Exercise In Hot Conditions Research Articles

The Effect of a Synthetic-Grass Sport Surface on Physiology and Perception During Intermittent Exercise in Hot Conditions.

The current study aimed to determine the effect of a synthetic-grass sport surface on core body temperature, skin temperature, heart rate, thermal sensation, thermal comfort, and rating of perceived exertion (RPE)during intermittent exercise in hot conditions. Using a randomized crossover design, 13 trained/developmental team-sport athletes completed two 50-minute standardized intermittent running protocols on a synthetic and anatural-grass surface, on separate days (control-condition air temperature 32.6 °C [1.3 °C], relative humidity 43.2% [5.3%]). Final skin temperature was significantly higher on synthetic compared with natural grass at the calf (40.1 °C [2.5 °C] vs 33.4 °C [0.6 °C]; P < .001), shoulder (36.6 °C [1.7 °C] vs 33.7 °C [0.7 °C]; P < .001), and chest (33.2 °C [1.1 °C] vs 31.8 °C [1.2 °C]; P = .02). Thermal sensation (median: 2.3; interquartile range [0.5] vs 2.2 [0.5], P = .03) and sweat rate (1.5 [0.4]L·h-1 vs 1.2 [0.3]L·h-1; P = .02) were also significantly higher on synthetic grass. While final core body temperature was significantly higher on the natural than synthetic grass (38.4 °C [0.3 °C] vs 38.2 °C [0.4 °C]),there were no significant differences in delta core temperature, as well as heart rate, thermal comfort, or RPE. Higher skin temperatures, thermal sensation, and sweat rates suggest that exercising on synthetic grass in hot conditions may increase some markers of heat strain during exercise. However, delta core body temperature, heart rate, thermal comfort, and RPE remained unaffected.

The Effects of Caffeine on Exercise in Hot Environments: A Bibliometric Study.

Caffeine is widely recognized as an ergogenic aid to enhance athletic performance, yet its effects in hot environments remain relatively underexplored. To provide a comprehensive overview of the research landscape and identify research themes in this field. We systematically searched the Web of Science (WoS) and SCOPUS databases using keywords related to caffeine (e.g., caffe*), hot environments (e.g., heat, hot, or therm*), and athletic performance (e.g., cardio, endurance, or strength). The Bibliometrix package in R was used for bibliometric analysis and result visualization, while a narrative review was subsequently performed to identify research themes. We found that studies examining the impact of caffeine on exercise in hot conditions are relatively sparse and have progressed slowly in recent years. Research in this domain has predominantly been concentrated within an academic network led by Professor Lawrence Armstrong. Recent contributions have been sporadically made by emerging scholars, with collaborations largely confined to a few research groups and countries. Key research themes identified include exercise performance, thermoregulation, fluid balance, physiological responses, immune responses, synergistic effects with other compounds, and the influence of individual differences. Of these, the first three themes-exercise performance, thermoregulation, and fluid balance-have received the most attention. Caffeine's effects on exercise performance in hot environments have not been thoroughly studied. The existing research themes are varied, and the conclusions show considerable inconsistencies. Our study highlights the need for further research into the effects of caffeine dosage, administration methods, and population-specific variables. We also call for increased collaboration among research groups to advance scientific understanding and address the gaps in this field.

Acute work rate adjustments during high-intensity interval training in a hot and temperate environment.

Heart rate drifts upward over time during interval exercise and during exercise in hot conditions. As such, work rate must be lowered to maintain target heart rate. The purpose was to characterize acute work rate adjustments during high-intensity interval training based on target heart rate. Seven humans (three females) completed five study visits: a graded exercise test on a cycle ergometer to measure maximal heart rate (HRmax) in ∼22°C and four trials performed in ∼22°C (TEMP) or ∼35°C (HOT), consisting of an 8 min warm-up at 70% HRmax followed by one (15TEMP and 15HOT) or five (43TEMP and 43HOT) rounds of high-intensity interval training (one round = 4min work at 90% HRmax and 3min recovery at 70% HRmax) totaling 15min or 43min of exercise, respectively. Work rate was lowered 33 ± 20W (p=0.005) in 43TEMP and 56 ± 30W (p=0.003) in 43HOT between the first and fifth work intervals. Thermal strain (0.2°C higher rectal temperature, p=0.01) and cardiovascular strain (6 beats·min-1 larger increase in heart rate from first to fifth recovery interval, p=0.01) were greater in 43HOT versus 43TEMP. Using target heart rate during high-intensity interval training may reduce the training stimulus, especially in hot environments, but it may also limit thermal strain and enable participants to complete the prescribed workout despite the heat.

