AUTONOMIC CONTROL OF HEART-RATE DURING EXERCISE STUDIED BY HEART-RATE-VARIABILITY SPECTRAL-ANALYSIS

Spectral analysis of heart rate variability (HRV) might provide an index of relative sympathetic (SNS) and parasympathetic nervous system (PNS) activity during exercise. Eight subjects completed six 17-min submaximal exercise tests and one resting measurement in the upright sitting position. During submaximal tests, work rate (WR) was increased for the initial 3 min in a ramp fashion until it reached constant WRs of 20 W, or 30, 60, 90, 100, and 110% of the predetermined ventilatory threshold (T(vent). Ventilatory profile and alveolar gas exchange were monitored breath by breath, and beat-to-beat HRV was measured as R-R intervals of an electrocardiogram. Spectral analysis was applied to the HRV from 7 to 17 min. Low-frequency (0-0.15 Hz) and high-frequency (0.15-1.0 Hz) areas under power spectra (LO and HI, respectively) were calculated. The indicator of PNS activity (HI) decreased dramatically (P < 0.05) when the subjects exercised compared with rest and continued to decrease until the intensity reached 60% T(vent). The indicator of SNS activity (LO/HI) remained unchanged up to 100% T(vent), whereas it increased abruptly (P < 0.05) at 110% T(vent). The results suggested that (cardiac) PNS activity decreased progressively from rest to a WR equivalent to 60% T(vent), and SNS activity increased only when exercise intensity exceeded T(vent).