Effects Of Environmental Heat Stress Research Articles

The Immuno-Suppressive Effects of Cyclic, Environmental Heat Stress in Broiler Chickens: Local and Systemic Inflammatory Responses to an Intradermal Injection of Lipopolysaccharide.

To assess effects of environmental heat stress (HS) on the local and systemic inflammatory responses to lipopolysaccharide (LPS), broilers were reared under thermoneutral (TN) or cyclic HS conditions. Thermoneutral temperatures followed commercial production settings, with HS broilers exposed to 35 °C for 14 h/day from 4 days onward. At 37 days, HS- and TN-broilers were assigned to either LPS (100 μg/mL) or endotoxin-free phosphate-buffered saline (PBS; vehicle) treatments, eight each to HS- and TN-LPS, four each to HS- and TN-PBS. Treatments were administered by intradermal injection of growing feather (GF) pulps; 10 μL/GF; 12 GF/broiler. Blood and GF were collected before and at 6 and 24 h post-injection to assess leukocyte population changes in GF-pulps and blood, reactive oxygen species (ROS) generation and cytokine expression in GF-pulps, and plasma concentrations of alpha-1 acid glycoprotein (AGP-1). HS-LPS broilers had lower (p ≤ 0.05) infiltration of heterophils and macrophages, ROS generation, and inflammatory cytokine expression in GF-pulps, and lacked the increases in heterophil, monocyte, and plasma AGP-1 concentrations observed in TN-LPS broilers. HS-broilers had similar or greater drops in blood lymphocytes 6 h post-LPS or -PBS injection, respectively, and lower baseline levels (p ≤ 0.05) of circulating T- and B-lymphocytes than TN-broilers. Results indicated that cyclic HS reduced the local and systemic acute inflammatory responses to LPS in broilers, likely impairing their innate defense against microbial infection.

Carbohydrate, but not fat, oxidation is reduced during moderate-intensity exercise performed in 33 vs. 18°C at matched heart rates.

Exposure to environmental heat stress increases carbohydrate oxidation and extracellular heat shock protein 70 (HSP70) concentrations during endurance exercise at matched absolute, external work rates. However, a reduction in absolute work rate typically occurs when unacclimated enduranceathletes train and/or compete in hot environments. We sought to determine the effect of environmental heat stress on carbohydrate oxidation rates and plasma HSP70 expression during exercise at matched heart rates (HR). Ten endurance-trained, male cyclists performed two experimental trials in an acute, randomised, counterbalanced cross-over design. Each trial involved a 90-min bout of cycling exercise at 95% of the HR associated with the first ventilatory threshold in either 18 (TEMP) or 33°C (HEAT), with ~ 60% relative humidity. Mean power output (17 ± 11%, P < 0.001) and whole-body energy expenditure (14 ± 8%, P < 0.001) were significantly lower in HEAT. Whole-body carbohydrate oxidation rates were significantly lower in HEAT (19 ± 11%, P = 0.002), while fat oxidation rates were not different between-trials. The heat stress-induced reduction in carbohydrate oxidation was associated with the observed reduction in power output (r = 0.64, 95% CI, 0.01, 0.91, P = 0.05) and augmented sweat rates (r = 0.85, 95% CI, 0.49, 0.96, P = 0.002). Plasma HSP70 and adrenaline concentrations were not increased with exercise in either environment. These data contribute to our understanding of how moderate environmental heat stress is likely to influence substrate oxidation and plasma HSP70 expression in an ecologically-valid model of endurance exercise.

L-serine modulates activities of antioxidant enzymes and behavioral responses in broiler chickens subjected to feed restriction during the hot-dry season

