Heat Stress Test Research Articles

Relationship between heat storage and parameters of thermotolerance and fatigue in exertional heat stress

Background/Aim. The risk assessment of heat illness and fatigue development is essential in military service. The aim of our study was to investigate the relationship between heat storage and various psychophysiological parameters of heat stress, as well as potential peripheral markers of fatigue in soldiers performing exertional heat stress tests. Methods. Fifteen young, healthy, and unacclimatized men underwent an exertional heat stress test (EHST) with the submaximal workload in warm conditions (WBGT 29?C) in a climatic chamber. Every 10 min, the following parameters of thermotolerance were measured or calculated: core temperature (Tc), mean skin (Tsk) and body temperature (Tb), heart rate (HR), heat storage (HS), physiological strain index (PSI), as well as peripheral markers of fatigue [blood concentrations of ammonia, urea nitrogen (BUN), lactate dehydrogenase (LDH), cortisol and prolactin] and subjective parameters: thermal sensation (TS) and rate of perceived exertion (RPE). Results. Tolerance time varied from 45 to 75 min (mean 63 ? 7.7 min). Average values of Tc, Tb, and HR constantly increased during EHST, while Tsk reached the plateau after 10 min. Concentrations of all investigated peripheral markers of fatigue were significantly higher after EHST compared to baseline levels (31.47 ? 7.29 vs. 11.8 ? 1.11 ?mol/L for ammonia; 5.92 ? 0.73 vs. 4.69 ? 0.74 mmol/L for BUN, 187.27 ?2 8.49 vs.152.7 3? 23.39 U/L for LDH, 743.43 ? 206.19 vs. 558.79 ? 113.34 mmol/L for cortisol, and 418.08 ? 157.14 vs. 138.79 ? 92.83 ?IU/mL for prolactin). Conclusions. This study demonstrates the relationship between heat storage and Tc, HR, TS, and RPE, but also with PSI. Concentrations of cortisol and especially prolactin showed a significant correlation with parameters of thermotolerance.

Coral heat tolerance under variable temperatures: Effects of different variability regimes and past environmental history vs. current exposure

AbstractExposure to high‐frequency temperature variability often but not always enhances coral heat tolerance, raising the question of whether this depends on the type of variability regime and past vs. current exposure. We collected corals from a macrotidal, highly fluctuating temperature environment and preconditioned them to either constant or variable daily temperatures for ~ 1.5 yr. Corals were then exposed to three new temperature variability regimes for ~ 1 month (constant control, symmetric variability, and tidal variability) to assess the effect of short‐term environmental history, followed by a 12‐d heat stress test. Measurements of visual coral health, photophysiology, photosynthesis, respiration, and calcification rates showed that preconditioning to constant vs. variable temperatures for 1.5 yr did not significantly impact coral physiology and heat tolerance. In contrast, environmental history experienced in the month prior to the heat stress test significantly influenced the physiological responses, with corals exposed to both types of variability having lower heat tolerance. Interestingly, corals in the tidal variability regime suffered greater health declines than in the symmetric variability regime although both treatments had the same cumulative heat exposure. Since heating rate and temperature amplitude were higher in the tidal variability regime (but time spent above the bleaching threshold was shorter), this suggests that short, extreme heat pulses may be more deleterious than longer but more moderate ones, though other factors likely also played a role. Overall, our findings demonstrate that daily temperature variability has significant potential to alter coral heat tolerance but only certain types of variability may enhance coral adaptive capacity.

Exercise heat acclimation causes human responses and safety performance improvements.

