2 Physical activity as a risk or protective factor | 19 BACKGROUND Falls are one of the major causes of mortality and morbidity in older adults of 65 years or older (67). More than one-third of the population aged 65 years or older falls at least once each year (67, 68). Major injuries, such as head trauma and fractures occur in 10-15% of falls (67, 68). Physical inactivity is suggested to be a risk factor for falls and fall-related fractures in older persons due to its negative effects on gait, balance control, mobility, and muscle strength (69-71). Previous research has indeed shown that high levels of physical activity could be associated with a lower risk of falls and fractures in older adults, because physical activity helps to maintain balance control and muscle and bone strength (72-75). However, high levels of physical activity may also increase the risk of both falls and fall-related fractures (28, 76). A longer duration (27, 77, 78) or intensity (79, 80) of physical activity increases exposure to situations where falls, and thus fractures, could occur. Thus, there is a paradox regarding the question whether physical activity is a risk factor or a protective factor for falls and fall-related fractures (28, 81). This paradox could possibly be explained by frailty status (82, 83). Frailty is a state of increased vulnerability to adverse outcomes resulting from low physiological reserves, low resistance to stressors and multisystem impairment (29, 84, 85). Whether and how frailty affects the association between physical activity and both falls and fractures is not clear and pathways may be complex considering the multidimensionality of frailty and the numerous risk factors for falls and fractures (82, 83). The frailty index is a measure of frailty and involves the accumulation of diseases, activities of daily living, and cognitive and psychological function disabilities, in which a greater number of health deficits indicates a higher frailty status (86). One possible explanation for an increased fall risk in frail older adults compared to non-frail older adults is that sarcopenia (i.e., lower muscle mass and strength) is a major component of the frailty status (87, 88). Therefore, physical activity in frail older adults is possibly associated with more falls compared to non-frail older adults, due to reduced muscle strength (82, 83, 89). Current clinical guidelines and health care policies for older persons recommend physical activity because of its beneficial effects on many health outcomes (90, 91), but these guidelines do not take frailty status into account. An adverse effect of these recommendations may be increased fall and fracture incidences among frail older adults. However, it is also possible that high physical activity is associated with more falls, but with less fractures among frail older adults, because of an increased bone strength (92). In that case, the positive health outcomes of physical activity may outweigh the consequences of a fall. Therefore, a better understanding of the complex relationship between physical activity and falls, and physical activity and fall-related fractures is warranted.
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