Biomedical Engineering Reference
In-Depth Information
The Nurses' Health Study is a good example of how prospective cohort studies
have provided insights on the associations of physical activity with hip fracture
risk [ 22 ]. The findings of this study suggest that postmenopausal women should
accrue at least 9-14.9 MET . h of physical activity per week to reduce fracture risk
(Fig. 2 , top panel) or at least 4 h/week of walking (Fig. 2 , middle panel). The
results further suggest that a fast walking pace is more likely to reduce fracture risk
than a slow walking pace (Fig. 2 , bottom panel).
2 Exercise and BMD
There have been numerous randomized controlled trials of the effects of exercise
training on BMD, which is a major determinant of fracture risk. Because many
of the intervention trials have been relatively small, it is not surprising that at
least 19 meta-analyses have been conducted to integrate the findings across
studies (Table 1 )[ 8 , 9 , 38 - 45 , 53 , 58 - 62 , 93 , 95 ]. Only four of these meta-
analyses reported no significant benefit of exercise training on BMD at any
skeletal region [ 8 , 41 , 42 , 53 ]. Both weight-bearing endurance exercise and
resistance exercise have been found to generate increases in BMD. Of the 15
meta-analyses that evaluated the effects of exercise training on lumbar spine
BMD, 11 reported significant benefits (see Table 1 ). In general, the weighted
mean differences between exercisers and controls indicate that exercise training
can improve lumbar spine BMD by 1-2%. The meta-analyses seemingly further
suggest that exercise training is more likely to improve spine BMD than hip
BMD. Only eight of 15 meta-analyses reported significant benefits of exercise
training on BMD of the femoral neck or some other region of the proximal
femur (see Table 1 ). The greater responsiveness of lumbar spine BMD to
exercise, as compared with the proximal femur, could be related to the high
proportion of cancellous bone in the spine, which has a higher turnover rate than
cortical bone. Alternatively, because the proximal femur is loaded during all
physical activities performed in a standing posture, it is possible that exercise
training interventions do not generate strains in the proximal femur that are
markedly different from peak strains that occur during usual daily activities. This
may be particularly true for resistance exercise interventions if the majority of
exercises are performed in a seated position.
2.1 Exercise and Bone Strength
BMD measured by dual-energy X-ray absorptiometry (DXA) is the best single
predictor of fracture risk, but it accounts for only about 60% of the variability in
bone strength [ 94 ]. Other characteristics of bone, such as the shape, distribution of
mass, and trabecular microarchitecture, are also determinants of the resistance to
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