Hyperthermia and central fatigue during prolonged exercise in humans

Effects of pre-exercise glycerol supplementation on dehydration, metabolic, kinematic, and thermographic variables in international race walkers

BACKGROUND

Due to the increase in global temperature, it is necessary to investigate solutions so that athletes competing in hot conditions can perform in optimal conditions avoiding loss of performance and health problems. Therefore, this study aims to evaluate the effect of pre-exercise glycerol supplementation during a rectangular test at ambient temperature mid (28.2ºC) on dehydration variables in international race walkers.

METHODS

Eight international male race walkers (age: 28.0 years (4.4); weight: 65.6 kg (6.6); height: 180.0 cm (5.0); fat mass: 6.72% (0.66); muscle mass: 33.3 kg (3.3); VO2MAX: 66.5 ml · kg-1·min-1 (1.9)) completed this randomized crossover design clinical trial. Subjects underwent two interventions: they consumed placebo (n = 8) and glycerol (n = 8) acutely, before a rectangular test where dehydration, RPE, metabolic, kinematic, and thermographic variables were analyzed before, during and after the test.

RESULTS

After the intervention, significant differences were found between groups in body mass in favor of the placebo (Placebo: -2.23 kg vs Glycerol: -2.48 kg; p = 0.033). For other variables, no significant differences were found.

CONCLUSION

Therefore, pre-exercise glycerol supplementation was not able to improve any dehydration, metabolic, kinematic, or thermographic variables during a rectangular test at temperature mid in international race walkers. Possibly, a higher environmental temperature could have generated a higher metabolic and thermoregulatory stress, generating differences between groups like other previous scientific evidence.

Does acute dehydration affect the neuromuscular function in healthy adults?-a systematic review

The effects of acute dehydration on neuromuscular function have been studied. However, whether the mechanisms underpinning such function are central or peripheral is still being determined, and the results are inconsistent. This systematic review aims to elucidate the influence of acute dehydration on neuromuscular function, including a novel aspect of investigating the central and peripheral neuromuscular mechanisms. Three databases were used for the article search: PubMed, Web of Science, and Scopus. Studies were included if they had objective measurements of dehydration, muscle performance, and electromyography data or transcranial magnetic stimulation or peripheral nerve stimulation measurements with healthy individuals aged 18-65 years. Twenty-three articles met the eligibility criteria. The studies exhibited considerable heterogeneity in the methods used to induce and quantify dehydration. Despite being inconsistent, the literature shows some evidence that acute dehydration does not affect maximal strength during isometric or moderate-speed isokinetic contractions. Conversely, acute dehydration significantly reduces maximal strength during slow-speed isokinetic contractions and fatigue resistance in response to endurance tasks. The studies report that dehydration does not affect the motor cortical output or spinal circuity. The effects occur at the peripheral level within the muscle.

Impact of Eccentric versus Concentric Cycling Exercise on Neuromuscular Fatigue and Muscle Damage in Breast Cancer Patients

INTRODUCTION

This study investigated the magnitude and etiology of neuromuscular fatigue and muscle damage induced by eccentric cycling compared with conventional concentric cycling in patients with breast cancer.

METHODS

After a gradual familiarization protocol for eccentric cycling, nine patients with early-stage breast cancer performed three cycling sessions in eccentric or concentric mode. The eccentric cycling session (ECC) was compared with concentric cycling sessions matched for power output (CON power ; 80% of concentric peak power output, 95 ± 23 W) or oxygen uptake ( ; 10 ± 2 mL·min·kg -1 ). Preexercise to postexercise changes (30-s through 10-min recovery) in knee extensor maximal voluntary contraction force (MVC), voluntary activation, and quadriceps potentiated twitch force ( Qtw ) were quantified to determine global, central, and peripheral fatigue, respectively. Creatine kinase and lactate dehydrogenase activities were measured in the plasma before and 24 h after exercise as markers of muscle damage.

RESULTS

Compared with CON power (-11% ± 9%) and (-5% ± 5%), the ECC session resulted in a greater decrease in MVC (-25% ± 12%) postexercise ( P < 0.001). Voluntary activation decreased only in ECC (-9% ± 6% postexercise, P < 0.001). The decrease in Qtw was similar postexercise between ECC and CON power (-39% ± 21% and -40% ± 16%, P > 0.99) but lower in ( P < 0.001). The CON power session resulted in twofold greater compared with the ECC and sessions ( P < 0.001). No change in creatine kinase or lactate dehydrogenase activity was reported from preexercise to 24 h postexercise.

CONCLUSIONS

The ECC session induced greater neuromuscular fatigue compared with the concentric cycling sessions without generating severe muscle damage. ECC is a promising exercise modality for counteracting neuromuscular maladaptation in patients with breast cancer.

Head-neck cooling effects on central and peripheral fatigue, motor accuracy, and blood markers of stress in men with multiple sclerosis and healthy men: A randomized crossover study

BACKGROUND

The present study aimed to investigate whether head and neck cooling (at 18 °C next to the skin) and fatiguing submaximal exercise at a thermoneutral ambient temperature can induce peripheral and central responses in healthy men and those with multiple sclerosis (MS).

METHODS

A local head-neck cooling (at 18 °C next to the skin) intervention in men with a relapsing-remitting form of MS (n = 18; age 30.9 ± 8.1 years) and healthy men (n = 22; age 26.7 ± 5.9 years) was assessed. Men in both groups performed 100 intermittent isometric knee extensions with 5 s contractions and 20 s of rest. The primary variables were measured before exercise, after 50 and 100 repetitions, and 1 h after recovery. The central activation ratio, maximal voluntary contraction, electrically induced force, electromyography, contractile properties, blood markers, muscle temperature, and perception of effort were measured.

RESULTS

Compared with noncooled conditions, head and neck cooling increased the central capacity to activate exercising muscles but resulted in greater exercise-induced peripheral fatigue in men with MS (p < 0.05). Local cooling did not affect motor accuracy or the amplitude of electromyography signals; however, these factors were related to the intensity of the motor task (p > 0.05). The changes in central and peripheral fatigability induced by local cooling during submaximal exercise were more pronounced in men with MS than in healthy men (p < 0.05).

