Exercise-heat Stress Research Articles

The influence of hydration status on heart rate variability after exercise heat stress

While exercise heat stress and hydration status are known to independently influence heart rate variability (HRV), the combined effect of these physiological stressors is unknown. Thus, heat-acclimated subjects ( n=5) performed exercise heat trials (40 °C, 20% relative humidity) in the euhydrated and hypohydrated state (3.9±0.7% body weight loss). During each trial, cardiac cycle R–R interval data were collected for 45 min at rest (pre-) and after (post-) completing 90 min of cycle ergometer exercise. Pre- and post-exercise RRI data were analyzed by Fast Fourier Power Spectral analysis to determine the high-frequency (HF), low-frequency (LF), very low-frequency (VLF), and total power (TP) components of HRV. Overall HRV was decreased by both hypohydration and exercise heat stress. Hypohydration reduced TP, LF, VLF, and LF:HF ratio ( P < 0.05 ) while HF was significantly higher. The change in both LF and HF power (pre- vs. post-exercise) were blunted during hypohydration compared to euhydration. These data suggest that dehydration alone positively influences the parasympathetic (HF) control of HRV, but the reduction in overall HRV and the blunted oscillations in LF and HF power following exercise heat stress support an overall deleterious effect of dehydration on autonomic cardiac stability.

Effects of heat acclimation on sweating during graded exercise until exhaustion

The aim of the present study was to test the hypothesis that the sweating during graded exercise until exhaustion in a temperate environment would be greater after heat acclimation. Six healthy young males performed an exercise–heat stress acclimation protocol during 9 days. Before (PRE) and after (POS) the acclimation protocol they performed a graded exercise until exhaustion and the sweat loss during exercise increased after acclimation (3.94±1.10, PRE, and 4.86±1.70 g m −2 min −1, POS; p<0.05). The results showed that daily prolonged exposures to exercise-heat stress increased sweating during a graded and short duration exercise in a temperate environment.

Sex Differences In Voluntary Fluid Intake By Older Adults During Exercise

Older adults are particularly susceptible to fluid/electrolyte imbalances due to impaired thirst sensitivity and renal sodium- and water-conserving ability. In addition, anecdotal evidence suggests that hyponatremia, which usually occurs due to over-consumption of sodium- free fluids such as water, most commonly occurs in women; however, little data exists on actual sex-related differences in fluid intake behavior of older adults during exercise-heat stress. PURPOSE This study compared the voluntary fluid intake behavior of older men and women (54–70 yrs) when provided cold, palatable beverages and ample opportunity to drink between repeated bouts of exercise in the heat. METHODS Thirteen men and 14 women performed 4 bouts of 15 min cycling at 65% VO2peak followed by 15 min rest, all at 30°C and 50% rh. In separate trials, subjects drank either a carbohydrate-electrolyte solution (CES) or water ad libitum during the rest periods and were unaware that their fluid intake was being measured. RESULTS Fluid intake behavior was repeatable (r=0.75, p< 0.05) and subjects drank enough of either beverage to match sweating rates and maintain their body mass (BM). Fluid intake per kg BM was greater with CES (18.7±2.2 vs. 15.1±2.1 mL/kg; p< 0.05) and plasma volume (PV) was better maintained during the CES trials (−1.3±1.1 vs. −4.2±1.1% during the second half of the session). Women drank significantly more water than the men on a per kg basis (17.2±2.9 vs. 12.8±1.7 mL/kg BM) and one woman (BM = 45.7 kg) became hyponatremic (Na+ = 126 mmol/L) with symptoms during the water trial. CONCLUSIONS Older adults drink enough to maintain fluid balance when palatable fluid is readily available; however, CES promotes greater voluntary fluid intake and restores PV losses faster than water. In addition, older women drink more water than men during interval exercise in the heat which may put smaller women at an increased risk for developing hyponatremia. Funded by the Gatorade Sports Science Institute