The aim of the study was to evaluate the effects of L-serine on catalase and glutathione peroxidase activities; latency, rest, number of lines crossed, attempts at jumping, vocalization, defecation, preening rate and tonic immobility of broiler chickens subjected to feed restriction during the hot-dry season. Broiler chicks (n =120) were divided into four groups of 30 each: Group A: feed restriction (FR); Group B: feed restriction + L-serine (FR + L-serine); Group C: feed ad libitum (AL); and Group D: feed ad libitum + L-serine (FR + L-serine). L-serine (200 mg/kg) was administered for 14 days orally from one-day old. Broiler chickens were subjected to 20%-feed restriction from day 7-14 of rearing. The temperature-humidity index recorded from the study period was 30.88 ± 0.81, indicating the chickens were subjected to environmental heat stress. At weeks 3, 4 and 5, serum catalase and glutathione peroxidase activities were evaluated. Tonic immobility and Open-field responses were assessed at weeks 4 and 5. Activities of catalase and glutathione peroxidase increased (P < 0.05) in FR + L-serine and AL + L-serine broiler chickens, compared to FR and AL broiler chickens. Latency duration, and rest and frequency of preening increased (P < 0.05) in FR and AL broiler chickens, compared to FR + L-serine and AL + L-serine broiler chickens. Tonic immobility decreased significantly (P < 0.05) in the following order: AL > FR > AL + L-serine > FR + L-serine broiler chickens. The results show that feed restriction and/or L-serine ameliorated the risk of adverse effects of environmental heat stress by increasing catalase and glutathione peroxidase activities; and by decreasing duration of rest, preening frequency, latency of movement and tonic immobility. In conclusion, L-serine administration and feed restriction, particularly in combination, alleviated the adverse heat stress-induced behavioral responses partly by enhancing activities of catalase and glutathione peroxidase.

Thermophysiological and Perceptual Responses of Amateur Healthcare Workers: Impacts of Ambient Condition, Inner-Garment Insulation and Personal Cooling Strategy

While personal protective equipment (PPE) protects healthcare workers from viruses, it also increases the risk of heat stress. In this study, the effects of environmental heat stress, the insulation of the PPE inner-garment layer, and the personal cooling strategy on the physiological and perceptual responses of PPE-clad young college students were evaluated. Three levels of wet bulb globe temperatures (WBGT = 15 °C, 28 °C, and 32 °C) and two types of inner garments (0.37 clo and 0.75 clo) were chosen for this study. In an uncompensable heat stress environment (WBGT = 32 °C), the effects of two commercially available personal cooling systems, including a ventilation cooling system (VCS) and an ice pack cooling system (ICS) on the heat strain mitigation of PPE-clad participants were also assessed. At WBGT = 15 °C with 0.75 clo inner garments, mean skin temperatures were stabilized at 31.2 °C, Hskin was 60-65%, and HR was about 75.5 bpm, indicating that the working scenario was on the cooler side. At WBGT = 28 °C, Tskin plateaued at approximately 34.7 °C, and the participants reported "hot" thermal sensations. The insulation reduction in inner garments from 0.75 clo to 0.37 clo did not significantly improve the physiological thermal comfort of the participants. At WBGT = 32 °C, Tskin was maintained at 35.2-35.7 °C, Hskin was nearly 90% RH, Tcore exceeded 37.1 °C, and the mean HR was 91.9 bpm. These conditions indicated that such a working scenario was uncompensable, and personal cooling to mitigate heat stress was required. Relative to that in NCS (no cooling), the mean skin temperatures in ICS and VCS were reduced by 0.61 °C and 0.22 °C, respectively, and the heart rates were decreased by 10.7 and 8.5 bpm, respectively. Perceptual responses in ICS and VCS improved significantly throughout the entire field trials, with VCS outperforming ICS in the individual cooling effect.

Assessing the Effect of Environmental Heat Stress on Maternal Physiology and Fetal Blood Flow in Pregnant Subsistence Farmers in West Africa

Background: Anthropogenic climate change has led to increasingly extreme temperatures worldwide. Sub-Saharan Africa (SSA), including West Africa, is considered especially vulnerable to the impacts of climate extremes. In SSA women make up 50% of agricultural workforce and often work throughout pregnancy, while maternal exposure to high temperatures increases the risk of adverse birth outcomes. Understanding the physiological mechanisms responsible is central to designing targeted interventions to reduce the risks of heat exposure. Method: This observational cohort study utilised the high heat stress in West Africa to gain understanding of the physiological impact of heat on maternal and fetal physiology. Pregnant subsistence farmers who performed manual tasks throughout pregnancy were observed during field visits where environmental measurements, maternal physiology (e.g. heat strain) and fetal stress (fetal heart rate > 160 or < 115, or increase in umbilical artery resistance index) were measured. Results: A total of 92 participants were included (122 field visits). Extreme heat stress was observed in 31% of field visits. Maternal temperature, heat strain and fetal heart rate were all significantly increased from baseline to working. Fetal stress occurred in 41/122 (33%) field visits. Multilevel modelling revealed that fetal stress was significantly associated with both Universal Thermal Climate Index (OR 1·17, CI 1·05;1·29, p=0.01) and gestational age (OR 1·12, CI 1·02;1·22, p=0.02). Interpretation: Decreasing maternal exposure to heat stress in later pregnancy may reduce fetal stress, a potential pathophysiological mechanism leading to adverse birth outcomes. There is an urgent need to explore this further. Funding Information: This project was funded by the Wellcome Trust through the Wellcome Trust Global Health PhD Fellowship awarded to AB (216336/Z/19/Z). Declaration of Interests: None declared. Ethics Approval Statement: The study was approved by the Gambia government/MRC Joint ethics committee and the London School of Hygiene and Tropical Medicine Ethics Advisory Board (ref: 16405) in accordance with the Declaration of Helsinki (2013).