CONCLUSION

Head and neck cooling enhances central activation of muscles during exercise, leading to improved exercise performance compared with noncooled conditions in men with MS.

Multidimensional Analysis of Physiological Entropy during Self-Paced Marathon Running

The pacing of a marathon is arguably the most challenging aspect for runners, particularly in avoiding a sudden decline in speed, or what is colloquially termed a "wall", occurring at approximately the 30 km mark. To gain further insight into the potential for optimizing self-paced marathon performance through the coding of comprehensive physiological data, this study investigates the complex physiological responses and pacing strategies during a marathon, with a focus on the application of Shannon entropy and principal component analysis (PCA) to quantify the variability and unpredictability of key cardiorespiratory measures. Nine recreational marathon runners were monitored throughout the marathon race, with continuous measurements of oxygen uptake (V˙O2), carbon dioxide output (V˙CO2), tidal volume (Vt), heart rate, respiratory frequency (Rf), and running speed. The PCA revealed that the entropy variance of V˙O2, V˙CO2, and Vt were captured along the F1 axis, while cadence and heart rate variances were primarily captured along the F2 axis. Notably, when distance and physiological responses were projected simultaneously on the PCA correlation circle, the first 26 km of the race were positioned on the same side of the F1 axis as the metabolic responses, whereas the final kilometers were distributed on the opposite side, indicating a shift in physiological state as fatigue set in. The separation of heart rate and cadence entropy variances from the metabolic parameters suggests that these responses are independent of distance, contrasting with the linear increase in heart rate and decrease in cadence typically observed. Additionally, Agglomerative Hierarchical Clustering further categorized runners' physiological responses, revealing distinct clusters of entropy profiles. The analysis identified two to four classes of responses, representing different phases of the marathon for individual runners, with some clusters clearly distinguishing the beginning, middle, and end of the race. This variability emphasizes the personalized nature of physiological responses and pacing strategies, reinforcing the need for individualized approaches. These findings offer practical applications for optimizing pacing strategies, suggesting that real-time monitoring of entropy could enhance marathon performance by providing insights into a runner's physiological state and helping to prevent the onset of hitting the wall.

Heat stress and the velocity-duration relationship in amateur runners

Tolerance to high-intensity constant power exercise can be characterized by the hyperbolic power-duration (or velocity-duration) relationship. The hyperbola is defined by the asymptote (critical power or velocity) and the curvature constant (W' or D'). The effects of thermoregulatory stress on middle-distance running performance are equivocal-possibly due to the complexities of the hyperbolic velocity-duration relationship for these relatively short duration events. We aimed to measure the effects of heat stress on the velocity-duration relationship in amateur runners. Fifteen participants (23 ± 6 years) completed a series of constant-velocity running bouts to intolerance in three heat indices (MILD: 20°C, VERY HOT: 38°C, EXTREME: 55°C). Critical velocity (CV) in MILD (3.52 ± 0.86 m/s) was higher than VERY HOT (3.39 ± 0.82 m/s) and EXTREME (3.29 ± 1.05 m/s; F[2.28] = 3.80, p < 0.035) with no effect of thermal stress on D' (F[2.28] = 2.48, p = 0.11). In amateur competitive/recreational runners, heat stress of ≥38°C heat index negatively affected CV. Thus, even during relatively short events, such as middle-distance running where fluid loss is not a primary concern, heat stress may negatively impact performance.

Caffeine ingestion compromises thermoregulation and does not improve cycling time to exhaustion in the heat amongst males

PURPOSE

Caffeine is a commonly used ergogenic aid for endurance events; however, its efficacy and safety have been questioned in hot environmental conditions. The aim of this study was to investigate the effects of acute caffeine supplementation on cycling time to exhaustion and thermoregulation in the heat.

METHODS

In a double-blind, randomised, cross-over trial, 12 healthy caffeine-habituated and unacclimatised males cycled to exhaustion in the heat (35 °C, 40% RH) at an intensity associated with the thermoneutral gas exchange threshold, on two separate occasions, 60 min after ingesting caffeine (5 mg/kg) or placebo (5 mg/kg).

RESULTS

There was no effect of caffeine supplementation on cycling time to exhaustion (TTE) (caffeine; 28.5 ± 8.3 min vs. placebo; 29.9 ± 8.8 min, P = 0.251). Caffeine increased pulmonary oxygen uptake by 7.4% (P = 0.003), heat production by 7.9% (P = 0.004), whole-body sweat rate (WBSR) by 21% (P = 0.008), evaporative heat transfer by 16.5% (P = 0.006) and decreased estimated skin blood flow by 14.1% (P < 0.001) compared to placebo. Core temperature was higher by 0.6% (P = 0.013) but thermal comfort decreased by - 18.3% (P = 0.040), in the caffeine condition, with no changes in rate of perceived exertion (P > 0.05).

CONCLUSION

The greater heat production and storage, as indicated by a sustained increase in core temperature, corroborate previous research showing a thermogenic effect of caffeine ingestion. When exercising at the pre-determined gas exchange threshold in the heat, 5 mg/kg of caffeine did not provide a performance benefit and increased the thermal strain of participants.

Assessment of the Omius™ cooling headband effectiveness during a 70-min submaximal running effort followed by a 5-km time-trial in hot/humid conditions

Exercise performed under hot/humid conditions can hinder endurance performance. The Omius™ headband (OH) is purported to reduce the perception of heat and improve performance. We examined the impact of OH on selected thermal and cardiovascular functions, subjective perceptions and running performance. Using a randomized crossover protocol, 10 trained male athletes (28 ± 4 years) completed two trials (OH and sham headband (SH), 35.0 ± 0.3 °C, 56 ± 3% relative humidity) comprising 70 min of running (60% V˙O2max) followed by a 5-km running time-trial (TT). Heart rate, perceived exertion and whole-body thermal comfort did not significantly differ between conditions during the submaximal running effort and TT. Rectal temperature was higher with OH (0.11 ± 0.16 °C, p = 0.052) than SH prior to the submaximal running effort, however, no significant differences were observed between conditions regarding the changes in rectal temperature from baseline during the submaximal running effort and TT. Forehead temperature was significantly lower with OH than SH during the submaximal running effort, but no significant differences were observed at the end of the TT. Scores of perceived forehead thermal comfort were only significantly lower with OH than SH during the submaximal running effort. TT performance did not significantly differ between OH (19.8 ± 1.2 min) and SH (20.2 ± 1.0 min). In conclusion, OH improves forehead thermal comfort and reduces forehead temperature but not rectal temperature, heart rate and perceived exertion during, nor 5-km TT performance following, 70 min of submaximal running in the heat.