Acute exercise in a hot environment increases heat shock protein 70 and peroxisome proliferator-activated receptor γ coactivator 1α mRNA in Thoroughbred horse skeletal muscle.

Heat acclimatization or acclimation training in horses is practiced to reduce physiological strain and improve exercise performance in the heat, which can involve metabolic improvement in skeletal muscle. However, there is limited information concerning the acute signaling responses of equine skeletal muscle after exercise in a hot environment. The purpose of this study was to investigate the hypothesis that exercise in hot conditions induces greater changes in heat shock proteins and mitochondrial-related signaling in equine skeletal muscle compared with exercise in cool conditions. Fifteen trained Thoroughbred horses [4.6 ± 0.4 (mean ± SE) years old; 503 ± 14 kg] were assigned to perform a treadmill exercise test in cool conditions [COOL; Wet Bulb Globe Temperature (WBGT), 12.5°C; n = 8] or hot conditions (HOT; WBGT, 29.5°C; n = 7) consisting of walking at 1.7 m/s for 1 min, trotting at 4 m/s for 5 min, and cantering at 7 m/s for 2 min and at 90% of VO2max for 2 min, followed by walking at 1.7 m/s for 20 min. Heart rate during exercise and plasma lactate concentration immediately after exercise were measured. Biopsy samples were obtained from the middle gluteal muscle before and at 4 h after exercise, and relative quantitative analysis of mRNA expression using real-time RT-PCR was performed. Data were analyzed with using mixed models. There were no significant differences between the two groups in peak heart rate (COOL, 213 ± 3 bpm; HOT, 214 ± 4 bpm; p = 0.782) and plasma lactate concentration (COOL, 13.1 ± 1.4 mmoL/L; HOT, 17.5 ± 1.7 mmoL/L; p = 0.060), while HSP-70 (COOL, 1.9-fold, p = 0.207; HOT, 2.4-fold, p = 0.045), PGC-1α (COOL, 3.8-fold, p = 0.424; HOT, 8.4-fold, p = 0.010), HIF-1α (COOL, 1.6-fold, p = 0.315; HOT, 2.2-fold, p = 0.018) and PDK4 (COOL, 7.6-fold, p = 0.412; HOT, 14.1-fold, p = 0.047) mRNA increased significantly only in HOT at 4 h after exercise. These data indicate that acute exercise in a hot environment facilitates protective response to heat stress (HSP-70), mitochondrial biogenesis (PGC-1α and HIF-1α) and fatty acid oxidation (PDK4).

Concurrent validity of the CORE wearable sensor with BodyCap temperature pill to assess core body temperature during an elite women’s field hockey heat training camp

ABSTRACT Wearable temperature sensors offer the potential to overcome several limitations associated with current laboratory- and field-based methods for core temperature assessment; however, their ability to provide accurate data at elevated core temperatures (Tc) has been questioned. Therefore, this investigation aimed to determine the concurrent validity of a wearable temperature sensor (CORE) compared to a reference telemetric temperature pill (BodyCAP) during a team-sport heat training camp prior to the 2020 Olympic Games. Female field hockey players (n = 19) in the Australian national squad completed 4 sessions in hot conditions where their temperature was monitored via CORE and BodyCAP. Concurrent validity of the wearable CORE device was determined with reference to the ingested BodyCAP pill. Lin’s Concordance Correlation Coefficients determined there was “poor” agreement between devices during all sessions. Mean bias demonstrated that CORE underestimated Tc in all sessions (−0.06°C to −0.34°C), with wide mean 95% confidence intervals (±0.35°C to ±0.56°C). Locally estimated scatterplot smoothing regression lines illustrated a non-linearity of error, with greater underestimation of Tc by the CORE device, as Tc increased. The two devices disagreed more than ±0.3°C for 41-60% of all data samples in each session. Our findings do not support the use of the CORE device as a valid alternative to telemetric temperature pills for Tc assessment, particularly during exercise in hot conditions where elevated Tc are expected.

Effects of wetted inner clothing on thermal strain in young and older males while wearing ventilation garments.

