For this review we have chosen to focus on the outcome of
fractures as an example of injury. Although osteoporosis is now recognised as a health outcome in its own right as a condition characterised by low mass and structural deterioration of bone tissue, it leads to bone fragility and increased risk of fracture. It was, therefore, conceptualised as a risk factor and excluded from this review.
It is well known that weight bearing physical activity and resistance training maintain the normal structure and functional strength of bone, and increase bone mineral density (BMD), thereby decreasing the risk of fracture (Drinkwater, 1993). However, risk of fracture is complex and it is difficult to assess the independent role of physical activity in risk reduction as there are complex interactions between physical activity and muscle strength, balance, BMD, use of medications (including hormone replacement therapy), calcium intake, eyesight and falls, all of which have been shown to have a role in the aetiology of fracture.
It is also likely that the skeletal effects of physical activity differ in younger and older women, with evidence to suggest that exercise during periods of high growth (ie around the time of the adolescent growth spurt) is associated with greater increases in bone density than occur at any other stage of the life cycle (Bailey, 2000). This makes the evidence from prospective cohort studies of adults difficult to interpret, as we cannot be sure that adults who report lower levels of physical activity now are not protected against fracture by earlier efforts to get
"bone in the bank" during childhood and adolescence (Bailey, 2000). Notwithstanding, the USSG report concluded that there was promising evidence to support the view that physical activity, including resistance training, is protective against falling and fractures among the elderly, as a result of increased muscle strength and improved balance (US Department of Health and Human Services, 1996).
For this review we found six new reports from cohort studies on the relationships between physical activity and fracture in women (see Appendix I). The French OFELY (Os des Femmes de Lyon) study tracked 672 healthy post-menopausal women for 5.3 years and found that low physical activity was independently associated with increased risk of hip fracture, after adjustment for smoking, alcohol, caffeine and calcium intake. (Albrand, Munoz, Sornay-Rendu, duBoeuf, & Delamas, 2003). Women who sustained a fracture had significantly lower BMD and grip strength and were likely to have had a previous history of fracture.
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The US Study of Osteoporotic Fractures (SOF) followed a cohort of almost ten thousand women aged over 65 years for 7.6 years (Gregg, Cauley, Seeley, Ensrud, & Bauer, 1998). Physical activity was assessed using a modified version of the Harvard Alumni questionnaire. The researchers found that each increasing quintile of physical activity was associated with reduced relative risk of hip fracture; with the greatest risk reduction in women who reported participation in aerobics, tennis or weight training, or at least two hours of moderate/vigorous physical activity per week. There was also a significant reduction of hip fracture in women who reported ten or more hours of vigorous household chores each week. In this study physical activity was not associated with wrist or vertebral fracture.
In contrast, data from the Blue Mountains Eye Study (which was established to explore risk factors for eye disease, and therefore has an interest in fracture through the links between visual acuity and falls), found that women who reported doing
no viorous physical activity had a
reduced risk of wrist fracture (Ivers, Cumming, Mitchell, & Peduto, 2002). These somewhat surprising data confirm previous data from the SOF that suggest that wrist fracture occurs in women who are active and healthy and presumably participating in the type of physical activity that would predispose them to falling.
The remaining studies in Appendix I focus on hip fracture. The most comprehensive data are from the NHS, which found that, in their cohort of more than 61,000 post-menopausal women, risk of hip fracture declined by 6% for every hour per week of walking at average pace (Feskanich, Willett, & Colditz, 2002). The effects were seen in both lean and heavy women, but the heavier women had lower risk of fracture. This is hypothesised to reflect both the increased BMD which is associated with higher body mass index, and potential protective effects of adipose tissue around the hips (Chaperlat, Bauer, Nevitt, Stone, & Cummings, 2003).
The NHS researchers estimated that if all the women had exercised at 9 MET.hours per week (2.3 hours of brisk walking) or more, 23% of hip fractures could have been prevented (Feskanich et al., 2002). If all women were active for 24 MET.hours per week (6 hours per week, or an hour on most days) there would be a 42% reduction in risk of hip fracture. Importantly, women who
only walked (ie reported no other form of activity) for 4 hours a week or more, had a 40% decreased risk fracture, and even those who reported standing at work for 10 hours or more each week had a 28-46% reduction in risk. This evidence suggests that occupations such as nursing and teaching that involve standing (rather than sitting) at work may decrease the risk of fracture, independent of body weight and time spent in leisure activities (Feskanich et al., 2002). The NHS researchers also reported that active women not taking oestrogen supplements had similar protection against hip fracture to that provided by hormone use (Feskanich et al., 2002). The Danish Nurse Cohort Study also found that hormone replacement therapy did not modify the beneficial effect of activity on hip fracture risk (Hundrup, Ekholm, Hoidrup, Davidson, & Obel, 2005; see Appendix I).