Steady-state sweating during exercise is determined by the evaporative requirement for heat balance independently of absolute core and skin temperatures.

KEY POINTS

When exercise was prescribed to elicit a fixed evaporative heat balance requirement (E ), no differences in steady-state sweat rates were observed with different absolute oesophageal and/or skin temperatures, secondary to differences in time of the day (i.e. morning (AM) vs. afternoon (PM)) and ambient temperature (i.e. 23°C vs. 33°C). Exercise at a fixed metabolic heat production (H ), but a different E (due to differences in air temperature), yielded higher steady-state sweat rates with a higher E , irrespective of absolute oesophageal temperature. Circadian rhythm did not alter the change in core temperature prior to the onset for sudomotor activation, nor the thermosensitivity, resulting in similar cumulative whole-body sweat rates irrespective of time of day at a fixed E . Collectively, these data indicate that during exercise in a compensable environment, steady-state sudomotor responses are influenced by E rather than absolute core and skin temperatures, or H .

ABSTRACT

The present study sought to determine whether absolute core temperature (modified via diurnal variation) and absolute skin temperature (modified by different air temperatures (T )) alters the steady-state sweating response to exercise at a fixed evaporative heat balance requirement (E ). Ten males exercised for 60 min on six occasions. Three T /heat production (H ) combinations (23°C/525 W, 33°C/400 W, 33˚C/525 W) were completed in the morning (08.00 h, AM) and afternoon (16.00 h, PM), to yield: (1) the same E (200 or 275 W·m ) with different absolute core temperatures (AM vs. PM); (2) the same E (200 W·m ) with different skin temperatures (T : 23˚C vs. 33˚C); (3) the same heat production (525 W) with different E (200 vs. 275 W·m ). Oesophageal temperature (T ), local sweat rate (LSR) on the arm and upper-back, and whole-body sweat rate (WBSR) were measured. Steady-state T was always higher in PM versus AM at an E of 200 W·m (23°C, P = 0.001; 33°C, P = 0.004) and 275 W·m , (33°C, P = 0.001). However steady-state mean LSR (200 W·m /23°C: P = 0.25; 200 W·m /33°C: P = 0.86; 275 W·m /33°C: P = 0.53) and WBSR (200 W·m /23°C: P = 0.79; 200 W·m /33°C: P = 0.48; 275W·m /33°C: P = 0.32) were similar. When E was matched (200 W·m ) with different T (23°C vs. 33°C), steady-state LSR (P > 0.17) and WBSR (P > 0.93) were similar despite different skin temperatures. For the same H (525 W) but different E (200 vs. 275 W·m ), mean LSR (P < 0.001), and WBSR (P < 0.001) were higher with a greater E . Collectively, steady-state sweating during exercise is altered by E but not T , skin temperature, or H .