Search

Purpose: The rising frequency of extreme heat events poses an escalating threat of heat-related illnesses and fatalities, placing an additional strain on global healthcare systems. Whether the risk of heat-related issues is sex specific, particularly among the elderly, remains uncertain., Methods: 16 men and 15 women of similar age (69 ± 5 years) were exposed to an air temperature of 39.1 ± 0.3 °C and a relative humidity (RH) of 25.1 ± 1.9%, during 20 min of seated rest and at least 40 min of low-intensity (10 W) cycling exercise. RH was gradually increased by 2% every 5 min starting at minute 30. We measured sweat rate, heart rate, thermal sensation, and the rise in gastrointestinal temperature (Tgi) and skin temperature (Tsk)., Results: Tgi consistently increased from minute 30 to 60, with no significant difference between females and males (0.012 ± 0.004 °C/min vs. 0.011 ± 0.005 °C/min; p = 0.64). Similarly, Tsk increase did not differ between females and males (0.044 ± 0.007 °C/min vs. 0.038 ± 0.011 °C/min; p = 0.07). Females exhibited lower sweat rates than males (0.29 ± 0.06 vs. 0.45 ± 0.14 mg/m 2 /min; p < 0.001) in particular at relative humidities exceeding 30%. No sex differences in heart rate and thermal sensation were observed., Conclusion: Elderly females exhibit significantly lower sweat rates than their male counterparts during low-intensity exercise at ambient temperatures of 39 °C when humidity exceeds 30%. However, both elderly males and females demonstrate a comparable rise in core temperature, skin temperature, and mean body temperature, indicating similar health-related risks associated with heat exposure., (© 2024. The Author(s).)

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.

Critical environmental limits are environmental thresholds above which heat gain exceeds heat loss and body core temperature (T c ) cannot be maintained at equilibrium. Those limits can be represented as critical wet-bulb globe temperature (WBGT crit ), a validated index that represents the overall thermal environment. Little is known about WBGT crit at rest and during low-to-moderate intensity exercise, or sex differences in WBGT crit , in unacclimated young adults. The following hypotheses were tested: 1 ) WBGT crit progressively decreases as metabolic heat production (M net ) increases, 2 ) no sex differences in WBGT crit occur at rest, and 3 ) WBGT crit is lower during absolute-intensity exercise but higher at relative intensities in women than in men. Thirty-six participants [19 men (M)/17 women (W); 23 ± 4 yr] were tested at rest, during light, absolute-intensity exercise (10 W), or during moderate, relative-intensity exercise [30% maximal oxygen consumption (V̇o 2max )] in an environmental chamber. Dry-bulb temperature was clamped as relative humidity or ambient water vapor pressure was increased until an upward inflection was observed in T c (rectal or esophageal temperature). Sex-aggregated WBGT crit was lower during 10 W (32.9°C ± 1.7°C, P < 0.0001) and 30% V̇o 2max (31.6°C ± 1.1°C, P < 0.0001) exercise versus at rest (35.3°C ± 0.8°C), and lower at 30% V̇o 2max versus 10 W ( P = 0.01). WBGT crit was similar between sexes at rest (35.6°C ± 0.8°C vs. 35.0°C ± 0.8°C, P = 0.83), but lower during 10 W (31.9°C ± 1.7°C vs. 34.1°C ± 0.3°C, P < 0.01) and higher during 30% V̇o 2max (32.4°C ± 0.8°C vs. 30.8°C ± 0.9°C, P = 0.03) exercise in women versus men. These findings suggest that WBGT crit decreases as M net increases, no sex differences occur in WBGT crit at rest, and sex differences in WBGT crit during exercise depend on absolute versus relative intensities.

