This study was conducted to assess the effects of bathing in still water and in flowing water on the heart rate variability. Eight healthy young males (age 20 to 28) bathed in still water at temperatures of 34°C, 38°C, and 41°C for 20 minutes each. The other eight healthy young males (age 22 to 28) bathed in flowing water at a temperature of 36°C for 30 minutes. Electrocardiograms were recorded before, during, and after the bathing. Subjects sat still for 20 minutes before bathing, and then bathed in water to the axilla in a sitting position. Subjects breathed freely during the experiment. Heart rate variability was estimated with the power spectral analysis using FFT. The power densities in the high frequency (0.15 to 0.50Hz) and low frequency (0.04 to 0.15Hz) areas as obtained from this frequency analysis (HF and LF) as well as the ratio of LF/HF were calculated, and HF was used as index of cardiac parasympathetic activity, LF as index of sympathetic activity with parasympathetic modulation, and LF/HF as index of sympathetic activity.
During bathing in still water at 34°C and 36°C, no significant change from the value before the bathing was found in heart rate, HF, LF, or LF/HF. HF and LF significantly decreased during the bathing in still water at 38°C and 41°C, LF/HF significantly increased during the bathing in still water at 38°C, During the bathing in still water at 41°C, we could not calculate LF/HF for many subjects because HF disappeared. During the bathing in water flowing at a moderate speed (1.0m/sec), LF/HF increased significantly. During the bathing in water flowing at a high speed (2.0m/sec), heart rate and LF/HF increased significantly while LF decreased significantly.
These results suggest that parasympathetic nervous activities are suppressed and sympathetic nervous activities are enhanced during bathing in still water at temperatures higher than the neutral temperature (34°C), and sympathetic nervous activity is enhanced during the bathing in flowing water at 36°C, However, the effects of respiration rate and tidal-volume on HF, and the validity of the HR variabilities as an index of autonomic nervous activities should be examined in further detail.