1. The Nielsen Companyhttp://www.nielsen.com/20172017-12-30Nielsen Total Audience Report - Q2 2017
http://www.nielsen.com/us/en/insights/reports/2017/the-nielsen-total-audience-q2-2017.html
2. European Commissionhttps://data.europa.eu/20142017-12-31Special Eurobarometer 412: Sport and Physical Activity 2014
https://data.europa.eu/euodp/data/dataset/S1116_80_2_412
3. Global physical activity levels: surveillance progress, pitfalls, and prospects
4. Health benefits of physical activity: the evidence
5. Walking in Relation to Mortality in a Large Prospective Cohort of Older U.S. Adults