of cardiac control. In agreement with the conclusion of Casadei et al. (Casadei, 1996), we hypothesize that HRV could be mainly generated by neural mechanisms at rest and also during moderate exercise whereas the residual HRV remaining during heavy exercise could be due to the mechanical effect of breathing on the sinus node. Therefore, the aim of the present study is twofold: 1) to compare HRV spectral components during steady state exercise, at two different exercise intensities corresponding to sub- (moderate exercise) and supra-ventilatory threshold (heavy exercise). 2) To study the effect of exercise duration on HRV and dynamic cardio-respiratory interactions by analyzing the instantaneous spectral parameters (amplitude, frequency, phase) and comparing spectral energies at the beginning and at the end of the heavy exercise test, since changes in BF during heavy exercise could entail a modification of the HRV components (Clark, 1983). Methods Beat-to-beat RR interval, VO2, VCO2, VE and blood lactate concentration of eleven healthy well-trained young subjects were collected during two exercise tests: 1) a moderate intensity test: fifteen minutes performed below the power at ventilatory threshold (pVT). 2) A heavy intensity test: above pVT until exhaustion. Fast Fourier Transform, Smoothed Pseudo Wigner-Ville Distribution and Complex Demodulation were applied to RR time series. Results Moderate exercise shows a prevalence of low frequency (LF) spectral energy compared to the high frequency (HF) one (LF = 80 ± 10 % vs. HF = 20 ± 10 %, p < 0.001), whereas the reverse is observed during heavy exercise (LF = 11 ± 8 % vs. HF = 89 ± 8 %, p < 0.001, Fig 1). During heavy exercise, the HF amplitude and the tidal volume (Vt) remained constant whereas the breathing frequency (BF) increased (BF: 0.70 ± 0.18 vs. 0.93 ± 0.31, p < 0.01, Fig 2) and mean RR decreased (342 ± 15 vs. 317 ± 16, p < 0.01). Despite the RR series and the breathing signal remaining synchronized, HR/BF ratio decreased and stabilized at 3 RR for 1 breathing cycle, whatever the initial ratio. Discussion/Conclusion Heart rate variability (HRV) allows to differentiate sub- from supra-ventilatory-threshold exercise. Determinants of the residual HRV observed at this intensity are in all probability of non-neural origin. Among the plausible determinants, a mechanical effect of breathing rate to the sinus node is very conceivable. Exercise duration at supra-threshold intensity does not alter the cardio-respiratory synchronization.
© Copyright 2004 Book of Abstracts - 9th Annual Congress European College of Sport Science, July 3-6, 2004, Clermont-Ferrand, France. All rights reserved.