The reliability of a heat acclimation state test prescribed from metabolic heat production intensities

Acclimation state indicates an individual’s phenotypic response to a thermally stressful environment, where changes in heat dissipation capacity are determined during a heat acclimation state test (HAST). Variations in thermoregulatory and sudomotor function are reported while exercising at intensities relative to maximal oxygen uptake. This inter-individual variation is not true when intensity is prescribed to elicit a fixed rate of metabolic heat production (Ḣprod). This study investigated the reliability of peak Tre and two composite measures (sweat gain and sweat setpoint) derived from indices of thermosensitivity during a HAST prescribed from Ḣprod intensities. Fourteen participants (mean ± SD; age 23 ± 3 years, stature 174 ± 7 cm, body mass 75.0 ± 9.4 kg, body surface area 1.9 ± 0.1 m2, peak oxygen consumption [V̇O2peak] 3.49 ± 0.53 L.min-1) completed a lactate threshold-V̇O2peak test and two duplicate Ḣprod HASTs on a cycle ergometer. The HAST consisted of three, 30-minute periods of exercise at fixed Ḣprod intensities relative to body mass (3, 4.5 and 6 W.kg-1), within hot dry conditions (44.7 ± 1.8°C and 18.1 ± 4.7 % relative humidity). Peak Tre (38.20 ± 0.36 vs 38.16 ± 0.42°C, p = 0.54), sweat setpoint (36.76 ± 0.34 and 36.79 ± 0.38°C, p = 0.68) and sweat gain (0.37 ± 0.14 and 0.40 ± 0.18 g.sec-1.°C-1, p = 0.40) did not differ between HASTs. Typical error of measurement (TEM), coefficient variation (CV) and intra-class coefficient of correlation (ICC) were 0.19°C, 0.5% and 0.80 for peak Tre, 0.21°C, 0.6% and 0.65 for sweat setpoint and 0.09 g.sec-1.°C-1, 28% and 0.68 for sweat gain, respectively. The use of fixed Ḣprod intensities relative to body mass is a reliable method for measuring Tre and ascertaining sweat setpoint during a HAST, whereas, sweat gain displays greater variability. A Ḣprod HAST appears sufficiently reliable for quantifying heat acclimation state, where TEM in peak Tre and sweat setpoint are small enough to identify physiologically meaningful improvements post intervention.