Progressive Heat Exposure and Physiological Strain

Participants will complete a 9-hour simulated occupational workday consisting of a 4-hour morning work period, a 1-hour seated lunch-recovery period, and a 4-hour afternoon work period. Wet-bulb globe temperature (WBGT) will remain constant at 28°C throughout the workday. During each work period, participants will perform moderate-intensity treadmill walking at a metabolic heat production of approximately 200 W·m-2. Participants will initially work continuously until reaching the initial stay-time threshold, defined as a core temperature of 38.0°C or an increase of 1.0°C above pre-work baseline, whichever occurs first. Thereafter, participants will complete 45-min work and 15-min seated rest cycles for the remainder of the work period.

Participants complete a simulated occupational workday under constant environmental conditions, with WBGT maintained at 28°C throughout the morning and afternoon work periods.

Participants will complete a 9-hour simulated occupational workday consisting of a 4-hour morning work period, a 1-hour seated lunch-recovery period, and a 4-hour afternoon work period. WBGT will progressively increase from morning to afternoon to simulate the changing environmental conditions of a hot summer workday, while maintaining a mean daily WBGT of 28°C. During each work period, participants will perform moderate-intensity treadmill walking at a metabolic heat production of approximately 200 W·m-2. Participants will initially work continuously until reaching the initial stay-time threshold, defined as a core temperature of 38.0°C or an increase of 1.0°C above pre-work baseline, whichever occurs first. Thereafter, participants will complete 45-min work and 15-min seated rest cycles for the remainder of the work period.

Participants complete a simulated occupational workday in which WBGT progressively increases from morning to afternoon while maintaining a mean daily WBGT of 28°C.

Total work completed during each 240-minute morning and afternoon work period, expressed as cumulative minutes of treadmill walking. Each work period consists of an initial continuous walking phase until core temperature reaches 38.0°C or increases by 1.0°C above the pre-work baseline, whichever occurs first, followed by intermittent 45-min work and 15-min seated rest cycles.

Time, in minutes, from the start of continuous treadmill walking until the initial stay-time threshold is reached, defined as a core temperature of 38.0°C or an increase of 1.0°C above the pre-work baseline, whichever occurs first. Participants who do not reach the threshold during the 240-minute work period will be identified as not having reached the threshold within 240 min.

Core temperature measured continuously using a rectal thermocouple temperature probe to assess thermoregulatory strain throughout the simulated workday.

Average core temperature during the intermittent 45-min work and 15-min seated rest cycles following attainment of the initial stay-time threshold.

Mean skin temperature calculated from temperatures measured at eight body sites using ISO 9886 weighting factors.

Heart rate measured continuously using a chest-strap heart rate monitor and expressed as absolute heart rate.

Heart rate expressed as a percentage of the participant's heart rate reserve to assess relative cardiovascular strain during heat exposure.

Systolic blood pressure measured at the brachial artery using automated oscillometry. Three measurements obtained at approximately 1-min intervals will be averaged for analysis.

Diastolic blood pressure measured at the brachial artery using automated oscillometry. Three measurements obtained at approximately 1-min intervals will be averaged for analysis.

Cardiac autonomic modulation assessed from consecutive R-R intervals obtained using a 5-lead Holter electrocardiogram to calculate the standard deviation of normal R-R intervals (SDNN), expressed in milliseconds.

Cardiac autonomic modulation assessed from consecutive R-R intervals obtained using a 5-lead Holter electrocardiogram to calculate the root mean square of successive differences between adjacent normal R-R intervals (RMSSD), expressed in milliseconds.

Urine specific gravity measured to assess hydration status upon arrival at the laboratory. Higher values indicate greater urine concentration, whereas lower values indicate lower urine concentration.

Cumulative fluid consumption determined from all fluid consumed during each experimental session.

Fluid loss calculated from changes in body mass during each daytime work exposure, corrected for food and fluid intake.

Thermal sensation assessed using an 8-point scale ranging from 0 (neutral) to 7 (extremely hot). Higher scores indicate a greater sensation of heat.

Thermal comfort assessed using the ISO 10551 scale ranging from 1 (comfortable) to 4 (very uncomfortable). Higher scores indicate greater thermal discomfort.

Thirst sensation assessed using a 9-point scale ranging from 1 (not thirsty at all) to 9 (very, very thirsty). Higher scores indicate greater perceived thirst

Affective state will be assessed using the 11-point Feeling Scale ranging from -5 (very bad) to +5 (very good), with 0 representing neutral. Higher scores indicate more positive affective state.

Perceived arousal assessed using a 7-point scale ranging from 1 (low arousal) to 7 (high arousal). Higher scores indicate greater perceived arousal.

Perceived exertion assessed using the Borg Rating of Perceived Exertion scale ranging from 6 (no exertion at all) to 20 (maximal exertion). Higher scores indicate greater perceived exertion.

Symptoms associated with heat exposure will be assessed using the Environmental Symptoms Questionnaire-IV (ESQ-IV). Responses will be combined to generate a total symptom score, with higher scores indicating greater environmental symptom burden.

Mood state will be assessed using the abbreviated 40-item Profile of Mood States questionnaire (POMS-40). Responses will be used to calculate a total mood disturbance score, with higher scores indicating greater overall mood disturbance.

Change in plasma volume from baseline values calculated from duplicate measurements of hemoglobin and hematocrit at the start and end of each exposure using the technique by Dill and Costill.