Heat acclimation (HA) is a widely recognized physiological phenomenon of human body in hot environments. HA has many benefits, such aspreventing hyperthermia responses, and is an efficient way to improve human responses to hyperthermal environments. However, it is not known whether HA is dependent on the environmental conditions. Moreover, its mechanism and effect on the safety performance remain unexplored. In this study, we created a climate chamber to simulate a hyperthermal environment. Thirty healthy males were recruited for this study, who were then trained under the same ambient conditions (temperature of 38°C and relative humidity (RH) of 40%). The training involved running on treadmills (at 5km/h) to simulate heavy manual labor, and performing heat stress tests (HST) under six different conditions (32°C/40% RH, 35°C/40%, 38°C/40%, 32°C/70%, 35°C/70%, and 38°C/70%). Their physiological indices (rectal temperature, heart rate, sweat loss and skin temperature) and one psychological index (thermal sensation) were measured. Furthermore, a hazard avoidance test device (HATD) was designed to evaluate the individual safety performance by detecting human errors. The results show that training and environmental conditions have different effects on HA. After HA, the physiological and psychological strain were significantly improved. More importantly, HA also helped improve the participants' awareness of the dangers and required emergency responses to face potential hazards. Overall, a reasonable HA training under proper conditions is helpful to ensure the safety of human beings. More research is needed to study the role of HA on safety performance.

A 5-day Heat Acclimation Program Improves Heat Stress Indicators While Maintaining Exercise Capacity.

Wardenaar, FC, Ortega-Santos, CP, Vento, K, Beaumont, JS, Griffin, SC, Johnston, C, and Kavouras, SA. A 5-day heat acclimation program improves heat stress indicators while maintaining exercise capacity. J Strength Cond Res 35(5): 1279-1286, 2021-This study aimed to evaluate whether a daily 60 minutes isothermic biking protocol during a 5-day period could improve physiological heat acclimation and exercise performance capacity in partially acclimated subjects. A quasi-experimental study consisted of an intervention (INT, n = 7) and control (CON, n = 7) group completing 2 12 minutes Cooper tests (pre-CT on day 1 and post-CT on day 7) and a heat stress test (HST, on day 9). INT performed additional intensive exercise 1 hour per day on days 1-5, whereas CON did not. During CTs and HST, core temperature (Tc, telemetric capsule), skin temperature (Tsk, sensors at neck, right shoulder, left hand, and right shin), and heart rate (HR, chest strap) were continuously monitored and baseline, average, peak, and increment were calculated. During the HST, the INT group showed a smaller baseline-peak Tc increment (INT 0.88 ± 0.27 vs. CON 1.64 ± 0.90° C, p = 0.02), a lower HR peak (150.2 ± 12.6 vs. 173.0 ± 16.8 b·min-1, p = 0.02), and lower Tsk peak (36.47 ± 0.62 vs. 36.54 ± 0.46° C, p = 0.04). There was a nonsignificant, but practical difference based on a moderate effect size for change in pre-CT to post-CT performance of nearly +2.7 ± 12.3% in INT and -3.0 ± 8.5% in CON (p = 0.32 and d = 0.51), and HST distance covered resulting in a nonsignificant difference of 464 ± 849 m between INT and CON (p = 0.38 and d = 0.44). In conclusion a short-term 5-day heat acclimation program including 300 minutes of extra exercise resulted in positive physiological adaptions to heat stress, as indicated by lower core temperature and HR in comparison with a control group.

Individual characteristics associated with the magnitude of heat acclimation adaptations

PurposeThe 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.MethodsTwenty-four participants (9 female, 15 male; maximum oxygen uptake [dot{{V}}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).ResultsBody dimensions (p > 0.3) and dot{{V}}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). dot{{V}}O2peak,kg did not differ among levels of responsiveness for WBSR and HR (p > 0.3).ConclusionIndividual body dimensions influenced the magnitude of sudomotor and cardiovascular adaptive responses, but did not differentiate Tre adaptations to HA. The influence of dot{{V}}O2peak,kg on the magnitude of adaptations was limited.

