The field study about the effects of artificial CO-rich cool-water immersion after outdoor sports activity in a hot environment.

Background/objective: In our previous laboratory experiment (room temperature of 25 °C), CO-rich cool-water immersion (CCWI) suppressed subjects' core body temperature even during repeated exercise. It is unclear whether the suppression of body temperature elevation would also continue after CCWI in a hot outdoor environment. Herein we investigated the thermal effects of CCWI after regular exercise training in heat on subjects' core temperature (T), three skin temperatures (T), heart rate (HR), and the rate of perceived ice (RPI).

Methods: Thirty-six subjects (25 males, 11 females) were randomly allocated into three groups (CCWI, CWI, and control). After training at their competitive clubs, each subject was immersed up to the chest in CCWI or CWI at 20 °C for 20 min, followed by a 60-min recovery period. T, T, HR, and RPI were measured at the initial rest, the end of immersion, and every 10 min during the recovery period.

Results: Compared to the control, the CCWI subjects' T was significantly lower at 50-60 min after the end of immersion (p < 0.05). T at abdominal and lower-leg regions during the recovery period was maintained at significantly lower values in the CWI and CCWI groups versus control (p < 0.05). The CCWI subjects maintained lower T for a longer time than the CWI subjects.

Conclusions: These findings indicate that CCWI suppresses the rise in body temperatures more than CWI, even in a hot environment, suggesting that CCWI may be a more effective countermeasure against increasing body temperature in a hot outdoor environment.