Endurance exercise improves heart rate complexity in the presence of vagal withdrawal in young adults

Sudden cardiac death (<i>SCD</i>) has been reported during and following physical activity. <i>SCD</i> may be due to vagal withdrawal and/or sympathetic dominance associated with the exercise occurring at any time during, immediately following, or up to several days after exercise. Heart rate variability (<i>HRV</i>) describes the influence of the autonomic nervous system on heart rate. We assessed the immediate post-exercise influence of endurance training on <i>HRV</i> in young adults following the morning exercise bout and also on the same day in the afternoon. Linear domain parameter root mean square of successive <i>RR</i> interval differences (<i>RMSSD</i>) showed vagal withdrawal when analysed both immediately after the <i>AM</i> session and also when pre exercise <i>HRV</i> was compared to post exercise <i>HRV</i> during the early evening (median average change 6.6%) suggesting a possible increased risk of adverse cardiac events. However multiscale Rényi entropy indicated either no change immediately following the exercise for all scaling factors or an increase in <i>HRV</i> complexity of the heart rate for the afternoon recordings. Despite decreased vagal influence, endurance training may be protective for some individuals that retain a higher heart rate complexity in the presence of vagal withdrawal.