Uticaj toplotnog stresa i fizičke aktivnosti na neke složene kognitivne funkcije

Introduction/Aim: Heat stress represents the important problem in military services. This study investigates the effects of exertional heat stress on complex cognitive functions. Methods: 40 male soldiers performed exertional heat stress test, of which 10 performed the test of the same intensity in cool environment. Cognitive functions were mesured by computerized battery CANTAB, before and immediately after exertional heat stress test. Results: Exertional heat stress led to impairment of some cognitive functions in unacclimatized group: decrease of accuracy in MTS test (from 92,6±4,2% towards 84,5±6,9%, p&lt;0,05) and PSRs (from 85,0±8,0% towards 77,0±9,6%, p&lt;0,05), while similar decreases were recorded in MTS test in passively acclimatized group (from 92,2±5,5% towards 87,7±5,6%, p&lt;0,05) i.e. in PSRs test in actively acclimatized group (from 83,3±6,3% towards 69,4±5,1%, p&lt;0,05). The reaction time was not affected in any group whatsoever. Discussion and conclusion: Exertional heat stress leads to mild impairment of complex cognitive functions, particularly in domain of accuracy. Physical strain itself, however, does not affect cognitive functions. Relatively resistance to heat stress in zoung soldiers may be contributed to their high aerobic level.

Short- and long-term effects of surface fires on heat stress protein content in Scots pine needles

Plants can minimise the damaging effects of high temperatures through numerous protective mechanisms; however, it is largely unknown how these mechanisms respond to extreme temperatures associated with wildfire. We investigated the effect of experimental burning (EB) on the accumulation of stress heat shock proteins (Hsps), which are one of the factors of thermotolerance in plants, in the needles of Scots pine (Pinus sylvestris L.). Previous fire exposure led not only to short- and long-term changes in the content of stress proteins in needles but also to changes in the accumulation of these proteins in response to reheating. The content of Hsp 101, Hsp 70 and Hsp 17.6 in the needles increased on the second day after EB (short-term effect of fire). Three years after EB, the content of Hsps in the fire-exposed needles was lower compared with the control needles. When these needles were subjected to the heat stress test at 45°C, the content of Hsps increased, whereas the content of Hsps in control needles decreased. Our results suggest that Scots pine needles retain a fairly long-term ‘stress memory’, expressed through proteomic defence mechanisms, to wildfire heat-induced damage.

Four-month operational heat acclimatization positively affects the level of heat tolerance 6\xa0months later

Benefits obtained after heat acclimation/acclimatization should be completely lost after an estimated period of 6 weeks. However, this estimate is still hypothetical. We evaluate the long-term effects of heat acclimatization on the level of heat tolerance. Physiological and subjective markers of heat tolerance were assessed during a heat stress test (HST: 3 × 8-min runs outdoors [~ 40 °C and 20% RH] at 50% of their estimated speed at VO2max) performed on the 2nd day upon arrival to the desert military base in the United Arab Emirates after a first day of mostly passive exposure to heat. Among the 50 male French soldiers, 25 partook in a 4-month military mission in countries characterized by a hot environment ~ 6 months prior to the study (HA). The other 25 participants were never heat acclimatized (CT). Rectal temperature (p = 0.023), heart rate (p = 0.033), and perceived exertion (p = 0.043) were lower in the HA than CT group at the end of HST. Soldiers who experienced a former 4-month period of natural heat acclimatization very likely had a higher level of heat tolerance during exercise in the heat, even 6 months after returning from the previous desert mission, than that of their non-acclimatized counterparts.

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

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. We examined whether the eccrine sweat glands' ion reabsorption rates improved following heat acclimation (HA) in older individuals. Ten healthy older adults (>65years) 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 7days' 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.

Sweat rate and sweat composition following active or passive heat re-acclimation: A pilot study

