Objectives: Studies including people with severe mental illness (SMI) have reported beneficial effects from physical activity (PA) on psychiatric symptoms, quality of life, and global functioning. However, it is important to be regularly physically active to obtain these effects. Using the motivational lens of self-determination theory, the aim of the current study was to explore the dynamic interplay between satisfaction of psychological needs (autonomy, competence, relatedness) in PA and the patients' weekly PA level.

Methods: The study had a longitudinal design, following 10 inpatients with du& diagnoses (SMI and addiction) over 12 weeks at a psychiatric ward offering physical activity as part of treatment Data were collected 14 times with a questionnaire measuring psychological need satisfaction, while an accelerometer was used to objectively count steps to reflect participants average PA-level each week. To analyse the week-to-week relationships between the basic psychological needs and PA-level, the Bayesian dynamic p-technique analysis was used to explore both cross-sectional-, autoregressive- as well as cross-lagged effects between the constructs.

Results: The results indicated credible and strong positive autoregressive effects for all three psychological needs as well as for PA-level, and positive credible cross-sectional associations between all three psychological needs and PA-level. However, the cross-lagged effects were small and not credible for all three psychological needs in relation to PA-level.

Conclusions: In total, these results support the established positive relation between basic psychological need satisfaction and PA-level yet failed in finding any predictive effects between need satisfaction and PA-level. © 2021 The Author(s).