Reductions in cardiac output, central blood volume, and stroke volume with thermal stress in normal men during exercise

Firefighter uncompensable heat stress results in excessive upper body temperatures measured by infrared thermography: Implications for cooling strategies

This research sought to evaluate the thermal zones of the upper body and firefighter personal protective equipment (PPE) immediately following uncompensable heat stress (0.03 °C increase/min). We hypothesized that the frontal portion of the head and the inside of the firefighter helmet would be the hottest as measured by infrared thermography. This hypothesis was due to previous research demonstrating that the head accounts for ∼8-10% of the body surface area, but it accounts for ∼20% of the overall body heat dissipation during moderate exercise. Twenty participants performed a 21-min graded treadmill exercise protocol (Altered Modified Naughton) in an environmental chamber (35 °C, 50 % humidity) in firefighter PPE. The body areas analyzed were the frontal area of the head, chest, abdomen, arm, neck, upper back, and lower back. The areas of the PPE that were analyzed were the inside of the helmet and the jacket. The hottest areas of the body post-exercise were the frontal area of the head (mean: 37.3 ± 0.4 °C), chest (mean: 37.5 ± 0.3 °C), and upper back (mean: 37.3 ± 0.4 °C). The coldest area of the upper body was the abdomen (mean: 36.1 ± 0.4 °C). The peak temperature of the inside of the helmet increased (p < 0.001) by 9.8 °C from 27.7 ± 1.6 °C to 37.4 ± 0.7 °C, and the inside of the jacket increased (p < 0.001) by 7.3 °C from 29.2 ± 1.7 °C to 36.5 ± 0.4 °C. The results of this study are relevant for cooling strategies for firefighters.

Heat acclimation improves exercise performance in hot conditions and increases heat shock protein 70 and 90 of skeletal muscles in Thoroughbred horses

This study aimed to determine whether heat acclimation could induce adaptations in exercise performance, thermoregulation, and the expression of proteins associated with heat stress in the skeletal muscles of Thoroughbreds. Thirteen trained Thoroughbreds performed 3 weeks of training protocols, consisting of cantering at 90% maximal oxygen consumption (VO2max) for 2 min 2 days/week and cantering at 7 m/s for 3 min 1 day/week, followed by a 20-min walk in either a control group (CON; Wet Bulb Globe Temperature [WBGT] 12-13°C; n = 6) or a heat acclimation group (HA; WBGT 29-30°C; n = 7). Before and after heat acclimation, standardized exercise tests (SET) were conducted, cantering at 7 m/s for 90 s and at 115% VO2max until fatigue in hot conditions. Increases in run time (p = 0.0301), peak cardiac output (p = 0.0248), and peak stroke volume (p = 0.0113) were greater in HA than in CON. Pulmonary artery temperature at 7 m/s was lower in HA than in CON (p = 0.0332). The expression of heat shock protein 70 (p = 0.0201) and 90 (p = 0.0167) increased in HA, but not in CON. These results suggest that heat acclimation elicits improvements in exercise performance and thermoregulation under hot conditions, with a protective adaptation to heat stress in equine skeletal muscles.

The effect of heat on lung diffusing capacity for carbon monoxide (DLCO) during cycling exercise

PURPOSE

This study aimed to quantify the combined effects of heat exposure and exercise of increasing intensity on pulmonary blood flow using lung diffusing capacity for carbon monoxide (DLCO) as an indirect measure. We hypothesized that, during exercise in the heat, the well-documented increase in skin blood flow for thermoregulation would lead to alterations in pulmonary blood flow and a subsequent fall in DLCO versus a thermoneutral condition.

METHODS

Nine healthy subjects (4 F/5 M, 20-45 years, VO2max 46.7 ± 5.8 mL/kg/min) completed three 15-min stages including rest and during cycling at 20 and 40% of maximum workload (Wmax) in either thermoneutral (TN; 22.2 ± 0.6 °C) or heat (HT; 39.4 ± 0.4 °C) conditions. DLCO, minute ventilation (VE), oxygen consumption ([Formula: see text]), heart rate (HR), and core (TC) and skin temperature (Tsk) were measured.

RESULTS

DLCO showed a significant interaction between exercise intensity and heat (P = 0.019); post hoc testing revealed that DLCO was higher at 40% of Wmax in HT vs. TN (53.2 ± 10.6 vs 50.0 ± 10.3 mL/min/mmHg, P = 0.003) only. VE and [Formula: see text] showed no difference in HT vs. TN. HR was higher in HT vs. TN (P < 0.001). TC and Tsk showed a significant interaction between temperature and intensity (P < 0.05).

CONCLUSION

The unexpected increase in DLCO during exercise in HT vs. TN conditions suggests a larger lung surface area for gas exchange, perhaps due to increased pulmonary capillary recruitment and/or distension secondary to a higher cardiac output (Q) in the heat. This study furthers our understanding of how heat exposure might impact pulmonary blood flow, specifically as assessed via DLCO.

Differential impact of heat and hypoxia on dynamic oxygen uptake and deoxyhemoglobin parameters during incremental exhaustive exercise

Purpose: This study aims to explore the relationship between the dynamic changes in oxygen uptake (V ˙ O 2 ) and deoxyhemoglobin (HHb) and peripheral fatigue in athletes during incremental exhaustive exercise under different environmental conditions, including high temperature and humidity environment, hypoxic environment, and normal conditions. Methods: 12 male modern pentathlon athletes were recruited and performed incremental exhaustive exercise in three different environments: normal condition (23°C, 45%RH, FiO2 = 21.0%, CON), high temperature and humidity environment (35°C, 70%RH, FiO2 = 21.0%, HOT), and hypoxic environment (23°C, 45%RH, FiO2 = 15.6%, HYP). Gas metabolism data of the athletes were collected, and muscle oxygen saturation (SmO2) and total hemoglobin content in the vastus lateralis muscles (VL) were measured to calculate the deoxyhemoglobin content. Linear and nonlinear function models were used to fit the characteristic parameters of V ˙ O 2 and HHb changes. Results: The results showed that compared to the CON, V ˙ O 2 , V ˙ CO 2 , and exercise time were decreased in the HOT and HYP (p < 0.05). Δ E V ˙ O 2 and OUES were reduced in the HOT and HYP compared to the CON (p < 0.05). The Gas exchange threshold in the CON corresponded to higher V ˙ O 2 than in the HYP and HOT (p < 0.05). Δ E V ˙ O 2 - 1 was reduced in the HOT compared to the HYP (p < 0.05). ΔEHHb was higher in the HOT compared to the CON (p < 0.05). ΔEHHb-1 was increased in the HYP compared to the CON (p < 0.05). There was a negative correlation between ΔEHHb and corresponding V ˙ O 2 ⁡ max in the HOT (r = -0.655, p < 0.05), and a negative correlation between ΔEHHb-1 and corresponding V ˙ O 2 ⁡ max in the HYP (r = -0.606, p < 0.05). Conclusion: Incremental exhaustive exercise in hypoxic environment and high temperature and humidity environments inhibits gas exchange and oxygen supply to skeletal muscle tissue in athletes. For athletes, the accelerated deoxygenation response of skeletal muscles during incremental exhaustive exercise in high temperature and humidity environments, as well as the excessive deoxygenation response before BP of deoxyhemoglobin in hypoxic environment, may be contributing factors to peripheral fatigue under different environmental conditions.

Delayed metabolic disturbances in the myocardium after exertional heat stroke: contrasting effects of exertion and thermal load

Epidemiological studies report higher risks of cardiovascular disease in humans exposed to heat stroke earlier in life. Previously, we explored mechanistic links between heat stroke and developing cardiac abnormalities using a preclinical mouse model of exertional heat stroke (EHS). Profound metabolic abnormalities developed in the ventricles of females but not males after 2 wk of recovery. Here we tested whether this lack of response in males could be attributed to the lower exercise performances or reduced thermal loads they experienced with the same running protocol. We systematically altered environmental temperature (Te) during EHS to manipulate heat exposure and exercise performance in the males. Three groups of adult C57BL/6 male mice were studied: "EHS-34" (Te = 34°C), "EHS-41" (Te = 41°C), and "EHS-39.5" (Te = 39.5°C). Mice ran until symptom limitation (unconsciousness), reaching max core temperature (Tc,max). After a 2-wk recovery, the mice were euthanized, and the ventricles were removed for untargeted metabolomics. Results were compared against age-matched nonexercise controls. The EHS-34 mice greatly elevated their exercise performance but reached lower Tc,max and lower thermal loads. The EHS-41 mice exhibited equivalent thermal loads, exercise times, and Tc,max compared with EHS-39.5. The ventricles from EHS-34 mice exhibited the greatest metabolic disturbances in the heart, characterized by shifts toward glucose metabolism, reductions in acylcarnitines, increased amino acid metabolites, elevations in antioxidants, altered TCA cycle flux, and increased xenobiotics. In conclusion, delayed metabolic disturbances following EHS in male myocardium appear to be greatly amplified by higher levels of exertion in the heat, even with lower thermal loads and max core temperatures.NEW & NOTEWORTHY Epidemiological data demonstrate greater cardiovascular risk in patients with previous heat stroke exposure. Using a preclinical mouse model of exertional heat stroke, male mice were exposed to one of three environmental temperatures (Te) during exercise. Paradoxically, after 2 wk, the mice in the lowest Te, exhibiting the largest exercise response and lowest heat load, had the greatest ventricular metabolic disturbances. Metabolic outcomes resemble developing left ventricular hypertrophy or stress-induced heart disease.