Ineffectiveness Of Torso Cooling Vests In Reducing Uncompensable Heat Stress

Introduction Personal cooling systems have been widely used by both civilian and military personal to reduce the effects of heat stress. Clothing designed to protect military, industrial, rescue personnel and racecar drivers in hazardous environments generally have high thermal insulation and low permeability. The lack of permeability greatly reduces the potential for evaporative and conductive heat loss thereby producing a condition known as uncompensable heat stress. Purpose The purpose of this investigation was to evaluate the physiological responses of individuals exposed to uncompensable heat stress when using a commercially available personal microclimate cooling system. Methods Ten recreationally active male subjects participated in this investigation (24.5 ± 4.3 yrs. and 76.2 ± 9.9 kg). In a randomized fashion, the subjects completed three uncompensable heat stress exposures while walking on a treadmill for up to 60 min at 4.1 km/h in a 43°C and 60% RH environment. The three heat stress treatment conditions were no cooling, cooling vest soaked in ice water, and cooling vest soaked in ice water with an ice water filled insert. Subjects wore a four layer fire-retardant race car driving suit and a closed-faced protective driving helmet during all heat stress exposures. Metabolic and heart rate data were continuously recorded during each heat stress exposure via a portable metabolic analyzer. Core temperature and RPE data were monitored at five min intervals during each exposure. Pre and post body weight was recorder prior to and following each heat stress exposure. Results Core temperature, heart rate, VO2, VE, and RPE all significantly increased (p<0.05) from rest to the cessation of each exercise heat stress exposure with no significant differences between no cooling and the cooling treatment conditions. Time to exhaustion was not significantly different between treatment conditions. Body weight significantly decreased (p<0.05) following each exposure with no significant differences detected between treatment conditions. Conclusion Cooling vests either soaked in ice water or soaked in ice water with an ice water filled insert are not effective in attenuating the rise in core temperature to critical levels during exercise while in an uncompensable heat stress environment.

Ineffectiveness Of Torso Cooling Vests In Reducing Uncompensable Heat Stress

Introduction Personal cooling systems have been widely used by both civilian and military personal to reduce the effects of heat stress. Clothing designed to protect military, industrial, rescue personnel and racecar drivers in hazardous environments generally have high thermal insulation and low permeability. The lack of permeability greatly reduces the potential for evaporative and conductive heat loss thereby producing a condition known as uncompensable heat stress. Purpose The purpose of this investigation was to evaluate the physiological responses of individuals exposed to uncompensable heat stress when using a commercially available personal microclimate cooling system. Methods Ten recreationally active male subjects participated in this investigation (24.5 ± 4.3 yrs. and 76.2 ± 9.9 kg). In a randomized fashion, the subjects completed three uncompensable heat stress exposures while walking on a treadmill for up to 60 min at 4.1 km/h in a 43°C and 60% RH environment. The three heat stress treatment conditions were no cooling, cooling vest soaked in ice water, and cooling vest soaked in ice water with an ice water filled insert. Subjects wore a four layer fire-retardant race car driving suit and a closed-faced protective driving helmet during all heat stress exposures. Metabolic and heart rate data were continuously recorded during each heat stress exposure via a portable metabolic analyzer. Core temperature and RPE data were monitored at five min intervals during each exposure. Pre and post body weight was recorder prior to and following each heat stress exposure. Results Core temperature, heart rate, VO2, VE, and RPE all significantly increased (p<0.05) from rest to the cessation of each exercise heat stress exposure with no significant differences between no cooling and the cooling treatment conditions. Time to exhaustion was not significantly different between treatment conditions. Body weight significantly decreased (p<0.05) following each exposure with no significant differences detected between treatment conditions. Conclusion Cooling vests either soaked in ice water or soaked in ice water with an ice water filled insert are not effective in attenuating the rise in core temperature to critical levels during exercise while in an uncompensable heat stress environment.

