Weston, M., Castagna, C., Impellizzeri, F. M., Rampinini, E., & Abt, G. (2007). Analysis of physical match performance in English Premier League soccer referees with particular reference to first half and player work rates. Journal of science and medicine in sport / Sports Medicine Australia, 10(6), 390-397. https://doi.org/10.1016/j.jsams.2006.09.001