Body surface profile in ambient and hot temperatures during a rectangular test in race walker champions of the World Cup in Oman 2022

There is current interest in infrared thermography as a method to assess changes in body surface temperature to determine thermoregulatory mechanisms, especially in endurance sports. The aim of this study was to evaluate the effect of two environmental temperatures (17 and 28°C) on body surface temperature in different anterior and posterior aspects of the body during a rectangular test in international walkers of the Spanish National Team. Three international walkers performed a rectangular test, where body temperature was measured at rest, and after the 5th, 10th and 15th run using an infrared thermographic camera in room temperatures at 17 and 28°C. In addition, oxygen consumption was measured simultaneously. ANOVA detected a group × time interaction in the chest and abdomen (right and left), left back and right calf (p = < 0.05), with a trend in the right hamstring (p = 0.053) when comparing 17°C and 28°C. ANOVA detected no significant group × time interaction (p = 0.853) but there was a significant group effect (p = 0.022). The eleven degrees increase in ambient temperature (17 to 28°C) produces changes in almost all anatomical zones, but not homogeneously in international walkers during a rectangular test. This indicates that metabolic and blood flow changes are different depending on the anatomical zone measured.

Influence and Mechanisms of Action of Environmental Stimuli on Work Near and Above the Severe Domain Boundary (Critical Power)

Abstract

The critical power (CP) concept represents the uppermost rate of steady state aerobic metabolism during work. Work above CP is limited by a fixed capacity (W′) with exercise intensity being an accelerant of its depletion rate. Exercise at CP is a considerable insult to homeostasis and any work done above it will rapidly become intolerable. Humans live and exercise in situations of hypoxia, heat, cold and air pollution all of which impose a new environmental stress in addition to that of exercise. Hypoxia disrupts the oxygen cascade and consequently aerobic energy production, whereas heat impacts the circulatory system’s ability to solely support exercise performance. Cold lowers efficiency and increases the metabolic cost of exercise, whereas air pollution negatively impacts the respiratory system. This review will examine the effects imposed by environmental conditions on CP and W′ and describe the key physiological mechanisms which are affected by the environment.

Graphical Abstract

Can ten days of heat acclimation training improve temperate-condition rowing performance in national-level rowers?

This study investigated whether heat acclimation (HA) could improve rowing performance in temperate conditions in national-level rowers. Using a parallel-group design, eleven rowers (3 female, 8 male, age: 21±3 years, height: 182.3±6.8cm, mass: 79.2±9.0kg, V˙O2peak: 61.4±5.1ml·kg·min^-1) completed either a HA intervention (HEAT, n = 5) or acted as controls (CON, n = 6). The intervention replaced usual cross-training sessions and consisted of an hour of submaximal cycling or rowing ergometry in either 34±0°C for HEAT or 14±1°C for CON daily over two five-day blocks (10 sessions total), separated by 72h. Participants performed the ‘10+4’ test that consists of 10-min submaximal rowing and a 4-min time-trial (TT) in temperate conditions (20±0°C) before and after the intervention. Heat acclimation following the 10-session intervention was evidenced by large significant (p<0.05) decreases in maximum tympanic temperature (d = -1.68) and rate of perceived exertion (RPE) (d = -2.26), and a large significant increase in sweat loss (d = 0.91). Large non-significant (p>0.05) decreases were seen in average tympanic temperature (d = -3.08) and average heart rate (d = -1.53) in HEAT from session 2 to session 10 of the intervention. Furthermore, a large significant increase was seen in plasma volume (d = 3.74), with large significant decreases in haemoglobin concentration (d = -1.78) and hematocrit (d = -12.9). Following the intervention, large non-significant increases in respiratory exchange ratio (d = 0.87) and blood lactate (d = 1.40) as well as a large non-significant decrease in RPE (d = -1.23) were seen in HEAT during the 10-min submaximal rowing. A large significant decrease in peak heart rate (d = -2.27), as well as a large non-significant decrease in relative V˙O2peak (d = -0.90) and large non-significant increases in respiratory exchange ratio (d = 1.18), blood lactate concentration (d = 1.25) and power output (d = 0.96) were seen in HEAT during the 4-min TT. This study suggests that a 10-session HA intervention may elicit HA in national-level rowers, with potential to improve 4-min TT performance in temperate conditions.

Cardiovascular responses to hot skin at rest and during exercise

Integrative cardiovascular responses to heat stress during endurance exercise depend on various variables, such as thermal stress and exercise intensity. This review addresses how increases in skin temperature alter and challenge the integrative cardiovascular system during upright submaximal endurance exercise, especially when skin is hot (i.e. >38°C). Current evidence suggests that exercise intensity plays a significant role in cardiovascular responses to hot skin during exercise. At rest and during mild intensity exercise, hot skin increases skin blood flow and abolishes cutaneous venous tone, which causes blood pooling in the skin while having little impact on stroke volume and thus cardiac output is increased with an increase in heart rate. When the heart rate is at relatively low levels, small increases in heart rate, skin blood flow, and cutaneous venous volume do not compromise stroke volume, so cardiac output can increase to fulfill the demands for maintaining blood pressure, heat dissipation, and the exercising muscle. On the contrary, during more intense exercise, hot skin does not abolish exercise-induced cutaneous venoconstriction possibly due to high sympathetic nerve activities; thus, it does not cause blood pooling in the skin. However, hot skin reduces stroke volume, which is associated with a decrease in ventricular filling time caused by an increase in heart rate. When the heart rate is high during moderate or intense exercise, even a slight reduction in ventricular filling time lowers stroke volume. Cardiac output is therefore not elevated when skin is hot during moderate intensity exercise.

A mathematical model for predicting cardiovascular responses at rest and during exercise in demanding environmental conditions

The present research describes the development and validation of a cardiovascular model (CVR Model) for use in conjunction with advanced thermophysiological models, where usually only a total cardiac output is estimated. The CVR Model detailed herein estimates cardio-dynamic parameters (changes in cardiac output, stroke volume, and heart rate), regional blood flow, and muscle oxygen extraction, in response to rest and physical workloads, across a range of ages and aerobic fitness levels, as well as during exposure to heat, dehydration, and altitude. The model development strategy was to first establish basic resting and exercise predictions for cardio-dynamic parameters in an "ideal" environment (cool, sea level, and hydrated person). This basic model was then advanced for increasing levels of altitude, heat strain, and dehydration, using meta-analysis and reaggregation of published data. Using the estimated altitude- and heat-induced changes in maximum oxygen extraction and maximum cardiac output, the decline in maximum oxygen consumption at high altitude and in the heat was also modeled. A validation of predicted cardiovascular strain using heart rate was conducted using a dataset of 101 heterogeneous individuals (1,371 data points) during rest and exercise in the heat and at altitude, demonstrating that the CVR Model performs well (R² = 0.82-0.84) in predicting cardiovascular strain, particularly at a group mean level (R² = 0.97). The development of the CVR Model is aimed at providing the Fiala thermal Physiology & Comfort (FPC) Model and other complex thermophysiological models with improved estimations of cardiac strain and exercise tolerance, across a range of individuals during acute exposure to environmental stressors.NEW & NOTEWORTHY The present research promotes the adaption of thermophysiological modeling to the estimation of cardiovascular strain in individuals exercising under acute environmental stress. Integration with advanced models of human thermoregulation opens doors for detailed numerical analysis of athletes' performance and physiology during exercise, occupational safety, and individual work tolerability. The research provides a simple-to-validate metric of cardiovascular function (heart rate), as well as a method to evaluate key principles influencing exercise- and thermoregulation in humans.

