Heat Exchange in Young and Older Men during Constant- and Variable-Intensity Work.

PURPOSE: Current occupational heat stress guidelines rely on time-weighted averaging to quantify the metabolic demands of variable-intensity work. However, variable-intensity work may be associated with impairments in whole-body total heat loss (dry+evaporative heat loss), especially in older workers, which exacerbate heat strain relative to constant-intensity work eliciting the same time-weighted average metabolic rate. We therefore used direct calorimetry to evaluate whether variable-intensity work would cause decrements in the average rate of whole-body total heat loss that augment body heat storage and core temperature compared to constant-intensity work in young and older men. METHODS: Eight young (19-31 years) and eight older (54-65 years) men completed four trials involving 90-min of work (cycling) eliciting an average metabolic heat production of ~200 W/m in dry-heat (40C, 20% relative humidity). One trial involved constant-intensity work (CON), while the others involved 10-min cycles of variable-intensity work: 5-min low- and 5-min high-intensity (VAR 5:5), 6-min low- and 4-min very high-intensity (VAR 6:4), and 7-min low- and 3-min very, very high-intensity (VAR 7:3). Metabolic heat production, total heat loss, body heat storage (heat production minus total heat loss) and core (rectal) temperature were measured throughout. RESULTS: When averaged over each 90-min work period, metabolic heat production, total heat loss, and heat storage were similar between groups and conditions (all p≥0.152). Peak core temperature (average of final 10-min) was also similar between groups and conditions (both p≥0.111). CONCLUSION: Whole-body total heat loss, heat storage and core temperature were not significantly influenced by the partitioning of work intensity in young or older men, indicating that time-weighted averaging appears to be appropriate for quantifying the metabolic demands of variable-intensity work to assess occupational heat stress.