Effect of environmental heat stress on Kıvırcık ram sperm parameters

The relationship between environmental conditions and reproduction has been in sight of research. Therefore, there has been a growing interest in effect of climate change, global warming and subsequently environmental heat stress on gametes in last decades. Thus, the present study was designed to investigate the effect of environmental heat stress on Kıvırcık ram sperm parameters. In this study, semen samples (n=72) were collected from six mature rams during a 12-month study period. The temperature and relative humidity of animal box were controlled daily. The mean monthly temperature humidity index (THI) values were calculated to determine heat stress and control periods. According to the mean monthly THI values, the 36 of 72 ejaculates, which were collected between May and October, were classified as heat stress period samples. The others (n=36), which were collected between November and April, were considered as control period samples. The heat stress period sperm samples had lower sperm concentration, motility, viability, membrane integrity and higher abnormal sperm rate compared to control period sperm samples (P&lt; 0.05). Semen volume, sperm DNA integrity and the ability of sperm to undergo acrosome reaction were detected similar between the heat stress and control periods. In conclusion, environmental heat stress was found deleterious for some sperm parameters in Kıvırcık rams.

Thermodynamics of standing and lying behavior in lactating dairy cows in freestall and parlor holding pens during conditions of heat stress

Heat load is a challenge for high-producing dairy cows, with adverse consequences on production, reproduction, and welfare. The objectives of this observational study in 2 commercial dairy herds were to determine the effects of environmental heat stress on standing and lying behavior, to monitor the changes in core body temperature (CBT) of cows during lying and standing bouts, and to compare changes in CBT during standing bouts in freestall pens versus standing in holding pens. High-producing cows were selected for data collection over a period of 6 d of increasing heat stress during a heat wave to which they were not acclimated. A total of 20 cows were fitted with leg accelerometers that recorded lying behavior and with vaginal temperature loggers that recorded CBT. These data were recorded at 30-s intervals. Time in the parlor holding pen was calculated from automated parlor software that recorded milking unit attachment and removal times. Mean daily temperature-humidity index in the pens increased from 68.5 to 79.0 during the 6-d trial, whereas mean daily lying time decreased from 9.5 to 6.2 h/d. The mean number of lying bouts per day remained similar at 11.1 to 12.2, but duration of lying bouts decreased from a high of 49.7 min on the coolest day to 32.8 min on the hottest day. During lying bouts, CBT increased at a mean rate of 0.50°C/h. In contrast, CBT changed at a mean rate of -0.25°C/h when standing in the freestall pens and only -0.09°C/h when standing in the milking-center holding pens. Explanatory models for the CBT at which cows ended either standing or lying bouts were derived from 6 selected lying bouts and 3 selected standing bouts for each cow on each day. The CBT at which a cow ended a lying bout was positively related to CBT and temperature-humidity index at the start of the bout, bout duration, and rate of CBT change during the bout. The CBT at which a cow ended a standing bout was negatively related to bout duration and positively related to start CBT, start temperature-humidity index, and rate of CBT change. Insights into the thermodynamics of standing and lying behavior in dairy cows during periods of heat stress provided by this study may contribute to the development of more effective strategies to mitigate heat load in dairy cattle.