Effect of Cyclic Heat Stress on Hypothalamic Oxygen Homeostasis and Inflammatory State in the Jungle Fowl and Three Broiler-Based Research Lines

Heat stress (HS) is devastating to poultry production sustainability due its detrimental effects on performance, welfare, meat quality, and profitability. One of the most known negative effects of HS is feed intake depression, which is more pronounced in modern high-performing broilers compared to their ancestor unselected birds, yet the underlying molecular mechanisms are not fully defined. The present study aimed, therefore, to determine the hypothalamic expression of a newly involved pathway, hypoxia/oxygen homeostasis, in heat-stressed broiler-based research lines and jungle fowl. Three populations of broilers (slow growing ACRB developed in 1956, moderate growing 95RB from broilers available in 1995, and modern fast growing MRB from 2015) and unselected Jungle fowl birds were exposed to cyclic heat stress (36°C, 9 h/day for 4 weeks) in a 2 × 4 factorial experimental design. Total RNAs and proteins were extracted from the hypothalamic tissues and the expression of target genes and proteins was determined by real-time quantitative PCR and Western blot, respectively. It has been previously shown that HS increased core body temperature and decreased feed intake in 95RB and MRB, but not in ACRB or JF. HS exposure did not affect the hypothalamic expression of HIF complex, however there was a line effect for HIF-1α (P = 0.02) with higher expression in JF under heat stress. HS significantly up regulated the hypothalamic expression of hemoglobin subunits (HBA1, HBBR, HBE, HBZ), and HJV in ACRB, HBA1 and HJV in 95RB and MRB, and HJV in JF, but it down regulated FPN1 in JF. Additionally, HS altered the hypothalamic expression of oxygen homeostasis- up and down-stream signaling cascades. Phospho-AMPK^Thr172 was activated by HS in JF hypothalamus, but it decreased in that of the broiler-based research lines. Under thermoneutral conditions, p-AMPK^Thr172 was higher in broiler-based research lines compared to JF. Ribosomal protein S6K1, however, was significantly upregulated in 95RB and MRB under both environmental conditions. HS significantly upregulated the hypothalamic expression of NF-κB2 in MRB, RelB, and TNFα in ACRB, abut it down regulated RelA in 95RB. The regulation of HSPs by HS seems to be family- and line-dependent. HS upregulated the hypothalamic expression of HSP60 in ACRB and 95RB, down regulated HSP90 in JF only, and decreased HSP70 in all studied lines. Taken together, this is the first report showing that HS modulated the hypothalamic expression of hypoxia- and oxygen homeostasis-associated genes as well as their up- and down-stream mediators in chickens, and suggests that hypoxia, thermotolerance, and feed intake are interconnected, which merit further in-depth investigations.

Physiological Function during Exercise and Environmental Stress in Humans-An Integrative View of Body Systems and Homeostasis

Claude Bernard's milieu intérieur (internal environment) and the associated concept of homeostasis are fundamental to the understanding of the physiological responses to exercise and environmental stress. Maintenance of cellular homeostasis is thought to happen during exercise through the precise matching of cellular energetic demand and supply, and the production and clearance of metabolic by-products. The mind-boggling number of molecular and cellular pathways and the host of tissues and organ systems involved in the processes sustaining locomotion, however, necessitate an integrative examination of the body's physiological systems. This integrative approach can be used to identify whether function and cellular homeostasis are maintained or compromised during exercise. In this review, we discuss the responses of the human brain, the lungs, the heart, and the skeletal muscles to the varying physiological demands of exercise and environmental stress. Multiple alterations in physiological function and differential homeostatic adjustments occur when people undertake strenuous exercise with and without thermal stress. These adjustments can include: hyperthermia; hyperventilation; cardiovascular strain with restrictions in brain, muscle, skin and visceral organs blood flow; greater reliance on muscle glycogen and cellular metabolism; alterations in neural activity; and, in some conditions, compromised muscle metabolism and aerobic capacity. Oxygen supply to the human brain is also blunted during intense exercise, but global cerebral metabolism and central neural drive are preserved or enhanced. In contrast to the strain seen during severe exercise and environmental stress, a steady state is maintained when humans exercise at intensities and in environmental conditions that require a small fraction of the functional capacity. The impact of exercise and environmental stress upon whole-body functions and homeostasis therefore depends on the functional needs and differs across organ systems.

Effect of Clothing Fabric on 20-km Cycling Performance in Endurance Athletes

Purpose: Examine the effect of synthetic fabrics (SYN, 60% polyester: 40% nylon) vs. 100% cotton fabric (CTN) on the 20-km cycling time trial (20 kmCTT) performance of competitive cyclists and triathletes.

Methods: In this randomized controlled crossover study, 15 adults (5 women) aged 29.6 ± 2.7 years (mean ± SE) with a peak rate of O2 consumption of 60.0 ± 2.0 ml/kg/min completed a 20 kmCTT under ambient laboratory conditions (24.3 ± 0.7°C and 17 ± 7% relative humidity) with a simulated wind of ~3 m/s while wearing SYN or CTN clothing ensembles. Both ensembles were of snowflake mesh bi-layer construction and consisted of a loose-fitting long-sleeved shirt with full-length trousers.

Results: Participants maintained a significantly (p &lt; 0.05) higher cycling speed and power output over the last 6-km of the 20 kmCTT while wearing the SYN vs. CTN ensemble (e.g., by 0.98 km/h and 18.4 watts at the 20-km mark). Consequently, 20 kmCTT duration was significantly reduced by 15.7 ± 6.8 sec or 0.8 ± 0.3% during SYN vs. CTN trials (p &lt; 0.05). Improved 20 kmCTT performance with SYN vs. CTN clothing could not be explained by concurrent differences in esophageal temperature, sweat rate, ratings of perceived exertion and/or cardiometabolic responses to exercise. However, it was accompanied by significantly lower mean skin temperatures (~1°C) and more favorable ratings of perceived clothing comfort and thermal sensation during exercise.

Conclusion: Under the experimental conditions of the current study, athletic clothing made of synthetic fabrics significantly improved the 20 kmCTT performance of endurance-trained athletes by optimizing selected thermoregulatory and perceptual responses to exercise.

The impact of elevated body core temperature on critical power as determined by a 3-min all-out test

The parameters of the power-duration relationship (critical power and W′) estimated by a 3-min all-out test were not altered by elevated body core temperature as compared with a thermoneutral condition.

Temperate performance and metabolic adaptations following endurance training performed under environmental heat stress

Endurance athletes are frequently exposed to environmental heat stress during training. We investigated whether exposure to 33°C during training would improve endurance performance in temperate conditions and stimulate mitochondrial adaptations. Seventeen endurance-trained males were randomly assigned to perform a 3-week training intervention in 18°C (TEMP) or 33°C (HEAT). An incremental test and 30-min time-trial preceded by 2-h low-intensity cycling were performed in 18°C pre- and post-intervention, along with a resting vastus lateralis microbiopsy. Training was matched for relative cardiovascular demand using heart rates measured at the first and second ventilatory thresholds, along with a weekly "best-effort" interval session. Perceived training load was similar between-groups, despite lower power outputs during training in HEAT versus TEMP (p < .05). Time-trial performance improved to a greater extent in HEAT than TEMP (30 ± 13 vs. 16 ± 5 W, N = 7 vs. N = 6, p = .04), and citrate synthase activity increased in HEAT (fold-change, 1.25 ± 0.25, p = .03, N = 9) but not TEMP (1.10 ± 0.22, p = .22, N = 7). Training-induced changes in time-trial performance and citrate synthase activity were related (r = .51, p = .04). A group × time interaction for peak fat oxidation was observed (Δ 0.05 ± 0.14 vs. -0.09 ± 0.12 g·min^-1 in TEMP and HEAT, N = 9 vs. N = 8, p = .05). Our data suggest exposure to moderate environmental heat stress during endurance training may be useful for inducing adaptations relevant to performance in temperate conditions.

Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies

A rise in body core temperature and loss of body water via sweating are natural consequences of prolonged exercise in the heat. This review provides a comprehensive and integrative overview of how the human body responds to exercise under heat stress and the countermeasures that can be adopted to enhance aerobic performance under such environmental conditions. The fundamental concepts and physiological processes associated with thermoregulation and fluid balance are initially described, followed by a summary of methods to determine thermal strain and hydration status. An outline is provided on how exercise-heat stress disrupts these homeostatic processes, leading to hyperthermia, hypohydration, sodium disturbances, and in some cases exertional heat illness. The impact of heat stress on human performance is also examined, including the underlying physiological mechanisms that mediate the impairment of exercise performance. Similarly, the influence of hydration status on performance in the heat and how systemic and peripheral hemodynamic adjustments contribute to fatigue development is elucidated. This review also discusses strategies to mitigate the effects of hyperthermia and hypohydration on exercise performance in the heat by examining the benefits of heat acclimation, cooling strategies, and hyperhydration. Finally, contemporary controversies are summarized and future research directions are provided.

Cardiovascular Drift and Maximal Oxygen Uptake during Running and Cycling in the Heat

INTRODUCTION

Greater cardiovascular (CV) drift occurs during cycling compared to running in temperate conditions. CV drift also corresponds to proportional reductions in maximal oxygen uptake (V˙O2max) during heat stress. Whether exercise mode differentially affects CV drift-and accompanying declines in V˙O2max-during heat stress is uncertain. The purpose of this study was to test the hypothesis that a greater magnitude of CV drift, accompanied by a greater decrement in V˙O2max, occurs during cycling compared to running in hot conditions.

METHODS

7 active men (mean ± SD; age = 25 ± 6 yr, percent body fat = 11.9% ± 2.4%) completed a control graded exercise test (GXT) on a cycle ergometer and treadmill. Then on separate, counterbalanced occasions they completed 15 or 45 min of cycling or running at 60% V˙O2max in 35°C, immediately followed by a GXT to measure V˙O2max (4 trials total). The separate 15- and 45-min trials were designed to measure CV drift and V˙O2max over the same time interval.

