Revisiting the influence of individual factors on heat exchange during exercise in dry heat using direct calorimetry.

New Findings: What is the central question of this study? The aim was to identify the greatest contributor(s) to the variation in whole-body heat exchange, as assessed using direct calorimetry, among young men and women with heterogeneous characteristics during exercise at increasing metabolic heat production rates in dry heat. What is the main finding and its importance? The evaporative heat loss requirement, body morphology and aerobic fitness made the greatest contributions to the individual variation in evaporative and dry heat exchange, with the variance explained being exercise intensity dependent. These findings provide a foundation on which to build our ability to explain the individual variation in heat exchange during exercise-induced heat stress.

Self-reported physical activity level does not alter whole-body total heat loss independently of aerobic fitness in young adults during exercise in the heat.

We evaluated whether self-reported physical activity (PA) level modulates whole-body total heat loss (WB-THL) as assessed using direct calorimetry in 10 young adults (aged 22 ± 3 years) matched for rate of peak oxygen consumption (an index for aerobic fitness), but of low and high self-reported PA, during 3 incremental cycling bouts (∼39%, 52%, and 64% peak oxygen consumption) in the heat (40 °C). We showed that level of self-reported PA does not appear to influence WB-THL independently of peak oxygen consumption.

Postexercise whole-body sweating increases during muscle metaboreceptor activation in young men.

We assessed the effect of metaboreceptor activation on whole-body evaporative heat loss (WB-EHL) in 12 men (aged 24 ± 4 years) in the early-to-late stages of a 60-min exercise recovery in the heat. Metaboreceptor activation induced by 1-min isometric-handgrip (IHG) exercise followed by 5-min forearm ischemia to trap metabolites increased WB-EHL by 25%-31% and 26%-34% during the ischemic period relative to IHG-only and control (natural recovery only), respectively, throughout recovery. We show that metaboreceptor activation enhances WB-EHL in recovery.

Fitness-related differences in the rate of whole-body total heat loss in exercising young healthy women are heat-load dependent.

New Findings: What is the central question of this study? Aerobic fitness modulates heat loss, albeit the heat load at which fitness-related differences occur in young healthy women remains unclear. What is the main finding and its importance? We demonstrate using direct calorimetry that fitness modulates heat loss in a heat-load dependent manner, with differences occurring between young women of low and high fitness and matched physical characteristics when the metabolic heat load is at least 400 W in hot, dry conditions. Although fitness has been known for some time to modulate heat loss, our findings define the metabolic heat load at which fitness-related differences occur.

Fitness-related differences in the rate of whole-body evaporative heat loss in exercising men are heat-load dependent.

What is the central question of this study? Aerobic fitness modulates heat loss, but the heat-load threshold at which fitness-related differences in heat loss occur in young healthy men remains unclear. What is the main finding and its importance? We demonstrate using direct calorimetry that aerobic fitness modulates heat loss in a heat-load-dependent manner, with fitness-related differences occurring between young men who have low and high fitness when the heat load is ∼≥500 W. Although aerobic fitness has been known for some time to modulate heat loss, our findings define the precise heat-load threshold at which fitness-related differences occur.

The recommended Threshold Limit Values for heat exposure fail to maintain body core temperature within safe limits in older working adults.

Purpose: The American Conference of Governmental and Industrial Hygienists (ACGIH®) Threshold Limit Values (TLV® guidelines) for work in the heat consist of work-rest (WR) allocations designed to ensure a stable core temperature that does not exceed 38°C. However, the TLV® guidelines have not been validated in older workers. This is an important shortcoming given that adults as young as 40 years demonstrate impairments in their ability to dissipate heat.

Temperature of Ingested Water during Exercise Does Not Affect Body Heat Storage.

Purpose: The objective of this study was to examine the effect of ingested water temperature on heat balance during exercise as assessed by direct calorimetry.

Methods: Ten healthy males (25 ± 4 yr) cycled at 50% V˙O2peak (equivalent rate of metabolic heat production (M-W) of 523 ± 84 W) for 75 min under thermocomfortable conditions (25°C, 25% relative humidity) while consuming either hot (50°C) or cold (1.5°C) water.

Adenosine receptor inhibition attenuates the suppression of postexercise cutaneous blood flow.

The time-dependent contributions of active vasodilation (e.g. nitric oxide) and noradrenergic vasoconstriction to the postexercise suppression of cutaneous perfusion despite persistent hyperthermia remain unknown.