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

•A new heat acclimation state test using metabolic heat production intensities.•This new heat acclimation state test provides reliable physiological measures.•A reliable 0.2 °C typical error of measurement in sweat setpoint occurred.•Sweat gain was more variable than sweat setpoint due to measurement error.

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-min 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 s−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 s−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.