The present study examined the effect of wearing a water-soaked inner t-shirt with a ventilation garment on thermal and cardiovascular strain in eight young (26 ± 4years) and eight older (67 ± 3years) men undertaking moderate-intensity work (metabolic rate: 200-230Wm-2) in a hot environment (37°C, 50% RH, 2.8kPa). While intermittent walking in hot conditions for 60min, as a control (CON), the subject wore a dry inner t-shirt (long-sleeved) without fanning of a ventilation jacket (single-layered cotton, 0.21 clo). On separate days, under a fanned ventilation jacket, the subject wore a dry inner t-shirt (DRY) or an inner t-shirt soaked with 350mL of tap water (WET). In the young group, increases in rectal temperature from pre-exercise baseline in the WET trial (0.7°C ± 0.2°C) were lower than in the CON (1.3°C ± 0.3°C) and DRY (1.1°C ± 0.2°C) (both p < 0.05) trials during exercise in hot conditions. In the older group, the increases were also attenuated in WET (0.7°C ± 0.4°C) compared with CON (1.3°C ± 0.4°C) and DRY (1.1°C ± 0.4°C) (both p < 0.05) without differences between age groups. Heart rate and whole-body sweat loss were lowest in the WET, followed by DRY, and then CON conditions in both groups (all p < 0.05). These findings demonstrate that wearing a water-soaked inner t-shirt while using a ventilation garment is an effective and practical cooling strategy to mitigate thermal and cardiovascular strains in young and older individuals during moderate-intensity work in hot conditions.

Post-exercise hot water immersion elicites heat acclimation responses in endurance and recreational athletes. A systematic review and meta-analysis

Aims This meta-analysis aims to evaluate the effectiveness of heat acclimation (HA) via hot water immersion protocols and their effect on time trial (TT) performance, heart rate (HRE, HRM and HRTT), rectal temperature (Tre), Rate of perceived exertion (RPE), psychological stress index (PhSI), thermal comfort (Tcomf), thermal sensation (Tsen) and maximum oxygen uptake (Vo2max). Methods Pubmed, Scopus, Sportdiscus and Web of Science databases were used alongside the grey matter sites Google Scholar and Researchgate. The databases were then searched for randomised control trials and mixed-method design studies. Two RTCs, six repeated measure design studies and one randomised crossover design study were included after screening a total of 50 titles and 28 full-text articles. Sample sizes range from 1 - 13 with all participants having not participated in any form of heat training 6 months before their inclusion in the study. Results The mean difference (MD) for Heart rate (HR) was -9.1125 BPM (95% CI p = 0.026) and was considered to be statistically significant. The MD for Rectal temperature (Tre) effect size was -0.3814 Tre (°C) (95% CI; p = 0.05). The MD for sweat rate was 0.085; (96% CI; p = 0.0179) The changes in RPE, PhSI, Tcomf and Tsen were too small to be considered statistically significant. There was no significant difference between pre and post HA for Vo2max and PV. This meta-analysis implies that HA via HWI may improve tolerance to discomfort during heat exposure and thus subsequently improve physical performance during exercise in hot conditions. Conclusion The primary finding of this meta-analysis is that athletic performance is improved with post-exercise hot water immersion heat acclimation training. HWI HA protocols should focus on the following guidelines: 40-50 minutes of submaximal exercise (&gt;65% of Vo2max) should be followed directly (within 10 minutes) by 40 minutes of hot water immersion at 40°C with the individual immersed up to their neck. The HA protocol should last between 6-9 days with a single bout of HWI every day for this period.

Analysis of Thermal Sensation and Wearing Comfort before and after Bikram Yoga Activity

This study evaluated the effect of functional characteristics of the fabric for Bikram yoga clothing and fit measured in clothing pressure on the wearer''s physiological response and perceptive sensation before and after exercise in hot conditions and during a resting period in standard conditions. The test garment consisted of two tops (T1, T2) and two pants (P1, P2) each with different functional characteristics of fabric. Using various combinations of the test garments, assessment of the thermal sensation and comfort was performed before/after yoga and after resting. This study revealed that thermal sensation and wearing comfort significantly changed based on the functionality of the fabric of top garment before the exercise period. In addition, the results showed that based on clothing pressure, the feeling of comfort was different between before yoga and after resting. The appropriate choice of fabric material was important when constructing Bikram yoga clothing worn in hot conditions; however, level of clothing pressure was also found to be an essential design factor for comfort during rest after exercise.

Heat acclimation does not affect maximal aerobic power in thermoneutral normoxic or hypoxic conditions.