Effect of Environmental Heat Stress on Performance and Carcass Yield of Broiler Chicks

Effect of Environmental Heat Stress on Performance and Carcass Yield of Broiler Chicks

Workers' health and productivity under occupational heat strain: a systematic review and meta-analysis

Occupational heat strain (ie, the effect of environmental heat stress on the body) directly threatens workers' ability to live healthy and productive lives. We estimated the effects of occupational heat strain on workers' health and productivity outcomes. Following PRISMA guidelines for this systematic review and meta-analysis, we searched PubMed and Embase from database inception to Feb 5, 2018, for relevant studies in any labour environment and at any level of occupational heat strain. No restrictions on language, workers' health status, or study design were applied. Occupational heat strain was defined using international health and safety guidelines and standards. We excluded studies that calculated effects using simulations or statistical models instead of actual measurements, and any grey literature. Risk of bias, data extraction, and sensitivity analysis were performed by two independent investigators. Six random-effects meta-analyses estimated the prevalence of occupational heat strain, kidney disease or acute kidney injury, productivity loss, core temperature, change in urine specific gravity, and odds of occupational heat strain occurring during or at the end of a work shift in heat stress conditions. The review protocol is available on PROSPERO, registration number CRD42017083271. Of 958 reports identified through our systematic search, 111 studies done in 30 countries, including 447 million workers from more than 40 different occupations, were eligible for analysis. Our meta-analyses showed that individuals working a single work shift under heat stress (defined as wet-bulb globe temperature beyond 22·0 or 24·8°C depending on work intensity) were 4·01 times (95% CI 2·45-6·58; nine studies with 11 582 workers) more likely to experience occupational heat strain than an individual working in thermoneutral conditions, while their core temperature was increased by 0·7°C (0·4-1·0; 17 studies with 1090 workers) and their urine specific gravity was increased by 14·5% (0·0031, 0·0014-0·0048; 14 studies with 691 workers). During or at the end of a work shift under heat stress, 35% (31-39; 33 studies with 13 088 workers) of workers experienced occupational heat strain, while 30% (21-39; 11 studies with 8076 workers) reported productivity losses. Finally, 15% (11-19; ten studies with 21 721 workers) of individuals who typically or frequently worked under heat stress (minimum of 6 h per day, 5 days per week, for 2 months of the year) experienced kidney disease or acute kidney injury. Overall, this analysis include a variety of populations, exposures, and occupations to comply with a wider adoption of evidence synthesis, but resulted in large heterogeneity in our meta-analyses. Grading of Recommendations, Assessment, Development and Evaluation analysis revealed moderate confidence for most results and very low confidence in two cases (average core temperature and change in urine specific gravity) due to studies being funded by industry. Occupational heat strain has important health and productivity outcomes and should be recognised as a public health problem. Concerted international action is needed to mitigate its effects in light of climate change and the anticipated rise in heat stress. EU Horizon 2020 research and innovation programme.

Effects of environmental heat stress on reproduction and its management in chickens

The effects of environmental heat stress on reproduction in chickens were reviewed to find out the associated physiological changes and the interplay of the physiological events, after which the role of certain measures in the management of heat stress were identified. The normal rectal temperature for chicken is 40.6 - 43.0 oC with 45 oC as the upper limit of safety. The rectal temperature begins to increase when the ambient temperature increases, but this is prevented by panting to increase heat loss by respiratory evaporation of water. Chickens are said to be heat stressed when their body heat builds up during high ambient temperature. The physiological responses associated with it include respiratory alkalosis from panting, reduced feed intake and efficiency, reduced absorption of calcium, decrease in secretion of thyroid hormones, estrogen, progesterone and testosterone, and increased secretion of glucocorticoids, luteinising hormone and follicle stimulating hormone. Consequently, heat stress produces decrease in anabolism, increase in catabolism and insulin resistance, loss in body weight, delayed sexual maturity, reduction in number and quality of eggs, infertility in male and female chickens and reduced hatchability of fertile eggs with the greatest effect being on the number and quality of eggs produced. The impact of environmental heat stress remains a challenge as it affects reproduction in chickens and researches in this area are necessary to support increased productivity of chickens in hot semi-arid regions.Keywords: Chickens, eggs, heat stress, management, reproduction

Novel Cooling Strategies for Military Training and Operations.

The deleterious effects of environmental heat stress, combined with high metabolic loads and protective clothing and equipment of the modern Warfighter, impose severe heat strain, impair task performance, and increase risk of heat illness, thereby reducing the chance for mission success. Despite the implementation of heat-risk mitigation procedures over the past decades, task performance still suffers and exertional heat illness remains a major military problem. We review 3 novel heat mitigation strategies that may be implemented in the training or operational environment to reduce heat strain and the risk of exertional heat illness. These strategies include ingestion of ice slurry, arm immersion cooling, and microclimate cooling. Each of these strategies is suitable for use in different scenarios and the choice of cooling strategy is contingent on the requirements, circumstances, and constraints of the training and operational scenario. Ingestion of ice slurry and arm immersion cooling are practical strategies that may be implemented during training scenarios; ice slurry can be ingested before and during exercise, whereas arm immersion cooling can be administered after exercise-heat exposure. In the operational environment, existing microclimate cooling can be implemented with retrofitted vehicles and as an unmounted system, and it has the potential for use in many military occupational scenarios. This review will discuss the efficacy, limitations, and practical considerations for field implementation of each strategy.