Exploring the Influence of Acid-Base Status on Athletic Performance during Simulated Three-Day 400 m Race

This study aimed to investigate the ability of highly trained athletes to consistently perform at their highest level during a simulated three-day 400 m race and to examine the impact of an alkaline diet associated with chronic consumption of bicarbonate-rich water or placebo on their blood metabolic responses before and after the three races. Twenty-two highly trained athletes, divided into two groups-one with an alkalizing diet and placebo water (PLA) and the other with an alkalizing diet and bicarbonate-rich water (BIC)-performed a 400 m race for three consecutive days. Performance metrics, urine and blood samples assessing acid-base balance, and indirect markers of neuro-muscular fatigue were measured before and after each 400 m race. The evolution of the Potential Renal Acid Load (PRAL) index and urinary pH highlights the combination of an alkalizing diet and bicarbonate-rich hydration, modifying the acid-base state (p < 0.05). Athletes in the PLA group replicated the same level of performance during three consecutive daily races without an increase in fatigue-associated markers. Athletes experienced similar levels of metabolic perturbations during the three 400 m races, with improved lactate clearance 20 min after the third race compared to the first two (p < 0.05). This optimization of the buffering capacity through ecological alkaline nutrition and hydration allowed athletes in the BIC group to improve their performance during the third 400 m race (p < 0.01). This study highlights athletes' ability to replicate high-level performances over three consecutive days with the same extreme level of metabolic disturbances, and an alkaline diet combined with bicarbonate-rich water consumption appears to enhance performance in a 400 m race.

Effect of hyperthermia on simulated muscle activation in female when crossing obstacle

It is well known that hyperthermia greatly impairs neuromuscular function and dynamic balance. However, whether a greater level of hyperthermia could potentially alter the lower limb simulated muscle activation when crossing an obstacle in female participants remains unknown. Therefore we examined the effect of a systematic increase in oral temperature on lower limb simulated muscle activation when crossing an obstacle in female participants. Eighteen female participants were recruited where they underwent a control trial (Con) and two progressive passive heating trials with Δ 1°C and Δ 2°C increase of oral temperature (Toral) using a 45°C water bath. In each trial, we assessed lower limb simulated muscle activation when crossing an obstacle height of 10%, 20%, and 30% of the participant's leg length and toe-off, toe-above-obstacle and heel-strike events were identified and analyzed. In all events, the lower limb simulated muscle activation were greater in Δ2°C than Δ1°C and Con when both leading and trailing limbs crossed the obstacle height of 20% and 30% leg length (all p < 0.001). However, the lower limb simulated muscle activation were not different between Δ1°C and Con across all obstacle heights (p > 0.05). This study concluded that a greater level of hyperthermia resulted in a greater lower limb simulated muscle activation to ensure safety and stability when females cross an obstacle height of 20% leg length or higher.

Effect of Heat Acclimatization, Heat Acclimation, and Intermittent Heat Training on Maximal Oxygen Uptake

BACKGROUND

Maximal oxygen uptake (VO2max) is an important determinant of endurance performance. Heat acclimation/acclimatization (HA/HAz) elicits improvements in endurance performance. Upon heat exposure reduction, intermittent heat training (IHT) may alleviate HA/HAz adaptation decay; however, corresponding VO2max responses are unknown.

HYPOTHESIS

VO2max is maintained after HAz/HA; IHT mitigates decrements in aerobic power after HAz/HA.

STUDY DESIGN

Interventional study.

LEVEL OF EVIDENCE

Level 3.

METHODS

A total of 27 male endurance runners (mean ± SD; age, 36 ± 12 years; body mass, 73.03 ± 8.97 kg; height, 178.81 ± 6.39 cm) completed VO2max testing at 5 timepoints; baseline, post-HAz, post-HA, and weeks 4 and 8 of IHT (IHT4, IHT8). After baseline testing, participants completed HAz, preceded by 5 days of HA involving exercise to induce hyperthermia for 60 minutes in the heat (ambient temperature, 39.13 ± 1.37°C; relative humidity, 51.08 ± 8.42%). Participants were assigned randomly to 1 of 3 IHT groups: once-weekly, twice-weekly, or no IHT. Differences in VO2max, velocity at VO2max (vVO2), and maximal heart rate (HRmax) at all 5 timepoints were analyzed using repeated-measure analyses of variance with Bonferroni corrections post hoc.

RESULTS

No significant VO2max or vVO2 differences were observed between baseline, post-HAz, or post-HA (P = 0.36 and P = 0.09, respectively). No significant group or time effects were identified for VO2max or vVO2 at post-HA, IHT4, and IHT8 (P = 0.67 and P = 0.21, respectively). Significant HRmax differences were observed between baseline and post-HA tests (P < 0.01). No significant group or time HRmax differences shown for post-HA, IHT4, and IHT8 (P = 0.59).

CONCLUSION

VO2max was not reduced among endurance runners after HA/HAz and IHT potentially due to participants' similar aerobic training status and high aerobic fitness levels.

CLINICAL RELEVANCE

HAz/HA and IHT maintain aerobic power in endurance runners, with HAz/HA procuring reductions in HRmax.

