Objectives: To examine bidirectional within- and between-person relations between physical activity and cognitive function across 15 years.

Methods: Participants (N = 1,722, age range 40–85 years, 55% women) were drawn from the Betula prospective cohort study. We included 4 waves of data. Bivariate latent curve models with structured residuals were estimated to examine bidirectional within- and between-person relations between physical activity and cognitive function (episodic memory recall, verbal fluency, visuospatial ability).

Results: We observed no statistically significant bidirectional within-person relations over time. Higher levels of physical activity at baseline were related to less decline in episodic memory recall. Positive occasion-specific within- and betweenperson relations were observed, with the most consistent being between physical activity and episodic memory recall.

Discussion: The lack of bidirectional within-person relations indicates that shorter time lags may be needed to capture time-ordered within-person relations. The link between higher physical activity at baseline and less decline in episodic memory recall over time may indicate a protective effect of physical activity on episodic memory recall.