Effect of Environmental Heat Stress on Reproduction Performance of Dairy Cows- A Review

Stress is the condition where there is undue demand for physical and mental energy due to excessive and aversive environmental factors (stressors) and cause deformations those are identifiable through physiological disequilibrium. Temperature stress imparts physical and economical losses to livestock production in temperate, subtropical and tropical regions of the world. Each species, breed or animal with its physiological state, has a comfort zone, in which the energy expenditure of the animal is minimal, constant and independent of environmental temperature. When environmental temperatures move out of the thermo-neutral zone (or comfort zone) dairy cattle begin to experience heat stress. Thermo-neutral zone depends on the age, breed, feed intake, diet composition, previous state of temperature acclimatization, production, housing and stall conditions, tissue (fat, skin) insulation and external (coat) insulation, and the behaviour of the animal. The ranges of thermo-neutral zone are from lower critical temperature (LCT) to upper critical temperature (UCT). LCT is the environmental temperature at which an animal needs to increase metabolic heat production to maintain body temperature. UCT is the environmental temperature at which the animal increases heat production as a consequence of a rise in body temperature resulting for inadequate evaporative heat loss. Heat stress negatively impacts a variety of dairy parameters including milk yield and reproduction and therefore is a significant financial burden in many dairy-producing areas of the world. Advances in management (i.e. cooling systems) and nutritional strategies have alleviated some of the negative impact of thermal stress on dairy cattle, but production continues to decrease during the summer. In this review an attempt has been made to bring forth the effect of heat stress and to discuss their impact on dairy cows.

Heat stress impairs repeated jump ability after competitive elite soccer games.

This study examined the effect of environmental heat stress on repeated jump performance after elite competitive soccer games. Male elite soccer players (n = 19) from 2 Scandinavian teams participated (age: 26.7 ± 1.0 years, height: 181.7 ± 1.1 cm, body mass: 75.8 ± 1.0 kg). The players had a Yo-Yo IR2 performance of 1,032 ± 42 m (range: 920-1,400 m). The players took part in the Champions League Qualification, where 6 games (3 home and 3 away) were played. The home games took place at an average ambient temperature of 12.2 ± 0.5° C (control game; CON) and the away games in hot conditions (30.0 ± 0.3° C; HOT). In the resting condition (Baseline) and immediately after CON and HOT, the players performed a repeated countermovement jump (CMJ) test consisting of 5 jumps separated by 5 seconds of recovery. Game-induced body mass loss was determined based on change in body mass after correction for fluid intake. The net loss of body mass was 3.1 ± 0.3% in HOT, which was higher (p < 0.05) than in CON (1.7 ± 0.2%). Mean CMJ performance after HOT was 37.9 ± 1.1 cm, which was 6.0% lower (p < 0.05) than Baseline (40.3 ± 1.1 cm) and tended (p = 0.08) to be lower than in the CON (39.6 ± 1.2 cm). The mean CMJ performance after CON was not different from Baseline. Peak CMJ performance after HOT was 41.1 ± 1.1 cm, which was not different from either Baseline or CON (42.0 ± 1.1 and 41.7 ± 1.2 cm, respectively). The relative decline in repeated CMJ performance from Baseline to after HOT correlated (r = 0.60; p < 0.05) to relative net loss in body mass during HOT. This study demonstrates that repeated CMJ performance deteriorates after a soccer game played in warm environmental settings, which is partly associated with severe dehydration.

