age-related thinning compromised the capacity of the femur to absorb energy independently of osteoporosis.
"Walking protects against hip fracture in old people, but it may not be through directly strengthening the hip. To preserve the stability of the cortex, regular loading targeted to the superolateral cortex might need to be lifelong, as occurs naturally when the femur is straightened (extended) at the hip from a flexed position," the investigators wrote.
Forms of exercise that might help to strengthen the femoral neck through regular extension and loading include cycling, sculling, gymnastics and weightlifting, they noted.
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