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.

Heat-stress memory enhances the acclimation of a migratory insect pest to global warming.

In the face of rising global temperatures, the mechanisms behind an organism's ability to acclimate to heat stress remain enigmatic. The rice leaf folder, Cnaphalocrocis medinalis, traditionally viewed as temperature-sensitive, paradoxically exhibits robust larval acclimation to heat stress. This study used the heat-acclimated strain HA39, developed through multigenerational exposure to 39°C during the larval stage, and the unacclimated strain HA27 reared at 27°C to unravel the transgenerational effects of heat acclimation and its regulatory mechanisms. Heat acclimation for larvae incurred a fitness cost in pupae when exposed to high temperature, yet a significant transgenerational effect surfaced, revealing heightened fitness benefit in pupae from HA39, even without additional heat exposure during larval recovery at 27°C. This transgenerational effect exhibited a short-term memory, diminishing after two recovery generations. Moreover, the effect correlated with increased superoxide dismutase (SOD) enzyme activity and expression levels of oxidoreductase genes, representing physiological and molecular foundations of heat acclimation. Heat-acclimated larvae displayed elevated DNA methylation levels, while pupae from HA39, in recovery generations, exhibited decreased methylation indicated by the upregulation of a demethylase gene and downregulation of two methyltransferase genes at high temperatures. In summary, heat acclimation induces DNA methylation, orchestrating heat-stress memory and influencing the expression levels of oxidoreductase genes and SOD activity. Heat-stress memory enhances the acclimation of the migratory insect pest to global warming.

Multiomics analyses reveal the central role of the nucleolus and its machinery during heat stress acclimation in Pinus radiata.

Global warming is causing rapid changes in mean annual temperature and more severe drought periods. These are major contributors of forest dieback, which is becoming more frequent and widespread. In this work, we investigated how the transcriptome of Pinus radiata changed during initial heat stress response and acclimation. To this end, we generated a high-density dataset employing Illumina technology. This approach allowed us to reconstruct a needle transcriptome, defining 12 164 and 13 590 transcripts as down- and up-regulated, respectively, during a time course stress acclimation experiment. Additionally, the combination of transcriptome data with other available omics layers allowed us to determine the complex inter-related processes involved in the heat stress response from the molecular to the physiological level. Nucleolus and nucleoid activities seem to be a central core in the acclimating process, producing specific RNA isoforms and other essential elements for anterograde-retrograde stress signaling such as NAC proteins (Pra_vml_051671_1 and Pra_vml_055001_5) or helicase RVB. These mechanisms are connected by elements already known in heat stress response (redox, heat-shock proteins, or abscisic acid-related) and with others whose involvement is not so well defined such as shikimate-related, brassinosteriods, or proline proteases together with their potential regulatory elements. This work provides a first in-depth overview about molecular mechanisms underlying the heat stress response and acclimation in P. radiata.

Quantitative Proteomics Provided Insights into the Protective Effects of Heat Acclimation on the Rat Hypothalamus after Exertional Heatstroke.

BACKGROUND: The effects of heat acclimation (HA) on the hypothalamus after exertional heatstroke (EHS) and the specific mechanism have not been fully elucidated, and this study aimed to address these questions. METHODS: In the present study, rats were randomly assigned to the control, EHS, HA, or HA + EHS groups (n = 9). Hematoxylin and eosin (H&E) staining was used to examine pathology. Tandem mass tag (TMT)-based proteomic analysis was utilized to explore the impact of HA on the protein expression profile of the hypothalamus after EHS. Bioinformatics analysis was used to predict the functions of the differentially expressed proteins. The differential proteins were validated by western blotting. An enzyme-linked immunosorbent assay was used to measure the expression levels of inflammatory cytokines in the serum. RESULTS: The H&E staining (n = 5) results revealed that there were less structural changes in hypothalamus in the HA + EHS group compared with the EHS group. Proteomic analysis (n = 4) revealed that proinflammatory proteins such as argininosuccinate synthetase (ASS1), high mobility group protein B2 (HMGB2) and vimentin were evidently downregulated in the HA + EHS group. The levels of interleukin (IL)-1ß, IL-1, and IL-8 were decreased in the serum samples (n = 3) from HA + EHS rats. CONCLUSIONS: HA may alleviate hypothalamic damage caused by heat attack by inhibiting inflammatory activities, and ASS1, HMGB2 and vimentin could be candidate factors involved in the exact mechanism.

