Fitness and gender-related differences in heart period variability

Objective: The purpose of this study was to elucidate the processes of autonomic nervous system control of the heart in high fit and low fit men and women by examining indices of vagally mediated cardiac control. Method: There were 40 participants (19 women and 21 men) classified as either high fit or low fit on the basis of scores from the University of Houston Non-Exercise Test (N-EX). Both time domain and frequency domain cardiovascular measures were calculated during a resting baseline period (BL), a facial cooling task (FCT) designed to elicit parasympathetic activity, a reaction time task (RT) designed to elicit primarily sympathetic activity, and a combination task (COMBO) designed to elicit a combination of both parasympathetic and sympathetic activation and recovery periods after each task. Results: Both time and frequency domain indices of heart period variability (HPV) indicated that high fit individuals exhibited greater vagal control of the heart relative to low fit individuals at baseline and across all tasks. In addition, the results suggest greater relative vagal control of the heart in women compared with men. Reactivity scores revealed significant main effects for condition for both time and frequency domain measures of HPV, but no main effects or interactions involving fitness or gender. Conclusions: First, small differences in aerobic capacity result in meaningful differences in cardiac autonomic control, with high fit individuals clearly demonstrating greater vagally mediated cardiac control. Second, important gender-related differences in HPV indicate that women exhibit greater vagal control and men exhibit more sympathetic cardiac control even though heart period (HP) did not differ. Third, the mechanisms underlying the beneficial effects of fitness and those placing women at a reduced risk for coronary heart disease (CHD) seem to be similar.