Physiological Strain in Young Adults During a Simulated Heat Wave

Peak rectal temperature (15 min average) during exposure. Rectal temperature is measured continuously throughout the daytime period of the simulated heat wave.

Area under the curve of rectal temperature (in degree-hours)

Rectal temperature measured at hour 10 of each daytime exposure to simulated heat wave (15-min average)

Heart rate during each daytime exposure to simulated heat wave (15-min average)

Heart rate measured during each daytime exposure to simulated heat wave (15-min average)

Cardiac response to standing evaluated as the ratio between the highest R-wave to R-wave interval (lowest heart rate) measured at the 30th heart beat after standing from supine (+/- 5 beats) and the lowest R-wave to R-wave interval (highest heart rate) measured at the 15th heart beat after standing (+/- 5 beats). Cardiac response to standing will be evaluated twice, during two lying-to-standing tests (separated by 10 min of supine rest). Assessment is perform in the morning prior to ( and following daytime exposure.

Participants will be provided with a tablet device with the mobile testing platform (Sway Medical). Participants will be complete the following cognitive tests below:

Reaction time: participants will be asked to initiate a movement of the device in response to a visual cue.

Impulse control: participants will be asked to respond to both "go" and "no-go" visual cues. In response to the "go" cue, participants will initiate a movement of the device.

Memory: participants complete both a delayed recall test and a working memory test.

Executive function: participants will complete the Cued Stroop test, in which they are presented a sequence of congruent, neutral and incongruent colour-word tasks.

Balance will be assessed in two ways:

Method 1: CDC 4-Stage Balance Test. Participants hold a tablet to their chest, then auditory cues guide participants through four consecutive stances, feet side by side, instep of one foot touching the big toe of the other foot, tandem stand with one foot in front of the other, heel touching toe, and stand on one foot. The balance assessment will be evaluated based on movement detected by an accelerometer integrated into the hardware of the tablet device (Sway Medical Inc).

Method 2: Participants will be asked to stand on a BTracks force plate with their feet spread out to shoulder width, hand on their hips and eyes closed. The assessment will comprise of four trials (one practice trial) lasting 20 seconds in length and 20 seconds between trials. Center of pressure (COP) vector data along vertical (y) and horizontal (x) axes will be summed for the total path excursion length (cm) during each trial.

Systolic blood pressure response to standing evaluated as the difference in blood pressure measured between the standing and supine. Standing systolic blood pressure will be taken as the lowest value of those measured after 60 and 120 seconds of standing. Systolic response to standing will be evaluated twice, during two lying-to-standing tests (separated by 10 min of supine rest).

Systolic blood pressure measured in triplicate via automated oscillometry (~60 seconds between measures)

Diastolic blood pressure measured in triplicate via automated oscillometry (~60 seconds between measures)

Rate pressure product, an index of myocardial work and strain, calculated as systolic blood pressure x heart rate.

Standard deviation of normal-to-normal R-wave to R-wave intervals (SDNN) measured during 5 minutes of paced breathing (15 breaths/min) with participants in the seated position. SDNN will be evaluated twice, during two paced breathing periods (separated by 4 min of seated rest).

Root mean squared standard deviation of normal-to-normal R-wave to R-wave intervals (RMSSD) measured during 5 minutes of paced breathing (15 breaths/min) with participants in the seated position. RMSSD will be evaluated twice, during two paced breathing periods (separated by 4 min of seated rest).

Average hourly fluid consumption calculated by weighing participant water intake at the start and end of each hour of exposure (normalized to the exposure duration).

Fluid loss calculated as the change in body mass during each exposure presented as a percentage of baseline body mass (corrected for food consumption)

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

Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") ranging from "extremely uncomfortable" to "extremely comfortable"(midpoint: neutral).

Thermal sensation assessed via a visual analog scale ("How hot do you feel?") ranging from "extremely hot" to "neutral"(midpoint: hot).

Thirst sensation scale assessed via a visual analog scale ("How thirsty are you?") ranging from "Very, very thirsty" to "Not thirsty at all" (midpoint: moderately thirsty).

