In a study of 427 children aged 10-11 in Bristol, UK, children who spent more time outdoors after school were more likely to be physically active. For every hour spent outside playing with friends, the children averaged 17 minutes of moderate to vigorous physical activity, while physical activity averaged only 6 minutes for every hour spent playing indoors with friends.
Pearce, Matthew, Angie S. Page, Tom P. Griffin, and Ashley R. Cooper. (2014) Who children spend time with after school: Associations with objectively recorded indoor and outdoor physical activity. International Journal of Behavioral Nutrition and Physical Activity. 11(1). 45. doi:10.1186/1479-5868-11-45