RESULTS

Heart rate increased 19% and 17% and stroke volume decreased 20% and 15% between 15 and 45 min during running and cycling, respectively, but modes were not different (all P > 0.05). Despite a 1.8°C larger core-to-skin thermal gradient during running, decrements in V˙O2peak were not different between exercise modes (95% CI for difference in change scores between 15 and 45 min: -0.2, 0.3).

CONCLUSIONS

CV strain (indexed as CV drift) during prolonged exercise in the heat corresponds to reduced V˙O2max, irrespective of exercise mode or the thermal gradient. As such, the upward drift in heart rate associated with CV drift reflects increased relative metabolic intensity (%V˙O2max) during prolonged cycling or running in the heat.

Short Term High-Repetition Back Squat Protocol Does Not Improve 5-km Run Performance

The purpose of this study was to evaluate the hypothesis that a novel high-repetition, low-resistance back squat training protocol, designed to stimulate high-intensity interval training, improves 5-km run performance. Fifteen runners [4 male, 11 female; 150 + minutes of endurance exercise/week; age = 22.7 ± 2.0 y; 21.5 ± 2.2 kg/m² BMI] in this single-group test-retest design completed two weeks of back squats consisting of three sets of 15-24 repetitions at 60% of estimated one-repetition max (1RM), three times per week (1-2 days of rest between sessions). Outcome tests included a 5-km outdoor timed run, laboratory indirect calorimetry to quantify substrate oxidation rates during steady-state submaximal exercise (60% and 70% heart rate max (HRmax)), and estimated 1RM for back squats. Back squat estimated 1RM increased by 20% (58.3 ± 18.5 to 70.2 ± 16.7 kg, P < 0.001). However, 5-km run times due to the back squat protocol did not significantly change (Pre-Squats: 23.9 ± 5.0 vs. Post-Squats: 23.7 ± 4.3 minutes, P = 0.71). Likewise, the squat training program did not significantly alter carbohydrate or lipid oxidation rates during steady-state submaximal exercise at 60% or 70% of HRmax (P values ranged from 0.36 - 0.99). Short term high-repetition back squat training does not appear to impact 5-km run performance or substrate utilization during submaximal exercise.

Menstrual cycle effects on cardiovascular drift and maximal oxygen uptake during exercise heat stress

AIM

Compared to other modulators of physiological strain associated with exercise heat stress, hyperthermia results in the greatest magnitude of cardiovascular (CV) drift and associated decrements in maximal oxygen uptake ([Formula: see text]).

PURPOSE

To determine if elevated core temperature in the luteal phase (LP) of the menstrual cycle results in greater CV drift and reductions in [Formula: see text] versus the follicular phase (FP).

METHODS

Seven women performed 15- and 45-min cycling bouts on separate occasions (60% [Formula: see text], 35 °C) followed by a [Formula: see text] test during the FP and LP. CV drift was measured between 15 and 45 min during the 45-min bout, and the 15-min bout was for measuring [Formula: see text] over the same time interval that CV drift occurred.

RESULTS

Core temperature during LP was ~ 0.3 °C higher than FP (P < 0.05), but changes from rest during exercise were similar between phases (all P > 0.05). Heart rate increased significantly over time but was not different between phases (P = 0.78). Stroke volume decreased more over time during LP compared to FP (P = 0.02), but the values were similar at the end of exercise between phases (both time points P > 0.05). [Formula: see text] decrements for FP (13%) and LP (16%) were also comparable (P = 0.97).

CONCLUSIONS

The LP-FP difference in core temperature in this study was not sufficient to amplify CV strain and decrements in [Formula: see text]. Greater differences in core temperature may be required to independently modulate CV drift and accompanying decrements in [Formula: see text] during prolonged exercise heat stress.

Exercise Thermoregulation with a Simulated Burn Injury: Impact of Air Temperature

The U.S. Army's Standards of Medical Fitness (AR 40-501) states: "Prior burn injury (to include donor sites) involving a total body surface area of 40% or more does not meet the standard." However, the standard does not account for the interactive effect of burn injury size and air temperature on exercise thermoregulation.

PURPOSE

To evaluate whether the detrimental effect of a simulated burn injury on exercise thermoregulation is dependent on air temperature.

METHODS

On eight occasions, nine males cycled for 60 min at a fixed metabolic heat production (6 W·kg) in air temperatures of 40°C or 25°C with simulated burn injuries of 0% (Control), 20%, 40%, or 60% of total body surface area (TBSA). Burn injuries were simulated by covering the skin with an absorbent, vapor-impermeable material to impede evaporation from the covered areas. Core temperature was measured in the gastrointestinal tract via telemetric pill.

RESULTS

In 40°C conditions, greater elevations in core temperature were observed with 40% and 60% TBSA simulated burn injuries versus Control (P < 0.01). However, at 25°C, core temperature responses were not different versus Control with 20%, 40%, and 60% TBSA simulated injuries (P = 0.97). The elevation in core temperature at the end of exercise was greater in the 40°C environment with 20%, 40%, and 60% TBSA simulated burn injuries (P ≤ 0.04).

CONCLUSIONS

Simulated burn injuries ≥20% TBSA exacerbate core temperature responses in hot, but not temperate, air temperatures. These findings suggest that the U.S. Army's standard for inclusion of burned soldiers is appropriate for hot conditions, but could lead to the needless discharge of soldiers who could safely perform their duties in cooler training/operational settings.

Heat, Hydration and the Human Brain, Heart and Skeletal Muscles

People undertaking prolonged vigorous exercise experience substantial bodily fluid losses due to thermoregulatory sweating. If these fluid losses are not replaced, endurance capacity may be impaired in association with a myriad of alterations in physiological function, including hyperthermia, hyperventilation, cardiovascular strain with reductions in brain, skeletal muscle and skin blood perfusion, greater reliance on muscle glycogen and cellular metabolism, alterations in neural activity and, in some conditions, compromised muscle metabolism and aerobic capacity. The physiological strain accompanying progressive exercise-induced dehydration to a level of ~ 4% of body mass loss can be attenuated or even prevented by: (1) ingesting fluids during exercise, (2) exercising in cold environments, and/or (3) working at intensities that require a small fraction of the overall body functional capacity. The impact of dehydration upon physiological function therefore depends on the functional demand evoked by exercise and environmental stress, as cardiac output, limb blood perfusion and muscle metabolism are stable or increase during small muscle mass exercise or resting conditions, but are impaired during whole-body moderate to intense exercise. Progressive dehydration is also associated with an accelerated drop in perfusion and oxygen supply to the human brain during submaximal and maximal endurance exercise. Yet their consequences on aerobic metabolism are greater in the exercising muscles because of the much smaller functional oxygen extraction reserve. This review describes how dehydration differentially impacts physiological function during exercise requiring low compared to high functional demand, with an emphasis on the responses of the human brain, heart and skeletal muscles.

Biochemical recovery from exertional heat stroke follows a 16-day time course

BACKGROUND

The aim of this study was to characterize the time-resolved progression of clinical laboratory disturbances days-following an exertional heat stroke (EHS). Currently, normalization of organ injury clinical biomarker values is the primary indicator of EHS recovery. However, an archetypical biochemical recovery profile following EHS has not been established.

METHODS

We performed a retrospective analysis of EHS patient records in US military personnel from 2008-2014 using the Military Health System Data Repository (MDR). We focused on commonly reported clinical laboratory analytes measured on the day of injury and all proceeding follow-up visits.

RESULTS

Over the prescribed period, there were 2,529 EHS episodes treated at 250 unique treatment locations. Laboratory results, including a standardized set of blood, serum and urine assays, were analyzed from 0-340 days following the initial injury. Indicators of acute kidney injury, including serum electrolyte disturbances and abnormal urinalysis findings, were most prevalent on the day of the injury but normalized within 24-48hours (creatinine, blood urea nitrogen, and blood and protein in urine). Muscle damage and liver function-associated markers peaked 0-4 days after injury and persisted outside their respective reference ranges for 2-16 days (alanine aminotransferase, aspartate aminotransferase, creatine phosphokinase, myoglobin, prothrombin time).

CONCLUSION

Biochemical recovery from EHS spans a 16-day time course, and markers of end-organ damage exhibit distinct patterns over this period. This analysis underscores the prognostic value of each clinical laboratory analyte and will assist in evaluating EHS patient presentation, injury severity and physiological recovery.