Impact of temperature on physical and cognitive performance in elite female football players during intermittent exercise

There is limited research on female football players, especially related to their physical and cognitive performance under different climactic conditions. We analyzed the impact of a hot environmental temperature on physical performance and anticipation in elite female football players during a fatigue-inducing intermittent protocol. Elite female players (n = 21) performed the countermovement jump (CMJ) and responded to filmed sequences of offensive play under two distinct environmental temperatures (i.e., mild environment temperature- 20°C and 30% rh versus hot environment temperature- 38°C and 80% rh), interspersed by 1-week interval. Linear mixed models were used. CMJ performance declined following the intermittent protocol on both temperature conditions (p < 0.05). Moreover, there were significant main effects for protocol on CMJ speed (m/s) (p = 0.001; ηp 2 = 0.12), CMJ power (p = 0.002; ηp 2 = 0.11), and CMJ Heightmax (p = 0.002; ηp 2 = 0.12). After performing the intermittent protocol, exposure to a hot temperature caused a greater decline in anticipation accuracy (mild temperature = 64.41% vs. hot temperature = 53.44%; p < 0.001). Our study shows impaired performance in elite female football players following an intermittent protocol under hot compared with mild environmental conditions. We report decreased performance in both CMJ and anticipation performance under hotter conditions. The results reveal that exposure to hot temperatures had a negative effect on the accuracy of their anticipatory behaviors. We consider the implication of the work for research and training interventions.

Elevated brain temperature under severe heat exposure impairs cortical motor activity and executive function

BACKGROUND

Excessive heat exposure can lead to hyperthermia in humans, which impairs physical performance and disrupts cognitive function. While heat is a known physiological stressor, it is unclear how severe heat stress affects brain physiology and function.

METHODS

Eleven healthy participants were subjected to heat stress from prolonged exercise or warm water immersion until their rectal temperatures (Tre) attained 39.5°C, inducing exertional or passive hyperthermia, respectively. In a separate trial, blended ice was ingested before and during exercise as a cooling strategy. Data were compared to a control condition with seated rest (normothermic). Brain temperature (Tbr), cerebral perfusion, and task-based brain activity were assessed using magnetic resonance imaging techniques.

RESULTS

Tbr in motor cortex was found to be tightly regulated at rest (37.3°C ± 0.4°C (mean ± SD)) despite fluctuations in Tre. With the development of hyperthermia, Tbr increases and dovetails with the rising Tre. Bilateral motor cortical activity was suppressed during high-intensity plantarflexion tasks, implying a reduced central motor drive in hyperthermic participants (Tre = 38.5°C ± 0.1°C). Global gray matter perfusion and regional perfusion in sensorimotor cortex were reduced with passive hyperthermia. Executive function was poorer under a passive hyperthermic state, and this could relate to compromised visual processing as indicated by the reduced activation of left lateral-occipital cortex. Conversely, ingestion of blended ice before and during exercise alleviated the rise in both Tre and Tbr and mitigated heat-related neural perturbations.

CONCLUSION

Severe heat exposure elevates Tbr, disrupts motor cortical activity and executive function, and this can lead to impairment of physical and cognitive performance.

Passive heating-induced changes in muscle contractile function are not further augmented by prolonged exposure in young males experiencing moderate thermal stress

Background: We investigated the impact of 1) passive heating (PH) induced by single and intermittent/prolonged hot-water immersion (HWI) and 2) the duration of PH, on muscle contractile function under the unfatigued state, and during the development of muscle fatigue. Methods: Twelve young males volunteered for this study consisting of two phases: single phase (SP) followed by intermittent/prolonged phase (IPP), with both phases including two conditions (i.e., four trials in total) performed randomly: control passive sitting (CON) and HWI (44-45°C; water up to the waist level). SP-HWI included one continuous 45-min bath (from 15 to 60 min). IPP-HWI included an initial 45-min bath (from 15 to 60 min) followed by eight additional 15-min baths interspaced with 15-min breaks at room temperature between 75 and 300 min. Intramuscular (Tmu; measured in the vastus lateralis muscle) and rectal (Trec) temperatures were determined. Neuromuscular testing (performed in the knee extensors and flexors) was performed at baseline and 60 min later during SP, and at baseline, 60, 90, 150 and 300 min after baseline during IPP. A fatiguing protocol (100 electrical stimulations of the knee extensors) was performed after the last neuromuscular testing of each trial. Results: HWI increased Tmu and Trec to 38°C-38.5°C (p < 0.05) during both SP and IPP. Under the unfatigued state, HWI did not affect electrically induced torques at 20 Hz (P20) and 100 Hz (P100). However, it induced a shift towards a faster contractile profile during both SP and IPP, as evidenced by a decreased P20/P100 ratio (p < 0.05) and an improved muscle relaxation (i.e., reduced half-relaxation time and increased rate of torque relaxation; p < 0.05). Despite a reduced voluntary activation (i.e., -2.63% ± 4.19% after SP-HWI and -5.73% ± 4.31% after IPP-HWI; condition effect: p < 0.001), HWI did not impair maximal isokinetic and isometric contraction torques. During the fatiguing protocol, fatigue index and the changes in muscle contractile properties were larger after HWI than CON conditions (p < 0.05). Finally, none of these parameters were significantly affected by the heating duration. Conclusion: PH induces changes in muscle contractile function which are not augmented by prolonged exposure when thermal stress is moderate.

Effect of sportswear on performance and physiological heat strain during prolonged running in moderately hot conditions

INTRODUCTION

This study examined the impact of different upper-torso sportswear technologies on the performance and physiological heat strain of well-trained and national-level athletes during prolonged running in moderately hot conditions.

METHODS

A randomized crossover design was employed in which 20 well-trained (n = 16) and national-level (n = 4) athletes completed four experimental trials in moderately hot conditions (35°C, 30% relative humidity). In each trial, participants ran at 70% of their peak oxygen uptake (70% V̇O2peak ) for 60 min, while wearing a different upper-body garment: cotton t-shirt, t-shirt with sweat-wicking fabric, compression t-shirt, and t-shirt with aluminum dots lining the inside of the upper back of the garment. Running speed was adjusted to elicit the predetermined oxygen consumption associated with 70% V̇O2peak . Physiological (core and skin temperatures, total body water loss, and urine specific gravity) and perceptual (thermal comfort and sensation, ratings of perceived exertion, and garment cooling functionality) parameters along with running speed at 70% V̇O2peak were continuously recorded.