What is the central question of this study? Controlled-hyperthermia heat-acclimation protocols induce an array of thermoregulatory and cardiovascular adaptations that facilitate exercise in hot conditions. We investigated whether this ergogenic potential can be transferred to thermoneutral normoxic or hypoxic exercise conditions. What is the main finding and its importance? We showed that heat acclimation did not affect maximal cardiac output or maximal aerobic power in thermoneutral normoxic or hypoxic conditions. Heat acclimation augmented the sweating response in thermoneutral normoxic conditions. The cross-adaptation theory, according to which heat acclimation could facilitate hypoxic exercise capacity, is not supported by our data. Heat acclimation (HA) mitigates heat-induced decrements in maximal aerobic power ( ) and augments exercise thermoregulatory responses in the heat. Whether this beneficial effect of HA is observed in hypoxic or thermoneutral conditions remains unresolved. We explored the effects of HA on cardiorespiratory and thermoregulatory responses to exercise in normoxic, hypoxic and hot conditions. Twelve men [ 54.7(standard deviation 5.7) mlkg-1 min-1 ] participated in a HA protocol consisting of 10 daily 90-min controlled-hyperthermia (target rectal temperature, Tre =38.5°C) exercise sessions. Before and after HA, we determined in thermoneutral normoxic (NOR), thermoneutral hypoxic (fractional inspired O2 =13.5%; HYP) and hot (35°C, 50% relative humidity; HE) conditions in a randomized and counterbalanced order. Preceding each maximal cycling test, a 30-min steady-state exercise bout at 40% of the NOR peak power output was used to evaluate thermoregulatory responses. Heat acclimation induced the expected adaptations in HE: reduced Tre and submaximal heart rate, enhanced sweating response and expanded plasma volume. However, HA did not affect or maximal cardiac output (P=0.61). The peak power output was increased post-HA in NOR (P<0.001) and HE (P<0.001) by 41±21 and 26±22 W, respectively, but not in HYP (P=0.14). Gross mechanical efficiency was higher (P=0.004), whereas resting Tre and sweating thresholds were lower (P<0.01) post-HA across environments. Nevertheless, the gain of the sweating response decreased (P=0.05) in HYP. In conclusion, our data do not support a beneficial cross-over effect of HA on in normoxic or hypoxic conditions.

Impact of Exercise‐Induced Dehydration on Perceived Sleep Quality

ObjectiveTo examine the effect of exercise‐induced dehydration on perceived sleep quality.MethodsTwenty‐three recreationally active males (mean±SD; age, 21±2 y; height, 179.8±6.4 cm; body mass, 75.18±8.09 kg; VO2max, 51.7±5.5 ml•kg−1•min−1; body fat, 14.4±4.6%) completed three randomized exercise trials: euhydrated + fluid replacement during exercise (EUR), euhydrated + no fluid replacement during exercise (EUD), and hypohydrated arrival via fluid restriction + no fluid replacement during exercise (HYPO) in hot conditions (ambient temperature, 35.3±0.6°C; relative humidity, 31.3±2.0%). Twenty‐four hours prior to (PRE24) and following (POST24) the exercise trial, a nude body mass (NBM) was recorded, and participants collected all urine in a sealed container to assess 24 h hydration status via urine osmolality (UOSMO) and percent body mass loss (%BML). Prior to exercise (PREEX) and following exercise (POSTEX), NBM was measured to calculate %BML. At PREEX and POST24 participants completed the Karolinska Sleep Diary (KSD) to obtain measures of overall perceived sleep quality (KSDTOT) from the previous night's sleep. Time spent sleeping (SLTM), sleep efficiency (SLEFF), overall sleep quality (SLQUAL), and total number of awakenings (AWTOT) was were also reported. Trial x time repeated measures ANOVAs with a Tukey post hoc test were used to evaluate differences in continuous dependent variables with significance set a priori p&lt;0.05.ResultsAt PRE24, UOSMO was significantly different (p&lt;0.05) in HYPO (605±212 mOsm•kg−1) versus EUD (441±176 mOsm•kg−1), and EUR (411±207 mOsm•kg−1). %BML was significantly greater (p&lt;0.001) in HYPO at PREEX (1.3±1.2%) and POSTEX (5.0±0.9%) versus EUD (−0.9±1.0%, 3.4±1.0%) and EUR (−0.8±1.0%, 0.2±1.0%). %BML in EUD was significantly greater than EUR at POSTEX, p&lt;0.001. At POST24, UOSMO was significantly higher (p=0.002) in HYPO (714±292 mOsm•kg−1) versus EUR (443±158 mOsm•kg−1); however, no significant differences (p&gt;0.05) were observed in %BML (HYPO, 0.3± 1.3%; EUD, −0.3± 0.9%; EUR, −0.0±1.1%). Hypohydration achieved prior to (PREEX) and following (POSTEX) exercise did not influence KSDTOT, SLTM, SLEFF, SLQUAL, KSDTOT, or AWTOT (p&gt;0.05). Independent of trial, SLTM (MD [95%CI], 34.6 min [7.7, 61.5], p=0.012) and SLEFF (1.4% [0.6, 2.78], p=0.041) were significantly greater at POST24 versus PREEX.ConclusionsFollowing exercise in hot conditions, participants reported an increased length and more efficient bout of sleep. The ability of participants to return to within ±1% of baseline body mass within 24 h may have negated any adverse effects of hydration state on perceived sleep quality, however future research is needed to investigate the effects of sustained hypohydration on sleep quality.Support or Funding InformationThis study was funded in part by Halo Wearables, LLC and General Electric.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Tumor Necrosis Factor‐alpha, TNFR1, and TNFR2 Responses to Exercise in the Heat

