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

Abstract

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