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

Sudden cardiac death (SCD) has been reported during and following physical activity. SCD 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 (HRV) describes the influence of the autonomic nervous system on heart rate. We assessed the immediate post-exercise influence of endurance training on HRV in young adults in the morning and also on the same day in the afternoon following the morning exercise session. Linear domain parameter root mean square of successive RR interval differences (RMSSD) showed vagal withdrawal when analysed both immediately after the AM session and also when pre exercise HRV was compared to post exercise HRV during the afternoon (median average change 6.6%). However multiscale Rényi entropy indicated either no change immediately following the exercise for all scaling factors or an increase in HRV complexity of the heart rate. Despite decreased vagal influence, endurance training may be protective for some individuals that retain a higher heart rate complexity as measured by Rényi entropy in the presence of vagal withdrawal.