A study of the effects of the covid-19 pandemic on the experience of back pain reported on twitter® in the united states: A natural language processing approach - Fiok, K.; Karwowski, W.; Gutierrez, E.; Saeidi, M.; Aljuaid, A.M.; Davahli, M.R.; Taiar, R.; Marek, T.; Sawyer, B.D. (International Journal of Environmental Research and Public Health, 2021)