Acute Kidney Injury Biomarker Responses to Short-Term Heat Acclimation

The combination of hyperthermia, dehydration, and strenuous exercise can result in severe reductions in kidney function, potentially leading to acute kidney injury (AKI). We sought to determine whether six days of heat acclimation (HA) mitigates the rise in clinical biomarkers of AKI during strenuous exercise in the heat. Twenty men completed two consecutive 2 h bouts of high-intensity exercise in either hot (n = 12, 40 °C, 40% relative humidity) or mild (n = 8, 24 °C, 21% relative humidity) environments before (PreHA) and after (PostHA) 4 days of 90–120 min of exercise per day in a hot or mild environment. Increased clinical biomarkers of AKI (CLINICAL) was defined as a serum creatinine increase ≥0.3 mg·dL⁻¹ or estimated glomerular filtration rate (eGFR) reduction >25%. Creatinine similarly increased in the hot environment PreHA (0.35 ± 0.23 mg·dL⁻¹) and PostHA (0.39 ± 0.20 mg·dL⁻¹), with greater increases than the mild environment at both time points (0.11 ± 0.07 mg·dL⁻¹, 0.08 ± 0.06 mg·dL⁻¹, p ≤ 0.001), respectively. CLINICAL occurred in the hot environment PreHA (n = 9, 75%), with fewer participants with CLINICAL PostHA (n = 7, 58%, p = 0.007), and no participants in the mild environment with CLINICAL at either time point. Percent change in plasma volume was predictive of changes in serum creatinine PostHA and percent changes in eGFR both PreHA and PostHA. HA did not mitigate reductions in eGFR nor increases in serum creatinine during high-intensity exercise in the heat, although the number of participants with CLINICAL was reduced PostHA.

Per-Cooling (Using Cooling Systems during Physical Exercise) Enhances Physical and Cognitive Performances in Hot Environments. A Narrative Review

There are many important sport events that are organized in environments with a very hot ambient temperature (Summer Olympics, FIFA World Cup, Tour de France, etc.) and in hot locations (e.g., Qatar). Additionally, in the context of global warming and heat wave periods, athletes are often subjected to hot ambient temperatures. It is known that exercising in the heat induces disturbances that may provoke premature fatigue and negatively affects overall performance in both endurance and high intensity exercises. Deterioration in several cognitive functions may also occur, and individuals may be at risk for heat illnesses. To train, perform, work and recover and in a safe and effective way, cooling strategies have been proposed and have been routinely applied before, during and after exercise. However, there is a limited understanding of the influences of per-cooling on performance, and it is the subject of the present review. This work examines the influences of per-cooling of different areas of the body on performance in terms of intense short-term exercises ("anaerobic" exercises), endurance exercises ("aerobic" exercises), and cognitive functioning and provides detailed strategies that can be applied when individuals train and/or perform in high ambient temperatures.

Optical and Optoacoustic Imaging

The present chapter summarizes progress with optical methods that go beyond human vision. The focus is on two particular technologies: fluorescence molecular imaging and optoacoustic (photoacoustic) imaging. The rationale for the selection of these two methods is that in contrast to optical microscopy techniques, both fluorescence and optoacoustic imaging can achieve large fields of view, i.e., spanning several centimeters in two or three dimensions. Such fields of views relate better to human vision and can visualize large parts of tissue, a necessary premise for clinical detection. Conversely, optical microscopy methods only scan millimeter-sized dimensions or smaller. With such operational capacity, optical microscopy methods need to be guided by another visualization technique in order to scan a very specific area in tissue and typically only provide superficial measurements, i.e., information from depths that are of the order of 0.05-1 mm. This practice has generally limited their clinical applicability to some niche applications, such as optical coherence tomography of the retina. On the other hand, fluorescence molecular imaging and optoacoustic imaging emerge as more global optical imaging methods with wide applications in surgery, endoscopy, and non-invasive clinical imaging, as summarized in the following. The current progress in this field is based on a volume of recent review and other literature that highlights key advances achieved in technology and biomedical applications. Context and figures from references from the authors of this chapter have been used here, as it reflects our general view of the current status of the field.

Fan cooling after cardiovascular drift does not reverse decrements in maximal oxygen uptake during heat stress

Cardiovascular (CV) drift, the progressive increase in heart rate (HR) and decrease in stroke volume (SV) during constant rate, moderate intensity exercise, is related to reduced maximal oxygen uptake (V̇O_2max) during heat stress. Once it has already occurred, it is unknown whether the detrimental effects of CV drift on V̇O_2max can be reversed. This study tested the hypothesis that fan cooling after CV drift has occurred attenuates decrements in V̇O_2max associated with CV drift. Eight men completed a control graded exercise test (GXT) in 22°C to measure V̇O_2max. Then on separate, counterbalanced occasions, they completed one 15-min (15MIN) and two 45-min bouts (45NF and 45FAN) of cycling in 35°C, 40% RH at 60% V̇O_2max, each immediately followed by a GXT to measure V̇O_2max. For one of the 45-min trials (45FAN), fan airflow (4.5 m/s) was directed at participants beginning ~5 min before the GXT and continuing throughout the remainder of exercise. The purpose of the separate 15- and 45-min trials was to measure V̇O_2max during the same time interval that CV drift occurred. HR increased (13.8% and 11.4%) and SV decreased (14.4% and 14.1%) for 45NF and 45FAN, respectively; trials were not different (all P > 0.05). Despite a decrease in mean skin temperature of ~1°C with fan use, V̇O_2max decreased similarly between conditions (17% vs. 15% for 45NF and 45FAN, P = 0.54). Fan cooling after CV drift was insufficient to reverse the negative consequences of CV drift on V̇O_2max after prolonged exercise in a hot environment. Abbreviations: 15MIN: 15-min trial; 45FAN: 45-min, fan trial; 45NF: 45-min, no fan trial; ANOVA: Analysis of variance; CV: Cardiovascular; GXT: Graded exercise test; HR: Heart rate; SV: Stroke volume; T̅_b: Mean body temperature; T_re: Rectal temperature; T̅_sk: Mean skin temperature; V̇O_2max: Maximal oxygen uptake.

Listening to motivational music mitigates heat-related reductions in exercise performance

PURPOSE

To examine whether listening to motivational music mitigates heat-related reductions in exercise performance, and leads to a greater increase in thermal and cardiovascular strain.

METHODS

Twelve participants (26 ± 5 y, 77.5 ± 17.0 kg, 49 ± 8 ml·min^-1·kg^-1) completed 30-min of cycling preload at 50% VO_2max followed by a 5-min rest and 15-min cycling time trial on seven separate occasions; three familiarisation sessions in a 20 °C room and four experimental trials in a climatic chamber regulated at either 21 °C, 50%RH (NEU) or 36 °C, 50%RH (HOT), each with and without the participant listening to self-selected motivational music during the 5-min rest and 15-min time trial. Measures of total work, core temperature and heart rate and blood pressure (from which rate-pressure product for cardiovascular strain was calculated), were recorded.

RESULTS

Without music, total work was lower (p < .001) in the HOT condition (168 ± 59 kJ) relative to the NEU condition (193 ± 60 kJ). With music, total work was greater relative to no music in both the NEU condition (203 ± 60 kJ vs 193 ± 60 kJ; p = .008) and HOT condition (183 ± 63 kJ vs 168 ± 60 kJ; p = .029). The greater total work in the HOT condition with music relative to no music resulted in a higher (p = .006) core temperature (38.7 ± 0.4 °C vs 38.6 ± 0.5 °C) and a higher (p < .001) rate-pressure product (34.8 ± 7.1 mmHg·beats·min^-1·10^-3 vs 27.8 ± 3.7 mmHg·beats·min^-1·10^-3).

CONCLUSION

Listening to motivational music mitigated heat-related reductions in exercise performance with an improvement in performance in the heat of ~10%. This improved exercise performance led to a greater increase in thermal and cardiovascular strain in the heat but did not exceed levels typically associated with an elevated health risk in a young, healthy population.

Validity of a noninvasive estimation of deep body temperature when wearing personal protective equipment during exercise and recovery

BACKGROUND

Deep body temperature is a critical indicator of heat strain. However, direct measures are often invasive, costly, and difficult to implement in the field. This study assessed the agreement between deep body temperature estimated from heart rate and that measured directly during repeated work bouts while wearing explosive ordnance disposal (EOD) protective clothing and during recovery.

METHODS

Eight males completed three work and recovery periods across two separate days. Work consisted of treadmill walking on a 1% incline at 2.5, 4.0, or 5.5 km/h, in a random order, wearing EOD protective clothing. Ambient temperature and relative humidity were maintained at 24 °C and 50% [Wet bulb globe temperature (WBGT) (20.9 ± 1.2) °C] or 32 °C and 60% [WBGT (29.0 ± 0.2) °C] on the separate days, respectively. Heart rate and gastrointestinal temperature (T_GI) were monitored continuously, and deep body temperature was also estimated from heart rate (ECTemp).

RESULTS

The overall systematic bias between T_GI and ECTemp was 0.01 °C with 95% limits of agreement (LoA) of ±0.64 °C and a root mean square error of 0.32 °C. The average error statistics among participants showed no significant differences in error between the exercise and recovery periods or the environmental conditions. At T_GI levels of (37.0-37.5) °C, (37.5-38.0) °C, (38.0-38.5) °C, and > 38.5 °C, the systematic bias and ± 95% LoA were (0.08 ± 0.58) °C, (- 0.02 ± 0.69) °C, (- 0.07 ± 0.63) °C, and (- 0.32 ± 0.56) °C, respectively.