Introduction: The aim of this study was to compare the effectiveness of exercise versus hot water immersion heat reacclimation (HRA) protocols., Methods: Twenty-four participants completed a heat stress test (HST; 33°C, 65% RH), which involved cycling at a power output equivalent to 1.5 W·kg-1 for 35 min whereby thermophysiological variables were measured. This was followed by a graded exercise test until exhaustion. HST1 was before a 10-d controlled hyperthermia (CH) heat acclimation (HA) protocol and HST2 immediately after. Participants completed HST3 after a 28-d decay period without heat exposure and were then separated into three groups to complete a 5-d HRA protocol: a control group (CH-CON, n = 8); a hot water immersion group (CH-HWI, n = 8), and a controlled hyperthermia group (CH-CH, n = 8). This was followed by HST4., Results: Compared with HST1, time to exhaustion and thermal comfort improved; resting rectal temperature (Tre), end of exercise Tre, and mean skin temperature (Tsk) were lower; and whole body sweat rate (WBSR) was greater in HST2 for all groups (P < 0.05). After a 28-d decay, only WBSR, time to exhaustion, and mean Tsk returned to pre-HA values. Of these decayed variables, only WBSR was reinstated after HRA; the improvement was observed in both the CH-CH and the CH-HWI groups (P < 0.05)., Conclusion: The data suggest that HRA protocol may not be necessary for cardiovascular and thermal adaptations within a 28-d decay period, as long as a 10-d CH-HA protocol has successfully induced these physiological adaptations. For sweat adaptations, a 5-d CH or HWI-HRA protocol can reinstate the lost adaptations., (Copyright © 2021 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the American College of Sports Medicine.)

Background: Although the acquisition of heat acclimation (HA) is well-documented, less is known about HA decay (HAD) and heat re-acclimation (HRA). The available literature suggests 1 day of HA is lost following 2 days of HAD. Understanding this relationship has the potential to impact upon the manner in which athletes prepare for major competitions, as a HA regimen may be disruptive during final preparations (i.e., taper)., Objective: The aim of this systematic review and meta-analysis was to determine the rate of HAD and HRA in three of the main physiological adaptations occurring during HA: heart rate (HR), core temperature (T c ), and sweat rate (SR)., Data Sources: Data for this systematic review were retrieved from Scopus and critical review of the cited references., Study Selection: Studies were included when they met the following criteria: HA, HAD, and HRA (when available) were quantified in terms of exposure and duration. HA had to be for at least 5 days and HAD for at least 7 days for longitudinal studies. HR, T c , or SR had to be monitored in human participants., Study Appraisal: The level of bias in each study was assessed using the McMaster critical review form. Multiple linear regression techniques were used to determine the dependency of HAD in HR, T c , and SR from the number of HA and HAD days, daily HA exposure duration, and intensity., Results: Twelve studies met the criteria and were systematically reviewed. HAD was quantified as a percentage change relative to HA (0% = HA, 100% = unacclimated state). Adaptations in end-exercise HR decreased by 2.3% (P < 0.001) for every day of HAD. For end-exercise T c , the daily decrease was 2.6% (P < 0.01). The adaptations in T c during the HA period were more sustainable when the daily heat exposure duration was increased and heat exposure intensity decreased. The decay in SR was not related to the number of decay days. However, protracted HA-regimens seem to induce longer-lasting adaptations in SR. High heat exposure intensities during HA seem to evoke more sustained adaptations in SR than lower heat stress. Only eight studies investigated HRA. HRA was 8-12 times faster than HAD at inducing adaptations in HR and T c , but no differences could be established for SR., Limitations: The available studies lacked standardization in the protocols for HA and HAD., Conclusions: HAD and HRA differ considerably between physiological systems. Five or more HA days are sufficient to cause adaptations in HR and T c ; however, extending the daily heat exposure duration enhances T c adaptations. For every decay day, ~ 2.5% of the adaptations in HR and T c are lost. For SR, longer HA periods are related to better adaptations. High heat exposure intensities seem beneficial for adaptations in SR, but not in T c . HRA induces adaptations in HR and T c at a faster rate than HA. HRA may thus provide a practical and less disruptive means of maintaining and optimizing HA prior to competition.

