Barriers and facilitators to the adoption of physical activity policies in elementary schools from the perspective of principals: An application of the consolidated framework for implementation research-A cross-sectional study. - Wendt J; Scheller DA; Flechtner-Mors M; Meshkovska B; Luszczynska A; Lien N; Forberger S; Banik A; Lobczowska K; Steinacker JM (Frontiers in public health, 2023)