Felt arousal scale assessed via a visual analog scale ("How worked up are you?") ranging from "high arousal" to "low arousal".

Cumulative sum of scores on 6 questions asking participant to rank symptoms associated with orthostatic intolerance during the lying to standing tests. All symptoms scored on a scale from 0 (none) to 10 (worst possible) and include feelings of: (1) "dizziness, lightheadedness, feeling faint, or feeling like you might black out"; (2) "Problems with vision (blurring, seeing spots, tunnel vision, etc.)"; (3) "Weakness"; (4) "Fatigue"; (5) "Trouble concentrating"; and (6) "Head and neck discomfort".

Peak rectal temperature (15 min average) during exposure. Rectal temperature is measured continuously throughout the daytime period of the simulated heat wave.

Area under the curve of rectal temperature (in degree-hours)

Rectal temperature measured at hour 10 of each daytime exposure to simulated heat wave (15-min average)

Heart rate during each daytime exposure to simulated heat wave (15-min average)

Heart rate measured during each daytime exposure to simulated heat wave (15-min average)

Standard deviation of normal-to-normal R-wave to R-wave intervals (SDNN) measured during 5 minutes of paced breathing (15 breaths/min) with participants in the seated position. SDNN will be evaluated twice, during two paced breathing periods (separated by 4 min of seated rest).

Root mean squared standard deviation of normal-to-normal R-wave to R-wave intervals (RMSSD) measured during 5 minutes of paced breathing (15 breaths/min) with participants in the seated position. RMSSD will be evaluated twice, during two paced breathing periods (separated by 4 min of seated rest).

Participants will be attached with 2 probes to measure tissue oxygenation on two locations via a non-invasive tool: near-infrared spectroscopy (NIRS). The device (Portalite, Artinis Medical, The Netherlands) is placed on the left prefrontal cortex and muscle belly of the vastus lateralis. It measures local oxy-, deoxy- and total hemoglobin concentration changes and tissue saturation index (TSI) on the specific location of the human body and provide valuable insights during a diverse set of tests.

Potential changes in mood (7 subscales of mood: tension, anger, depression, fatigue, confusion, vigor and esteem-related affect). The POMS-40 is a validated, self-administered questionnaire that examines seven distinct aspects of mood state across two positive subscales (Esteem-Related Affect, and Vigor) and five negative subscales (Fatigue, Tension, Confusion, Anger, and Depression), which are described across 40 distinct adjectives (reference). For each individual item, participants were asked to describe "how you feel right now" by responding using a 5-point Likert scale (0 = "Not at all", 1 = "A little", 2 = "Moderately", 3 = "Quite a lot", or 4 = "Extremely"). The values of items associated with a specific subscale (e.g., Fatigue) were summed to calculate its score.

Self-reported environmental reactions and medical symptomatology associated with prolonged heat exposure. The ESQ-IV is a validated 68-item, self-administered questionnaire that has been used successfully in identifying symptomatology during exposure to a wide variety of environmental conditions, including heat exposure [24, 25]. Participants are asked to assess and described "how you have been feeling today" by responding to each item using a 6-point Likert scale (0 = "Not at all", 1 = "Slight", 2 = "Somewhat", 3 = "Moderate", 4 = "Quite a bit", or 5 = "Extreme"). Total Symptom Score was calculated from this data by taking the sum of the intensity ratings from all 68 individual items using reverse scores for the three positive items from the list ("I Felt Good", "I Felt Alert", and "I Felt Wide Awake").

The following questionnaires will be administered:

• Consensus Sleep Diary (CSD) to provide an overview of the sleep quantity and quality of the previous night.
• Richards-Campbell Sleep Questionnaire to evaluate the perception of sleep .
• Leeds Sleep evaluation questionnaire which is 10-item, subjective, questionnaire designed to assess changes in sleep quality over the course of the intervention.

The following questionnaires will be administered to evaluate the participants sleepiness during the daytime exposure period:

• Karolinska sleepiness scale measures the subjective level of sleepiness at a particular time during the day.
• Stanford sleepiness scale consists of only one item, where participant must identify one of seven statements that best represents their level of perceived sleepiness