Abstract

Aims of the study were to investigate autonomic nervous system influence on heart rate during physical exercise and to examine the relationship between the fractal component in heart rate variability (HRV) and the system's response. Ten subjects performed incremental exercise on a cycle ergometer with work rate increasing 2.0 W/min until exhaustion. Alveolar gas exchange, plasma norepinephrine (NE) and epinephrine (E) responses, and beat-to-beat HRV were monitored. HRV data were analyzed by "coarse-graining spectral analysis" to break down their total power (Pt) into harmonic and nonharmonic (fractal) components. The harmonic component was further divided into low-frequency and high-frequency components, from which low-frequency and high-frequency power (Pl and Ph, respectively) were calculated. Parasympathetic (PNS) and sympathetic (SNS) nervous system activity indicators were evaluated by Ph/Pt and Pl/Ph, respectively. From the fractal component, the fractal dimension (DF) and the spectral exponent (beta) were calculated. The PNS indicator decreased significantly when exercise intensity exceeded 50% VO2peak. Conversely, the SNS indicator initially increased at 50-60% VO2peak and further increased significantly at >60% VO2peak when there were also more pronounced increases in NE and E. The decreased PNS and the increased SNS indicators were accompanied by an increase in beta (a decrease in DF) to the level of beta>2 (DF<2). Results suggest that fractal component in HRV might be coupled to those in sympathovagal balance. Verf.-Ref.