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 (ΔT _re ) 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 m ² ; large (LG): 100.0 ± 13.1 kg, 2.3 ± 0.1 m ² ], 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 (E _max ) 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 VO _2max , (2) a heat production (H _prod ) of 520 W, (3) H _prod relative to mass (6 W·kg ^-1 ), and (4) H _prod relative to mass above E _max (3 W·kg ^-1 >E _max ). E _max 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). ΔT _re 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 VO _2max (SM: 1.4 ± 0.5; LG 0.9 ± 0.3°C, P  <   0.001). However, ΔT _re was similar between groups when H _prod 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 >E _max (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 ΔT _re and LSR between groups unmatched for body size during uncompensable heat stress can be avoided by a fixed H _prod in W·kg ^-1 or W·kg ^-1 >E _max .
(© 2017 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of The Physiological Society and the American Physiological Society.)