Aim: The overarching aim of this thesis was to gain understanding of the meaning and experience of sedentary behavior, its relation to physical activity, physical function, and health in older adults.

Method: This thesis utilizes both qualitative and quantitative data. Two qualitative studies, involving in-depth interviews, investigated the lived experiences of sedentary behavior (n = 16, Study I) and the influence of lifelong physical activity experiences (n = 14, Study III) among community-dwelling older adults receiving initial support. Two quantitative studies examined physical activity and its correlates. Study II (n = 72) used questionnaires and accelerometers to measure daily step counts and its predictors in the same population as Studies I and III. Study IV (n = 819) leveraged data from the Swedish National Study on Aging and Care to investigate sit-to-stand time, a measure of lower body strength, in relation to physical function and health. Qualitative data were analyzed using a phenomenological hermeneutic approach (Study I) and qualitative content analysis (Study III). Quantitative data analysis involved descriptive statistics and multiple linear regression models (Studies II & IV).

Result: Study I showed that, for older adults, sedentary behavior is more than just sitting still. They experience it as an adaptation to an aging body, influenced by choices and a sense of frailty. Although some view it as unnatural, others see it as a necessary part of recovery and rest. Study II showed that daily step counts among older adults are influenced by a combination of predictors, including time spent in long bouts of sitting, independent outdoor walking ability, and the intention to be more active. Interestingly, the multiple linear regression model explained 47.8% of the variance in daily step counts. Of seven predictors included, time spent sitting in bouts of at least 60 minutes per day (R2 = 19.6%), independence when walking outdoors (R2 = 9.2%), and intention to become more physically active (R2 = 13.0%) explained most daily step count variance. Study III showed that a lifetime of physical activity shapes how older adults approach physical activity later in life. They rely on an embodied “activity compass” based on past experiences and their body's current limitations to navigate daily routines and make choices about activity levels and support. Study IV showed predictors explaining sit-to-stand time. The multiple linear regression model explained 26.8% of the variance in sit-to-stand time. Out of seven predictors, grip strength (R2 = 12.0%), age (R2 = 7.2%), and generic quality of life (R2 = 2.8%) explained most sit-to-stand time variance.

Conclusion: This thesis, grounded in the lived experiences of older adults, examined sedentary behavior, providing a deeper understanding of how to address sedentary behavior among older adults. Sedentary behavior is considered unnatural but still a conscious choice and sitting down for longer bouts during the day does not necessarily influence daily physical activity. Sedentary behavior among older adults can combine well-needed rest and engagement in meaningful activities. Furthermore, analyses of predictors of sit-to-stand time and daily step counts revealed potential disruptions to the natural motion of rising from a chair and walking. The findings suggest that older adults' lifelong experiences with physical activity and sedentary behavior become embodied knowledge, helping them navigate daily life.