Plasma levels of Tumor Necrosis Factor‐alpha (TNF‐α) have been shown to increase in response to exercise in hot environments, however, the effects of exercise in the heat on surface expression of TNF receptors (TNFR) 1 and 2 have not been investigated.OBJECTIVETo observe the effects of moderate‐ and high‐intensity exercise in the heat on circulating TNF‐α and surface expression of TNFR1 and TNFR2 on human classical monocytes (CD14++CD16–).METHODSEight recreationally active Caucasian men (24.8 ± 3.0 yrs; 179.8 ± 7.4 cm; 84.4 ± 2.9 kg; 46.8 ± 5.9 ml·kg−1·min−1) completed four exercise trials. The first trial consisted of a VO2max assessment, conducted on a cycle ergometer, while the remaining trials were completed in three environmental conditions: high temperature/low humidity (HTLH; 35°C, 20% RH); high temperature/moderate humidity (HTMH; 35°, 45% RH); and moderate temperature/moderate humidity (MTMH; 22°C; 45%RH). The exercise protocol consisted of a 60‐minute cycling trial at 60% of their previously determined VO2max, a 15‐minute rest period, and a time to exhaustion trail at 90% VO2max (TTE). Blood samples were obtained before (T1) and after (T2) the 60‐minute trial; immediately after TTE (T3), and one hour post‐TTE (T4). Plasma concentrations of TNF‐α were measured via commercially available ELISA. TNFR1 and TNFR2 expression was measured on a subset of participants (n= 5) using flow cytometry. Natural log (LN) transformation was used on non‐normally distributed data and area under the curve (AUC) was calculated using the standard trapezoidal technique. Changes were analyzed using a two factor (time × trial) within‐subjects repeated measures ANOVA and one‐way repeated measures ANOVA.RESULTSA significant interaction was observed for circulating TNF‐α (F= 3.912, p = 0.034, η2p = 0.358). During HTLH, TNF‐α increased from T1 to T2 (p = 0.026) and T3 (p = 0.036), and decreased from T3 to T4 (p = 0.036). During HTMH, TNF‐α increased from T1 to T2 (p = 0.002), T3 (p = 0.006), and T4 (p = 0.011). No significant time effect was observed during MTMH (p &gt; 0.05). Circulating TNF‐α at T1 was significantly lower during HTMH compared to MTMH (p = 0.040). No significant main effect of trial (F= 1.906, p = 0.185, η2p = 0.214) was observed for TNF‐α AUC. No significant interaction (F= 1.422, p = 0.247, η2p = 0.262), nor main effects of time (F= 0.465, p = 0.712, η2p = 0.104) or trial (F= 1.165, p = 0.360, η2p = 0.226) were observed for TNFR1 expression. Additionally, no significant main effect of trial (F= 1.681, p = 0.246, η2p = 0.296) was observed for TNFR1 AUC. No significant interaction (F= 0.998, p = 0.449, η2p = 0.200), nor main effects of time (F= 0.205, p = 0.681, η2p = 0.049) or trial (F= 1.659, p = 0.250, η2p = 0.293) were observed for TNFR2 expression. Additionally, no significant main effect of trial (F= 1.411, p = 0.299, η2p = 0.261) was observed for TNFR2 AUC.CONCLUSIONSModerate‐ and high‐intensity exercise in hot conditions elicits an increase in plasma TNF‐α concentrations, though moderate temperature does not. Furthermore, the environmental condition during exercise did not affect TNFR1 and TNFR2 expression on classical monocytes.Support or Funding InformationThis study was partially funded by the Kent State University Research CouncilThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Topical and Ingested Cooling Methodologies for Endurance Exercise Performance in the Heat.

