Effects of different water temperature on rectal temperature and cardiac autonomic nervous system during supine floating after exercise with a cycle ergometer

The purpose of this study was to determine the effects of different water temperature on rectal temperature and cardiac parasympathetic reactivation during supine floating after exercise with a cycle ergometer. Six healthy Japanese males volunteered for this study. All subjects signed an informed consent form prior to participation in this study. Subjects performed exercise with cycle ergometer by 50%VO2max for 15 min, and recovered for 30 min at supine floating and supine position on land (Control (C)-condition). Water temperature set up 25 (WT-25), 30(WT-30) and 35 degrees Celsius (WT-35). Rectal temperature, heart rate and cardiac autonomic nervous system activity were measured throughout experimental period. Cardiac autonomic nervous system activity was calculated using Maximum Entropy Calculation Methodology (MemCalc) and evaluated by using high frequency (HF ; 0.15-0.40 Hz) in R-R intervals. HF domain was transformed into logarithmic values. We used the slope in regression line calculated by appearance time and peak of HE as cardiac parasympathetic reactivation. Heart rate increased up to about 160 bpm during exercise period and was not significantly different among these conditions. During the recovery process, delta rectal temperature (point 0-0 is end of exercise) was significantly reduced under the WT-25 and WT-30, as compared to the WT-35. Rectal temperature was reduced by the heat loss such as heat dissipation and increase of the skin blood flow. The heat loss of WT-25 and WT-30 are greater than that of WT-35 and C-condition. The log HF was significantly increased under the WT-25 and WT-30, as compared to the WT-35 and C-condition. The slope in regression lines in the WT-25 and the WT-30 were significantly steeper than the WT-35 and the C-condition. This was caused by cold stimulus in skin. Our data suggest when water temperature is greatly different from increased body temperature with exercise, reduced rectal temperature and reactivation in cardiac parasympathetic nervous system induced by during supine floating after exercise with a cycle ergometer was pronounced.