The influence of body morphology on changes in core temperature during exercise in an uncompensable environment

Introduction

Evidence demonstrates that for unbiased comparisons of changes in core temperature (ΔT_core) between groups unmatched for body morphology, exercise should be performed using a fixed heat production (H_prod) 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 ΔT_core [2], however in addition to not accounting for differences in H_prod 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 (E_max). We examined the best method for performing unbiased comparisons of ΔT_core 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. E_max 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) H_prod of 6 W.kg^-1, or (iii) H_prod of 3 W.kg^-1 above E_max.

Results

E_max 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 ΔT_re 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 H_prod of 6 W.kg^-1 (P < 0.001; Figure 1B). This systematic difference in ΔT_re between SM and LG groups was abolished at a fixed H_prod of 3 W.kg^-1 above E_max (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, ΔT_re in an uncompensable environment should be determined by the rate of heat storage per unit mass, which is presently expressed as the difference between H_prod and E_max in W.kg^-1. At a fixed absolute workload of 100 W, ΔT_re and H_prod-E_max in W.kg^-1 were greater in SM. At a fixed H_prod of 6 W.kg^-1, ΔT_re and H_prod-E_max in W.kg^-1 as greater in LG due to a smaller surface area-to-mass ratio. When H_prod-E_max in W.kg^-1 was fixed between SM and LG, ΔT_re was the same despite a different H_prod in W.kg^-1.

Conclusion

Preliminary results suggest that over a fixed exercise duration in an uncompensable environment, unbiased comparisons of ΔT_re between groups/individuals of different body size (mass and BSA) may be best attained using an exercise intensity at a fixed H_prod-E_max in W.kg^-1.