This systematic review and meta-analysis aimed to assess studies which have investigated cooling methodologies, their timing and effects, on endurance exercise performance in trained athletes (Category 3; VO2max ≥ 55 mL·kg·min−1) in hot environmental conditions (≥28 °C). Meta-analyses were performed to quantify the effects of timings and methods of application, with a narrative review of the evidence also provided. A computer-assisted database search was performed for articles investigating the effects of cooling on endurance performance and accompanying physiological and perceptual responses. A total of 4129 results were screened by title, abstract, and full text, resulting in 10 articles being included for subsequent analyses. A total of 101 participants and 310 observations from 10 studies measuring the effects of differing cooling strategies on endurance exercise performance and accompanying physiological and perceptual responses were included. With respect to time trial performance, cooling was shown to result in small beneficial effects when applied before and throughout the exercise bout (Effect Size: −0.44; −0.69 to −0.18), especially when ingested (−0.39; −0.60 to −0.18). Current evidence suggests that whilst other strategies ameliorate physiological or perceptual responses throughout endurance exercise in hot conditions, ingesting cooling aids before and during exercise provides a small benefit, which is of practical significance to athletes’ time trial performance.

The effect of ice slurry ingestion on body temperature and cycling performance in competitive athletes

The effects of pre cooling on endurance performance are widely known. In contrast, the approach of cooling during endurance exercise in combination with pre-exercise cooling has been poorly understood. The purpose of the present study was to determine whether the effects of precooling and cooling during exercise enhance exercise performance compared to the ingestion of a thermo-neutral beverage (20 °C) or precooling alone in cycling performance. This was an experimental study using a randomised crossover design in which 7 cyclists underwent three trials comprising of 45 min steady state cycling (SS) at 70% VO2 max and a subsequent 10 km time trial (TT) in hot conditions (32 °C, 50% relative humidity). Rectal temperature (Tre), heat storage (HS), heart rate (HR), blood lactate concentration (BLA) and thermal sensation (TS) were measured. The intervention consisted of: (1) ingestion of thermo-neutral beverage before and during SS cycling (TN), (2) ingestion of ice slurry beverage and application of iced towels (precooling) prior to exercise, and then ingestion of thermo-neutral beverage during SS (PRE) and (3) precooling strategy as above plus ice slurry ingestion during SS cycling (PRE + MID). The intake of thermo-neutral or ice slurry beverage (14 g/kg) occurred over 30 min before and every 15 min during SS cycling. There was no significant difference in TT performance between all the conditions (P =0.72). However, PRE and PRE + MID caused a significant decrease in Tre (P < 0.05) from TN during exercise. Accordingly, both precooling and a combination of precooling and mid-cooling during exercise in hot conditions may be a practical and effective way of reducing core temperature. Future studies should investigate longer distance events and timing of ice slurry ingestion.

Aerobic Fitness Modulates Whole‐body Heat Loss Responses in Young Adult Males during Exercise in Hot Dry Conditions

It is well established that aerobic fitness (as defined by peak oxygen uptake) improves the body's ability to dissipate heat during exercise in hot conditions. However, it currently remains unclear at what level of heat stress differences in aerobic fitness may alter the body's physiological capacity to dissipate heat. Therefore, in this pilot work, we examined whole‐body heat loss using direct calorimetry in endurance (n=4, 63.5±1.5 mlO2/kg/min) and non‐endurance trained (n=4; 37.3±0.3 mlO2/kg/min, P=0.002) healthy young adult males (22±4 years) matched for body surface area (endurance: 1.89±0.14; non‐endurance: 1.87±0.16 m2, P=0.104). Participants performed three 30‐min bouts of semi‐recumbent cycling at fixed rates of metabolic heat production equal to 300 (Ex1), 400 (Ex2), and 500 W (Ex3) (an equivalent external workload of approximately 45, 75 and 100 W, respectively), each separated by a 15‐min recovery period in dry heat (40°C, 14% relative humidity). These fixed rates of heat production were employed to ensure a similar stimulus for whole‐body sweating. The rates of whole‐body heat loss (evaporative heat loss and dry heat exchange) and metabolic heat production were measured by direct and indirect calorimetry, respectively. Whole‐body heat content was measured as the cumulative summation of heat production and heat loss. We show that the rate of dry heat gain (i.e., from the environment), and therefore the net heat load (i.e., metabolic heat production plus dry heat gain), was similar between groups throughout the intermittent exercise protocol (Ex1, Ex2, Ex3, all P≥0.207). Although not statistically significant at all exercise bouts, the rate of whole‐body evaporative heat loss tended to be lower in the non‐endurance males during Ex1 (non‐endurance: 357±17 vs. endurance: 378±9 W, P=0.047), Ex2 (non‐endurance: 438±17 vs. endurance: 481±18 W, P=0.059), and Ex3 (non‐endurance: 513±29 vs. endurance: 573±26 W, P=0.113) compared to their endurance trained counterparts. Consequently, whole‐body heat content trended towards being greater in the non‐endurance group during Ex1 (non‐endurance: 127±50 vs. 112±36 kJ, P=0.163), Ex2 (non‐endurance: 138±38 vs. 86±25 kJ, P=0.038), and Ex3 (non‐endurance: 227±47 vs. 157±34 kJ, P=0.122) relative to the endurance trained group. Our early findings demonstrate that aerobic fitness may modulate the body's physiological capacity to dissipate heat at metabolic heat loads ≥300 W in young adult males differing in peak oxygen uptake by as much as ~26 mlO2/kg/min. Therefore, our data supports that maintaining a high level of aerobic fitness may facilitate an enhanced capacity to dissipate heat and thereby minimize the incidence of experiencing heat related illness during exercise in the heat.Support or Funding InformationThis study was supported by grants from the Natural Sciences and Engineering Research Council of Canada (Discover grant, RGPIN‐06313‐2014; Discovery Grants Program ‐ Accelerator Supplement, RGPAS‐462252‐2014; funds held by Dr. Glen P. Kenny).

