While it is inarguable that exercise builds more and better muscle, and the outcomes are easily measured, it is much harder to determine the degree to which exercise improves bone. There is strong evidence to show that it does produce improvements to a degree that appears to taper with age, but present measurement technologies leave a lot of ambiguity and room for debate in that statement. Separately there is the question of the degree to which exercise slows the changes of aging, a related but quite different set of mechanisms. Both muscle and bone decline with age, so there is interest in the research community in exploring the biochemistry of reliable interventions, in search of ways to slow down that degeneration. This review paper is an introduction to present thinking on the effects of exercise on the quality and strength of bone tissue, and the challenges of measuring those outcomes:
In the United States, over 1.5 million osteoporotic fractures occur annually. The majority of these occur in the latter decades of life when rates of bone loss and microarchitectural deterioration are at their greatest. Exercise is a commonly recommended intervention for preventing bone fragility; however, human and animal studies suggest that the anabolic effect of exercise is much less potent in the mature and post-mature vs. the immature skeleton. These observations raised the question: is exercise a worthwhile strategy for promoting bone health in mature and elderly individuals? One author cites minuscule gains in bone density reported from exercise trials in adult populations and concludes that "exercise has little or no effect on bone strength." This conclusion, however, is based on studies that do not take into account recent advances in non-invasive technologies for measuring bone density and structure or new strategies to make exercise more potent, or osteogenic, in aging populations.
Human studies have demonstrated an age-related decline in the responsiveness of bone mineral density (BMD), as measured by dual-energy X-ray absorptiometry (DXA), to exercise interventions. Nevertheless, a recent meta-analysis in older adults revealed small but statistically significant increases in BMD at the lumbar spine and femoral neck. It is important to consider, however, the inherent limitations of DXA that may lead to underestimation of the mechanical benefits of exercise. Based on attenuation of photons by bone and soft tissue, DXA provides a precise estimate of the amount of bone located within an area; however, it does not reveal bone structure. Recent introduction of quantitative computed tomography (QCT), DXA-derived hip structural analysis (HSA), and magnetic resonance imaging (MRI) has afforded the ability to assess bone geometry, bone macro- and micro-structure, three-dimensional bone density, and estimates of bone strength using engineering analyses. A systematic review of studies using either peripheral QCT (pQCT) or HSA to measure exercise effects on bone strength across various age groups reported improvements of 1-8% in children and adolescents, with either no change or very modest improvements in middle-aged and older adults. Nevertheless, like DXA, HSA and pQCT have inherent limitations that make exercise studies difficult to interpret.
Multiple lines of evidence from human and animal studies indicate that the aging skeleton remains modestly responsive to exercise interventions. Animal and human studies demonstrate that mature and senescent bone retains the ability to respond to loading with osteogenesis. Clearly, further mechanistic research is needed to fully elucidate why bone becomes less sensitive to exercise with age. Innovative paradigms such as rest insertion and "non-customary" loading may be necessary to maximize the osteogenic potential of exercise. Further, studies suggest that the sympathetic nervous system may be an important mechanistic link between physical activity and bone health. This emerging evidence raises the possibility that pharmacological intervention may be able to augment the benefits of exercise on bone health in humans across the lifespan.