There is a need for easy-to-use methods to assess training progress in sports and rehabilitation research. The present review investigated whether cardiac acceleration at the onset of physical exercise (HRonset) can be used as a monitoring variable. The digital databases of Scopus and PubMed were searched to retrieve studies investigating HRonset. In total 652 studies were retrieved. These articles were then classified as having emphasis on HRonset in a sports or rehabilitation setting, which resulted in 8 of 112 studies with a sports application and 6 of 68 studies with a rehabilitation application that met inclusion criteria. Two co-existing mechanisms underlie HRonset: feedforward (central command) and feedback (mechanoreflex, metaboreflex, baroreflex) control. A number of studies investigated HRonset during the first few seconds of exercise (HRonsetshort), in which central command and the mechanoreflex determine vagal withdrawal, the major mechanism by which heart rate (HR) increases. In subsequent sports and rehabilitation studies, interest focused on HRonset during dynamic exercise over a longer period of time (HRonsetlong). Central command, mechanoreflexes, baroreflexes, and possibly metaboreflexes contribute to HRonset during the first seconds and minutes of exercise, which in turn leads to further vagal withdrawal and an increase in sympathetic activity. HRonset has been described as the increase in HR compared with resting state (delta HR) or by exponential modeling, with measurement intervals ranging from 0-4 s up to 2 min. Delta HR was used to evaluate HRonsetshort over the first 4 s of exercise, as well as for analyzing HRonsetlong. In exponential modeling, the HR response to dynamic exercise is biphasic, consisting of fast (parasympathetic, 0-10 s) and slow (sympathetic, 1-4 min) components. Although available studies differed largely in measurement protocols, cross-sectional and longitudinal training studies showed that studies analyzing HRonset in relation to physical training primarily incorporated HRonsetlong. HRonsetlong slowed in athletes as well as in patients with a coronary disease, who have a relatively fast HRonsetlong. It is advised to include both HRonsetlong and HRonsetshort in further studies. The findings of this review suggest that HRonset is a potential tool for monitoring and titrating training in sports as well as in rehabilitation settings, particularly in patients with ventricular fibrillation. Monitoring HRonset in the early phase of training can help optimize the effectiveness of training and therapy. More research is needed to gain a better understanding of the mechanisms underlying HRonset in relation to their application in sports and rehabilitation settings.

There is concern that the values provided by devices using infrared thermometry in emergency departments (EDs) do not reflect body core temperature accurately. Evaluation of three thermometers commonly used in the ED. Two infrared ear thermometers and an infrared forehead thermometer were evaluated using 1) the Voltcraft IRS-350 calibration device, 2) comparing temperature values to a rectal end-exercise temperature (T-RECT) of 38.1°C in 12 participants, and 3) comparing temperature values to rectal temperature in 133 ED patients. Calibration across the human core temperature range revealed that the ear thermometers underestimated radiant temperature by 0.77 ± 0.39°C and 1.84 ± 0.26°C, respectively, whereas the forehead thermometer overestimated radiant temperature by 0.90 ± 0.51°C. After cycling exercise, all thermometers underestimated T-RECT (0.54 ± 0.27°C and 1.03 ± 0.48°C for the ear thermometers and 1.14 ± 0.38°C for the forehead thermometer). In the ED, the ear thermometers underestimated T-RECT by 0.31 ± 0.37°C and 0.46 ± 0.50°C, whereas the forehead thermometer exhibited a nonsignificant overestimation of 0.04 ± 0.46°C. If the threshold for fever in all systems had been set to 37.5°C instead of 38.0°C, the sensitivity and specificity of the systems for real fever (T-RECT ≥ 38°C) are, respectively, 71% and 96% (ear thermometer 1), 57% and 97% (ear thermometer 2), and 86% and 90% (forehead thermometer). We conclude that the investigated thermometers are not reliable as devices to measure radiant temperature, cannot be used to assess body core temperature during exercise, but may be used as a screening device, with 37.5°C as a threshold for fever in emergency care settings. [ABSTRACT FROM AUTHOR]

