Gallo Villegas, J. A., D. L. Alvarez, and J. Farbiarz. “Time and Frequency Analysis of R-R Variability in Athletes and Sedentary People”. Acta Medica Colombiana, Vol. 26, no. 2, 1, pp. 65-72, https://actamedicacolombiana.com/ojs/index.php/actamed/article/view/3457.