Effects of Short-term Exercise in the Heat on Thermoregulation, Blood Parameters, Sweat Secretion and Sweat Composition of Tropic-dwelling Subjects

This study investigates the effects of a short-term aerobic training program in a hot environment on thermoregulation, blood parameters, sweat secretion and composition in tropic-dwellers who have been exposed to passive heat. Sixteen healthy Malaysian-Malay male volunteers underwent heat acclimation (HA) by exercising on a bicycle ergometer at 60% of VO2max for 60 min each day in a hot environment (Ta: 31.1+/-0.1 degrees C, rh: 70.0+/-4.4%) for 14 days. All parameters mentioned above were recorded on Day 1 and at the end of HA (Day 16). On these two days, subjects rested for 10 min, then cycled at 60% of VO2max for 60 min and rested again for 20 min (recovery) in an improvised heat chamber. Rectal temperature (Tre), mean skin temperature (Tsk) heart rate (HR), ratings of perceived exertion (RPE), thermal sensation (TS), local sweat rate and percent dehydration were recorded during the test. Sweat concentration was analysed for sodium [Na+]sweat and potassium. Blood samples were analysed for biochemical changes, electrolytes and hematologic indices. Urine samples were collected before and after each test and analysed for electrolytes.After the period of acclimation the percent dehydration during exercise significantly increased from 1.77+/-0.09% (Day 1) to 2.14+/-0.07% (Day 16). Resting levels of hemoglobin, hematocrit and red blood cells decreased significantly while [Na+]sweat increased significantly. For Tre and Tsk there were no differences at rest. Tre, HR, RPE, TS, plasma lactate concentration, hemoglobin and hematocrit at the 40th min of exercise were significantly lower after the period of acclimation but mean corpuscular hemoglobin and serum osmolality were significantly higher while no difference was seen in [Na+]sweat and Tsk. It can be concluded that tropic-dwelling subjects, although exposed to prolonged passive heat exposure, were not fully heat acclimatized. To achieve further HA, they should gradually expose themselves to exercise-heat stress in a hot environment.

Role of Gastrointestinal Permeability in Exertional Heatstroke

Reduced splanchnic blood flow and hyperthermia during exercise-heat stress can produce gastrointestinal barrier dysfunction and increased gastrointestinal permeability. This may allow endotoxin to enter the internal environment, causing local and systemic immune responses. These responses may be involved in the cause and outcome of exertional heatstroke. Countermeasures may reduce gastrointestinal permeability and possibly exertional heatstroke occurrence and outcome.

Daily Body Mass Variability and Stability in Active Men Undergoing Exercise-Heat Stress

The purpose of this study was to quantify the variability and stability of 1st morning body mass (BM) fluctuations during daily exercise in the heat while following traditional fluid intake guidance. Data from 65 men were examined retrospectively. BM fluctuations were monitored over 4 to 15 consecutive days. Group daily variation in BM was 0.51+/-0.20 kg. Group coefficient of variation was 0.66+/-0.24%, normally distributed, and not related to either absolute BM (r = 0.04) or number of measurements (r = 0.34). Three days resulted in a similar variability estimate compared to 6 or 9 d, although precision was improved with 9 d. In conclusion, 3 consecutive BM measurements provide an accurate assessment of daily BM variability, which is less than 1% for active men when replacing 100% of sweat losses during exercise. The data also suggest that daily BM is a sufficiently stable physiological parameter for potential daily fluid balance monitoring.

Acute effects of dehydration on sweat composition in men during prolonged exercise in the heat.

