The Association Between Physical Activity and Osteoporotic Fractures: A Review of the Evidence and Implications for Future Research
Introduction
About half of all women and one-third of all men will sustain a fragility fracture during their lifetime (1). Despite the introduction of several new treatments in the past decade, osteoporosis still affects more than 200 million women throughout the world and is responsible for more than 1.5 million fractures annually (2). The annual healthcare costs of osteoporotic fractures in the United States are estimated at 7 to 10 billion dollars (3). Increased morbidity, mortality, and costs associated with the increased incidence of fractures make it imperative to implement prevention strategies in the community. Even if prevention efforts succeed in decreasing the rate of new fractures, the overall aging of worldwide populations will mean that the number of fractures will increase substantially (4), increasing the already-enormous public health burden they impose.
The risk for osteoporotic fracture is mainly determined by three factors: the risk of falling, bone strength, and force of impact in the event of a fall. Established risk factors for falls include older age, impaired balance and orthostatic hypotension, decreased reaction time, impaired vision and cognition, weakness in lower-extremity muscles, decreased lean body mass, and overall impaired mobility 5, 6, 7, 8. Medications, particularly sedative and psychotropic drugs; alcohol intake; inappropriate footwear; and physical factors in the environment, such as stairs, lighting, and streets, also have been cited as important factors 5, 8. Acute situational factors, including the force of movement, body position, location of impact, and protective responses during a fall also influence whether an injury will occur. Aside from the risk of falling, primary risk factors for osteoporotic fractures include low bone mineral density (BMD), architectural deterioration of bone, older age, female sex, white race, and lower body weight (9).
Physical activity has been identified as a lifestyle factor that may influence the risk of falls and fractures among older adults. Physical activity is likely to influence the risk for fractures, mainly through the musculoskeletal and neuromuscular systems and by direct influence on three main risk determinants of fracture (falls, bone density, and bone quality) 10, 11. It is also important to consider that physical activity could increase risk for injurious falls because physical activities involve skeletal muscle movement that displaces the body's center of gravity and balance. Not surprisingly, walking and going up and down stairs are the most common circumstances of nonsyncopal falls, accounting for 39% and 20% of events, respectively, among older adults (8). However, as is the case with risk for sudden cardiac death, physical activity could have multiple long-term protective effects while simultaneously increasing acute risk for an event. It should be noted that hip and wrist fractures risk is thought to be influenced by both the tendency to fall and bone strength, whereas vertebral fractures have not been causally related to falls and may be more solely related to bone and muscle strength (12).
In this article, I review the epidemiological evidence related to the association between physical activity and the risk of osteoporotic fractures among older adults. The association between physical activity and intermediate outcomes (namely, falls, BMD, and bone quality) is summarized from other review papers and the implications for future research are discussed from an epidemiological perspective.
Section snippets
Methods
Given the enormous number of studies evaluating the effects of physical activity on bones, the literature search was restricted to find randomized controlled trials (RCTs), prospective studies, and review articles on the topic. Peer-reviewed articles were identified in the PUBMED Central with the use of the MeSH (Medical Subject Headings) terms “Motor Activity” and “Exercise” for the exposure and the MeSH terms “Fractures, Bone,” “Osteoporosis,” “Bone density,” “Densitometry, X-ray,”
Physical Activity and Hip Fractures
Numerous studies have evaluated the association between physical activity and bone health using different end points like fractures, risk of falls, and BMD. Hip fracture, as the most important type of osteoporotic fractures, has attracted considerable attention among researchers. Practically all patients with hip fracture seek clinical attention and this point facilitates use of hip fracture as an end point for epidemiological studies (3). Most of the prospective studies evaluating the
Discussion
A thorough search of literature on the topic did not reveal any RCT specifically designed to evaluate the role of physical activity in reduction of fracture rates. Moreover, no protocol for such an RCT is registered in the U.S. National Library of Medicine registry for clinical trials ClinicalTrials.gov) and the Cochrane Central Register of Controlled Trials. Obviously, the main factor that has prevented the research community so far from conducting an RCT with fracture end points is the
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