Background: Physical activity is associated with reductions in fatigue in breast cancer survivors. However, mechanisms underlying this relationship are not well-understood. The purpose of this study was to longitudinally test a model examining the role of self-efficacy and depression as potential mediators of the relationship between physical activity and fatigue in a sample of breast cancer survivors using both self-report and objective measures of physical activity. Methods: All participants (N = 1,527) completed self-report measures of physical activity, self-efficacy, depression, and fatigue at baseline and 6 months. A subsample was randomly selected to wear an accelerometer at both time points. It was hypothesized that physical activity indirectly influences fatigue via selfefficacy and depression. Relationships among model constructs were examined over the 6-month period using panel analysis within a covariance modeling framework. Results: The hypothesized model provided a good model-data fit (x2=599.66, df=105, P<0.001; CFI=0.96; SRMR = 0.02) in the full sample when controlling for covariates. At baseline, physical activity indirectly influenced fatigue via self-efficacy and depression. These relationships were also supported across time. In addition, the majority of the hypothesized relationships were supported in the subsample with accelerometer data (x2 = 387.48, df = 147, P < 0.001, CFI = 0.94, SRMR = 0.04). Conclusions: This study provides evidence to suggest the relationship between physical activity and fatigue in breast cancer survivors may be mediated by more proximal, modifiable outcomes of physical activity participation. Impact: Recommendations are made relative to future applications and research concerning these relationships. Cancer Epidemiol Biomarkers Prev; 22(5); 773-81.
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