To determine whether acute exercise-heat-induced dehydration affects sweat composition, eight males cycled for 2 h at 39.5 +/- 1.6% VO2peak on two separate occasions in a hot-humid environment (38.0 +/- 0.0 degrees C, 60.0 +/- 0.1% relative humidity). During exercise, subjects ingested either no fluid (dehydration) or a 20 mmol L(-1) sodium chloride solution (euhydration). The volume of solution, calculated from whole-body sweat loss and determined in a familiarization trial, was ingested at 0 min and every 15 min thereafter. Venous blood was collected at 0, 60 and 120 min of exercise and sweat was aspirated from a patch located on the dominant forearm at 120 min. Following the 2-h cycling exercise, sweat [Na+] and [Cl-] was greater (P < 0.05) in the dehydration trial (Na+ 91.1 +/- 6.8 mmol L(-1); Cl- 73.3 +/- 3.5 mmol L(-1)) compared with the euhydration trial (Na+ 81.1 +/- 5.9 mmol L(-1); Cl- 68.5 +/- 3.3 mmol L(-1)). In addition, dehydration invoked a greater serum [Na+] (142.2 +/- 0.7 mmol L(-1); P < 0.05), [Cl-] (105.8 +/- 0.6 mmol L(-1); P < 0.05) and [K+] (5.27 +/- 0.2 mmol L(-1); P < 0.05) over the euhydration values for [Na+], [Cl-] and [K+], respectively (138.9 +/- 0.6, 102.9 +/- 0.5 and 4.88 +/- 0.1 mmol L(-1)). Plasma aldosterone was also significantly higher during exercise in the dehydration trial compared with the euhydration trial (53.8 +/- 3.8 vs. 40.0 +/- 4.3 ng dL(-1); P < 0.05). Acute exercise-heat stress without fluid replacement resulted in a greater sweat [Na+] and [Cl-] which was potentially related to greater extracellular fluid [Na+], plasma aldosterone or sympathetic nervous activity.

Upper Body Cooling During Exercise-Heat Stress Wearing the Improved Toxicological Agent Protective System for HAZMAT Operations

Upper Body Cooling During Exercise-Heat Stress Wearing the Improved Toxicological Agent Protective System for HAZMAT Operations

PERCEPTUAL COLD INDEX VS. PHYSIOLOGICAL COLD STRAIN INDEX DURING EXERCISE-COLD STRESS

Recent studies have proposed the use of a perceptual index using thermal sensation (TS) and rating of perceived exertion (RPE) as an analog to the physiological strain index during exercise-heat stress (Tikuisis et al. MSSE 34:1454–1461, 2002). Whether a similar perceptual construct, analogous to the cold strain index (CSI), directly relates to physiological strain during exercise-cold stress is unknown. PURPOSE Develop a new Perceptual Cold Index (PCI) and determine: 1) the relationship of PCI to CSI and 2) whether PCI discriminates physiological strain during exercise-cold stress. METHODS Ten men exercised in cold-wet conditions (CW) for 6-h before (D0) and after 3 days of exhaustive exercise (D3). Each hour of CW consisted of 10-min standing in rain (5.4 cm·hr−1, 5°C air) followed by 45-min walking (1.34m·s−1, 5.4 m·s−1 wind, 5°C air). Exhaustive exercise consisted of 4-h/day of aerobic, anaerobic, and resistive exercise. PCI was defined as: PCI = 6.67*((TS-4.0)/-4.0)+3.33*(RPE/20) and directly analogous to CSI, defined as: CSI = 6.67*((Tcore – initial Tcore)/(35 – initial Tcore))+3.33*((Tsk – initial Tsk)/(20 – initial Tsk)). PCI used a modified Gagge TS scale (0–8) and the Borg RPE scale (6–20). TS and RPE were measured at the end of each 45-min walking period. RESULTS Rectal temperature was 0.3°C lower and skin temperature 1.3°C higher on D3 vs. D0 (P < 0.05), with no differences between trials for heart rate and ventilation. CSI increased during CW but was similar between D3 (4.6 ± 0.6) and D0 (4.2 ± 0.8) at the end of CW. PCI also increased over time during CW but values were the same (P = 0.09) between D3 (7.4 ± 0.5) and D0 (6.8 ± 0.4) with PCI values significantly higher than CSI on both D3 and D0. CONCLUSION People subjectively perceive higher strain over time during prolonged exercise-cold stress, but 1.7 times higher than indicated by the physiological index. PCI also did not discriminate between the higher heat loss and greater decline in rectal temperature during CW that occurred after 3 days of exhaustive exercise.

