Julian Wienert

• With an increasing age levels of physical activity tend to decrease, though maintaining a healthy and active lifestyle is a crucial factor in the prevention of severe chronic diseases. Due to this it is of great importance to understand age-related changes and maintenance of behaviors such as physical activity. Effects of chronological age on processes of health behavior change are well documented, but probably fail to demonstrate the impact of aging comprehensively. Therefore, other age-related factor should be investigated which might provide new insights in the adoption and maintenance of physical activity throughout the life-span. The primary goal of this dissertation was to introduce subjective physical age as a facet of subjective age and investigate its additional value in the prediction of physical activity while focusing on intentional processes. The second goal was to highlight self-regulatory processes which aid the transition of intentions into behavior, in the light of subjective physical age. Furthermore, differences in social-cognitive predictors and their interplay were investigated. Results from this dissertation suggest that subjective physical age contributes to in the explanation of physical activity, even when controlling for strong correlates of subjective age (health status & chronological age). Those who feel physically younger indicate higher levels of physical activity. Feeling physically younger is also related to better self-regulatory planning skills which help to translate intentions in actual behavior. Finally, results provide insights in differing cognitions regarding health behavior change predictors and mechanisms between those who feel physically younger, as old as they are, and older. These findings are of special interest, as subjective physical age can be intervened upon to change cognitions about own ageing and age-related, physical capabilities in health behavior change interventions. They also provide a new proxy to tailor content towards the characteristics of participants in tailored, web-based health behavior change interventions to further personalize content (e.g., in web-based interventions for rehabilitation patients).