Environmental heat stress offsets adaptation associated with carbohydrate periodization in trained male triathletes.

PURPOSE: Carbohydrate (CHO) intake periodization via the sleep low train low (SL-TL) diet-exercise model increases fat oxidation during exercise and may enhance endurance-training adaptation and performance. Conversely, training under environmental heat stress increases CHO oxidation, but the potential of combined SL-TL and heat stress to enhance metabolic and performance outcomes is unknown. METHODS: Twenty-three endurance-trained males were randomly assigned to either control (n = 7, CON), SL-TL (n = 8, SLTemp ) or SL-TL + heat stress (n = 8, SLHeat ) groups and prescribed identical 2-week cycling training interventions. CON and SLTemp completed all sessions at 20°C, but SLHeat at 35°C. All groups consumed matched CHO intake (6 g·kg-1 ·day-1 ) but timed differently to promote low CHO availability overnight and during morning exercise in both SL groups. Submaximal substrate utilization was assessed (at 20°C), and 30-min performance tests (at 20 and 35°C) were performed Pre-, Post-, and 1-week post-intervention (Post+1). RESULTS: SLTemp improved fat oxidation rates at 60% MAP (~66% VO2peak ) at Post+1 compared with CON (p < 0.01). Compared with SLTemp , fat oxidation rates were significantly lower in SLHeat at Post (p = 0.02) and Post+1 (p < 0.05). Compared with CON, performance was improved at Post in SLTemp in temperate conditions. Performance was not different between any groups or time points in hot conditions. CONCLUSION: SL-TL enhanced metabolic adaptation and performance compared with CON and combined SL-TL and heat stress. Additional environmental heat stress may impair positive adaptations associated with SL-TL.

Glucose Dehydrogenases-Mediated Acclimation of an Important Rice Pest to Global Warming.

Global warming is posing a threat to animals. As a large group of widely distributed poikilothermal animals, insects are liable to heat stress. How insects deal with heat stress is worth highlighting. Acclimation may improve the heat tolerance of insects, but the underlying mechanism remains vague. In this study, the high temperature of 39 °C was used to select the third instar larvae of the rice leaf folder Cnaphalocrocis medinalis, an important insect pest of rice, for successive generations to establish the heat-acclimated strain (HA39). The molecular mechanism of heat acclimation was explored using this strain. The HA39 larvae showed stronger tolerance to 43 °C than the unacclimated strain (HA27) persistently reared at 27 °C. The HA39 larvae upregulated a glucose dehydrogenase gene, CmGMC10, to decrease the reactive oxygen species (ROS) level and increase the survival rate under heat stress. The HA39 larvae maintained a higher activity of antioxidases than the HA27 when confronted with an exogenous oxidant. Heat acclimation decreased the H2O2 level in larvae under heat stress which was associated with the upregulation of CmGMC10. The rice leaf folder larvae may acclimate to global warming via upregulating CmGMC10 to increase the activity of antioxidases and alleviate the oxidative damage of heat stress.

Heat acclimation ameliorated heat stress-induced acute kidney injury and prevented changes in kidney macrophages and fibrosis.

Heatstroke can cause acute kidney injury (AKI), which reportedly progresses to chronic kidney disease. Kidney macrophages may be involved in such injury. Although heat acclimation (HA) provides thermal resilience, its renoprotective effect and mechanism remain unclear. To investigate heat stress-induced kidney injuries in mice and the mitigating effect of HA on them, male C57/BL6J mice were exposed to heat stress (40°C, 1 h) with or without 5-day HA (38°C, 3 h/day) prior to heat stress. Heat stress damaged kidney proximal tubules with an elevation of urinary kidney injury molecule-1. Kidney fibrosis was observed on day 7 and correlated with urinary kidney injury molecule-1 levels on day 3. Kidney resident macrophages decreased on day 1, whereas the number of infiltrating macrophages in the kidney did not change. Both subsets of macrophages polarized to the proinflammatory M1 phenotype on day 1; however, they polarized to the anti-inflammatory M2 phenotype on day 7. HA significantly ameliorated heat stress-induced proximal tubular damage and kidney fibrosis. HA substantially increased heat shock protein 70 expression in the tubules before heat stress and reduced the elevation of cleaved caspase-3 expression after heat stress. HA also induced heat shock protein 70 expression of resident macrophages and prevented heat stress-induced changes in both subsets of kidney macrophages. These results provide pathophysiological data supporting the renoprotective effect of HA. Further studies are needed to confirm that HA can prevent kidney damage due to heat stress in humans.NEW & NOTEWORTHY Heat stress could induce acute kidney injury. Although heat acclimation (HA) reportedly provides thermal tolerance, its effect on heat stress-induced kidney damage remains unclear. This study showed that 5-day HA ameliorates mouse kidney tubular damage and subsequent fibrosis caused by heat stress. It also demonstrated that HA enhances intracellular heat shock protein 70 expression in tubular cells and prevents a decrease in kidney resident macrophages, which explains the renoprotective effect of HA.