Total tract digestibility of nitrogen in pigs exposed to high environmental temperatures1

The effect of environmental heat stress on N metabolism was studied using 7 gilts (50.5 ± 1.7 kg initial BW). Pigs were housed in metabolism cages in a climate-controlled room. After a 4-d adaptation, two 14-d experimental periods followed during which the animals were exposed to thermo-neutral temperature (NT; 20.61 ± 0.1°C) or high environmental temperature (HT; 30.41 ± 0.4°C). In each experimental period, two 24-h balance periods were carried out to collect feces and urine (via bladder catheters) for N analysis. Based on N intake and N excretion, N balance was calculated. At HT conditions, N intake was lower (P = 0.028), urinary N excretion increased (P = 0.040), and N retention decreased (P = 0.001) in comparison with NT conditions. Exposure of pigs to HT tended to reduce (P = 0.070) digestibility of N as compared to NT conditions. When pigs are exposed long term to continuous HT, N retention decreases presumably due to decreased use of ME during respiration.

Effects of Environmental Heat Stress (35°C) with Simulated Air Movement on the Thermoregulatory Responses during a 4-km Cycling Time Trial

Effects of Environmental Heat Stress (35°C) with Simulated Air Movement on the Thermoregulatory Responses during a 4-km Cycling Time Trial

Aerobic Performance Is Degraded, Despite Modest Hyperthermia, in Hot Environments

Environmental heat stress degrades aerobic performance; however, little research has focused on performance when the selected task elicits modest elevations in core body temperature (<38.5 degrees C). To determine the effect of environmental heat stress, with modest hyperthermia, on aerobic performance and pacing strategies. After a 30-min cycling preload at 50% VO2peak, eight euhydrated men performed a 15-min time trial on a cycle ergometer in temperate (TEMP; 21 degrees C, 50% RH) and hot (HOT; 40 degrees C, 25% RH) environments. Core and skin temperature (Tc and Tsk, respectively) and HR were continuously monitored. Performance was assessed by the total work (kJ) completed in 15 min. Pacing was quantified by comparing the percent difference in actual work performed in each of five 3-min blocks normalized to the mean work performed per 3-min block. Pace over the final 2 min was compared with the average pace from minutes 0 to 13 for end spurt analysis. Tc and HR rose continually throughout both time trials. Peak Tc remained modestly elevated in both environments [mean (range): HOT = 38.20 degrees C (37.97-38.42 degrees C); TEMP = 38.11 degrees C (38.07-38.24 degrees C)], whereas Tsk was higher in HOT (36.19 +/- 0.40 degrees C vs 31.14 +/- 1.14 degrees C), and final HR reached approximately 95% of age-predicted maximum in both environments. Total work performed in HOT (147.7 +/- 23.9 kJ) was approximately 17% less (P < 0.05) than TEMP (177.0 +/- 25.0 kJ). Pace was evenly maintained in TEMP, but in HOT, volunteers were unable to maintain initial pace, slowing progressively over time. A significant end spurt was produced in both environments. During a brief aerobic exercise time trial where excessive hyperthermia is avoided, total work is significantly reduced by heat stress because of a gradual slowing of pace over time. These findings demonstrate how aerobic exercise performance degrades in hot environments without marked hyperthermia.

Last Word on Viewpoint: Current evidence does not support an anticipatory regulation of exercise intensity mediated by rate of body heat storage

to the editor: The main purpose of our Viewpoint article critiquing the primary “evidence” that the rate of body heat storage mediates an anticipatory reduction in exercise intensity was to clearly demonstrate the erroneous nature of using thermometry to estimate the rate of body heat storage

Prior Heat Stress

The impact of prior heat stress on subsequent aerobic exercise-heat performance has not been studied. To determine whether prior heat stress degrades subsequent aerobic exercise performance in the heat. Eighteen nonheat acclimated males were trained (four practice trials) on an aerobic exercise performance test in 22 degrees C and then divided into two (n = 8) groups. One group (EUHPH; (.)VO2peak = 44 +/- 7 mL x kg x min(-1)) was tested after 90 min of recovery (in 22 degrees C) from 3 h of intermittent light-intensity (<30% (.)VO2peak) exercise-heat (50 degrees C) stress, where sweat losses were matched with fluid intake (3.5 +/- 0.5 L) to maintain euhydration. The other group (EUH; (.)VO2peak = 45 +/- 5 mL x kg x min(-1)) was tested while euhydrated without prior exercise-heat stress. Aerobic performance was determined from a 30-min cycling preload (50% (.)VO2peak) followed by a 15-min time trial in 40 degrees C. Total work during the 15-min performance time trial in EUH and EUHPH was compared, as were the percent changes from the best practice trials. Volunteers were euhydrated (plasma osmolality < 290 mOsm x kg(-1)) and normothermic before each exercise-heat trial. Heart rate and core temperature were not different (P > 0.05) between groups at any time point during exercise. Total work was not different (P > 0.05) at baseline or between EUH (150.5 +/- 28.3 kJ; 2.0 +/- 0.3 kJ x kg(-1)) and EUHPH (160.3 +/- 24.0 kJ; 1.8 +/- 0.2 kJ x kg(-1)). The percent change in total work relative to baseline was not different (P > 0.05) between EUH (-18.7% +/- 9.2%) and EUHPH (-15.0% +/- 7.8%). If hydration and body temperatures recover, prior exercise-heat stress does not result in a greater degradation in aerobic time trial performance in the heat compared with heat exposure alone.

