Gridnev, V., A. Kiselev, and O. Posnenkova. “Using of Spectral Analysis of Heart Rate Variability for Increasing Reliability of Bicycle Ergometry Results”. Journal of Advanced Scientific Research, Vol. 2, no. 03, Aug. 2011, pp. 62-65, https://sciensage.info/index.php/JASR/article/view/46.