Cross-adaptation between heat and hypoxia: mechanistic insights into aerobic exercise performance.

Acclima(tiza)tion to heat or hypoxia enhances work capacity in hot and hypoxic environmental conditions, respectively; an acclimation response is considered to be mediated by stimuli-specific molecular/systemic adaptations and potentially facilitated by the addition of exercise sessions. Promising findings at the cellular level provided the impetus for recent studies investigating whether acclimation to one stressor will ultimately facilitate whole body performance when exercise is undertaken in a different environmental condition. The present critical Mini-Review examines the theory of cross-adaptation between heat and hypoxia with particular reference to the determinants of aerobic performance. Indeed, early functional adaptations (improved exercise economy and enhanced oxyhemoglobin saturation) succeeded by later morphological adaptations (increased hemoglobin mass) might aid acclimatized humans perform aerobic work in an alternative environmental setting. Longer-term acclimation protocols that focus on the specific adaptation kinetics (and further allow for the adaptation reversal) will elucidate the exact physiological mechanisms that might mediate gains in aerobic performance or explain the lack thereof.

Cuticular protein genes involve heat acclimation of insect larvae under global warming.

Cuticular proteins (CPs) play important roles in insect growth and development. However, it is unknown whether CPs are related to heat tolerance. Cnaphalocrocis medinalis, a serious pest of rice, occurs in summer and exhibits strong adaptability to high temperature, but the underlying mechanism is unclear. Here, the role of CP genes in heat acclimation was studied. Heat tolerance of the heat-acclimated larvae was significantly stronger than the unacclimated larvae. The cuticular protein content in the heat-acclimated larvae was higher than that of the unacclimated larvae. 191 presumed CP genes of C. medinalis (CmCPs) were identified. Expression patterns of 14 CmCPs were different between the heat acclimated (S39) and unacclimated (S27) larvae under heat stress. CmCPs were specifically expressed in epidermis and the head except CmCPR20 mainly expressed in Malpighian tubules. CmCPR20 was upregulated in S39 while downregulated in S27, but CmTweedle1 and CmCPG1 were upregulated in S27 and downregulated in S39. RNAi CmTweedle1 or CmCPG1 remarkably decreased heat tolerance and cuticular protein content of the heat-acclimated larvae but not the unacclimated larvae. RNAi CmCPR20 decreased heat tolerance and cuticular protein content of the unacclimated larvae but not the heat-acclimated larvae. CmTweedle1 and CmCPG1 genes involve heat acclimation of C. medinalis.

A crossover control study of three methods of heat acclimation on the magnitude and kinetics of adaptation.