Perceptual versus physiological heat strain during exercise-heat stress.

The physiological strain index (PSI) has been proposed as a universally applicable measure of exercise-heat strain. Unknown is whether this index, based on normalized increases in core temperature and heart rate, is matched by its perceptual analog. By using a similar mathematical construct to the PSI, the perceptions of thermal sensation and perceived exertion were combined, and the resultant index, PeSI, was compared with its physiological counterpart, denoted as PhSI, for the exercise-heat stress specific to this study. Twenty-six young and healthy subjects wore semi-impermeable clothing and walked (3.5 km.h(-1)) under hot conditions (40 degrees C and 30% RH) until exhaustion or when their core temperature reached 39.5 degrees C. Subjects were divided into two fitness groups [endurance trained (T) and untrained (U)] comprised of 10 men and 3 women each. U subjects had a higher level of body fatness (mean +/- SD 18.1 +/- 5.3 vs 12.6 +/- 4.5%; P=0.010) and a lower level of aerobic fitness ((.)VO(2max)= 43.6 +/- 3.8 +/- vs 59.0 +/- 6.2 mL.min(-1).kg(-1); P<0.001). During the first hour of exposure, there was no group difference in PhSI, yet T perceived their physiological strain (PeSI) lower than U (P=0.002). Further, the indices were not different for U whereas PhSI was higher than PeSI for T (P=0.008). At the end of the exposure, T had a higher value of PhSI than U (8.23 +/- 0.72 vs 6.74 +/- 1.47; = 0.002), but there was no group difference in PeSI. Although the indices were again not different for U, PhSI at the end was higher than PeSI for T (6.14 +/- 1.68; P<0.001). T underestimated and U consistently perceived their physiological strain, as defined by PhSI, in accordance with the measured increases in core temperature and heart rate throughout an exposure to uncompensable exercise-heat stress.

EFFECT OF HYPOHYDRATION ON MOTOR UNIT RECRUITMENT AND ENDURANCE DURING ADDUCTOR POLLICIS EXERCISE

Hypohydration reduces muscular endurance but it remains unclear whether the premature fatigue is due to peripheral mechanisms or altered central motor drive. PURPOSE: To assess the impact of hypohydration on endurance, rate of muscle fatigue, and central motor drive during exhaustive adductor pollicis exercise. METHODS: Fourteen subjects (12 men, 2 women) performed intermittent isometric exercise to exhaustion when hypo-hydrated (HY) by 4% of normal body mass and when euhydrated (EU). Hypohydation was produced by not drinking during an exercise-heat stress protocol completed 3 h before testing. For EU, the same exercise-heat stress protocol was performed but fluids were consumed to maintain body water. The endurance test consisted of 5 sec contractions at 50% MVC followed by 5 sec rest until exhausted. Maximal strength of the thumb was measured before and every minute of exercise. EMG was measured throughout exercise. RESULTS: Hypohydration did not reduce endurance (HY = 754 ± 255; EU = 714 ± 318 sec), muscle strength, or increase the rate of fatigue. The adductor pollicis EMG root mean square increased during exercise (P < 0.05) but was not different between HY and EU. CONCLUSION: The effects of hypohydration on endurance may be dependent on the exercise protocol as there were no deterimental effects of hypohydration on adductor pollicis performance.

Aging and assessment of physiological strain during exercise-heat stress.