Heat acclimation (HA) protocols repeatedly expose individuals to heat stress. As HA is typically performed close to the pinnacle event, it is essential that the protocol does not compromise immune status, health, or wellbeing. The purpose of this study was to examine the effect of HA on resting salivary immunoglobulin-A (s-IgA) and salivary cortisol (s-cortisol), self-reported upper-respiratory tract symptoms, and self-reported wellness parameters. Seventeen participants (peak oxygen uptake 53.2 ± 9.0 mL·kg−1·min−1) completed a 10-day controlled-hyperthermia HA protocol, and a heat stress test both before (HST1) and after (HST2) HA (33°C, 65% relative humidity). Resting saliva samples were collected at HST1, day 3 and 7 of the HA protocol, HST2, and at 5 ± 1 days post-HA. Upper-respiratory tract symptom data were collected weekly from one week prior to HA until three weeks post HA, and wellness ratings were reported daily throughout HA. HA successfully induced physiological adaptations, with a lower end-exercise rectal temperature and heart rate and higher whole-body sweat rate at HST2 compared to HST1. In contrast, resting saliva flow rate, s-IgA concentration, s-cortisol concentration, and s-cortisol secretion rate remained unchanged (n = 11–14, P = 0.10–0.48). Resting s-IgA secretion rate increased by 39% from HST1 to HST2 (n = 14, P = 0.03). No changes were observed in self-reported upper respiratory tract symptoms and wellness ratings. In conclusion, controlled-hyperthermia HA did not negatively affect resting s-IgA and s-cortisol, self-reported upper-respiratory tract symptoms, and self-reported wellness parameters in recreational athletes. [ABSTRACT FROM AUTHOR]

Purpose: To determine the effect of thermal state and thermal comfort on cycling performance in the heat. Methods: Seven well-trained male triathletes completed 3 performance trials consisting of 60 min cycling at a fixed rating of perceived exertion (14) followed immediately by a 20-km time trial in hot (30°C) and humid (80% relative humidity) conditions. In a randomized order, cyclists either drank ambient-temperature (30°C) fluid ad libitum during exercise (CON), drank ice slurry (-1°C) ad libitum during exercise (ICE), or precooled with iced towels and ice slurry ingestion (15g/kg) before drinking ice slurry ad libitum during exercise (PC+ICE). Power output, rectal temperature, and ratings of thermal comfort were measured. Results: Overall mean power output was possibly higher in ICE (+1.4% ± 1.8% [90% confidence limit]; 0.4 > smallest worthwhile change [SWC]) and likely higher PC+ICE (+2.5% ± 1.9%; 1.5 > SWC) than in CON; however, no substantial differences were shown between PC+ICE and ICE (unclear). Time-trial performance was likely enhanced in ICE compared with CON (+2.4% ± 2.7%; 1.4 > SWC) and PC+ICE (+2.9% ± 3.2%; 1.9 > SWC). Differences in mean rectal temperature during exercise were unclear between trials. Ratings of thermal comfort were likely and very likely lower during exercise in ICE and PC+ICE, respectively, than in CON. Conclusions: While PC+ICE had a stronger effect on mean power output compared with CON than ICE did, the ICE strategy enhanced late-stage time-trial performance the most. Findings suggest that thermal comfort may be as important as thermal state for maximizing performance in the heat. [ABSTRACT FROM AUTHOR]

The environmental conditions during the Tokyo Olympic and Paralympic Games are expected to be challenging, which increases the risk for participating athletes to develop heat-related illnesses and experience performance loss. To allow safe and optimal exercise performance of Dutch elite athletes, the Thermo Tokyo study aimed to determine thermoregulatory responses and performance loss among elite athletes during exercise in the heat, and to identify personal, sports-related, and environmental factors that contribute to the magnitude of these outcomes. For this purpose, Dutch Olympic and Paralympic athletes performed two personalized incremental exercise tests in simulated control (15°C, relative humidity (RH) 50%) and Tokyo (32°C, RH 75%) conditions, during which exercise performance and (thermo)physiological parameters were obtained. Thereafter, athletes were invited for an additional visit to conduct anthropometric, dual-energy X-ray absorptiometry (DXA), and 3D scan measurements. Collected data also served as input for a thermophysiological computer simulation model to estimate the impact of a wider range of environmental conditions on thermoregulatory responses. Findings of this study can be used to inform elite athletes and their coaches on how heat impacts their individual (thermo)physiological responses and, based on these data, advise which personalized countermeasures (i.e. heat acclimation, cooling interventions, rehydration plan) can be taken to allow safe and maximal performance in the challenging environmental conditions of the Tokyo 2020 Olympic and Paralympic Games. [ABSTRACT FROM AUTHOR]

