Dissociating Biophysical and Training-Related Determinants of Core Temperature

Dear Editor-in-Chief: In a recent review, Dr. Mora-Rodriguez (5) concluded that core temperature is predicted by the percentage of peak oxygen uptake (%V˙O2peak) in physiologically compensable conditions and absolute heat production in uncompensable conditions (see Fig. 4 in (5)). Heat balance calculations (3) and recent evidence from our laboratory (4) suggest otherwise. High (HI) and low (LO) V˙O2peak groups matched for mass and body surface area (BSA), exercising at 540 W heat production in compensable conditions, showed similar changes in rectal temperature (Tre) and whole-body sweat losses despite vastly different relative intensities (39.7% vs 57.6% V˙O2peak) (4). Furthermore, absolute end-exercise Tre was ∼0.2°C lower in the HI group simply because of lower preexercise values. In contrast, exercise at 60% V˙O2peak (heat production, 844 vs 600 W) yielded greater changes in Tre and absolute end-exercise Tre values in the HI group, and whole-body sweat losses were greater in the HI group because of higher evaporative heat balance requirements (Ereq) (4). In compensable conditions, these findings suggest the following after eliminating differences in mass and BSA: (i) changes in Tre are determined by heat production, not %V˙O2peak; (ii) any differences in end-exercise absolute Tre between fitness groups only arise because of differences in preexercise Tre; and (iii) sweating is not altered by a high V˙O2peak. We further suggested that groups heterogeneous for body morphology may be compared for changes in Tre using a fixed heat production per unit mass (W·kg–1) in compensable environments. This approach explains the greater Tre changes in trained subjects at 40%V˙O2peak (8.2 vs 6.1 W·kg–1) (6), with these greater changes compensated by different preexercise Tre values, leading to similar absolute end-exercise temperatures between training groups. By definition, uncompensable conditions arise when Ereq exceeds the maximum possible evaporation rate (Emax). Dr. Mora-Rodriguez suggests that Ereq > Emax at a similar %V˙O2peak in trained and untrained groups (see Fig. 4 in (5)). However, at a given %V˙O2peak, Ereq is lower in untrained individuals because of their lower heat production, and the primary reason that Ereq > Emax at the same V˙O2peak in the proposed model is the lower maximum skin wettedness (ωmax) assigned to untrained individuals (ωmax = 0.85). Although maximum sweat rate is probably different (1), such large ωmax adjustments as a function of training status do not seem justified by the literature. A ωmax of 0.85 and 1.00 were proposed originally for nonheat-acclimated and heat-acclimated individuals, respectively (2), but physical training only imparts partial acclimation (7). Even if ωmax differences between training groups are as large as proposed, heat balance calculations (3) show the %V˙O2peak at which Ereq > Emax still should be greater in unfit/untrained subjects with the same BSA/mass ratio. The %V˙O2peak at which Ereq > Emax declines with decreasing BSA/mass ratio. Because the BSA/mass ratio of the author’s untrained group (6) was lower, it appears that a combination of different physical characteristics and assigned ωmax values led to a conclusion with restricted validity. A more robust descriptor of the reported differences in Tre between training groups at high relative exercise intensities (6) may be the difference between Ereq and Emax expressed in W·kg–1. Matthew N. Cramer Nathan B. Morris Ollie Jay Thermal Ergonomics Laboratory School of Human Kinetics University of Ottawa Ottawa, Ontario, Canada

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