ORIGINAL ARTICLE I.K.M. Brenner á S. Thomas á R.J. Shephard Spectral analysis of heart rate variability during heat exposure and repeated exercise Accepted: 10 February 1997 Abstract This study examined indices of parasympa- thetic (PNS) and sympathetic (SNS) nerve activity dur- ing exposure to heat and/or two successive bouts of exercise. Seven healthy males [age = 27.1 (3.6) years; mean (SD), maximum oxygen consumption  _ V O 2 max = 48.1 (7.6) ml á kg )1 á min )1 ] were assigned to each of four experimental conditions according to a random- ized-block design. While in a thermoneutral (23°C) or heated (40°C, 30% relative humidity) climatic chamber subjects performed exercise on a cycle ergometer (two 30-min bouts at 50% _ V O 2 max , separated by a 45-min recovery period, (CEx and HEx, respectively) or re- mained seated (CS and HS, respectively) for 2 h. The R- R intervals of the subjects' ECGs were analyzed for se- lected near-steady-state time periods [termed Phase I (25±40 min) and Phase II (100±115 min)] according to the method of Yamamoto and Hughson (J Appl Physiol 71:1143±1150, 1991). Total (P T ), low-frequency (P LF = 0±0.15 Hz) and high-frequency (P HF = 0.15±0.5 Hz) power spectra were calculated using coarse-graining spectral analysis. Heat exposure alone did not alter au- tonomic balance or levels of circulating catecholamines signi®cantly. Exercise in both environmental conditions induced a signi®cant decrease in an index of PNS tone (P HF :P T ) [PNS indicator for CS = 0.084 (0.04) vs CEx = 0.023 (0.015) and HS = 0.065 (0.027) vs HEx = 0.015 (0.009)], with an increase in catecholamine con- centrations. Although the index of SNS activity (P LF :P HF ) tended to rise with exercise in both environ- mental conditions, increments reached levels of signi®- cance only during exercise in the heat [SNS indicator for CS = 8.22 (5.58) vs CEx = 34.06 (21.73) and HS = 8.94 (5.49) vs HEx = 54.29 (49.80)]. The relative magnitudes of SNS and PNS indicators did not dier signi®cantly between the ®rst and second bouts of exercise. These results indicate the substantial contribution of vagal withdrawal and catecholamine secretion to the increase in heart rate that occurs during repeated moderate ex- ercise at room temperature and the additional contri- bution from SNS activity during such exercise in the heat. Key words Heat á Repeated exercise á Heart rate variability á Coarse-graining spectral analysis á Vagal and sympathetic activities Introduction Minimal information is available on the sympathetic (SNS) and parasympathetic (PNS) nervous system re- sponses to either heat exposure or repeated bouts of exercise, although both situations are important to the occupational physiologist. Spectral analysis of heart rate variability (HRV) oers a non-invasive method of examining the autonomic response to such stressful stimuli. The power spectrum of HRV reduces variations in the ECG R-R interval into component sine waves of diering amplitude and frequency; amplitude (variance) is displayed as a function of frequency, and the power (cumulative variance) is calculated over speci®ed fre- quency ranges (Freeman et al. 1991). The HRV power spectrum of a normal healthy individual shows three major peaks: (1) a very-low-frequency (VLF) peak (<0.03 Hz), (2) a low-frequency (LF) peak (0.03± 0.15 Hz) and (3) a high-frequency (HF) peak (0.15± 0.5 Hz) (Akselrod et al. 1985; Freeman et al. 1991; Saul Eur J Appl Physiol (1997) 76: 145±156 Ó Springer-Verlag 1997 R. J. Shephard (&) University of Toronto, School of Physical and Health Education, 320 Huron Street, Toronto, Ontario M5S 1A1, Canada I.K.M. Brenner á S. Thomas á R.J. Shephard Program in Exercise Sciences, Graduate Department of Community Health, University of Toronto, Toronto, Ontario, Canada S. Thomas Department of Physical Therapy, University of Toronto, Toronto, Ontario, Canada