NEW FINDINGS: What is the central question to the study? Are primary indices of heat adaptation (e.g., expansion of plasma volume and reduction in resting core temperature) differentially affected by the three major modes of short-term heat acclimation, that is, exercise in the heat, hot water immersion and sauna? What it the main finding and its importance? The three modes elicited typical adaptations expected with short-term heat acclimation, but these were not significantly different between modes. This comparison has not previously been made and highlights that individuals can expect similar adaptation to heat regardless of the mode used. ABSTRACT: Heat acclimation (HA) can improve heat tolerance and cardiovascular health. The mode of HA potentially impacts the magnitude and time course of adaptations, but almost no comparative data exist. We therefore investigated adaptive responses to three common modes of HA, particularly with respect to plasma volume. Within a crossover repeated-measures design, 13 physically active participants (five female) undertook four, 5-day HA regimes (60 min/day) in randomised order, separated by ≥4 weeks. Rectal temperature (Tre ) was clamped at neutrality via 36.6°C (thermoneutral) water immersion (TWI; i.e., control condition), or raised by 1.5°C via heat stress in 40°C water, sauna (55°C, 52% relative humidity), or exercise in humid heat (40°C, 52% relative humidity; ExH). Adaptation magnitude was assessed as the pooled response across days 4-6, while kinetics was assessed via the 6-day time series. Plasma volume expansion was similar in all heated conditions but only higher than TWI in exercise in the heat (ExH) (by 4%, P = 0.036). Approximately two-thirds of the expansion was attained within the initial 24 h and was moderately related to that present on day 6, regardless of HA mode (r = 0.560-0.887). Expansion was mediated by conservation of both sodium and albumin content, with little evidence for these having differential roles between modes (P = 0.706 and 0.320, respectively). Resting Tre decreased by 0.1-0.3°C in all heated conditions, and systolic blood pressure decreased by 4 mmHg, but not differentially between conditions (P ≥ 0.137). In conclusion, HA mode did not substantially affect the magnitude or rate of adaptation in key resting markers of short-term HA.

Developing extreme heat acclimation in Bemisia tabaci Mediterranean (Hemiptera: Aleyrodidae).

Bemisia tabaci Mediterranean (MED) is a highly invasive cryptic species complex found in the world's tropical, subtropical, and temperate regions. It is a severe pest of various crops and a vector of plant pathogenic viruses, particularly geminiviruses. Thermal acclimation of insects is a critical for the survival in unfavorable temperature condition. We observed that great survival rate of B. tabaci MED at the uncontrolled greenhouse which had fluctuating temperature condition (FTC) from 10°C to 60°C in spring and summer season. Our study showed that while B. tabaci MED reared under FTC for 10 weeks from April to June, its survival rate was gradually increased when heat shock was treated 50°C for 0.5 h. In contrast, the same heat shock treatment was lethal in the colony reared under constant temperature condition (CTC) at the controlled insectary. After being acclimated, the lethal temperatures LT50 , LT95 , and LT100 under CTC were 47.7°C, 50.1°C, and 50.3°C, whereas those under FTC were 59.8°C, 62.7°C, and 63.0°C, respectively. In addition, we observed that the transcript levels of three investigated heat shock protein (HSP) genes (hsp20, hsp70, and hsp90) were lower under FTC than under CTC. This study suggests that B. tabaci MED retains high heat acclimation ability, making it tolerant of extreme thermal conditions.

Taking the plunge: When is best for hot water immersion to complement exercise in heat and hypoxia.

This investigation assessed the psycho-physiological and performance effects of hot water immersion (HWI) implemented either before or after a repeated-sprint training in hypoxia (RSH) session conducted in the heat. Ten participants completed three RSH trials (3 × 10 × 5-s sprints), conducted at 40°C and simulated altitude of 3000 m. A 30-min monitoring period preceded and followed all exercise sessions. In PRE, the pre-exercise period was HWI, and the post-exercise period was seated rest in temperate conditions. This combination was reversed in POST. In CON, participants were seated in temperate conditions for both periods. Compared to CON, PRE elicited a reduction in power output during each repeated-sprint set (14.8-16.2%, all p < 0.001), and a significantly higher core temperature (Tc) during the pre-exercise period and throughout the exercise session (p < 0.001 and p = 0.025, respectively). In POST, power output and Tc until the end of exercise were similar to CON, with Tc higher at the conclusion of the post-exercise period (p < 0.001). Time across the entire protocol spent ≥38.5°C Tc was significantly longer in PRE (48.1 ± 22.5 min) than POST (31.0 ± 11.3 min, p = 0.05) and CON (15.8 ± 16.3 min, p < 0.001). Employing HWI following RSH conducted in the heat provides effective outcomes regarding physiological strain and cycling performance when compared to pre-exercise or no HWI.

The effect of heat acclimation on the sweat lactate concentration vs. sweat rate relationship.