RESULTS

No significant differences were observed between the four garments for running speed at 70% V̇O2peak , physiological heat strain, and perceptual responses (all p > 0.05). The tested athletes reported larger areas of perceived suboptimal cooling functionality in the cotton t-shirt and the t-shirt with aluminum dots relative to the sweat-wicking and compression t-shirts (d: 0.43-0.52).

CONCLUSION

There were not differences among the tested garments regarding running speed at 70% V̇O2peak , physiological heat strain, and perceptual responses in well-trained and national-level endurance athletes exercising in moderate heat.

Whole-body passive heating at moderate hyperthermic state impairs static and dynamic balance in healthy females

BACKGROUND

Whether static and dynamic balances can be impaired with increasing core temperature in female participants remains unknown.

PURPOSE

We tested the hypothesis that static and dynamic balances can be further impaired with systematic increases of core temperature by Δ1 °C and Δ 2 °C using whole-body passive heating.

METHOD

Eighteen female participants underwent a control trial (Con) and two progressive passive heating trials with Δ 1 °C and Δ 2 °C increase of oral temperature (TOral) using 45 °C water bath. In each trial, we assessed static balance with both eye open and closed and assessed dynamic balance using obstacle crossing at 10 %, 20 % and 30 % of the participant's leg length.

RESULTS

Static balance was not different between Con and Δ1 °C but was different between Δ1 °C and Δ 2 °C in an eye closed condition. Furthermore, Δ 2 °C greatly impaired both static and dynamic balances when compared to Con. The joint angles and toe clearance increased while leading heel-obstacle distance decreased during crossing obstacles at the height of 20 % and 30 % leg length with leading limbs in the Δ2 °C compared to Δ 1 °C and Con (All P < 0.05). However, no differences in joint kinematics and toe clearance with trailing limbs were observed (All P > 0.05).

CONCLUSION

In female participants, static and dynamic balances only became impaired when TOral increased 2 °C from baseline.

Exposures to Elevated Core Temperatures during Football Training: The Impact on Autonomic Nervous System Recovery and Function

Exercising with elevated core temperatures may negatively affect autonomic nervous system (ANS) function. Additionally, longer training duration under higher core temperatures may augment these negative effects. This study evaluated the relationship between exercise training duration and 24 h ANS recovery and function at ≥37 °C, ≥38 °C and ≥39 °C core temperature thresholds in a sample of male Division I (D1) collegiate American football athletes. Fifty athletes were followed over their 25-week season. Using armband monitors (Warfighter MonitorTM, Tiger Tech Solutions, Inc., Miami, FL, USA), core temperature (°C) and 24 h post-exercise baseline heart rate (HR), HR recovery and heart rate variability (HRV) were measured. For HRV, two time-domain indices were measured: the root mean square of the standard deviation of the NN interval (rMSSD) and the standard deviation of the NN interval (SDNN). Linear regression models were performed to evaluate the associations between exercise training duration and ANS recovery (baseline HR and HRV) and function (HR recovery) at ≥37 °C, ≥38 °C and ≥39 °C core temperature thresholds. On average, the athletes were 21.3 (± 1.4) years old, weighed 103.0 (±20.2) kg and had a body fat percentage of 15.4% (±7.8%, 3.0% to 36.0%). The duration of training sessions was, on average, 161.1 (±40.6) min and they ranged from 90.1 to 339.6 min. Statistically significant associations between training duration and 24 h ANS recovery and function were observed at both the ≥38.0 °C (baseline HR: β = 0.10 ± 0.02, R2 = 0.26, p < 0.0000; HR recovery: β = -0.06 ± 0.02, R2 = 0.21, p = 0.0002; rMSSD: β = -0.11 ± 0.02, R2 = 0.24, p < 0.0000; and SDNN: β = -0.16 ± 0.04, R2 = 0.22, p < 0.0000) and ≥39.0 °C thresholds (β = 0.39 ± 0.05, R2 = 0.62, p < 0.0000; HR recovery: β = -0.26 ± 0.04, R2 = 0.52, p < 0.0000; rMSSD: β = -0.37 ± 0.05, R2 = 0.58, p < 0.0000; and SDNN: β = -0.67 ± 0.09, R2 = 0.59, p < 0.0000). With increasing core temperatures, increases in slope steepness and strengths of the associations were observed, indicating accelerated ANS deterioration. These findings demonstrate that exercise training under elevated core temperatures (≥38 °C) may negatively influence ANS recovery and function 24 h post exercise and progressively worsen.

Neuromuscular and gene signaling responses to passive whole-body heat stress in young adults

This study aimed to investigate whether acute passive heat stress 1) decreases muscle Maximal Voluntary Contraction (MVC); 2) increases peripheral muscle fatigue; 3) increases spinal cord excitability, and 4) increases key skeletal muscle gene signaling pathways in skeletal muscle. Examining the biological and physiological markers underlying passive heat stress will assist us in understanding the potential therapeutic benefits. MVCs, muscle fatigue, spinal cord excitability, and gene signaling were examined after control or whole body heat stress in an environmental chamber (heat; 82 °C, 10% humidity for 30 min). Heart Rate (HR), an indicator of stress response, was correlated to muscle fatigue in the heat group (R = 0.59; p < 0.05) but was not correlated to MVC, twitch potentiation, and H reflex suppression. Sixty-one genes were differentially expressed after heat (41 genes >1.5-fold induced; 20 < 0.667 fold repressed). A strong correlation emerged between the session type (control or heat) and principal components (PC1) (R = 0.82; p < 0.005). Cell Signal Transduction, Metabolism, Gene Expression and Transcription, Immune System, DNA Repair, and Metabolism of Proteins were pathway domains with the largest number of genes regulated after acute whole body heat stress. Acute whole-body heat stress may offer a physiological stimulus for people with a limited capacity to exercise.

