Effects of daily post‐training, high‐concentration <scp>CO₂</scp>‐water immersion on thermoregulatory function in young male baseball players

Among the different forms of hydrotherapy, carbon dioxide (CO₂) water immersion improves peripheral vasodilation and blood flow compared with tap water immersion; however, the heat stress placed on the body through CO₂ water immersion and the appropriate immersion protocols are uncertain. Therefore, this study aimed to compare the thermoregulatory responses during CO₂ and tap water immersions. The participants were 10 male college baseball players. After daily training was completed, intervention was performed for 15 min under three conditions: (1) CO₂ water immersion (CO₂; 40°C, 1000 ppm), (2) tap water immersion (TAP; 40°C), and (3) seated at rest at room temperature (control [CON]; at 25°C and relative humidity of 60%). Core body temperature (Tcore), skin temperature (Tsk), heart rate (HR), skin blood flow, local sweat rate (LSR), and blood pressure were measured. The Tcore, Tsk, thermal sensation (TS), and HR were significantly higher in the CO₂ and TAP trials than in the CON trial. Compared to tap water immersion, CO₂ water immersion resulted in higher Tcore and LSR values, with moderate to large effect sizes (Tcore: d = 0.52, LSR: d = 0.80). However, thermal discomfort did not increase, suggesting that CO₂ water immersion may increase heat stress without causing any negative effects.