The purpose of this study was to evaluate the physiological strain index (PSI) for different age groups during exercise-heat stress (EHS). PSI was applied to three different databases. First, from young and middle-age men (21 +/- 2 and 46 +/- 5 yr, respectively) matched (n = 9 each, P > 0.05) for maximal aerobic power. Subjects were heat acclimated by daily treadmill walking for two 50-min bouts separated by 10-min rest for 10 days in a hot-dry environment [49 degrees C, 20% relative humidity (RH)]. The second database involved a group (n = 8) of young (YA) and a group (n = 7) of older (OA) men (26 +/- 1 and 69 +/- 1 yr, respectively) who underwent 16 wk of aerobic training and two control groups (n = 7 each) who were matched for age to YA and OA. These four groups performed EHS at 36 degrees C, 40% RH on a cycle ergometer for 60 min at 60% maximal aerobic power before and after training. The third database was obtained from three groups of postmenopausal women and a group of 10 men. Two groups of women (n = 8 each) were undergoing hormone replacement therapy, estrogen or estrogen plus progesterone, and the third group (n = 9) received no hormone replacement. Subjects were over 50 yr and performed the same EHS: exercising at 36 degrees C, 40% RH on a cycle ergometer for 60 min. PSI assessed the strain for all three databases and reported differences were significant at P < 0.05. This index rated the strain in rank order, whereas the postacclimation and posttraining groups were assessed as having less strain than the preacclimation and pretraining groups. Furthermore, middle-aged women on estrogen replacement therapy had less strain than estrogen + progesterone and no hormone therapy. PSI evaluation was extended for men and women of different ages (50-70 yr) during acute EHS, heat acclimation, after aerobic training, and inclusive of women undergoing hormone replacement therapy.

The cumulative heat strain index : a novel approach to assess the physiological strain induced by exercise-heat stress. Frank, A., Belokopytov, M., Shapiro, Y., and Epstein, Y.: Eur. J. Appl. Physiol., 84(6): 527-532, 2001.(Abstracts of foreign literature)

The cumulative heat strain index : a novel approach to assess the physiological strain induced by exercise-heat stress. Frank, A., Belokopytov, M., Shapiro, Y., and Epstein, Y.: Eur. J. Appl. Physiol., 84(6): 527-532, 2001.(Abstracts of foreign literature)

Human surface to mass ratio and body core temperature in exercise heat stress—a concept revisited

The effects of morphological factors (body surface area ( A D), body mass and surface to mass ratio ( A D/ M)) on exercise heat stress responses in a hot wet (HW) (35°C 80% rh) and hot dry (HD) climate (45°C, 20% rh) of equal WBGT (≃31.6°C) were studied in 30 and 25 subjects respectively. In both climates there were positive correlations of T re with A D/ M, negative with A D and mass, giving the bigger subjects an “advantage” that remained ( A D/ M) when data were corrected for differences in V ̇ O 2 max , which accounted for a substantial part of the variance in T re. The observed A D/ M effect contradicts earlier studies but is consistent with model calculations.

The cumulative heat strain index--a novel approach to assess the physiological strain induced by exercise-heat stress.

The cumulative heat strain index (CHSI) is a new approach for assessing the total physiological strain experienced by subjects exposed to an exercise-heat stress. The index is based on inherent physiological logic that combines the thermoregulatory strain, which is described by the area under the hyperthermic curve, and the circulatory strain, which is characterized by heart-beat count. According to this model, the index reflects the dynamics of changes in the thermoregulatory and cardiovascular components and accounts for the complementary nature of the interaction between them. Mathematically, the index is calculated as follows: CHSI =[ sigma(0-t) hb-fc(0) x t] x l0(-3) x [ integral (0-t) Tre x dt-Tre(0) x t] (units) Where: hb=heart beats, fc(0)=initial lowest heart rate (bpm), Tre = rectal temperature (Tre(0) = baseline Tre) (degrees C) and t = time (min) from the onset of measurements. Four sets of data, from various former studies, have been used to demonstrate the index's applicability and its sensitivity to differentiate between levels of stain under various stressful conditions (e.g. clothing insulation, acclimation to heat and levels of tolerance to heat). In all cases, the index was found to be a sensitive tool for assessing the level of strain. Furthermore, the CHSI can be used to predict potential strain. The index's high sensitivity arises from its nature, which reflects miniature differences in the pattern of changes in the dynamics of physiological responses and therefore is a powerful and practical tool for evaluating even minor changes in strain.

Function of human eccrine sweat glands during dynamic exercise and passive heat stress.