Effect of acute heat exposure on the determination of critical power and W' in women and men

The goal of this study was to investigate to what extent acute heat exposure would affect the parameters of the power-duration relationship, i.e. CP and W', using multiple constant workload tests to task failure, in women and men. Twenty four young physically active participants (12 men, 12 women) performed 3-5 constant load tests to determine CP and W', both in temperate (TEMP; 18°C) and hot (HOT; 36°C) environmental conditions. A repeated-measures ANOVA was executed to find differences between TEMP and HOT, and between women and men. In HOT, CP was reduced by 6.5% (227 ± 50 vs. 212 ± 47 W), while W' increased 12.4% (16.4 ± 4.4 vs. 18.5 ± 5.6 kJ). No significant two-way sex × temperature interactions were observed, indicating that the environmental conditions did not have a different effect in men compared with women. The intersection of the average curvatures in TEMP and HOT occurred at 137 s and 280 W in women, and 153 s and 397 W in men. Acute heat exposure had an impact on the parameters CP and W', i.e. CP decreased whereas W' increased. The increase in W' might be a consequence of the mathematical modelling for the used test methodology, rather than a physiological accurate value of W' in HOT. No differences induced by heat exposure were observed between women and men.

Core Body Temperatures in Intermittent Sports: A Systematic Review

BACKGROUND

Hyperthermia (and associated health and performance implications) can be a significant problem for athletes and teams involved in intermittent sports. Quantifying the highest thermal strain (i.e. peak core body temperature [peak Tc]) from a range of intermittent sports would enhance our understanding of the thermal requirements of sport and assist in making informed decisions about training or match-day interventions to reduce thermally induced harm and/or performance decline.

OBJECTIVE

The objective of this systematic review was to synthesise and characterise the available thermal strain data collected in competition from intermittent sport athletes.

METHODS

A systematic literature search was performed on Web of Science, MEDLINE, and SPORTDiscus to identify studies up to 17 April 2023. Electronic databases were searched using a text mining method to provide a partially automated and systematic search strategy retrieving terms related to core body temperature measurement and intermittent sport. Records were eligible if they included core body temperature measurement during competition, without experimental intervention that may influence thermal strain (e.g. cooling), in healthy, adult, intermittent sport athletes at any level. Due to the lack of an available tool that specifically includes potential sources of bias for physiological responses in descriptive studies, a methodological evaluation checklist was developed and used to document important methodological considerations. Data were not meta-analysed given the methodological heterogeneity between studies and therefore were presented descriptively in tabular and graphical format.

RESULTS

A total of 34 studies were selected for review; 27 were observational, 5 were experimental (2 parallel group and 3 repeated measures randomised controlled trials), and 2 were quasi-experimental (1 parallel group and 1 repeated measures non-randomised controlled trial). Across all included studies, 386 participants (plus participant numbers not reported in two studies) were recruited after accounting for shared data between studies. A total of 4 studies (~ 12%) found no evidence of hyperthermia, 24 (~ 71%) found evidence of 'modest' hyperthermia (peak Tc between 38.5 and 39.5 °C), and 6 (~ 18%) found evidence of 'marked' hyperthermia (peak Tc of 39.5 °C or greater) during intermittent sports competition.

CONCLUSIONS

Practitioners and coaches supporting intermittent sport athletes are justified to seek interventions aimed at mitigating the high heat strain observed in competition. More research is required to determine the most effective interventions for this population that are practically viable in intermittent sports settings (often constrained by many competing demands). Greater statistical power and homogeneity among studies are required to quantify the independent effects of wet bulb globe temperature, competition duration, sport and level of competition on peak Tc, all of which are likely to be key modulators of the thermal strain experienced by competing athletes.

REGISTRATION

This systematic review was registered on the Open Science Framework ( https://osf.io/vfb4s ; https://doi.org/10.17605/OSF.IO/EZYFA , 4 January 2021).

Reduced running performance and greater perceived exertion, but similar post-exercise neuromuscular fatigue in tropical natives subjected to a 10 km self-paced run in a hot compared to a temperate environment

Environmental heat stress impairs endurance performance by enhancing exercise-induced physiological and perceptual responses. However, the time course of these responses during self-paced running, particularly when comparing hot and temperate conditions, still needs further clarification. Moreover, monitoring fatigue induced by exercise is paramount to prescribing training and recovery adequately, but investigations on the effects of a hot environment on post-exercise neuromuscular fatigue are scarce. This study compared the time course of physiological and perceptual responses during a 10 km self-paced treadmill run (as fast as possible) between temperate (25°C) and hot (35°C) conditions. We also investigated the changes in countermovement jump (CMJ) performance following exercise in these two ambient temperatures. Thirteen recreational long-distance runners (11 men and 2 women), inhabitants of a tropical region, completed the two experimental trials in a randomized order. Compared to 25°C, participants had transiently higher body core temperature (TCORE) and consistently greater perceived exertion while running at 35°C (p < 0.05). These changes were associated with a slower pace, evidenced by an additional 14 ± 5 min (mean ± SD) to complete the 10 km at 35°C than at 25°C (p < 0.05). Before, immediately after, and 1 h after the self-paced run, the participants performed CMJs to evaluate lower limb neuromuscular fatigue. CMJ height was reduced by 7.0% (2.3 ± 2.4 cm) at 1 h after the race (p < 0.05) compared to pre-exercise values; environmental conditions did not influence this reduction. In conclusion, despite the reduced endurance performance, higher perceived exertion, and transiently augmented TCORE caused by environmental heat stress, post-exercise neuromuscular fatigue is similar between temperate and hot conditions. This finding suggests that the higher external load (faster speed) at 25°C compensates for the effects of more significant perceptual responses at 35°C in inducing neuromuscular fatigue.

Practical internal and external cooling methods do not influence rapid recovery from simulated taekwondo performance

Background/Objectives

The influence of post-exercise cooling on recovery has gained much attention in the empirical literature, however, data is limited in regards to optimizing recovery from taekwondo performance when combat is repeated in quick succession within the same day. The aim of this study was therefore to compare the effects of external and internal cooling after simulated taekwondo combat upon intestinal temperature (T_int), psychomotor skills (reaction time, response time, movement time), and neuromuscular function (peak torque, average power, time to reach peak torque).