ABSTRACT The purpose of this study was to investigate local sweat rate (LSR) and sweat composition before and after active or passive heat re-acclimation (HRA). Fifteen participants completed four standardized heat stress tests (HST): before and after ten days of controlled hyperthermia (CH) heat acclimation (HA), and before and after five days of HRA. Each HST consisted of 35 min of cycling at 1.5W·kg−1 body mass (33°C and 65% relative humidity), followed by a graded exercise test. For HRA, participants were re-exposed to either CH (CH-CH, n = 6), hot water immersion (water temperature ~40°C for 40 min; CH-HWI, n = 5) or control (CH-CON, n = 4). LSR, sweat sodium, chloride, lactate and potassium concentrations were determined on the arm and back. LSR increased following HA (arm +18%; back +41%, P ≤ 0.03) and HRA (CH-CH: arm +31%; back +45%; CH-HWI: arm +65%; back +49%; CH-CON arm +11%; back +11%, P ≤ 0.021). Sweat sodium, chloride and lactate decreased following HA (arm 25–34; back 21–27%, P < 0.001) and HRA (CH-CH: arm 26–54%; back 20–43%; CH-HWI: arm 9–49%; back 13–29%; CH-CON: arm 1–3%, back 2–5%, P < 0.001). LSR increases on both skin sites were larger in CH-CH and CH-HWI than CH-CON (P ≤ 0.010), but CH-CH and CH-HWI were not different (P ≥ 0.148). Sweat sodium and chloride conservation was larger in CH-CH than CH-HWI and CH-CON on the arm and back, whilst CH-HWI and CH-CON were not different (P ≥ 0.265). These results suggest that active HRA leads to similar increases in LSR, but more conservation of sweat sodium and chloride than passive HRA. Abbreviations: ANOVA: Analysis of variance; ATP: Adenosine triphosphate; BSA (m2): Body surface area; CH: Controlled hyperthermia; CH-CH: Heat re-acclimation by controlled hyperthermia; CH-CON: Control group (no heat re-acclimation); CH-HWI: Heat re-acclimation by hot water immersion; CV (%): Coefficient of variation; dt (min): Duration of a stimulus; F: Female; GEE: Generalized estimating equations; HA: Heat acclimation; HRA : Heat re-acclimation; HST: Heat stress test; LSR (mg·cm−2·min−1) : Local sweat rate; LOD (mmol·L−1): Limit of detection; M: Male; (mg): Mass of x; RH (%): Relative humidity; RT: Recreationally trained; SA (cm2): Surface area; t (min): Time; T: Trained; Tsk (°C): Skin temperature; Tre (°C): Rectal temperature; USG : Urine specific gravity; VO2peak (mL·kg−1·min−1): Peak oxygen uptake; WBSL (L): Whole-body sweat loss; WBSR (L·h−1): Whole-body sweat rate

Influence of aerobic fitness on gastrointestinal barrier integrity and microbial translocation following a fixed-intensity military exertional heat stress test.

Exertional-heat stress adversely disrupts gastrointestinal (GI) barrier integrity, whereby subsequent microbial translocation (MT) can result in potentially serious health consequences. To date, the influence of aerobic fitness on GI barrier integrity and MT following exertional-heat stress is poorly characterised. Ten untrained (UT; VO2max = 45 ± 3ml·kg-1·min-1) and ten highly trained (HT; VO2max = 64 ± 4ml·kg-1·min-1) males completed an ecologically valid (military) 80-min fixed-intensity exertional-heat stress test (EHST). Venous blood was drawn immediately pre- and post-EHST. GI barrier integrity was assessed using the serum dual-sugar absorption test (DSAT) and plasma Intestinal Fatty-Acid Binding Protein (I-FABP). MT was assessed using plasma Bacteroides/total 16S DNA. UT experienced greater thermoregulatory, cardiovascular and perceptual strain (p < 0.05) than HT during the EHST. Serum DSAT responses were similar between the two groups (p = 0.59), although Δ I-FABP was greater (p = 0.04) in the UT (1.14 ± 1.36ng·ml-1) versus HT (0.20 ± 0.29ng·ml-1) group. Bacteroides/Total 16S DNA ratio was unchanged (Δ; -0.04 ± 0.18) following the EHST in the HT group, but increased (Δ; 0.19 ± 0.25) in the UT group (p = 0.05). Weekly aerobic training hours had a weak, negative correlation with Δ I-FABP and Bacteroides/total 16S DNA responses. When exercising at the same absolute workload, UT individuals are more susceptible to small intestinal epithelial injury and MT than HT individuals. These responses appear partially attributable to greater thermoregulatory, cardiovascular, and perceptual strain.

