Cousins, J. M., M. J. Peterson, A. N. Christopher, A. P. Francis, and H. H. Betz. “Grit-Passion and Grit-Perseverance in Ultramarathon Runners”. Journal of Human Sport and Exercise, vol. 16, no. 4, Nov. 2021, pp. 965-72, doi:10.14198/jhse.2021.164.18.