Why does physical activity alleviate depression? : Identifying potential mediators and understanding the process of change

Abstract

Physical activity has been found to alleviate depression, but little is known about why or how it 'works'. Combining quantitative and qualitative research methods, the overall aims of this thesis were to address this gap, identify potential mediators and develop an understanding of the process of change. Self-esteem, physical self-concept, physical self-efficacy, and the independent mood dimensions of depression, negative affect (NA) and positive affect (PA), were identified from theory as potential mediators. PA and NA have not previously been suggested, and represent a novel explanation. In particular, it was proposed that PA may be a stronger candidate mediator than NA. These factors were examined in an initial, longitudinal study in which individuals with elevated depression scores increased their physical activity over an eight week period. The temporal relation of change suggested that improvement in P A, NA and physical self-efficacy may be stronger candidate mediators than physical self-concept or self-esteem, at least for mediating change in depression in the early stages of increased activity. Larger effects were found for change in P A than NA or any other potential mediator. A qualitative, grounded theory study offered insight that depressed individuals may experience a sense of pleasurable engagement in life from physical activity. The accounts also suggested the importance of embodied experience and knowledge in the process of change. Direct (bodily) experiences of the physical activity - especially enjoyment - seemed to be related to experiencing benefit, changing motivations and eventually the active use of physical activity to self-control symptoms by some. It was consequently hypothesised that state mood changes (especially an increase in PA - i.e. movement away from anhedonia) may be responsible for initial reductions in depression and that coping self-efficacy may be a mechanism responsible for longer term effects. A final, cross-sectional, postal questionnaire study further examined the potential mediators identified from the other studies in a multiple mediation analysis. PA and NA were the only significant direct mediators and statistical comparisons suggested an equal mediating role. However, post-hoc analyses suggested that physical self-efficacy may indirectly mediate improvement in depression through improvement in P A but not NA. From the findings, a new, tentative theoretical model for understanding the process of change is proposed. Distinguishing between P A and NA forms a fundamental basis of the model. It is suggested that embodied experiences of physical activity (e.g. physical self-efficacy) may indirectly mediate higher-level response in depression through PA and / or NA, and that improvement in the mediators may dynamically reinforce future physical activity. The value of distinguishing between PA and NA in understanding mediators of at least initial response in depression to physical activity is discussed.

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