Aerobic exercise leads to reduced sympathetic and increased cardiac vagal modulation, providing an antiarrhythmic effect. The optimal exercise intensity to promote this adaptation remains undefined. The aims of the present investigation were twofold. First, to examine differences in heart rate variability (HRV) measures in participants with different levels of objectively measured physical activity (PA). Second, to identify the characteristic of PA which most influences the cardiac autonomic nervous system (cANS) function in young adults. Cross-sectional evaluation of 84 adults examining relationships between PA amount and intensities, measured by accelerometry, cANS function derived from HRV. Groups were created based on tertiles of PA and analysis of covariance was used to assess between-group differences in HRV. Stepwise regression analysis was used to determine the characteristic of PA, which best predicted vagal HRV indices. There were significantly higher levels of vagal HRV indices in the most active group compared with the least active group. Regression analysis revealed that the number of bouts of vigorous PA undertaken was the best predictor of the vagal HRV indices assessed. This study suggests that vagal modulation is enhanced with high levels of PA and that it is the number of bouts of vigorous PA that is most closely associated with cANS function.