CONCLUSIONS

The findings demonstrate acceptable validity of the ECTemp up to 38.5 °C. Conducting work within an ECTemp limit of 38.4 °C, in conditions similar to the present study, would protect the majority of personnel from an excessive elevation in deep body temperature (> 39.0 °C).

Relationship between climate and hemodynamics according to echocardiography

Studies performed in controlled laboratory conditions have shown that environmental thermal application may induce various circulatory changes. We aimed to demonstrate the effect of local climate on hemodynamics according to echocardiography. Echocardiographic studies conducted in ambulatory patients, 18 yr of age or older, between January 2012 and July 2016, at our medical center, for whom climate data on the day of the echocardiogram study were available, were retrospectively included in case climate data. Discomfort index, apparent temperature, temperature-humidity index, and thermal index were computed. Echocardiograms conducted in hotter months (June-November) were compared with those done in colder months (December-May). The cohort consisted of 11,348 individuals, 46.2% women, and mean age of 57.9 ± 18.1 yr. Climate indexes correlated directly with stroke volume ( r = 0.039) and e' (lateral r = 0.047; septal r = 0.038), and inversely with systolic pulmonary artery pressure (SPAP; r = -0.038) (all P values < 0.05). After adjustment for age and sex, echocardiograms conducted during June-November had a lower chance to show e' septal < 7 cm/s (odds ratio 0.88, 95% confidence interval 0.78-0.98, P = 0.017) and SPAP > 40 mmHg (odds ratio 0.81, 95% confidence interval 0.67-0.99, P = 0.04) compared with those conducted in other months. The authors concluded that climate may affect hemodynamics, according to echocardiographic assessment in ambulatory patients. NEW & NOTEWORTHY In the present study, we examined 11,348 individuals who underwent ambulatory echocardiography. Analyses of the echocardiographic studies demonstrated that environmental thermal stress, i.e., climate, may affect hemodynamics. Most notably were the effects on diastolic function. Higher values of mitral e', stroke volume, as well as ejection fraction, and lower values of systolic pulmonary artery pressure and tricuspid regurgitation were demonstrated on hotter days and seasons.

The effect of hot and cold drinks on thermoregulation, perception, and performance: the role of the gut in thermoreception

PURPOSE

Hot compared to cold drinks alter sweating responses during very low intensity exercise in temperate conditions. The thermoregulatory, perceptual, and performance effects of hot compared to cold drinks in hot, dry conditions during high-intensity exercise have not been examined.

METHOD

Ten participants [mean ± SD characteristics age 25 ± 5 years, height 1.81 ± 0.07 m, body mass 73.5 ± 10.6 kg, maximal power output (P_Max) 350 ± 41 W] completed two conditions, where they drank four boluses (ingested at - 9, 15, 30, and 45 min, respectively) of 3.2 mL kg^- 1 (~ 960 mL total) of either a COLD (5.3 °C) or a HOT drink (49.0 °C), which were contrasted to a no-drink CONTROL. They cycled for 60-min [55% P_Max in hot (34.4 °C) dry (34% RH)] ambient conditions followed by a test to exhaustion (TTE; 80% P_Max). The thermoregulatory, performance, and perceptual implications of drink temperature were measured.

RESULTS

TTE was worse in the CONTROL (170 ± 132 s) than the COLD drink (371 ± 272 s; p = 0.021) and HOT drink conditions (367 ± 301 s; p = 0.038) which were not different (p = 0.965). Sweat responses [i.e., reflex changes in mean skin temperature (T_msk) and galvanic skin conductance] indicated transient reductions in sweating response after COLD drink ingestion. The COLD drink improved thermal comfort beyond the transient changes in sweating.

CONCLUSION

Only COLD drink ingestion changed thermoregulation, but improved perceptual response. Accordingly, we conclude a role for gut thermoreception in thermal perception during exercise in hot, dry conditions.

Effect of a Cooling Kit on Physiology and Performance Following Exercise in the Heat

Context: Exercising in the heat leads to an increase in body temperature that can increase the risk of heat illness or cause detriments in exercise performance. Objective: To examine a phase change heat emergency kit (HEK) on thermoregulatory and perceptual responses and subsequent exercise performance following exercise in the heat. Design: Two randomized crossover trials that consisted of 30 minutes of exercise, 15 minutes of treatment (T1), performance testing (5-10-5 pro-agility test and 1500-m run), and another 15 minutes of treatment (T2) identical to T1. Setting: Outdoors in the heat (wet-bulb globe temperature: 31.5°C [1.8°C] and relative humidity: 59.0% [5.6%]). Participants: Twenty-six (13 men and 13 women) individuals (aged 20–27 y). Interventions: Treatment was performed with HEK and without HEK (control, CON) modality. Main Outcome Measures: Gastrointestinal temperature, mean skin temperature, thirst sensation, and muscle pain. Results: Maximum gastrointestinal temperature following exercise and performance was not different between trials (P > .05). Cooling rate was faster during T1 CON (0.053°C/min [0.049°C/min]) compared with HEK (0.043°C/min [0.032°C/min]; P = .01). Mean skin temperature was lower in HEK during T1 (P < .001) and T2 (P = .05). T2 thirst was lower in CON (P = .02). Muscle pain was lower in HEK in T2 (P = .03). Performance was not altered (P > .05). Conclusions: HEK improved perception but did not enhance cooling or performance following exercise in the heat. HEK is therefore not recommended to facilitate recovery, treat hyperthermia, or improve performance.

Cardiovascular responses to exercise when increasing skin temperature with narrowing of the core-to-skin temperature gradient

The decline in stroke volume (SV) during exercise in the heat has been attributed to either an increase in cutaneous blood flow (CBF) that reduces venous return or an increase in heart rate (HR) that reduces cardiac filling time. However, the evidence supporting each mechanism arises under experimental conditions with different skin temperatures (T_sk; e.g., ≥38°C vs. ≤36°C, respectively). We systematically studied cardiovascular responses to progressively increased T_sk (32°C-39°C) with narrowing of the core-to-skin gradient during moderate intensity exercise. Eight men cycled at 63 ± 1% peak oxygen consumption for 20-30 min. T_sk was manipulated by having subjects wear a water-perfused suit that covered most of the body and maintained T_sk that was significantly different between trials and averaged 32.4 ± 0.2, 35.5 ± 0.1, 37.5 ± 0.1, and 39.5 ± 0.1°C, respectively. The graded heating of T_sk ultimately produced a graded elevation of esophageal temperature (T_es) at the end of exercise. Incrementally increasing T_sk resulted in a graded increase in HR and a graded decrease in SV. CBF reached a similar average plateau value in all trials when T_es was above ~38°C, independent of T_sk. T_sk had no apparent effect on forearm venous volume (FVV). In conclusion, the CBF and FVV responses suggest no further pooling of blood in the skin when T_sk is increased from 32.4°C to 39.5°C. The decrease in SV during moderate intensity exercise when heating the skin to high levels appears related to an increase in HR and not an increase in CBF. NEW & NOTEWORTHY This study systematically investigated the effect of increasing skin temperature (T_sk) to high levels on cardiovascular responses during moderate intensity exercise. We conclude that the declines in stroke volume were related to the increases in heart rate but not the changes in cutaneous blood flow (CBF) and forearm venous volume (FVV) during moderate intensity exercise when T_sk increased from ~32°C to ~39°C. High T_sk (≥38°C) did not further elevate CBF and FVV compared with lower T_sk during moderate intensity exercise.

Whole body hyperthermia, but not skin hyperthermia, accelerates brain and locomotor limb circulatory strain and impairs exercise capacity in humans

Cardiovascular strain and hyperthermia are thought to be important factors limiting exercise capacity in heat‐stressed humans, however, the contribution of elevations in skin (T_sk) versus whole body temperatures on exercise capacity has not been characterized. To ascertain their relationships with exercise capacity, blood temperature (T_B), oxygen uptake (V̇O₂), brain perfusion (MCA V_mean), locomotor limb hemodynamics, and hematological parameters were assessed during incremental cycling exercise with elevated skin (mild hyperthermia; HYP_mild), combined core and skin temperatures (moderate hyperthermia; HYP_mod), and under control conditions. Both hyperthermic conditions increased T_sk versus control (6.2 ± 0.2°C; P < 0.001), however, only HYP_mod increased resting T_B, leg blood flow and cardiac output (Q̇), but not MCA V_mean. Throughout exercise, T_sk remained elevated in both hyperthermic conditions, whereas only T_B was greater in HYP_mod. At exhaustion, oxygen uptake and exercise capacity were reduced in HYP_mod in association with lower leg blood flow, MCA V_mean and mean arterial pressure (MAP), but similar maximal heart rate and T_B. The attenuated brain and leg perfusion with hyperthermia was associated with a plateau in MCA and two‐legged vascular conductance (VC). Mechanistically, the falling MCA VC was coupled to reductions in PaCO₂, whereas the plateau in leg vascular conductance was related to markedly elevated plasma [NA] and a plateau in plasma ATP. These findings reveal that whole‐body hyperthermia, but not skin hyperthermia, compromises exercise capacity in heat‐stressed humans through the early attenuation of brain and active muscle blood flow.

