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doi:10.1152/japplphysiol.00248.2014116:1121-1122, 2014. First published 20 March 2014;J Appl Physiol

Samuel N. Cheuvrontarea

surfaceresponses in groups of different body mass andMatch maker: how to compare thermoregulatory

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Match maker: how to compare thermoregulatory responses in groups of

different body mass and surface area

Samuel N. Cheuvront

U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts

THERE IS A LONG AND RICH HISTORY of research comparingthermoregulatory responses between groups of different sex,age, and health. Direct measurements of body temperature,whole body, and local sudomotor activity are the basic toolsused by thermal and exercise physiologists to understand andinterpret group differences in central and peripheral thermo-regulation. A qualitative appreciation for how differences inphysical characteristics and fitness between groups can impactthermoregulation drove the historical adoption of imperfectmatching solutions meant to isolate factors of interest. Apopular example, such as matching relative exercise intensityto control for fitness, does not seem necessary for making

unbiased comparisons (6). The

30 yr time span of excellentreviews on the topic of sex differences in thermoregulation (5,7) represents well the enduring problem; how should thermo-regulatory responses be compared between any two groupswith inherent or chance differences in physical characteristicsthat influence thermoregulation?

In this issue of the Journal of Applied Physiology, Cramerand Jay (1) provide for the first time a clear quantitativeexplanation for the important independent influences of bodymass and body surface area on thermoregulatory outcomemeasures of change in body core temperature (Tre) and localsweating rate (LSR) during exercise in two compensable en-vironments (2535C, 35% relative humidity). They also pro-vide the necessary methodology for selecting an exercise

intensity that remedies the historically confounding effects ofbody morphology. Briefly, the greater heat sink of individualswith larger body mass in their study (92 kg, LG) resulted ina smaller Tre at any absolute rate of metabolic heat produc-tion (Hprod, W) compared with smaller individuals (68 kg,SM). Despite this fact, whole body sweating rates (WBSR)were the same because the absolute evaporative sweatingrequirement (Ereq, W) was identical for both groups, as previ-ously described (4). As a consequence of having a smallerbody surface area in SM (1.8 m2) compared with LG (2.1 m2),LSR was significantly higher in SM (LSR expressed as a ratein surface area units by convention). On the surface, outcomesmake it appear that LG is less susceptible to heat stress than

SM (lower Tre), but whether LG sweats at the same rate(WBSR) or less (LSR) than SM depends on which sweatingmeasure is chosen for comparison. It is equally puzzling todecide how best to fairly compare the LSR threshold (Treonset) and sensitivity (LSR slope) between LG and SM in thisscenario. The remedy that Cramer and Jay (1) provide for theinconsistent measurement outcomes is to prescribe exerciseintensity as either watts per kilogram to control for the effectsof body mass on Tre(heat sink) or as Ereq(W/m

2) to control

for the effects of surface area on LSR. When LG and SM werecompared by matching for watts per kilogram and watts persquare meter, differences in Treand LSR disappeared. Figure 1summarizes how to select exercise intensity to control for Tre(Hprod; W/kg), WBSR (Ereq; W), or LSR (Ereq; W/m

2) when afactor distinct from body mass and surface area is suspected toindependently influence central or peripheral thermoregulation.

One obvious application of this research is that it will allowan improved isolation of the factors known or suspected togenuinely influence thermoregulation, thus ultimately enhanc-ing the science and understanding of group differences inhuman thermoregulation. Good examples include comparisonsbetween groups of different body size, such as men vs. womenand especially children vs. adults. A less obvious application isthat their methods might also be used to help fairly isolate andunderstand factors responsible for heat illness susceptibility.For example, absolute workload tests (Hprod, W) for assessingheat intolerance (i.e., Tre response) (2) might further beimproved by use of a more standardized test (Hprod, W/kg) thatremoves the influence of body mass from the observation.

A minor limitation of this study is that not everyone inter-ested in comparing thermoregulation between groups will bewell versed or practiced in the use of the biophysics equationsnecessary for applying some of the methods. For example,although absolute Hprod expressed in watts or watts per kilo-gram is a fairly straightforward set of calculations, the string of

necessary biophysical calculations needed to use the requiredevaporative heat loss, or Ereq, can be daunting for anyoneunaccustomed to their use and meaning. However, an excellentreview on this subject (3) will educate readers and provides allthe formulas necessary for easy input and automated outputusing an Excel module or similar software package. In addi-tion, although it is likely that the approach described (Fig. 1)will improve comparisons between groups made duringuncompensable heat stress also, it remains possible thatsmaller vapor pressure gradients between skin and air willsuppress sweating and alter the anticipated group differ-ences that otherwise require correction during compensableheat stress (1, 3, 5).

Address for reprint requests and other correspondence: S. N. Cheuvront,U.S. Army Research Institute of Environmental Medicine, 15 Kansas St., Bldg.42, Natick, MA 01760-5007 (e-mail:samuel.n.cheuvront.civ@mail.mil).

Thermoregulatory Responses

Exercise Intensity Selection

Tre

Hprod(W/kg)

WBSR

Ereq(W)

LSR

Ereq(W/m

2

)

Fig. 1. Correct exercise intensity selection for unbiased comparisons ofthermoregulatory responses in groups of different body mass and surface area.Tre, change in body core temperature; Hprod, metabolic heat production;WBSR, whole body sweating rates; Ereq, evaporative sweating requirement;LSR, local sweating rate.

J Appl Physiol 116: 11211122, 2014;doi:10.1152/japplphysiol.00248.2014. Invited Editorial

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In summary, thermoregulatory responses between any twogroups with inherent or chance differences in body mass andsurface area can be fairly compared in accordance with thematching methods described by Cramer and Jay (1) (Fig. 1).Widespread application of these techniques is recommendedwhen attempting to isolate and describe the unique role(s) that sex,age, health, and other factors play in affecting thermoregulation.

ACKNOWLEDGMENTS

The opinions or assertions contained herein are the private views of theauthors and should not be construed as official or reflecting the views of theArmy the Department of Defense.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the author(s).

AUTHOR CONTRIBUTIONS

Author contributions: S.N.C. drafted manuscript; S.N.C. edited and revisedmanuscript; S.N.C. approved final version of manuscript.

REFERENCES

1. Cramer MN, Jay O. Selecting the correct exercise intensity for unbiasedcomparisons of thermoregulatory responses between groups of different massand surface area. J Appl Physiol; doi:10.1152/japplphysiol.01312.2013.

2. Druyan A, Ketko I, Yanovich R, Epstein Y, Heled Y. Refining thedistinction between heat tolerant and intolerant individuals during a Heattolerance test. J Therm Biol 38: 539542, 2013.

3. Gagge A, Gonzalez R. Mechanisms of heat exchange: biophysics and

physiology. In: Handbook of Physiology: Environmental Physiology.Bethesda, MD: American Physiological Society, 1996, p. 4584.

4. Gagnon D, Jay O, Kenny GP. The evaporative requirement for heatbalance determines whole-body sweat rate during exercise under conditionspermitting full evaporation. J Physiol 591: 29252935, 2013.

5. Gagnon D, Kenny GP. Does sex have an independent effect on thermoef-fector responses during exercise in the heat? J Physiol 590: 59635973,2012.

6. Jay O, Bain A, Deren T, Sacheli M, Cramer M.Large differences in peakoxygen uptake do not independently alter changes in core temperature andsweating during exercise. Am J Physiol Regul Integr Comp Physiol 301:R832R841, 2011.

7. Nunneley S.Physiological responses of women to thermal stress: a review.Med Sci Sports 10: 250255, 1978.

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