Methods

Using a randomized counterbalanced crossover design, 10 well-trained male taekwondo athletes completed four recovery methods on separate occasions: passive recovery (CON), a 5-minute thermoneutral water immersion (35°C) (TWI), a 5-min cold water immersion (15°C) (CWI), and ice slurry ingestion (-1°C) (ICE; consumed every 5 min for 30 min). Heart rate (HR), blood lactate (Blac) concentrations, and T_int were determined at rest, immediately after combat, and at selected intervals during a 90-min recovery period. Neuromuscular functional (measured with isokinetic dynamometer) and psychomotor indices were assessed at baseline and after the recovery period.

Results

ICE led to a significantly lower T_int at 30 min (P<0.01) and 45 min (P<0.01) after simulated combat; 15-30 min after cessation of ingesting ice slurry, compared with the CON and TWI conditions, respectively. However, there were no differences in T_int across time points between the other conditions (P>0.05). Psychomotor skills and neuromuscular function indices returned to baseline values after the 90 min recovery period (P>0.05) with no differences observed between conditions (P>0.05).

Conclusion

The present findings suggest that internal (ICE) and external (CWI) recovery methods appear to have little impact on physiological and functional indices over the time course required to influence repeated taekwondo combat performance.

Effects of ice slurry ingestion on body temperature and softball pitching performance in a hot environment: a randomized crossover trial

BACKGROUND

Although softball players are often required to play in hot environments, scarce evidence is available regarding the effects of ice slurry ingestion on body temperature and pitching performance in softball pitchers in a hot environment. Thus, this study investigated the effects of ice slurry ingestion before and between innings on body temperature and softball pitching performance in a hot environment.

METHODS

In a randomized crossover design, seven heat-acclimatized amateur softball pitchers (four males and three females) completed simulated softball games consisting of 15 best-effort pitches per inning for seven innings with between-pitch rest intervals of 20 s. Participants were assigned to either a control trial (CON: ingestion of 5.0 g·kg^-1 of cool fluid [9.8 ± 2.2 °C] before simulated softball games and 1.25 g·kg^-1 of cool fluid between inning intervals) or an ice trial (ICE: ingestion of ice slurry [- 1.2 ± 0.1 °C] based on the same timings and doses as the CON). Participants performed both trials in an outdoor ground during the summer season (30.8 ± 2.7 °C, 57.0 ± 7.9% relative humidity).

RESULTS

Ice slurry ingestion before the simulated softball game (pre-cooling) resulted in a greater reduction in rectal temperature compared with cool fluid ingestion (p = 0.021, d = 0.68). No significant differences were observed between the trials in rectal temperature changes during the simulated softball game (p > 0.05). Compared to the CON, heart rate during the game was significantly decreased (p < 0.001, d = 0.43), and handgrip strength during the game was significantly increased (p = 0.001, d = 1.16) in the ICE. Ratings of perceived exertion, thermal comfort, and thermal sensation were improved in the ICE compared to those in the CON (p < 0.05). Ball velocity and pitching accuracy were not affected by ICE.

CONCLUSIONS

Ice slurry ingestion before and between innings reduced thermal, cardiovascular, and perceptual strain. However, it did not affect softball pitching performance compared to cool fluid ingestion.

Predicting the body core temperature of recreational athletes at the end of a 10 km self-paced run under environmental heat stress

NEW FINDINGS

What is the central question of this study? The aim was to identify the factors predicting the body core temperature of athletes at the end of a 10 km self-paced run in a hot environment. What is the main finding and its importance? Hyperthermia in athletes subjected to self-paced running depends on several factors, highlighting the integrated control of core temperature during exercise under environmental heat stress. Five of the seven variables that significantly predicted core temperature are not invasive and, therefore, practical for use outside the laboratory environment: heart rate, sweat rate, wet-bulb globe temperature, running speed and maximal oxygen consumption.

ABSTRACT

Measurement of body core temperature (Tcore ) is paramount to determining the thermoregulatory strain of athletes. However, standard measurement procedures of Tcore are not practical for extended use outside the laboratory environment. Therefore, determining the factors that predict Tcore during a self-paced run is crucial for creating more effective strategies to minimize the heat-induced impairment of endurance performance and reduce the occurrence of exertional heatstroke. The aim of this study was to identify the factors predicting Tcore values attained at the end of a 10 km time trial (end-Tcore ) under environmental heat stress. Initially, we extracted data obtained from 75 recordings of recreationally trained men and women. Next, we ran hierarchical multiple linear regression analyses to understand the predictive power of the following variables: wet-bulb globe temperature, average running speed, initial Tcore , body mass, differences between Tcore and skin temperature (Tskin ), sweat rate, maximal oxygen uptake, heart rate and change in body mass. Our data indicated that Tcore increased continuously during exercise, attaining 39.6 ± 0.5°C (mean ± SD) after 53.9 ± 7.5 min of treadmill running. This end-Tcore value was primarily predicted by heart rate, sweat rate, differences between Tcore and Tskin , wet-bulb globe temperature, initial Tcore , running speed and maximal oxygen uptake, in this order of importance (β power values corresponded to 0.462, -0.395, 0.393, 0.327, 0.277, 0.244 and 0.228, respectively). In conclusion, several factors predict Tcore in athletes subjected to self-paced running under environmental heat stress. Moreover, considering the conditions investigated, heart rate and sweat rate, two practical (non-invasive) variables, have the highest predictive power.