Over high or low dietary protein levels depressed the growth, TOR signaling, apoptosis, immune and anti-stress of abalone Haliotis discus hannai

A 120-day feeding trial was conducted to investigate the effects of relative higher and lower dietary protein levels on the growth, immunity and anti-stress of abalone Haliotis discus hannai fed diets with 17.64% (low), 30.49% (normal) and 43.27% (high) of proteins, respectively. The results showed that compared with 30.49% of dietary protein, 17.64% and 43.27% of dietary protein levels significantly decreased the weight gain rate and the activities of α-amylase, trypsin, alanine aminotransferase and aspartate aminotransferase in the hepatopancreas and serum of abalone (P < 0.05). Abalone fed 30.49% of dietary protein had the highest activity of superoxidase, acid phosphatase, alkaline phosphatase, lysozyme and the total anti-oxidative capacity, and the lowest content of malondialdehyde in the serum and hepatopancreas (P < 0.05). The gene expressions of TOR, S6k, Bcl-2, IκB, NfκB, TNF-α and Nrf2 were significantly up-regulated in the group with 30.49% of dietary protein (P < 0.05). Pathological abnormalities in hepatocyte cells of abalone were found in the groups with 17.64% and 43.27% of dietary protein. Meanwhile, accumulative mortalities of abalone after the Vibrio parahaemolyticus challenge test and heat stress test were significantly increased within these two groups (P < 0.05). In conclusion, the excessive (43.27) or deficient (17.64) dietary protein levels depressed the growth and immunity of abalone. Combined with the stress tests results, 17.63% or 43.27% of dietary protein contents are not recommended to the abalone facing the stress of vibriosis or high-water temperature (≥28 °C).

Evidence for age-related differences in heat acclimatisation responsiveness.

What is the central question of this study? Repeated heat exposure during the summer months can enhance heat loss in humans (seasonal heat acclimatisation), but does the magnitude of that enhancement differ between young and older adults when assessed during passive heat exposure? What is the main finding and its importance? While seasonal heat acclimatisation enhanced evaporative heat loss (i.e. sweating) in both young and older adults, those improvements led to a greater reduction in body heat storage in older adults. These outcomes indicate that heat acclimatisation may confer greater thermoregulatory benefits with increasing age. Repeated heat exposure throughout summer can enhance heat loss in humans (seasonal heat acclimatisation), although the effect of ageing on those improvements remains unclear. We therefore sought to assess thermoregulatory function in young and older adults during environmental heat exposure prior to and following seasonal heat acclimatisation, hypothesizing that the magnitude of adaptation would be greater in older relative to young adults. To achieve this, 14 young (19-27years) and 10 older adults (55-72years), who resided in a temperate humid-continental climate, completed a 3h resting heat exposure (44°C, ∼30% relative humidity) in the winter-spring months as part of a larger investigation (pre-acclimatisation), before being re-evaluated using the same heat stress test following the summer months (post-acclimatisation). Whole-body dry and evaporative heat exchange, and metabolic rate were measured throughout using direct and indirect calorimetry (respectively), and used to quantify body heat storage (metabolic rate + dry heat gain - evaporative heat loss). Evaporative heat loss increased in both groups following acclimatisation, but those improvements led to a decrease in body heat storage in older (mean difference (95% CI); 213 (295, 131)kJ; P<0.001), but not young adults (-25 (-94, 44)kJ; P=0.458). Thus, body heat storage was greater in older compared to young adults before (222 (123, 314)kJ; P<0.001), but not following acclimatisation (34(-55, 123)kJ; P=0.433). Although there is a need for larger and more controlled confirmatory studies, our findings indicate that seasonal heat acclimatisation may induce greater thermoregulatory adaptation in older compared to young adults.

The Impact Of Short-Term Hot Water Immersion On Heat Acclimation And Thermotolerance

