repetitions of 6–8 muscle groups
a Thus far proven only in premenopausal women or when combined with resistance training in older adults.
Physical activity and bone health
A wealth of animal and human data provide evidence for a relationship between physical activity and bone health at all ages. Mechanical loading of the skeleton generally leads to favourable site‐specific changes in bone density, morphology, or strength, whereas unloading (in the form of bed rest, immobilisation, casting, spinal cord injury, or space travel) produces rapid and sometimes dramatic resorption of bone, increased biochemical markers of bone turnover, changes in morphology such as increased osteoclast surfaces, and increased susceptibility to fracture.
Comparative studies of athletic and non‐athletic populations usually demonstrate significantly higher bone density in the active cohorts, ranging from 5 to 30% higher, depending on the type, intensity, and duration of exercise training undertaken and the characteristics of the athletes studied. Exceptions occur with non‐weight‐bearing activities such as swimming, cycling, or amenorrhoeic or competitive distance runners, whose bone density appears similar to or lower than that of controls. Similarly, on a smaller scale, differences are often observed between habitually active and sedentary non‐athletic individuals. Experimental evidence in animal models and also some human data suggest that changes in bone strength not directly correlated with density may contribute to the overall benefits of mechanical loading for skeletal integrity and resistance to fracture (e.g., increased bone volume or altered trabecular morphology) so that evaluating bone density changes alone likely significantly underestimates the skeletal benefits of loading.
Consistent with the bone density findings noted above, hip fracture incidence has been observed to be as much as 30–50% lower in older adults with a history of higher levels of physical activity in daily life, compared with age‐matched, less active individuals. For example, in the prospective Epidemiology of Osteoporosis Study (EPIDOS) study of 6901 white women over the age of 75 followed for 3.6 years, investigators found that a low level of physical activity increased the risk for proximal humerus fracture by more than twofold. The relative risk of fracture in sedentary women (RR = 2.2) was greater than that attributable to low bone density (RR = 1.4), maternal history of hip fracture (RR = 1.8), or impaired balance (RR = 1.8). The interaction of these risk factors is indicated by the fracture rate, which rose from about 5 per 1000 woman‐years in individuals with either bone fragility or high fall risk to 12 per 1000 woman‐years for women with both types of risk factors. Such data suggest the great potential utility of multifactorial prevention programmes for osteoporotic fracture that can address both bone density and fall risk (sedentary behaviour, sarcopenia, poor balance, polypharmacy, etc.) simultaneously.
Exercise intervention trials in postmenopausal women and older men
Significant changes in the femur, lumbar spine, and radius have been seen following high‐impact aerobic training, resistance training, and combined programmes of aerobic and resistive exercise. Unlike results obtained in younger women, isolated high‐impact training (jumping, skipping, heel drops) has not yet been found to be effective in studies of postmenopausal women. High dropout rates (30–50%) are problematic in these trials, raising the issue of generalisability and sustainability of the outcomes observed. This is particularly relevant to fracture prevention efficacy of exercise, as several studies have shown complete or partial reversal of gains in bone mineral density (BMD) after the cessation of training.
For older men and women, a combination of decreased anabolic hormones (oestrogen, testosterone, growth hormone), increased catabolic milieu (higher leptin and cortisol associated with visceral adipose tissue), the emergence of musculoskeletal and other diseases, retirement, and reduced recreational activities have a major negative impact on both bone and muscle tissue. The majority of studies demonstrating the efficacy of exercise on bone density have been conducted in women between 50 and 70 years of age, and it is not yet known if efficacy would be similar in women over 80 with multiple comorbidities, who have often been excluded from such trials. However, the most recent studies suggest that optimal adaptations continue to accrue with high‐intensity resistance and power training in older adults.70
Optimal exercise modality and intensity for bone health
The predominant exercise training factor that influences bony adaptation is the intensity and novelty of the load, rather than the number of repetitions, sets, or days per week or even the total duration of the programme. This observation is also true for animal models of mechanical loading, in which bone is most sensitive to short periods of loading characterised by unusual strain distribution, high strain magnitudes, and rapid rate of loading.
The relative efficacy of aerobic versus resistive exercise regimens for postmenopausal women may perhaps be best assessed via studies that have directly compared various intensities of these two exercise modalities in randomised subjects. Kohrt et al.71 found that both aerobic activities with high ground‐reaction forces (walking, jogging, stair climbing) and exercises with high joint‐reaction forces (weight‐lifting, rowing) significantly increased the BMD of the whole body, lumbar spine, and Ward’s triangle, whereas only the ground‐reaction group increased BMD at the femoral neck.71 The weight‐lifting group preserved femoral‐neck BMD relative to controls, as has been seen in other resistance training studies. However, lean mass and muscle strength increased only in the weight‐lifting group, leaving overall benefits of these two types of exercise for ultimate fall and fracture prevention still unresolved. Kerr et al.72 randomised 126 postmenopausal women to two years of high‐intensity weight‐lifting exercise, moderate‐intensity aerobic training (circuit training and stationary cycling), or sedentary control condition. Total hip and intertrochanteric BMD were improved only by strength training and were significantly different from aerobic training or control groups (+3.2% at two years). As most comparative studies other than those of Kohrt et al.71 and Kerr et al.72 have not sought to optimise both exercise modalities, it is still not possible to choose definitively one best modality for all bone sites. In general, the older the individual, the more favourable the resistance training appears, due to its broader benefits on muscle, bone, balance, and fall risk, relative to aerobic training. If aerobic training is used, however, activities that are weight‐bearing and higher impact have greater efficacy for bone health than non‐weight‐bearing or low‐impact aerobic activities.
It is important to consider not only the optimal modality of exercise but also the relative intensity, as the skeletal adaptation is critically linked to the intensity of the loading (whether due to increased amount of weight lifted during resistance training
