Whole‐body Heat Exchange in Young and Middle‐Aged Men during Constant‐ and Variable‐Intensity Work of Equivalent Metabolic Demand in Dry Heat

Abstract

Current occupational heat stress guidelines incorporate time‐weighted averaging to quantify a workers’ metabolic rate during tasks of varied intensity. Since such guidelines originate from data obtained during work at a constant (fixed) intensity, their validity relies on the assumption that constant‐ and variable‐intensity work eliciting the same time‐weighted average metabolic rate will evoke similar heat strain. We recently confirmed the veracity of this assumption in young men. However, it remains uncertain whether this holds for the rising number of middle‐aged workers, who display age‐related impairments in whole‐body total heat loss (evaporative+dry heat loss) during constant, moderate work that worsen at higher work intensities. It is possible that variable‐intensity work involving brief periods at higher intensities may exacerbate age‐related impairments in heat loss in these individuals, thereby worsening body heat storage relative to constant‐intensity work. To evaluate this hypothesis, 7 middle‐aged (mean (SD): 60 (4) years) and 8 young (25 (4) years) men completed four trials involving 90‐min of work (cycling) at an average metabolic heat production of 200 W/m2 in dry‐heat (40°C, 20% relative humidity). Trials were administered in a random order (each separated by >48 hours) and differed only in the pattern of work performed, with one involving constant external work (40 W/m2) and the others involving 10‐min cycles of varying external work: (i) 5 min at 15 W/m2 then 5‐min at 60 W/m2; (ii) 6 min at 15 W/m2 then 4 min at 70 W/m2; and (iii) 7 min at 15 W/m2 then 3 min at 80 W/m2. Metabolic heat production and whole‐body total heat loss were measured via indirect and direct calorimetry (respectively), and used to derive body heat storage (summation of heat production and total heat loss). Data were averaged over each 90‐min period and compared between groups and across conditions using a mixed‐model ANOVA. Heat production was similar between groups and across conditions (group: p=.80; condition: p=.53; interaction: p=.84), averaging 205 (9) W/m2 and 206 (7) W/m2 in middle‐aged and young men, respectively. However, contrary to our hypothesis, total heat loss did not differ significantly between groups or across conditions (group: p=.23; condition: p=.87; interaction: p=.97), averaging 180 (8) W/m2 and 185 (12) W/m2 in middle‐aged and young men, respectively. As such, body heat storage was also similar between groups and across conditions (group: p=.22; condition: p=.35; interaction: p=.75), averaging 25 (9) W/m2 and 21 (9) W/m2 in middle‐aged and young men, respectively. In conclusion, whole‐body total heat loss and body heat storage were not significantly influenced by the partitioning of work intensity in either middle‐aged or young men, indicating that time‐weighted averaging appears to be an appropriate means of quantifying metabolic demand to assess occupational heat stress.Support or Funding InformationThe Government of Ontario, Canada.