Purpose: The best way to apply precooling for endurance exercise in the heat is still unclear. The authors analyzed the effect of different preparation regimens on pacing during a 15-km cycling time trial in the heat. Methods: Ten male subjects completed four 15-km time trials (30°C), preceded by different preparation regimes: 10 min cycling (WARM-UP), 30 min scalp cooling of which 10 min was cycling (SC+WARM-UP), ice-slurry ingestion (ICE), and ice slurry ingestion + 30 min scalp cooling (SC+ICE). Results: No differences were observed in finish time and mean power output, although power output was lower for WARM-UP than for SC+ICE during km 13-14 (17 ± 16 and 19 ± 14 W, respectively) and for ICE during km 13(16± 16 W). Rectal temperature at the start of the time trial was lower for both ICE conditions (~36.7°C) than both WARM- UP conditions (~37.1°C) and remained lower during the first part of the trial. Skin temperature and thermal sensation were lower at the start for SC+ICE. Conclusions: The preparation regimen providing the lowest body-heat content and sensation of coolness at the start (SC+ICE) was most beneficial for pacing during the latter stages of the time trial, although overall performance did not differ. [ABSTRACT FROM AUTHOR]

Exercising in a hot and humid environment increases core body temperature, which may limit exercise performance. The risk of exercise-induced hyperthermia and associated performance decrement in Olympic sailing athletes is largely unknown. Therefore, this study aimed to compare thermoregulatory responses and performance indicators of elite sailors in a cool versus hot and humid environment. Seven elite athletes from four different Olympic sailing classes (Laser, RS:X, Finn, 470) performed cycling and/or rowing exercise in a cool (18°C) and hot (33°C) environment, while core body temperature (TC), skin temperature (TSK), exercise performance (covered distance), and rating of perceived exertion were measured continuously. TC increased significantly more in the hot environment (37.6 ± 0.2°C to 39.1 ± 0.1°C) compared to the cool environment (37.5 ± 0.1°C to 38.5 ± 0.2°C; p = 0.002), but the increase in TC between conditions differed substantially within individuals (range: 0.3°C - 0.9°C). Exercise performance decreased by 6.2 ± 2.9% in the hot environment (p = 0.013, range: 2.3%-9.5%), but more importantly, exercise performance was strongly inversely related to peak TC (R = −0.78, p = 0.039). Rating of perceived exertion (cool: 14.2 ± 0.6; hot: 13.9 ± 1.2) and increase in TSK (cool: 0.5 ± 1.0°C; hot: 0.9 ± 0.3°C) did not differ between conditions (p = 0.59 and p = 0.36, respectively). To conclude, a larger increase in TC and substantial exercise performance decrement were observed in the hot versus cool environment. As a further matter, large inter-individual differences were observed across athletes with an inverse relationship between TC and exercise performance, which stresses the importance of appropriate and personalized interventions to reduce thermoregulatory burden of elite sailors during exercise in the heat. [ABSTRACT FROM AUTHOR]

Background The efficacy of hair loss prevention by scalp cooling to prevent chemotherapy induced hair loss has been shown to be related to scalp skin temperature. Scalp skin temperature, however, is dependent not only on local cooling but also on the thermal status of the body. Objectives This study was conducted to investigate the effect of body temperature on scalp skin temperature. Methods We conducted experiments in which 13 healthy subjects consumed ice slurry to lower body temperature for 15 minutes after the start of scalp cooling and then performed two 12-minute cycle exercise sessions to increase body core temperature. Esophageal temperature ( Tes), rectal temperature ( Tre), mean skin temperature (eight locations, Tskin), and mean scalp temperature (five locations, Tscalp) were recorded. Results During the initial 10 minutes of scalp cooling, Tscalp decreased by >15 °C, whereas Tes decreased by 0.2 °C. After ice slurry ingestion, Tes, Tre, and Tskin were 35.8, 36.5, and 31.3 °C, respectively, and increased after exercise to 36.3, 37.3, and 33.0 °C, respectively. Tscalp was significantly correlated to Tes ( r = 0.39, P < 0.01): an increase of 1 °C in Tes corresponded to an increase of 1.6 °C in Tscalp. Conclusions Slight cooling of patients with an elevated body temperature during scalp cooling contributes to the decrease in scalp temperature and may improve the prevention of hair loss. This may be useful if the desired decrease of scalp temperature cannot be obtained by scalp cooling systems. [ABSTRACT FROM AUTHOR]