It is well known that there is a high concentration of lactate in sweat. Interest in measuring sweat lactate has arisen from its potential role in several clinical and sport performance applications. However, the effect of heat acclimation on sweat lactate concentration is still under debate. This is partly because sweat lactate concentration is greatly affected by sweat rate, which is known to increase during heat acclimation. Thus, to better understand this issue it is necessary to account for sweat rate - which has not been done previously in the literature. Therefore, the purpose of the current study was to determine the effect of heat acclimation on the relationship between sweat rate vs. sweat lactate concentration. Six subjects completed a 7-day heat acclimation protocol. The daily 2-h exercise bout was split into three 40-min intervals with exercise intensity increasing with each successive interval. This was done to induce three different sweat rates to determine the sweat rate vs. sweat lactate concentration relationship before and after heat acclimation for each participant. A 2 (heat acclimation) x 3 (sweat rate) repeated measures ANOVA was conducted to determine statistical significance. There was a significant (p < 0.05) decrease in the grand mean sweat lactate concentration over the course of seven days of heat acclimation from 17.0 ± 5.0 to 11.3 ± 1.1 mmol/L (p < 0.05). A significant (p < 0.05) heat acclimation x sweat rate ordinal interaction was also found. The results of the current study show that heat acclimation significantly decreases the sweat lactate concentration. In addition, there was a significant ordinal interaction which suggests that the impact of sweat rate on sweat lactate concentration is decreased following heat acclimation.

Exercise during hot-water immersion in divers habituated to hot-dry and hot-wet conditions.

Purpose: Diving in warm water increases thermal risk during exercise compared to thermoneutral waters. The purpose of this study was to evaluate exercise endurance in warm- and hot-water conditions in divers habituated to wet or dry heat. Methods: Nineteen male divers completed this study at the Navy Experimental Diving Unit. Subjects were assigned DRY or WET heat habituation groups. The DRY group (n=9) cycled at 125-150W for one hour in a non-immersed condition (34.4˚C, 50%RH), while the WET group (n=10) cycled at 50W for one hour while immersed in 34.4˚C water. Exercise time to exhaustion was tested on an underwater cycle ergometer in 35.8˚C (WARM) and 37.2˚C (HOT) water at 50W. Core temperature (Tc) was continuously recorded and for all dives. Results: Time to exhaustion was reduced in HOT compared to WARM water (p ≺0.01) in both DRY (92.7 ± 41.6 minutes in 35.8°C vs. 43.4 ± 17.5 minutes in 37.2°C) and WET (95.9 ± 39.2 minutes in 35.8°C vs. 53.4 ± 27.5 minutes in 37.2°C) groups, but did not differ between groups (p=0.62). Rate of Tc rise was greater with higher water temperature (p ≺0.01), but was not different between groups (p=0.68). Maximum Tc (p=0.94 and p=0.95) and Tc change from baseline (p=0.38 and p=0.34) was not different between water temperatures or habituation group, respectively. Conclusion: Endurance decreased with increased water temperature but was not different between WET and DRY. Divers became exhausted at a similar core temperature during WARM- and HOT-water exercise. Mechanisms and applications of heat acclimation for warm-water diving should be further explored.

Insights into the role of alternative splicing in plant temperature response.

Alternative splicing occurs in all eukaryotic organisms. Since the first description of multiexon genes and the splicing machinery, the field has expanded rapidly, especially in animals and yeast. However, our knowledge about splicing in plants is still quite fragmented. Though eukaryotes show some similarity in the composition and dynamics of the splicing machinery, observations of unique plant traits are only starting to emerge. For instance, plant alternative splicing is closely linked to their ability to perceive various environmental stimuli. Due to their sessile lifestyle, temperature is a central source of information allowing plants to adjust their development to match current growth conditions. Hence, seasonal temperature fluctuations and day-night cycles can strongly influence plant morphology across developmental stages. Here we discuss the available data about temperature-dependent alternative splicing in plants. Given its fragmented state it is not always possible to fit specific observations into a coherent picture, yet it is sufficient to estimate the complexity of this field and the need of further research. Better understanding of alternative splicing as a part of plant temperature response and adaptation may also prove to be a powerful tool for both, fundamental and applied sciences.

The sweat glands' maximum ion reabsorption rates following heat acclimation in healthy older adults.