Effects of Environmental Heat Stress (35 °C) with Simulated Air Movement on the Thermoregulatory Responses during a 4‐km Cycling Time Trial

The aim of the present investigation was to examine the influence of environmental heat stress (35 degrees C) on 4-km cycling time trial performance using simulated environmental conditions and facing air velocities that closely reflect competitive situations. Nine competitive cyclists (age 34 +/- 5 years, maximal oxygen uptake 61.7 +/- 8.6 ml . kg (-1) . min (-1)) completed a simulated 4-km cycling time trial in laboratory ambient temperatures (dry bulb temperatures) of 35 degrees C and 13 degrees C (relative humidity 60 %, air velocity 5.6 m/s). Mean performance time was reduced in 35 degrees C (390.1 +/- 19.6 s) compared to 13 degrees C (382.8 +/- 18.2 s) (95 % CI of difference = 4.0 to 10.6 s; p < 0.01). This was consistent with a decline in mean power output throughout the duration of exercise in 35 degrees C compared with 13 degrees C (p < 0.01). Mean skin temperature and mean body temperatures were elevated at rest and throughout the duration of exercise in 35 degrees C (p < 0.01). A higher level of muscle temperature was also observed at the onset and cessation of exercise in 35 degrees C (p < 0.01). The rate of heat storage (35 degrees C, 413.6 +/- 130.8 W . m (-2); 13 degrees C, 153.1 +/- 112.5 W . m (-2)) representative of the entire 4-km time trial was greater in the heat (p < 0.01). When expressed per kilometre, however, difference in the rate of heat storage between conditions declined during the final kilometre of exercise (p = 0.06). We conclude that the current decrements in self-selected work-rate in the heat are mediated to some extent through afferent feedback arising from changes in heat storage at rest and during the early stages of exercise which serve to regulate the subsequent exercise intensity in attempt to preserve thermal homeostasis.

The Effects of Environmental Heat Stress (35 °C) on Simulated 4 km Cycling Time -Trial Performance

The influence of environmental heat stress on the capacity to perform short and prolonged modes of exercise has been extensively studied. Limited research to date has evaluated the effects of thermoregulatory responses to moderate duration, high intensity exercise, particularly using self-paced performance models that replicate competitive events. PURPOSE: To determine the influence of environmental heat stress (35 °C) on simulated 4 km cycling time trial performance METHODS: Nine competitive cyclists (age 34 ± 5 years, maximal oxygen uptake 61.7 ± 8.6 ml·kg−1·min1) completed a simulated 4 km cycling time trial in laboratory ambient temperatures (dry bulb temperatures) of 35°C and 13°C (RH 60%, air velocity 5.6 m/s). RESULTS: Mean performance time in 35°C was 390.1 ± 19.6 s compared to 382.8 ± 18.2 sin 13°C(95%CIofdifference = 4.0 to 10.6 s; p = 0.01). A significant interaction between distance and condition was observed for oesophageal temperature (p = 0.009) and mean body temperature (p <0.0005) with the largest heat induced increase in oesophageal temperature observed at the cessation of exercise (35°C, 38.3 ± 0.3 °C; 13°C, 37.9 ± 0.4 °C). The rate of heat storage (35°C, 413.6 ± 130.8 W·m2; 13°C, 153.1 ± 112.5 W·m2; p = 0.002), muscle temperature (35°C, 39.3 ±.5 °C; 13°C, 38.4 ±0.4 °C), heart rate and perception of thermal comfort were significantly elevated during exercise in 35°C compared with 13 °C (p <0.0005). Total sweat loss, ratings of perceived exertion, oxygen uptake and RER were not significantly different between conditions. CONCLUSION: These results demonstrate that high environmental temperatures reduce the capacity to perform self-paced, moderate duration high intensity exercise.