Elderly indoors can be subject to considerable heat strain during heat waves. We investigated if a short indoor acclimation (HA) program leads to improved resilience to heat. Although full HA takes about ten days, the main changes occur in the first days, leading to reduced heat strain with similar stress. This study investigates changes after 3 days HA in 8 elderly (>75 y) and 8 young (20–30 y) females. The pre-test (Monday) and post-test (Friday) was a 20-min 50 W cycle exercise test in 35 °C/40% relative humidity (RH); the indoor preparation program or HA consisting of one hour exercise in the same climate aimed at reaching a gastrointestinal temperature ( T gi ) of 38 °C. HA did not result in any changes in T gi , mean skin temperature, heart rate, weight loss or thermal sensation. The elderly felt more fatigued but evaporated 28% less sweat than the young group during the tests. We conclude that 3-day HA for one hour daily is insufficient to cause physiological benefits in young and elderly females. [ABSTRACT FROM AUTHOR]

The goal of this study was to investigate the effects of different durations of skin temperature manipulation on pacing patterns and performance during a 15-km cycling time trial. Nineteen well-trained men completed three 15-km cycling time trials in 18°C and 50% relative humidity with 4.5-km (short-heat), 9.0-km (long-heat) or without (control) radiant heat exposure applied by infrared heaters after 1.5 km in the time trial. During the time trials, power output, mean skin temperature, rectal temperature, heart rate and rating of perceived exertion were assessed. The radiant heat exposure resulted in higher mean skin temperature during the time trial for short-heat (35.0 ± 0.6°C) and long-heat (35.3 ± 0.5°C) than for control (32.5 ± 1.0°C;P < 0.001), whereas rectal temperature was similar (P = 0.55). The mean power output was less for short-heat (273 ± 8 W;P = 0.001) and long-heat (271 ± 9 W;P = 0.02) than for control (287 ± 7 W), but pacing patterns did not differ (P = 0.55). Heart rate was greatest in control (177 ± 9 beats · min−1;P < 0.001), whereas the rating of perceived exertion remained similar. We concluded that a radiant heat exposure and associated higher skin temperature reduced overall performance, but did not modify pacing pattern during a 15-km cycling time trial, regardless of the duration of the exposure. [ABSTRACT FROM PUBLISHER]

Compared with the induction of heat acclimation (HA), studies investigating the decay and re-induction of HA (RA) are relatively sparse and have yielded conflicting results. Therefore, 16 semi-nude men were acclimated to dry-heat by undertaking an exercise protocol in a hot chamber (dry-bulb temperature 46.1 ± 0.1°C; relative humidity 17.9 ± 0.1%) on 10 consecutive days (HA1–10) in winter UK. Thereafter, the subjects were divided into two groups and re-exposed to the work-in-heat tests after 12 and 26 days until RA was attained (RA12, n = 8; RA26, n = 8). The exercise protocol consisted of 60 min of treadmill walking (1.53 m s−1) at an incline individually set to induce a rectal temperature ( T re) of ∼38.5°C during HA1 (equating to 45 ± 4% peak oxygen uptake), followed by 10 min of rest and 40 min of further treadmill exercise, the intensity of which was increased across HA to maintain T re at ∼38.5°C. T re, mean skin temperature, heart rate and rate of total water loss measured at 60 min did not change after HA7, and HA was taken as the mean of the responses during HA8–10. For both groups, there was no decay in T re and for all measured variables RA was attained after 2 and 4 days in RA12 and RA26, respectively. It is concluded that once adaptation to heat has been attained, the time that individuals may spend in cooler conditions before returning to a hot environment could be as long as one month, without the need for extensive re-adaptation to heat. [ABSTRACT FROM AUTHOR]