BACKGROUND: Environmental heat stress increases physiological strain during exercise in non-acclimated individuals. Heat acclimation (HA) protocols are often used as countermeasures to preserve physiological function during exercise in the heat. Passive heat strategies could be a potential method of HA that reduces excess physical exertion prior to activity or relocation. PURPOSE: To determine the effect of hot water immersion (HWI) on heat acclimation and thermotolerance. METHODS: 6 males (Age: 23.8 ± 1.5; VO2max: 45.0 ± 7.5 mL/kg/min.) participated in a crossover, counterbalanced study with a four-week washout between conditions. Heat stress tests (HST) were performed PRE and POST acclimation sessions and consisted of 45 min of cycling at 50% of VO2max in 40 °C, 40% RH. Acclimation sessions were either three consecutive bouts of HWI or traditional heat-exercise training (TRAD). HWI sessions consisted of 40 min of submersion at 40 °C. TRAD sessions consisted of 40 min of cycling at 50% VO2max in 40 °C, 40% RH. Core body temperature (Tcore), heart rate (HR), rate of perceived exertion (RPE), and thermal sensation (TSS) were recorded during HSTs. Blood was drawn PRE and POST HST to determine change in plasma volume. Nude body mass was recorded before and after HSTs to calculate whole body sweat loss (WBSL). Tcore and HR were used to calculate physiological strain index (PSI). RESULTS: HWI decreased average HR (PRE: 158 ± 7; POST: 149 ± 7; p = < 0.05), peak HR (176 ± 7; 164 ± 5; p = < 0.05), and end exercise RPE (15.3 ± 1.8; 13.2 ± 1.9; p = < 0.05). HWI had no significant effect on resting TCore (37.2 ± 0.4; 36.8 ± 0.3; p = 0.66), end exercise TCore (38.7 ± 0.4; 38.4 ± 0.3; p = 0.20), PSI (7.8 ± 1.1; 7.0 ± 0.8; p = 0.56), or TSS (10.8 ± 1.0; 9.5 ± 1.2; p = 0.09). TRAD resulted in no significant changes in average HR (PRE: 153 ± 11; POST: 149 ± 11; p = 0.82), peak HR (170 ± 9; 165 ± 10; p = 0.51), end exercise RPE (15.1 ± 1.2; 13.3 ± 2.0; p = 0.87), PSI (9.3 ± 2.1; 8.6 ± 1.4; p = 0.73), or TSS (10.5 ± 0.8; 10.7 ± 1.5; p = 0.77). Plasma volume expansion (%) was observed in both groups (HWI: 6.6 ± 6.4; TRAD: 4.3 ± 2.0; p = 0.41). CONCLUSION: Three consecutive HWI sessions are effective in lowering HR during submaximal exercise in the heat. Compared to heat-exercise exposures, HWI is a method in which to more rapidly elicit a HA phenotype.

Mixed-Mode Heat Training: A Practical Alternative for Enhancing Aerobic Capacity in Team Sports

Purpose: Heat training can be implemented to obtain performance improvements in hot and temperate environments. However, the effectiveness of these interventions for team sports during discrete periods of the season remains uncertain.Methods: We compared the effects of a short pre-season heat training intervention on fitness and thermal tolerance. In a counterbalanced crossover design, eleven state-level male football players undertook 6 × 60 min sessions in HEAT (35°C, 50% RH) and TEMP (18°C, 50% RH) conditions over 12 days. Running performance pre- and post-intervention was assessed via the Yo-Yo Interment Recovery Test Level 1 (YYIR1), and thermal adaptation using a submaximal (4 × 4 min @ 9–13 km·h−1) treadmill heat stress test in 35°C, 50% RH.Results: Running distance increased by 9, ±9% in HEAT (standardized mean, ±90% confidence limits) and 13, ±6% in TEMP, the difference in the mean change between conditions was unclear (0.24, ±0.64 standardized mean, ±90% confidence limits). Irrespective of training interventions, there was an order effect indicated by a substantial 476 ± 168 m increase in running distance between the first and final YYIR1 tests. There were trivial to small reductions in heart rate, blood lactate, RPE and thermal sensation after both interventions. Differences in mean core and skin temperature were unclear.Conclusions: Supplementary conditioning sessions in heat and temperate environments undertaken in addition to sports-specific field-based training were effective in enhancing player fitness during the pre-season. However, few clear differences between HEAT and TEMP conditions indicate conditioning in the heat appeared to offer no additional benefit to that of training in temperate conditions.