The effects of ice vest pre-cooling on skin blood flow at rest and during exercise in the heat

Ice vest pre-cooling has been show to lower rectal tem- perature during intermittent exercise in hot conditions but only after 40 min of exercise [1]. The authors sug- gested that the ice vest may have initiated a strong local cutaneous vasoconstrictor response reducing skin blood flow [2] and thus the cooling potential, until increases in body temperature and skin blood flow occurred later in exercise. Therefore, the purpose of this study was to determine whether ice vest pre-cooling reduces skin blood flow during intermittent exercise in the heat com- pared to a no cooling control.

Warm skin alters cardiovascular responses to cycling after preheating and precooling.

Exercise in hot conditions increases core (TC) and skin temperature (TSK) and can lead to a progressive rise in HR and decline in stroke volume (SV) during prolonged exercise. Thermoregulatory-driven elevations in skin blood flow (SkBF) adds complexity to cardiovascular regulation during exercise in these conditions. Presently, the dominant, although debated, view is that raising TSK increases SkBF and reduces SV through diminished venous return; however, this scenario has not been rigorously investigated across core and skin temperatures. We tested the hypothesis that high TSK would raise HR and reduce SV during exercise after precooling (cold water bath) and preheating (hot water bath) and that no relationship would exist between SkBF and SV during exercise. Non-endurance-trained individuals cycled for 20 min at 69% ± 1% VO₂peak on four occasions: cool skin-cool core (SkCCC), warm skin-cool core (SkWCC), cool skin-warm core (SkCCW), and warm skin-warm core (SkWCW) on separate days. After precooling of TC, the rise in HR was greater in SkWCC than in SkCCC (P < 0.001), yet SV was similar (P = 0.26), which resulted in higher QC at min 20 in SkWCC (P < 0.01). Throughout exercise after preheating of TC, HR was higher (P < 0.001), SV was reduced (P < 0.01), and QC was similar (P = 0.40) in SkWCW versus SkCCW. When all trials were compared, there was no relationship between SkBF and SV (r = -0.08, P = 0.70); however, there was an inverse relationship between HR and SV (r = -0.75, P < 0.001). These data suggest that when TSK is elevated during exercise, HR and TC will rise but SV will only be reduced when TC is also elevated above 38°C. Furthermore, changes in SV are not related to changes in SkBF.

Downhill running and exercise in hot environments increase leukocyte Hsp72 (HSPA1A) and Hsp90α (HSPC1) gene transcripts.

Stressors within humans and other species activate Hsp72 and Hsp90α mRNA transcription, although it is unclear which environmental temperature or treadmill gradient induces the largest increase. To determine the optimal stressor for priming the Hsp system, physically active but not heat-acclimated participants (19.8 ± 1.9 and 20.9 ± 3.6 yr) exercised at lactate threshold in either temperate (20°C, 50% relative humidity; RH) or hot (30°C, 50% RH) environmental conditions. Within each condition, participants completed a flat running (temperate flat or hot flat) and a downhill running (temperate downhill or hot downhill) experimental trial in a randomized counterbalanced order separated by at least 7 days. Venous blood samples were taken immediately before (basal), immediately after exercise, and 3 and 24 h postexercise. RNA was extracted from leukocytes and RT-quantitative PCR conducted to determine Hsp72 and Hsp90α mRNA relative expression. Leukocyte Hsp72 mRNA was increased immediately after exercise following downhill running (1.9 ± 0.9-fold) compared with flat running (1.3 ± 0.4-fold; P = 0.001) and in hot (1.9 ± 0.6-fold) compared with temperate conditions (1.1 ± 0.5-fold; P = 0.003). Leukocyte Hsp90α mRNA increased immediately after exercise following downhill running (1.4 ± 0.8-fold) compared with flat running (0.9 ± 0.6-fold; P = 0.002) and in hot (1.6 ± 1.0-fold) compared with temperate conditions (0.9 ± 0.6-fold; P = 0.003). Downhill running and exercise in hot conditions induced the largest stimuli for leukocyte Hsp72 and Hsp90α mRNA increases.

