The optimal exercise intensity for the unbiased comparison of thermoregulatory responses between groups unmatched for body size during uncompensable heat stress

We sought to identify the appropriate exercise intensity for unbiased comparisons of changes in rectal temperature (ΔTre) and local sweat rates (LSR) between groups unmatched for body size during uncompensable heat stress. Sixteen males vastly different in body morphology were separated into two equal groups [small (SM): 65.8 ± 6.2 kg, 1.8 ± 0.1 m2; large (LG): 100.0 ± 13.1 kg, 2.3 ± 0.1 m2], but matched for sudomotor thermosensitivity (SM: 1.3 ± 0.6; LG: 1.1 ± 0.4 mg·cm-2·min-1·°C-1). The maximum potential for evaporation (Emax) for each participant was assessed using an incremental humidity protocol. On separate occasions, participants then completed 60 min of cycling in a 35°C and 70% RH environment at (1) 50% of VO2max, (2) a heat production (Hprod) of 520 W, (3) Hprod relative to mass (6 W·kg-1), and (4) Hprod relative to mass above Emax (3 W·kg-1>Emax). Emax was similar between LG (347 ± 39 W, 154 ± 15 W·m-2) and SM (313 ± 63 W, 176 ± 34 W·m-2, P > 0.12). ΔTre was greater in SM compared to LG at 520 W (SM: 1.5 ± 0.5; LG 0.8 ± 0.3°C, P < 0.001) and at 50% of VO2max (SM: 1.4 ± 0.5; LG 0.9 ± 0.3°C, P < 0.001). However, ΔTre was similar between groups when Hprod was either 6 W·kg-1 (SM: 0.9 ± 0.3; LG 0.9 ± 0.2°C, P = 0.98) and 3 W·kg-1>Emax (SM: 1.4 ± 0.5; LG 1.3 ± 0.4°C, P = 0.99). LSR was similar between LG and SM irrespective of condition, suggesting maximum LSR was attained (SM: 1.10 ± 0.23; LG: 1.07 ± 0.35 mg·cm-2·min-1, P = 0.50). In conclusion, systematic differences in ΔTre and LSR between groups unmatched for body size during uncompensable heat stress can be avoided by a fixed Hprod in W·kg-1 or W·kg-1>Emax.