. . . Published ahead of Print Medicine & Science in Sports & Exercise ® Published ahead of Print contains articles in unedited manuscript form that have been peer reviewed and accepted for publication. This manuscript will undergo copyediting, page composition, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered that could affect the content. Copyright © 2018 American College of Sports Medicine A Longitudinal Study of the Effect of Organized Physical Activity on Free Active Play John Cairney 1,2 , Rheanna Bulten 1 , Sara King-Dowling 2,3 , and Kelly Arbour-Nicitopoulos 1,4 1 Faculty of Kinesiology and Physical Education, University of Toronto, Toronto, ON, Canada; 2 Infant and Child Health Lab, Department of Family Medicine, McMaster University, Hamilton, ON, Canada; 3 Department of Kinesiology, McMaster University, Hamilton, ON, Canada; 4 Holland Bloorview Kids Rehabilitation Hospital, Toronto, ON, Canada Accepted for Publication: 2 April 2018 ACCEPTED