Compared with the induction of heat acclimation (HA), studies investigating the decay and re-induction of HA (RA) are relatively sparse and have yielded conflicting results. Therefore, 16 semi-nude men were acclimated to dry-heat by undertaking an exercise protocol in a hot chamber (dry-bulb temperature 46.1 +/- 0.1 degrees C; relative humidity 17.9 +/- 0.1%) on 10 consecutive days (HA1-10) in winter UK. Thereafter, the subjects were divided into two groups and re-exposed to the work-in-heat tests after 12 and 26 days until RA was attained (RA(12), n = 8; RA(26), n = 8). The exercise protocol consisted of 60 min of treadmill walking (1.53 m s(-1)) at an incline individually set to induce a rectal temperature (T (re)) of approximately 38.5 degrees C during HA1 (equating to 45 +/- 4% peak oxygen uptake), followed by 10 min of rest and 40 min of further treadmill exercise, the intensity of which was increased across HA to maintain T(re )at approximately 38.5 degrees C. T(re), mean skin temperature, heart rate and rate of total water loss measured at 60 min did not change after HA7, and HA was taken as the mean of the responses during HA8-10. For both groups, there was no decay in T(re) and for all measured variables RA was attained after 2 and 4 days in RA(12) and RA(26), respectively. It is concluded that once adaptation to heat has been attained, the time that individuals may spend in cooler conditions before returning to a hot environment could be as long as one month, without the need for extensive re-adaptation to heat. [ABSTRACT FROM AUTHOR]

Time-trial performance deteriorates in the heat. This might potentially be the result of a temperature-induced decrease in gross-efficiency (GE). The effect of high ambient temperature on GE during cycling will be studied, with the intent of determining if a heat-induced change in GE could account for the performance decrements in time trial exercise found in literature. Ten well-trained male cyclists performed 20-min cycle ergometer exercise at 60% $$ P_{V{\text{O}}_{{\text{2max}}} }$$ (power output at which VO2max was attained) in a thermo-neutral climate (N) of 15.6 ± 0.3°C, 20.0 ± 10.3% RH and a hot climate (H) of 35.5 ± 0.5°C, 15.5 ± 3.2% RH. GE was calculated based on VO2 and RER. Skin temperature ( T sk), rectal temperature ( T re) and muscle temperature ( T m) (only in H) were measured. GE was 0.9% lower in H compared to N (19.6 ± 1.1% vs. 20.5 ± 1.4%) ( P < 0.05). T sk (33.4 ± 0.6°C vs. 27.7 ± 0.7°C) and T re (37.4 ± 0.6°C vs. 37.0 ± 0.6°C) were significantly higher in H. T m was 38.7 ± 1.1°C in H. GE was lower in heat. T m was not high enough to make mitochondrial leakage a likely explanation for the observed reduced GE. Neither was the increased T re. Increased skin blood flow might have had a stealing effect on muscular blood flow, and thus impacted GE. Cycling model simulations showed, that the decrease in GE could account for half of the performance decrement. GE decreased in heat to a degree that could explain at least part of the well-established performance decrements in the heat. [ABSTRACT FROM AUTHOR]

This study examined the effect of active pre-warming on speed and quality of performance during simulated firefighting exercise. Twelve male firefighters performed two trials in counterbalanced order. They were either pre-warmed by 20-min cycling at 1.5 Watt kg − 1 body mass (WARM) or remained thermoneutral (CON) prior to a simulated firefighting activity. After the pre-warming, gastrointestinal temperature ( P < 0.001), skin temperature ( P = 0.002), and heart rate ( P < 0.001) were higher in WARM than in CON. During the firefighting activity, rating of perceived exertion, thermal sensation and discomfort were higher for WARM than for CON. Finish time of the firefighting activity was similar, but the last task of the activity was completed slower in WARM than in CON ( P = 0.04). In WARM, self-reported performance quality was lower than in CON ( P = 0.04). It is concluded that pre-warming reduces the speed during the last part of simulated firefighting activity and reduces self-reported quality of performance. [ABSTRACT FROM AUTHOR]