Cycling-based repeat sprint training in the heat enhances running performance in team sport players

Applying heat training interventions in a team sports setting remains challenging. This study investigated the effects of integrating short-term, repeat sprint heat training with passive heat exposure on running performance and general conditioning in team sport players. Thirty male club-level Australian Football players were assigned randomly to: Passive + Active Heat (PAH; n = 10), Active Heat (AH; n = 10) or Control (CON; n = 10) to complete 6 × 40 min high-intensity cycling training sessions over 12 days in 35°C (PAH and AH) or 18°C (CON), 50% RH in parallel with mid-season sports-specific training and games. Players in PAH were exposed to 20 min pre-exercise passive heat. Physiological adaptation and running capacity were assessed via a treadmill submaximal heat stress test followed by a time-to-exhaustion run in 35°C, 50% RH. Running capacity increased by 26% ± 8% PAH (0.88, ±0.23; standardised mean, ± 90% confidence limits), 29% ± 12% AH (1.23, ±0.45) and 10% ± 11% CON (0.45, ±0.48) compared with baseline. Both PAH (0.52, ±0.42; standardised mean, ± 90% confidence limits) and AH (0.35, ±0.57) conditions yielded a greater improvement in running capacity than CON. Physiological and perceptual measures remained relatively unchanged between baseline and post-intervention heat stress tests, within and between conditions. When thermal adaptation is not a direct priority, short-term, repeat effort high-intensity cycling in hot conditions combined with sports-specific training can further enhance running performance in team sport players. Six heat exposures across 12-days should improve running performance while minimising lower limb load and cumulative fatigue for team sports players.

A basal heat stress test to detect military operational readiness after a 14-day operational heat acclimatization period

ABSTRACT A basal heat stress test (HST) to predict the magnitude of adaptive responses during heat acclimatization (HA) would be highly useful for the armed forces. The aim was to identify physiological markers assessed during a HST (three 8-min running sets at 50% of the speed at VO2max) performed just before a 14-day HA period that would identify participants still at “risk” at the end of HA. Individuals that responded poorly (large increases in rectal temperature [Trec] and heart rate [HR]) during the initial HST were more likely to respond favorably to HA (large reductions in Trec and HR). However, they were also more likely to exhibit lower tolerance to HST at D15. Basal Trec was found to efficiently discriminate participants showing a Trec > 38.5°C after HA, who are considered to be “at risk”. Finally, participants were classified by quartiles based on basal Trec and HR at the end of the HST and physiological strain index (PSI). Most of the participants “at risk” were among the upper quartile (i.e. the least tolerant) of Trec and PSI (p = 0.011 for both). Overall, these results show that the individuals who are less tolerant to a basal HST are very likely to benefit the most from HA but they also remain less tolerant to heat at the end of HA than those who better tolerated the basal HST. A basal HST could therefore theoretically help the command to select the most-ready personnel in hot conditions while retaining those who are less tolerant 6.

Effect of regular precooling on adaptation to training in the heat.

This study investigated whether regular precooling would help to maintain day-to-day training intensity and improve 20-km cycling time trial (TT) performed in the heat. Twenty males cycled for 10 day × 60min at perceived exertion equivalent to 15 in the heat (35°C, 50% relative humidity), preceded by no cooling (CON, n = 10) or 30-min water immersion at 22°C (PRECOOL, n = 10). 19 participants (n = 9 and 10 for CON and PRECOOL, respectively) completed heat stress tests (25-min at 60% [Formula: see text] and 20-km TT) before and after heat acclimation. Changes in mean power output (∆MPO, P = 0.024) and heart rate (∆HR, P = 0.029) during heat acclimation were lower for CON (∆MPO - 2.6 ± 8.1%, ∆HR - 7 ± 7bpm), compared with PRECOOL (∆MPO + 2.9 ± 6.6%, ∆HR - 1 ± 8bpm). HR during constant-paced cycling was decreased from the pre-acclimation test in both groups (P < 0.001). Only PRECOOL demonstrated lower rectal temperature (Tre) during constant-paced cycling (P = 0.002) and lower Tre threshold for sweating (P = 0.042). However, skin perfusion and total sweat output did not change in either CON or PRECOOL (all P > 0.05). MPO (P = 0.016) and finish time (P = 0.013) for the 20-km TT were improved in PRECOOL but did not change in CON (P = 0.052 for MPO, P = 0.140 for finish time). Precooling maintains day-to-day training intensity and does not appear to attenuate adaptation to training in the heat.

