EFFECTS OF VARIED AIR VELOCITY ON SWEATING AND EVAPORATIVE RATES DURING EXERCISE

This study was designed to determine the extent to which changes in the evaporative power of the environment (E(max)) affect sweating an evaporative rates. Six male subjects undertook four 60-min bouts of cycle ergometer exercise at 56% maximal O2 uptake (VO2max). E(max) was varied by differences in ambient temperature and airflow; two exercise bouts took place at 24-degrees-C and two at 35-degrees-C, with air velocity at <0.2 and 3.0 m/s in both. Total sweat production was estimated from body weight loss, whereas whole body evaporative rate was measured continuously from a Potter beam balance. Body core temperature was measured continuously from a thermocouple in the esophagus (T(es)), with mean skin temperature (T(sk)BAR) computed each minute from thermocouples at eight sites. Total body sweat loss was significantly greater (P < 0.05) in the 0.2- than in the 3.0-m/s condition at both 24 and 35-degrees-C. T(sk)BAR was higher (P < 0.05) in the still-air conditions at both temperatures, but final T(es) was significantly higher (P < 0.05) in still air only in the 35-degrees-C environment. Thus the reduced E(max) in still air caused a greater heat storage, thereby stimulating a greater total sweat loss. However, in part because of reduced skin wettedness, the slope of the sweat rate-to-T(es) relation at 35-degrees-C in the 3.0-m/s condition was 118% that at 0.2 m/s (P < 0.005). Heart rate at 60 min was significantly higher (P < 0.05) only in the still-air condition at 35-degrees-C (155 vs. 137 beats/min), when convection and radiation heat exchange were negligible and E(max) became insufficient to prevent an increased rate of heat storage and continuous heart rate drift. Neither occurred in the still-air condition at 24-degrees-C because of the combination of a large convection plus radiation heat loss and the intrinsic airflow-induced increase in E(max) produced by leg movement during cycling.