Rain influences the physiological and metabolic responses to exercise in hot conditions

Outdoor exercise often proceeds in rainy conditions. However, the cooling effects of rain on human physiological responses have not been systematically studied in hot conditions. The present study determined physiological and metabolic responses using a climatic chamber that can precisely simulate hot, rainy conditions. Eleven healthy men ran on a treadmill at an intensity of 70% VO2max for 30 min in the climatic chamber at an ambient temperature of 33°C in the presence (RAIN) or absence (CON) of 30 mm · h−1 of precipitation and a headwind equal to the running velocity of 3.15 ± 0.19 m · s−1. Oesophageal temperature, mean skin temperature, heart rate, rating of perceived exertion, blood parameters, volume of expired air and sweat loss were measured. Oesophageal and mean skin temperatures were significantly lower from 5 to 30 min, and heart rate was significantly lower from 20 to 30 min in RAIN than in CON (P < 0.05 for all). Plasma lactate and epinephrine concentrations (30 min) and sweat loss were significantly lower (P < 0.05) in RAIN compared with CON. Rain appears to influence physiological and metabolic responses to exercise in heat such that heat-induced strain might be reduced.

The effect of ice ingestion on female athletes performing intermittent exercise in hot conditions

Studies have reported the benefits of pre-cooling prior to exercise in the heat for male athletes, but at this time no research has investigated female athletes. The aim of the following study was to test the effects of pre-cooling on female repeat sprint performance in hot, humid conditions; namely is ice ingestion effective in reducing core temperature (Tc) and does this reduced Tc lead to improved repeat sprint performance in female athletes? Nine female team sport athletes with mean age (21.0 ± 1.2 y), height (169.8 ± 4.1 cm) and body mass (62.3 ± 5.0 kg) participated in this study. Participants completed 72 min of an intermittent sprint protocol (ISP) consisting of 2 × 36 min halves in hot, humid conditions (33.1 ± 0.1°C, 60.3 ± 1.5% RH) on a cycle ergometer. This was preceded by 30 min of either ice ingestion (ICE) or water consumption (CON) in a randomised order. At the end of the pre-cooling period, Tc significantly decreased following ICE (−0.7 ± 0.3°C) compared to CON (−0.1 ± 0.2°C; p = 0.001). Tc also remained lower in ICE compared to CON during the ISP (p = 0.001). Ratings of perceived thermal sensation were lower in ICE compared to CON (p = 0.032) at the beginning (p = 0.022) and mid-point (p = 0.035) of the second half. No differences in work, mean power, peak power, rating of perceived exertion, heart rate or sweat loss between conditions were recorded (p > 0.05). Ice ingestion significantly reduced female Tc prior to intermittent exercise in the heat and reduced thermal sensation; however, this did not coincide with improved performance.

Efficacy of a novel phase change material for microclimate body cooling

The present study was conducted in order to evaluate the efficiency of personal body microclimate cooling systems based on a phase change materials (PCM) and its effects on physiological strain in soldiers during exertional heat stress in hot environment. The results are obtained in the experiment conducted in the climatic chamber in the Institute of Hygiene, Military Medical Academy in Belgrade. Ten male soldiers were voluntarily subjected to exertional heat stress tests (EHST) consisted of walking on treadmill (5.5 km/h) in hot conditions (40?C) in climatic chamber. The subjects performed first test while wearing a field camouflage uniform without any cooling system ("CONTROL" group) and in second test they used additional microclimate cooling system with paraffin wax consist of n-hexadecane (C16H34), in a form of cooling packs ("COOL" group). As indicators of thermal strain, mean skin (Tsk) and tympanic (Tty) temperature were determined. Simultaneously, thermal effects of PCM were measured by thermal imaging camera. The exercise in hot conditions induced a physiological response to heat stress, manifested through increased body core and skin temperatures. The results confirmed that the cooling vest worn over the field uniform was able to attenuate the physiological strain during exercise, compared to the identical exposure in the ''control'' group. The results of thermal imaging also indicate that heat generated inside the body is the main factor that will affect the phase change material melting time.