Reliability of gastrointestinal barrier integrity and microbial translocation biomarkers at rest and following exertional heat stress.

PurposeExertional heat stress adversely distrupts (GI) barrier integrity and, through subsequent microbial translocation (MT), negativly impacts health. Despite widespread application, the temporal reliability of popular GI barrier integity and MT biomarkers is poorly characterised.MethodFourteen males completed two 80‐min exertional heat stress tests (EHST) separated by 7–14 days. Venous blood was drawn pre, immediately‐ and 1‐hr post both EHSTs. GI barrier integrity was assessed using the serum Dual‐Sugar Absorption Test (DSAT), Intestinal Fatty‐Acid‐Binding Protein (I‐FABP) and Claudin‐3 (CLDN‐3). MT was assessed using plasma Lipopolysaccharide Binding Protein (LBP), total 16S bacterial DNA and Bacteroides DNA.ResultsNo GI barrier integrity or MT biomarker, except absolute Bacteroides DNA, displayed systematic trial order bias (p ≥ .05). I‐FABP (trial 1 = Δ 0.834 ± 0.445 ng ml−1; trial 2 = Δ 0.776 ± 0.489 ng ml−1) and CLDN‐3 (trial 1 = Δ 0.317 ± 0.586 ng ml−1; trial 2 = Δ 0.371 ± 0.508 ng ml−1) were increased post‐EHST (p ≤ .01). All MT biomarkers were unchanged post‐EHST. Coefficient of variation and typical error of measurement post‐EHST were: 11.5% and 0.004 (ratio) for the DSAT 90‐min postprobe ingestion; 12.2% and 0.004 (ratio) at 150‐min postprobe ingestion; 12.1% and 0.376 ng ml−1 for I‐FABP; 4.9% and 0.342 ng ml−1 for CLDN‐3; 9.2% and 0.420 µg ml−1 for LBP; 9.5% and 0.15 pg µl−1 for total 16S DNA; and 54.7% and 0.032 for Bacteroides/total 16S DNA ratio.ConclusionEach GI barrier integrity and MT translocation biomarker, except Bacteroides/total 16S ratio, had acceptable reliability at rest and postexertional heat stress.

Short-term isothermic heat acclimation elicits beneficial adaptations but medium-term elicits a more complete adaptation

PurposeTo investigate the effects of 60 min daily, short-term (STHA) and medium-term (MTHA) isothermic heat acclimation (HA) on the physiological and perceptual responses to exercise heat stress.MethodsSixteen, ultra-endurance runners (female = 3) visited the laboratory on 13 occasions. A 45 min sub-maximal (40% Wmax) cycling heat stress test (HST) was completed in the heat (40 °C, 50% relative humidity) on the first (HSTPRE), seventh (HSTSTHA) and thirteenth (HSTMTHA) visit. Participants completed 5 consecutive days of a 60 min isothermic HA protocol (target Tre 38.5 °C) between HSTPRE and HSTSTHA and 5 more between HSTSTHA and HSTMTHA. Heart rate (HR), rectal (Tre), skin (Tsk) and mean body temperature (Tbody), perceived exertion (RPE), thermal comfort (TC) and sensation (TS) were recorded every 5 min. During HSTs, cortisol was measured pre and post and expired air was collected at 15, 30 and 45 min.ResultsAt rest, Tre and Tbody were lower in HSTSTHA and HSTMTHA compared to HSTPRE, but resting HR was not different between trials. Mean exercising Tre, Tsk, Tbody, and HR were lower in both HSTSTHA and HSTMTHA compared to HSTPRE. There were no differences between HSTSTHA and HSTMTHA. Perceptual measurements were lowered by HA and further reduced during HSTMTHA.ConclusionA 60 min a day isothermic STHA was successful at reducing physiological and perceptual strain experienced when exercising in the heat; however, MTHA offered a more complete adaptation.