Effects of chronic physical activity on cognition across the lifespan: a systematic meta-review of randomized controlled trials and realist synthesis of contextualized mechanisms.

In the surging field of chronic physical activity (PA) and cognition research, problems arise that prevent us from ‘seeing the forest for the trees’. The first aim was to identify them and propose solutions. Moreover, inconsistencies in conclusions of a rising amount of systematic reviews render necessary ‘an umbrella for a rain of evidence’. The second aim was to obtain a differentiated picture of moderators that may explain inconsistencies using the wide-angle lens of a systematic meta-review. We especially addressed the role of the PA context in causation mechanisms, complementing the meta-review with a realist approach that is best suited to identify context-mechanism-outcome (CMO) configurations. Main review outcomes are: (1) inconsistent grading of methodological quality and low consideration of external validity; (2) role of multiple moderators at participant-, intervention-, study-level but relative neglect of context-level moderators; (3) explanatory potential of CMO configurations, in which specific conditions of the context may trigger different mechanisms that generate PA effects on cognition. The majority of the proposed mechanisms converge on the concept of enrichment. Conclusions highlight the need for future research, in which PA interventions and their contexts are designed to mobilize the different mechanisms underpinning their individual and joint effects on cognition.