An exertional heat illness triage tool for a jungle training environment

This article introduces a practical triage tool designed to assist commanders, jungle training instructors (JTIs) and medical personnel to identify Defence Personnel (DP) with suspected exertional heat illness (EHI). The challenges of managing suspected EHI in a jungle training environment and the potential advantages to stratifying the urgency of evacuation are discussed. This tool has been designed to be an adjunct to the existing MOD mandated heat illness recognition and first aid training.

Defining the determinants of endurance running performance in the heat

In cool conditions, physiologic markers accurately predict endurance performance, but it is unclear whether thermal strain and perceived thermal strain modify the strength of these relationships. This study examined the relationships between traditional determinants of endurance performance and time to complete a 5-km time trial in the heat. Seventeen club runners completed graded exercise tests (GXT) in hot (GXTHOT; 32°C, 60% RH, 27.2°C WBGT) and cool conditions (GXTCOOL; 13°C, 50% RH, 9.3°C WBGT) to determine maximal oxygen uptake (V̇O2max), running economy (RE), velocity at V̇O2max (vV̇O2max), and running speeds corresponding to the lactate threshold (LT, 2 mmol.l-1) and lactate turnpoint (LTP, 4 mmol.l-1). Simultaneous multiple linear regression was used to predict 5 km time, using these determinants, indicating neither GXTHOT (R2 = 0.72) nor GXTCOOL (R2 = 0.86) predicted performance in the heat as strongly has previously been reported in cool conditions. vV̇O2max was the strongest individual predictor of performance, both when assessed in GXTHOT (r = -0.83) and GXTCOOL (r = -0.90). The GXTs revealed the following correlations for individual predictors in GXTHOT; V̇O2maxr = -0.7, RE r = 0.36, LT r = -0.77, LTP r = -0.78 and in GXTCOOL; V̇O2maxr = -0.67, RE r = 0.62, LT r = -0.79, LTP r = -0.8. These data indicate (i) GXTHOT does not predict 5 km running performance in the heat as strongly as a GXTCOOL, (ii) as in cool conditions, vV̇O2max may best predict running performance in the heat.

Short-term heat acclimation improves the determinants of endurance performance and 5-km running performance in the heat

This study investigated the effect of 5 days of controlled short-term heat acclimation (STHA) on the determinants of endurance performance and 5-km performance in runners, relative to the impairment afforded by moderate heat stress. A control group (CON), matched for total work and power output (2.7 W·kg−1), differentiated thermal and exercise contributions of STHA on exercise performance. Seventeen participants (10 STHA, 7 CON) completed graded exercise tests (GXTs) in cool (13 °C, 50% relative humidity (RH), pre-training) and hot conditions (32 °C, 60% RH, pre- and post-training), as well as 5-km time trials (TTs) in the heat, pre- and post-training. STHA reduced resting (p = 0.01) and exercising (p = 0.04) core temperature alongside a smaller change in thermal sensation (p = 0.04). Both groups improved the lactate threshold (LT, p = 0.021), lactate turnpoint (LTP, p = 0.005) and velocity at maximal oxygen consumption (vV̇O2max; p = 0.031) similarly. Statistical differences between training methods were observed in TT performance (STHA, −6.2(5.5)%; CON, −0.6(1.7)%, p = 0.029) and total running time during the GXT (STHA, +20.8(12.7)%; CON, +9.8(1.2)%, p = 0.006). There were large mean differences in change in maximal oxygen consumption between STHA +4.0(2.2) mL·kg−1·min−1 (7.3(4.0)%) and CON +1.9(3.7) mL·kg−1·min−1 (3.8(7.2)%). Running economy (RE) deteriorated following both training programmes (p = 0.008). Similarly, RE was impaired in the cool GXT, relative to the hot GXT (p = 0.004). STHA improved endurance running performance in comparison with work-matched normothermic training, despite equality of adaptation for typical determinants of performance (LT, LTP, vV̇O2max). Accordingly, these data highlight the ergogenic effect of STHA, potentially via greater improvements in maximal oxygen consumption and specific thermoregulatory and associated thermal perception adaptations absent in normothermic training.

Prolonged self-paced exercise in the heat - environmental factors affecting performance

In this review we examine how self-paced performance is affected by environmental heat stress factors during cycling time trial performance as well as considering the effects of exercise mode and heat acclimatization. Mean power output during prolonged cycling time trials in the heat (≥30°C) was on average reduced by 15% in the 14 studies that fulfilled the inclusion criteria. Ambient temperature per se was a poor predictor of the integrated environmental heat stress and 2 of the prevailing heat stress indices (WBGT and UTCI) failed to predict the environmental influence on performance. The weighing of wind speed appears to be too low for predicting the effect for cycling in trained acclimatized subjects, where performance may be maintained in outdoor time trials at ambient temperatures as high as 36°C (36°C UTCI; 28°C WBGT). Power output during indoor trials may also be maintained with temperatures up to at least 27°C when humidity is modest and wind speed matches the movement speed generated during outdoor cycling, whereas marked reductions are observed when air movement is minimal. For running, representing an exercise mode with lower movement speed and higher heat production for a given metabolic rate, it appears that endurance is affected even at much lower ambient temperatures. On this basis we conclude that environmental heat stress impacts self-paced endurance performance. However, the effect is markedly modified by acclimatization status and exercise mode, as the wind generated by the exercise (movement speed) or the environment (natural or fan air movement) exerts a strong influence.

The influence of a menthol and ethanol soaked garment on human temperature regulation and perception during exercise and rest in warm, humid conditions

This study assessed whether donning a garment saturated with menthol and ethanol (M/E) can improve evaporative cooling and thermal perceptions versus water (W) or nothing (CON) during low intensity exercise and rest in warm, humid conditions often encountered in recreational/occupational settings. It was hypothesised there would be no difference in rectal (Tre) and skin (Tsk) temperature, infra-red thermal imagery of the chest/back, thermal comfort (TC) and rating of perceived exertion (RPE) between M/E, W and CON, but participants would feel cooler in M/E versus W or CON.

METHODS

Six volunteers (mean [SD] 22 [4] years, 72.4 [7.4] kg and 173.6 [3.7] cm) completed (separate days) three, 60-min tests in 30°C, 70%rh, in a balanced order. After 15-min of seated rest participants donned a dry (CON) or 80mL soaked (M/E, W) long sleeve shirt appropriate to their intervention. They then undertook 30-min of low intensity stepping at a rate of 12steps/min on a 22.5cm box, followed by 15-min of seated rest. Measurements included heart rate (HR), Tre, Tsk (chest/back/forearm), thermal imaging (back/chest), thermal sensation (TS), TC and RPE. Data were reported every fifth minute as they changed from baseline and the area under the curves were compared by condition using one-way repeated measures ANOVA, with an alpha level of 0.05.

RESULTS

Tre differed by condition, with the largest heat storage response observed in M/E (p<0.05). Skin temperature at the chest/back/forearm, and thermal imaging of the chest all differed by condition, with the greatest rate of heat loss observed in W and M/E respectively (p<0.01). Thermal sensation differed by condition, with the coolest sensations observed in M/E (p<0.001). No other differences were observed.

CONCLUSIONS

Both M/E and W enhanced evaporative cooling compared CON, but M/E causes cooler sensations and a heat storage response, both of which are likely mediated by menthol.

Heat stress redistributes blood flow in arteries of the brain during dynamic exercise

We hypothesized that heat stress would decrease anterior and posterior cerebral blood flow (CBF) during exercise, and the reduction in anterior CBF would be partly associated with large increase in extracranial blood flow (BF). Nine subjects performed 40 min of semirecumbent cycling at 60% of the peak oxygen uptake in hot (35°C; Heat) and thermoneutral environments (25°C; Control). We evaluated BF and conductance (COND) in the external carotid artery (ECA), internal carotid artery (ICA), and vertebral artery (VA) using ultrasonography. During the Heat condition, ICA and VA BF were significantly increased 10 min after the start of exercise ( P < 0.05) and thereafter gradually decreased. ICA COND was significantly decreased ( P < 0.05), whereas VA COND remained unchanged throughout Heat. Compared with the Control, either BF or COND of ICA and VA at the end of Heat tended to be lower, but not significantly. In contrast, ECA BF and COND at the end of Heat were both higher than levels in the Control condition ( P < 0.01). During Heat, a reduction in ICA BF appears to be associated with a decline in end-tidal CO2 tension ( r = 0.84), whereas VA BF appears to be affected by a change in cardiac output ( r = 0.87). In addition, a change in ECA BF during Heat was negatively correlated with a change in ICA BF ( r = −0.75). Heat stress resulted in modification of the vascular response of head and brain arteries to exercise, which resulted in an alteration in the distribution of cardiac output. Moreover, a hyperthermia-induced increase in extracranial BF might compromise anterior CBF during exercise with heat stress.