NEW FINDINGS: What is the central question to this study? Do the sweat glands' maximum ion reabsorption rates increase following heat acclimation in healthy older individuals and is this associated with elevated aldosterone concentrations? What is the main finding and its importance? Sweat gland maximum ion reabsorption rates improved heterogeneously across body sites, which occurred without any changes in aldosterone concentration following a controlled hyperthermic heat acclimation protocol in healthy older individuals. ABSTRACT: We examined whether the eccrine sweat glands' ion reabsorption rates improved following heat acclimation (HA) in older individuals. Ten healthy older adults (>65 years) completed a controlled hyperthermic (+0.9°C rectal temperature, Tre ) HA protocol for nine non-consecutive days. Participants completed a passive heat stress test (lower leg 42°C water submersion) pre-HA and post-HA to assess physiological regulation of sweat gland ion reabsorption at the chest, forearm and thigh. The maximum ion reabsorption rate was defined as the inflection point in the slope of the relation between galvanic skin conductance and sweat rate (SR). We explored the responses again after a 7-day decay. During passive heating, the Tb thresholds for sweat onset on the chest and forearm were lowered after HA (P < 0.05). However, sweat sensitivity (i.e. the slope), the SR at a given Tre and gross sweat loss did not improve after HA (P > 0.05). Any changes observed were lost during the decay. Pilocarpine-induced sudomotor responses to iontophoresis did not change after HA (P ≥ 0.801). Maximum ion reabsorption rate was only enhanced at the chest (P = 0.001) despite unaltered aldosterone concentration after HA. The data suggest that this adaptation is lost after 7 days' decay. The HA protocol employed in the present study induced partial adaptive sudomotor responses. Eccrine sweat gland ion reabsorption rates improved heterogeneously across the skin sites. It is likely that aldosterone secretion did not alter the chest sweat ion reabsorption rates observed in the older adults.

Heat acclimation enhances the cold-induced vasodilation response.

PURPOSE: It has been reported that the cold-induced vasodilation (CIVD) response can be trained using either regular local cold stimulation or exercise training. The present study investigated whether repeated exposure to environmental stressors, known to improve aerobic performance (heat and/or hypoxia), could also provide benefit to the CIVD response. METHODS: Forty male participants undertook three 10-day acclimation protocols including daily exercise training: heat acclimation (HeA; daily exercise training at an ambient temperature, Ta = 35 °C), combined heat and hypoxic acclimation (HeA/HypA; daily exercise training at Ta = 35 °C, while confined to a simulated altitude of ~ 4000 m) and exercise training in normoxic thermoneutral conditions (NorEx; no environmental stressors). To observe potential effects of the local acclimation on the CIVD response, participants additionally immersed their hand in warm water (35 °C) daily during the HeA/HypA and NorEx. Before and after the acclimation protocols, participants completed hand immersions in cold water (8 °C) for 30 min, followed by 15-min recovery phases. The temperature was measured in each finger. RESULTS: Following the HeA protocol, the average temperature of all five fingers was higher during immersion (from 13.9 ± 2.4 to 15.5 ± 2.5 °C; p = 0.04) and recovery (from 22.2 ± 4.0 to 25.9 ± 4.9 °C; p = 0.02). The HeA/HypA and NorEx protocols did not enhance the CIVD response. CONCLUSION: Whole-body heat acclimation increased the finger vasodilatory response during cold-water immersion, and enhanced the rewarming rate of the hand, thus potentially contributing to improved local cold tolerance. Daily hand immersion in warm water for 10 days during HeA/Hyp and NorEx, did not contribute to any changes in the CIVD response.

Intermittent post-exercise sauna bathing improves markers of exercise capacity in hot and temperate conditions in trained middle-distance runners.