The purpose of this study was to identify the pattern of change in the density of activated sweat glands (ASG) and sweat output per gland (SGO) during dynamic constant-workload exercise and passive heat stress. Eight male subjects (22.8 +/- 0.9 yr) exercised at a constant workload (117.5 +/- 4.8 W) and were also passively heated by lower-leg immersion into hot water of 42 degrees C under an ambient temperature of 25 degrees C and relative humidity of 50%. Esophageal temperature, mean skin temperature, sweating rate (SR), and heart rate were measured continuously during both trials. The number of ASG was determined every 4 min after the onset of sweating, whereas SGO was calculated by dividing SR by ASG. During both exercise and passive heating, SR increased abruptly during the first 8 min after onset of sweating, followed by a slower increase. Similarly for both protocols, the number of ASG increased rapidly during the first 8 min after the onset of sweating and then ceased to increase further (P > 0.05). Conversely, SGO increased linearly throughout both perturbations. Our results suggest that changes in forearm sweating rate rely on both ASG and SGO during the initial period of exercise and passive heating, whereas further increases in SR are dependent on increases in SGO.

EXERCISE AND THERMOREGULATORY RESPONSES TO LOWER LIMB AND WHOLE BODY PRECOOLING

Lower limb immersion precooling (LL) has been shown to be an effective means to delay thermal strain associated with exercise. The goal of the present investigation was to compare thermoregulatory responses of LL versus whole body immersion precooling (WB) to an exercise-heat stress. Eleven males (age = 25.4 ± 1.7 yrs, ht = 179.7 ± 8.1 cm, wt = 78.0 ± 10.6 kg, VO2peak = 52.5 ± 5.9 ml/kg/min) participated in a randomized, cross over design with trials separated by one week. The protocol consisted of four 30 min intervals: baseline (Ta = 22°C, rh = 20%), immersion (TH2O = 20°C) to hip level (LL) or neck (WB), exercise at 60% of VO2peak (Ta = 30°C, rh = 30%), and recovery (Ta = 22°C, rh = 20%). Metabolic heat production was elevated (p ≤ 0.05) during WB water immersion compared to LL. Core temperature, mean body temperature, and mean skin temperature were lower (p ≤ 0.05) until 24, 16, and 14 min into exercise, respectively, for WB compared to LL. Body heat storage (S) was lower (p ≤ 0.05) during WB immersion (−58.9 ± 35.4 W/m2) compared to LL (−15.1 ± 26.0 W/m2). S during exercise was greater (p ≤ 0.05) for WB (189.2 ± 37.6 W/m2) compared to LL (148.8 ± 33.9 W/m2). No net difference between trials was observed in S summed across the entire protocol. LL and WB resulted in beneficial thermoregulatory responses over the protocol. However, LL resulted in less metabolic heat production and was associated with less thermal discomfort.

Hyperhydration and Glycerol: Thermoregulatory Effects During Exercise in Hot Climates

Hyperhydration or increasing body water content above normal (euhydration) level was thought to have some benefit during exercise heat-stress; however, attempts to overdrink have been minimized by a rapid diuretic response. The perception that hyperhydration might be beneficial for exercise performance and for thermoregulation arose from the adverse consequences of hypohydration. Many studies had examined the effects of hyperhydration on thermoregulation in the heat; however, most of them suffer from design problems that confound their results. The design problems included control conditions not representing euhydration but hypohydration, control conditions not adequately described, cold fluid ingestion that reduced core temperature, and/or changing heat acclimation status. Several investigators reported lower core temperatures during exercise after hyperhydration, while other studies do not. Some investigators reported higher sweating rates with hyperhydration, while other studies do not. Recent research that controlled for these confounding variables reported that hyperhydration (water or glycerol) did not alter core temperature, skin temperature, whole body sweating rate, local sweating rate, sweating threshold temperature, sweating sensitivity, or heart rate responses compared to euhydration trial. If euhydration is maintained during exercise-heat stress then hyperhydration appears to have no meaningful advantage.