Introduction
Evidence demonstrates that for unbiased comparisons of changes in core temperature (ΔTcore) between groups unmatched for body morphology, exercise should be performed using a fixed heat production (Hprod) per unit mass in physiologically compensable environments [1]. In uncompensable conditions, it has been suggested that a fixed external workload is the primary determinant of ΔTcore [2], however in addition to not accounting for differences in Hprod relative to mass, such an approach excludes the influence of differences the surface area-to-mass ratio on the absolute maximum rate of evaporative heat loss (Emax). We examined the best method for performing unbiased comparisons of ΔTcore between groups unmatched for body morphology during exercise in an uncompensable environment.
Methods
Six small (mean(SD) SM: 64.4(7.2) kg, 1.78(0.10) m2, 276(21) cm2.kg-1) and four large (LG: 94.2(7.2) kg, 2.19(0.09) m2, 233(8) cm2.kg-1) participants were recruited. Emax for each participant was first assessed [3]. Participants then completed three trials, during which they cycled for 75 min at 35 °C, 70 % RH, at a target (i) absolute workload of 100 W, (ii) Hprod of 6 W.kg-1, or (iii) Hprod of 3 W.kg-1 above Emax.
Results
Emax at 35 °C, 70 % RH was similar between SM and LG in W.m-2 (167 [27] vs. 146 [9] W.m-2), but lower in LG in W/kg (3.4 (0.2) vs. 4.6 (0.1) W.kg-1) by virtue of a difference in surface area-to-mass ratio. A systematically greater ΔTre was observed in the SM group at an external workload of 100 W (P = 0.036; Figure 1A); and in the LG group at an Hprod of 6 W.kg-1 (P < 0.001; Figure 1B). This systematic difference in ΔTre between SM and LG groups was abolished at a fixed Hprod of 3 W.kg-1 above Emax (P = 0.999; Figure 1C).
Figure 1.
The change in rectal temperature (Tre) during exercise at a fixed: external workload of 100 W (A), Hprod of 6 W.kg-1 (B), and Hprod of 3 W.kg-1 above Emax (C). SM small; LG large. *Significantly different between groups within condition (P < 0.05).
Discussion
Theoretically, ΔTre in an uncompensable environment should be determined by the rate of heat storage per unit mass, which is presently expressed as the difference between Hprod and Emax in W.kg-1. At a fixed absolute workload of 100 W, ΔTre and Hprod-Emax in W.kg-1 were greater in SM. At a fixed Hprod of 6 W.kg-1, ΔTre and Hprod-Emax in W.kg-1 as greater in LG due to a smaller surface area-to-mass ratio. When Hprod-Emax in W.kg-1 was fixed between SM and LG, ΔTre was the same despite a different Hprod in W.kg-1.
Conclusion
Preliminary results suggest that over a fixed exercise duration in an uncompensable environment, unbiased comparisons of ΔTre between groups/individuals of different body size (mass and BSA) may be best attained using an exercise intensity at a fixed Hprod-Emax in W.kg-1.