Temperature and blood flow distribution in the human leg during passive heat stress

The ability of direct heat stress to increase limb blood flow is well known, but the magnitude and profile of hemodynamic responses within the major vessels of the leg have not been explored. Here, we systematically characterize these responses through a wide range of heat stress levels and show that isolated leg heating confers potentially beneficial hemodynamic changes equivalent to those of moderate whole body hyperthermia, with these hemodynamic adjustments being predominantly driven by local temperature-sensitive mechanisms.

The influence of temperature on the hemodynamic adjustments to direct passive heat stress within the leg's major arterial and venous vessels and compartments remains unclear. Fifteen healthy young males were tested during exposure to either passive whole body heat stress to levels approaching thermal tolerance [core temperature (T_c) + 2°C; study 1; n = 8] or single leg heat stress (T_c + 0°C; study 2; n = 7). Whole body heat stress increased perfusion and decreased oscillatory shear index in relation to the rise in leg temperature (T_leg) in all three major arteries supplying the leg, plateauing in the common and superficial femoral arteries before reaching severe heat stress levels. Isolated leg heat stress increased arterial blood flows and shear patterns to a level similar to that obtained during moderate core hyperthermia (T_c + 1°C). Despite modest increases in great saphenous venous (GSV) blood flow (0.2 l/min), the deep venous system accounted for the majority of returning flow (common femoral vein 0.7 l/min) during intense to severe levels of heat stress. Rapid cooling of a single leg during severe whole body heat stress resulted in an equivalent blood flow reduction in the major artery supplying the thigh deep tissues only, suggesting central temperature-sensitive mechanisms contribute to skin blood flow alone. These findings further our knowledge of leg hemodynamic responses during direct heat stress and provide evidence of potentially beneficial vascular alterations during isolated limb heat stress that are equivalent to those experienced during exposure to moderate levels of whole body hyperthermia.

Relieving thermal discomfort: Effects of sprayed L-menthol on perception, performance, and time trial cycling in the heat

L-menthol stimulates cutaneous thermoreceptors and induces cool sensations improving thermal comfort, but has been linked to heat storage responses; this could increase risk of heat illness during self-paced exercise in the heat. Therefore, L-menthol application could lead to a discrepancy between behavioral and autonomic thermoregulatory drivers. Eight male participants volunteered. They were familiarized and then completed two trials in hot conditions (33.5 °C, 33% relative humidity) where their t-shirt was sprayed with CONTROL-SPRAY or MENTHOL-SPRAY after 10 km (i.e., when they were hot and uncomfortable) of a 16.1-km cycling time trial (TT). Thermal perception [thermal sensation (TS) and comfort (TC)], thermal responses [rectal temperature (Trec ), skin temperature (Tskin )], perceived exertion (RPE), heart rate, pacing (power output), and TT completion time were measured. MENTHOL-SPRAY made participants feel cooler and more comfortable and resulted in lower RPE (i.e., less exertion) yet performance was unchanged [TT completion: CONTROL-SPRAY 32.4 (2.9) and MENTHOL-SPRAY 32.7 (3.0) min]. Trec rate of increase was 1.40 (0.60) and 1.45 (0.40) °C/h after CONTROL-SPRAY and MENTHOL-SPRAY application, which were not different. Spraying L-menthol toward the end of self-paced exercise in the heat improved perception, but did not alter performance and did not increase heat illness risk.

Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports

Exercise heat acclimation induces physiological adaptations that improve thermoregulation, attenuate physiological strain, reduce the risk of serious heat illness, and improve aerobic performance in warm-hot environments and potentially in temperate environments. The adaptations include improved sweating, improved skin blood flow, lowered body temperatures, reduced cardiovascular strain, improved fluid balance, altered metabolism, and enhanced cellular protection. The magnitudes of adaptations are determined by the intensity, duration, frequency, and number of heat exposures, as well as the environmental conditions (i.e., dry or humid heat). Evidence is emerging that controlled hyperthermia regimens where a target core temperature is maintained, enable more rapid and complete adaptations relative to the traditional constant work rate exercise heat acclimation regimens. Furthermore, inducing heat acclimation outdoors in a natural field setting may provide more specific adaptations based on direct exposure to the exact environmental and exercise conditions to be encountered during competition. This review initially examines the physiological adaptations associated with heat acclimation induction regimens, and subsequently emphasizes their application to competitive athletes and sports.

Restoring heat stress-associated reduction in middle cerebral artery velocity does not reduce fatigue in the heat

Heat-induced hyperventilation may reduce PaCO2 and thereby cerebral perfusion and oxygenation and in turn exercise performance. To test this hypothesis, eight volunteers completed three incremental exercise tests to exhaustion: (a) 18 °C ambient temperature (CON); (b) 38 °C (HEAT); and (c) 38 °C with addition of CO2 to inspiration to prevent the hyperventilation-induced reduction in PaCO2 (HEAT + CO2 ). In HEAT and HEAT + CO2 , rectal temperature was elevated prior to the exercise tests by means of hot water submersion and was higher (P < 0.05) than in CON. Compared with CON, ventilation was elevated (P < 0.01), and hence, PaCO2 reduced in HEAT. This caused a reduction (P < 0.05) in mean cerebral artery velocity (MCAvmean ) from 68.6 ± 15.5 to 53.9 ± 10.0 cm/s, which was completely restored in HEAT + CO2 (68.8 ± 5.8 cm/s). Cerebral oxygenation followed a similar pattern. V ˙ O 2   m a x was 4.6 ± 0.1 L/min in CON and decreased (P < 0.05) to 4.1 ± 0.2 L/min in HEAT and remained reduced in HEAT + CO2 (4.1 ± 0.2 L/min). Despite normalization of MCAvmean and cerebral oxygenation in HEAT + CO2 , this did not improve exercise performance, and thus, the reduced MCAvmean in HEAT does not seem to limit exercise performance.

Thermal and Cardiovascular Strain Mitigate the Potential Benefit of Carbohydrate Mouth Rinse During Self-Paced Exercise in the Heat

Purpose: To determine whether a carbohydrate mouth rinse can alter self-paced exercise performance independently of a high degree of thermal and cardiovascular strain.

Methods: Eight endurance-trained males performed two 40-km cycling time trials in 35°C, 60% RH while swilling a 20-ml bolus of 6.5% maltodextrin (CHO) or a color- and taste-matched placebo (PLA) every 5 km. Heart rate, power output, rectal temperature (T_re), and mean skin temperature (T_sk) were recorded continuously; cardiac output, oxygen uptake (VO₂), mean arterial pressure (MAP), and perceived exertion (RPE) were measured every 10 min.

Results: Performance time and mean power output were similar between treatments, averaging 63.9 ± 3.2 and 64.3 ± 2.8 min, and 251 ± 23 and 242 ± 18 W in CHO and PLA, respectively. Power output, stroke volume, cardiac output, MAP, and VO₂ decreased during both trials, increasing slightly or remaining stable during a final 2-km end-spurt. T_re, T_sk, heart rate, and RPE increased throughout exercise similarly with both treatments. Changes in RPE correlated with those in T_re (P < 0.005) and heart rate (P < 0.001).

Conclusions: These findings suggest that carbohydrate mouth rinsing does not improve ~1-h time trial performance in hot-humid conditions, possibly due to a failure in down-regulating RPE, which may be influenced more by severe thermal and cardiovascular strain.

Simulated Firefighting Task Performance and Physiology Under Very Hot Conditions

PURPOSE

To assess the impact of very hot (45°C) conditions on the performance of, and physiological responses to, a simulated firefighting manual-handling task compared to the same work in a temperate environment (18°C).

METHODS

Ten male volunteer firefighters performed a 3-h protocol in both 18°C (CON) and 45°C (VH). Participants intermittently performed 12 × 1-min bouts of raking, 6 × 8-min bouts of low-intensity stepping, and 6 × 20-min rest periods. The area cleared during the raking task determined work performance. Core temperature, skin temperature, and heart rate were measured continuously. Participants also periodically rated their perceived exertion (RPE) and thermal sensation. Firefighters consumed water ad libitum. Urine specific gravity (USG) and changes in body mass determined hydration status.

RESULTS

Firefighters raked 19% less debris during the VH condition. Core and skin temperature were 0.99 ± 0.20 and 5.45 ± 0.53°C higher, respectively, during the VH trial, and heart rate was 14-36 beats.min(-1) higher in the VH trial. Firefighters consumed 2950 ± 1034 mL of water in the VH condition, compared to 1290 ± 525 in the CON trial. Sweat losses were higher in the VH (1886 ± 474 mL) compared to the CON trial (462 ± 392 mL), though both groups were hydrated upon protocol completion (USG < 1.020). Participants' average RPE was higher in the VH (15.6 ± 0.9) compared to the CON trial (12.6 ± 0.9). Similarly, the firefighers' thermal sensation scores were significantly higher in the VH (6.4 ± 0.5) compared to the CON trial (4.4 ± 0.4).

CONCLUSIONS

Despite the decreased work output and aggressive fluid replacement observed in the VH trial, firefighters' experienced increases in thermal stress, and exertion. Fire agencies should prioritize the health and safety of fire personnel in very hot temperatures, and consider the impact of reduced productivity on fire suppression efforts.