1. Bizzini M, Impellizzeri FM, Dvorak J, Bortolan L, Schena F, Modena R, Junge A (2013) Physiological and performance responses to the “FIFA 11+” (part 1): is it an appropriate warm-up? J Sports Sci 31(13):1481–1490.
https://doi.org/10.1080/02640414.2013.802922
2. Dvorak J, Junge A (2005) F-MARC football medicine manual: 1994–2005. Federation Internationale de Football Association, Zurich
3. FIFA (2006) 270 Million people active in football 2006.
http://www.fifa.com/mm/document/fifafacts/bcoffsurv/bigcount.statspackage_7024.pdf
4. Bureau U (2012) Participation in selected sports activities: 2009.
https://www.census.gov/compendia/statab/2012/tables/12s1249.pdf
5. Morris B (2015) Why is the U.S. so good at women’s soccer?
http://fivethirtyeight.com/datalab/why-is-the-u-s-so-good-at-womens-soccer/