Parfin, Agnieszka, et al. “An Influence of Social Isolation on the Level of Physical Activity As Well As on Well-Being and Mental State of People During the Coronavirus CO VID-19 Pandemic”. Polish Journal of Public Health, vol. 129, no. 4, Sept. 2020, pp. 127-31, https://doi.org/10.2478/pjph-2019-0029.