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 req ), 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 prod ), but a different E req (due to differences in air temperature), yielded higher steady‐state sweat rates with a higher E req , 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 req . Collectively, these data indicate that during exercise in a compensable environment, steady‐state sudomotor responses are influenced by E req rather than absolute core and skin temperatures, or H prod . 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 a )) alters the steady‐state sweating response to exercise at a fixed evaporative heat balance requirement ( E req ). Ten males exercised for 60 min on six occasions. Three T a /heat production ( H prod ) 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 req (200 or 275 W·m −2 ) with different absolute core temperatures (AM vs . PM); (2) the same E req (200 W·m −2 ) with different skin temperatures ( T a : 23˚C vs . 33˚C); (3) the same heat production (525 W) with different E req (200 vs . 275 W·m −2 ). Oesophageal temperature ( T oes ), local sweat rate (LSR) on the arm and upper‐back, and whole‐body sweat rate (WBSR) were measured. Steady‐state T oes was always higher in PM versus AM at an E req of 200 W·m −2 (23°C, P = 0.001; 33°C, P = 0.004) and 275 W·m −2 , (33°C, P = 0.001). However steady‐state mean LSR (200 W·m −2 /23°C: P = 0.25; 200 W·m −2 /33°C: P = 0.86; 275 W·m −2 /33°C: P = 0.53) and WBSR (200 W·m −2 /23°C: P = 0.79; 200 W·m −2 /33°C: P = 0.48; 275W·m −2 /33°C: P = 0.32) were similar. When E req was matched (200 W·m −2 ) with different T a (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 prod (525 W) but different E req (200 vs . 275 W·m −2 ), mean LSR ( P < 0.001), and WBSR ( P < 0.001) were higher with a greater E req . Collectively, steady‐state sweating during exercise is altered by E req but not T oes , skin temperature, or H prod .

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