32 | Chapter 2 sex as a covariate in the analyses, this appeared not to affect the association between physical activity and falls. We also did not find an association between physical activity and fall-related fractures in our study, in contrary to previous research (111-113). However, other studies also did not find a significant correlation between physical activity and fall-related fractures (76, 114, 115). These results may be explained by only long-term physical activity for more than one year leading to a reduction in fracture risk (76). Moreover, in this study we did not take the intensity of the physical activity into account. Possibly, being physical active on a high intensity results in more falls and thus fractures compared to physical activity on a low intensity. On the other hand, when exercising on a higher intensity, the bone density could increase and thus reduces the fall risk (116). Therefore, it would be interesting to investigate the intensity of physical activity on falls and fractures in further research. To our best knowledge, no previous research has been conducted on the modifying effect of frailty on the relationship between physical activity and both falls and fall-related fractures. Moreover, frailty did not modify the association of physical activity with fall risk and fall-related fractures. However, similar as found in this study, frail older adults have been shown less physically active and have a higher risk of falls and fall-related fractures compared to non-frail adults (30, 117-119). Moreover, high physical activity has been shown related to more falls, but only among women impaired in their instrumental activities in daily living tasks (120). This suggests that frailty is possibly a more important factor than physical activity when predicting falls and fall-related fractures. Further research should investigate this. A strength of this study is the participation of a large sample of nationally representative data from a large study among community-dwelling older adults in the Netherlands of 75 years or older. Another strength of this study is that physical activity was assessed using inertial sensors, resulting in objective measures of the duration of physical activity, in contrast to questionnaires that were frequently used in previous research and often result in an overestimation. Furthermore, falls and fractures were prospectively determined by keeping a fall and fracture diary, and were assessed every 9 months by telephone calls with the researchers. Last, two sensitivity analyses were conducted; both showed similar results as the primary analyses, which substantiated the results found in this study. The current study is limited by the absence of physical activity and frailty data at the moment of follow-up. It may be that the level of physical activity and frailty status changed during the three-year follow-up period, affecting the fall risk. Second, the amount of falls and fractures in this study population with 311 participants was limited. Therefore, it is more difficult to adequately test for interaction effects, because of a limited power. Further research is
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