Kocjan, J. “Effect of a C1-C2 Mulligan Sustained Natural Apophyseal Glide (SNAG) in the Treatment of Cervicogenic Headache”. Journal of Education, Health and Sport, vol. 5, no. 6, June 2015, pp. 79-86, https://apcz.umk.pl/JEHS/article/view/2725.