PURPOSE: This study investigated whether intermittent post-exercise sauna bathing across three-weeks endurance training improves exercise heat tolerance and exercise performance markers in temperate conditions, compared to endurance training alone. The subsidiary aim was to determine whether exercise-heat tolerance would further improve following 7-Weeks post-exercise sauna bathing. METHODS: Twenty middle-distance runners (13 female; mean ± SD, age 20 ± 2 years, [Formula: see text]O2max 56.1 ± 8.7 ml kg-1 min-1) performed a running heat tolerance test (30-min, 9 km h-1/2% gradient, 40 °C/40%RH; HTT) and temperate (18 °C) exercise tests (maximal aerobic capacity [[Formula: see text]O2max], speed at 4 mmol L-1 blood lactate concentration ([La-]) before (Pre) and following three-weeks (3-Weeks) normal training (CON; n = 8) or normal training with 28 ± 2 min post-exercise sauna bathing (101-108 °C, 5-10%RH) 3 ± 1 times per week (SAUNA; n = 12). Changes from Pre to 3-Weeks were compared between-groups using an analysis of co-variance. Six SAUNA participants continued the intervention for 7 weeks, completing an additional HTT (7-Weeks; data compared using a one-way repeated-measures analysis of variance). RESULTS: During the HTT, SAUNA reduced peak rectal temperature (Trec; - 0.2 °C), skin temperature (- 0.8 °C), and heart rate (- 11 beats min-1) more than CON at 3-Weeks compared to Pre (all p < 0.05). SAUNA also improved [Formula: see text]O2max (+ 0.27 L-1 min-1; p = 0.02) and speed at 4 mmol L-1 [La-] (+ 0.6 km h-1; p = 0.01) more than CON at 3-Weeks compared to Pre. Only peak Trec (- 0.1 °C; p = 0.03 decreased further from 3-Weeks to 7-Weeks in SAUNA (other physiological variables p > 0.05). CONCLUSIONS: Three-weeks post-exercise sauna bathing is an effective and pragmatic method of heat acclimation, and an effective ergogenic aid. Extending the intervention to seven weeks only marginally improved Trec.

Sex differences in the physiological adaptations to heat acclimation: a state-of-the-art review.

Over the last few decades, females have significantly increased their participation in athletic competitions and occupations (e.g. military, firefighters) in hot and thermally challenging environments. Heat acclimation, which involves repeated passive or active heat exposures that lead to physiological adaptations, is a tool commonly used to optimize performance in the heat. However, the scientific community's understanding of adaptations to heat acclimation are largely based on male data, complicating the generalizability to female populations. Though limited, current evidence suggests that females may require a greater number of heat acclimation sessions or greater thermal stress to achieve the same magnitude of physiological adaptations as males. The underlying mechanisms explaining the temporal sex differences in the physiological adaptations to heat acclimation are currently unclear. Therefore, the aims of this state-of-the-art review are to: (i) present a brief yet comprehensive synthesis of the current female and sex difference literature, (ii) highlight sex-dependent (e.g. anthropometric, menstrual cycle) and sex-independent factors (e.g. environmental conditions, fitness) influencing the physiological and performance adaptations to heat acclimation, and (iii) address key avenues for future research.

Individual characteristics associated with the magnitude of heat acclimation adaptations.

PURPOSE: The magnitude of heat acclimation (HA) adaptations varies largely among individuals, but it remains unclear what factors influence this variability. This study compared individual characteristics related to fitness status and body dimensions of low-, medium-, and high responders to HA. METHODS: Twenty-four participants (9 female, 15 male; maximum oxygen uptake [[Formula: see text]O2peak,kg] 52 ± 9 mL kg-1 min-1) completed 10 daily controlled-hyperthermia HA sessions. Adaptations were evaluated by heat stress tests (HST; 35 min cycling 1.5 W  kg-1; 33 °C, 65% relative humidity) pre- and post-HA. Low-, medium-, and high responder groups were determined based on tertiles (n = 8) of individual adaptations for resting rectal temperature (Tre), exercise-induced Tre rise (ΔTre), whole-body sweat rate (WBSR), and heart rate (HR). RESULTS: Body dimensions (p > 0.3) and [Formula: see text]O2peak,kg (p > 0.052) did not differentiate low-, medium-, and high responders for resting Tre or ΔTre. High WBSR responders had a larger body mass and lower body surface area-to-mass ratio than low responders (83.0 ± 9.3 vs 67.5 ± 7.3 kg; 249 ± 12 vs 274 ± 15 cm2 kg-1, respectively; p < 0.005). Conversely, high HR responders had a smaller body mass than low responders (69.2 ± 6.8 vs 83.4 ± 9.4 kg; p = 0.02). [Formula: see text]O2peak,kg did not differ among levels of responsiveness for WBSR and HR (p > 0.3). CONCLUSION: Individual body dimensions influenced the magnitude of sudomotor and cardiovascular adaptive responses, but did not differentiate Tre adaptations to HA. The influence of [Formula: see text]O2peak,kg on the magnitude of adaptations was limited.