Abstract
Heat acclimation (HA) in air confers adaptations that improve exercise capabilities in hot and possibly temperate air. Swimmers may benefit from HA, yet immersion may constrain adaptation. Therefore, we examined whether warm-water swimming constitutes effective HA. In a randomized-crossover study, eight male swimmers swam 60min/day on 7 days in 33 degrees C (HA) or 28 degrees C (CON) water. They performed 20-min distance trials before and after each regime: in 33 degrees C water (Warm); 28 degrees C water (Temperate); and cycling in 29 degrees C air (Terrestrial) following standardized exercise. Rectal temperature (T-re) rose approximate to 1 degrees C in HA sessions, and sweat loss averaged 1.4L/h. After accounting for CON, HA did not confer any clear expansion of plasma volume [1.9% (95% CI: 7.7)], reduction in heart rate during standardized cycling exercise [1b/min (9)], reduction in T-re during rest [+0.1 degrees C (0.1)] or exercise, or change in sudomotor function. Only perceived temperature and discomfort tended to improve. Performance was clearly not improved for Warm [+0.3% (1.8)] or Temperate [+0.3% (1.9)], was unclear for Terrestrial [+0.4% (17.7)], and was unrelated to changes in resting plasma volume (r<0.3). In conclusion, short-term HA using swimming in 33 degrees C water confers little adaptation and is not ergogenic for warm or temperate conditions.