Determinants of body core temperatures at fatigue in rats subjected to incremental-speed exercise: The prominent roles of ambient temperature, distance traveled, initial core temperature, and measurement site

Understanding the factors that underlie the physical exercise-induced increase in body core temperature (T_CORE) is essential to developing strategies to counteract hyperthermic fatigue and reduce the risk of exertional heatstroke. This study analyzed the contribution of six factors to T_CORE attained at fatigue in Wistar rats (n = 218) subjected to incremental-speed treadmill running: ambient temperature (T_AMB), distance traveled, initial T_CORE, body mass, measurement site, and heat loss index (HLI). First, we ran hierarchical multiple linear regression analyses with data from different studies conducted in our laboratory (n = 353 recordings). We observed that T_AMB, distance traveled, initial T_CORE, and measurement site were the variables with predictive power. Next, regression analyses were conducted with data for each of the following T_CORE indices: abdominal (T_ABD), brain cortex (T_BRAIN), or colonic (T_COL) temperature. Our findings indicated that T_AMB, distance traveled (i.e., an exercise performance-related variable), initial T_CORE, and HLI predicted the three T_CORE indices at fatigue. Most intriguingly, HLI was inversely related to T_ABD and T_BRAIN but positively associated with T_COL. Lastly, we compared the temperature values at fatigue among these T_CORE indices, and the following descendent order was noticed - T_COL, T_ABD, and T_BRAIN - irrespective of T_AMB where experiments were conducted. In conclusion, T_CORE in rats exercised to fatigue depends primarily on environmental conditions, performance, pre-exercise T_CORE, and measurement site. Moreover, the influence of cutaneous heat loss on T_COL is qualitatively different from the influence on T_ABD and T_BRAIN, and the temperature values at fatigue are not homogenous within the body core.

Voluntary torque production is unaffected by changes in local thermal sensation during normothermia and hyperthermia

NEW FINDINGS

What is the central question of this study? Hyperthermia reduces the human capacity to produce muscular force, which is associated with decreased neural drive: does mitigating a reduction in neural drive by altering localised thermal sensation help to preserve voluntary force output? What is the main finding and its importance? Altering thermal sensation by cooling and heating the head independent of core temperature did not change neural drive or benefit voluntary force production. Head cooling did slow the rate of rise in core temperature during heating, which may have practical applications in passive settings.

ABSTRACT

This study investigated altered local head and neck thermal sensation on maximal and rapid torque production during voluntary contractions. Nine participants completed four visits in two environmental conditions: at rectal temperatures ∼39.5°C in hot (HOT; ∼50°C, ∼39% relative humidity) and ∼37°C in thermoneutral (NEU; ∼22°C, ∼46% relative humidity) conditions. Local thermal sensation was manipulated by heating in thermoneutral conditions and cooling in hot conditions. Evoked twitches and octets were delivered at rest. Maximum voluntary torque (MVT), normalised surface electromyography (EMG) and voluntary activation (VA) were assessed during brief maximal isometric voluntary contractions of the knee extensors. Rate of torque development (RTD) and EMG were measured during rapid voluntary contractions. MVT (P = 0.463) and RTD (P = 0.061) were similar between environmental conditions despite reduced VA (-6%; P = 0.047) and EMG at MVT (-31%; P = 0.019). EMG in the rapid voluntary contractions was also lower in HOT versus NEU during the initial 100 ms (-24%; P = 0.035) and 150 ms (-26%; P = 0.035). Evoked twitch (+70%; P < 0.001) and octet (+27%; P < 0.001) RTD during the initial 50 ms were greater in the HOT compared to NEU conditions, in addition to a faster relaxation rate of the muscle (-33%; P < 0.001). In conclusion, hyperthermia reduced neural drive without affecting voluntary torque, likely due to the compensatory effects of improved intrinsic contractile function and faster contraction and relaxation rates of the knee extensors. Changes in local thermal perception of the head and neck whilst hyperthermic or normothermic did not affect voluntary torque.

Effect of Work-to-Rest Cycles on Cardiovascular Strain and Maximal Oxygen Uptake during Heat Stress

Cardiovascular drift-a progressive increase in heart rate (HR) and decrease in stroke volume (SV) during prolonged exercise-is exacerbated by heat stress and thermal strain, and often accompanied by a decrease in work capacity (indexed as maximal oxygen uptake [V.O2max]). To attenuate physiological strain during work in the heat, use of work:rest ratios is recommended by the National Institute for Occupational Safety and Health. The purpose of this study was to test the hypothesis that during moderate work in hot conditions, utilizing the recommended 45:15 min work:rest ratio would result in cardiovascular drift 'accumulating' over consecutive work:rest cycles and accompanying decrements in V.O2max. Eight people (5 women; (mean ± SD) age = 25 ± 5 y; body mass = 74.8 ± 11.6 kg; V.O2max = 42.9 ± 5.6 mL·kg-1·min-1) performed 120 min of simulated moderate work (201-300 kcal·h-1) in hot conditions (indoor wet-bulb globe temperature = 29.0 ± 0.6 °C). Participants completed two 45:15 min work:rest cycles. Cardiovascular drift was evaluated at 15 and 45 min of each work bout; V.O2max was measured after 120 min. On a separate day, V.O2max was measured after 15 min under identical conditions for comparison before and after cardiovascular drift occurred. HR increased 16.7% (18 ± 9 beats·min-1, p = 0.004) and SV decreased 16.9% (-12.3 ± 5.9 mL, p = 0.003) between 15 and 105 min, but V.O2max was unaffected after 120 min (p = 0.14). Core body temperature increased 0.5 ± 0.2 °C (p = 0.006) over 2 h. Recommended work:rest ratios preserved work capacity but did not prevent the accumulation of cardiovascular and thermal strain.

The effect of cold water immersion on the recovery of physical performance revisited: A systematic review with meta-analysis

This review evaluated the effect of CWI on the temporal recovery profile of physical performance, accounting for environmental conditions and prior exercise modality. Sixty-eight studies met the inclusion criteria. Standardised mean differences were calculated for parameters assessed at <1, 1-6, 24, 48, 72 and ≥96 h post-immersion. CWI improved short-term recovery of endurance performance (p = 0.01, 1 h), but impaired sprint (p = 0.03, 1 h) and jump performance (p = 0.04, 6h). CWI improved longer-term recovery of jump performance (p < 0.01-0.02, 24 h and 96 h) and strength (p < 0.01, 24 h), which coincided with decreased creatine kinase (p < 0.01-0.04, 24-72 h), improved muscle soreness (p < 0.01-0.02, 1-72 h) and perceived recovery (p < 0.01, 72 h). CWI improved the recovery of endurance performance following exercise in warm (p < 0.01) and but not in temperate conditions (p = 0.06). CWI improved strength recovery following endurance exercise performed at cool-to-temperate conditions (p = 0.04) and enhanced recovery of sprint performance following resistance exercise (p = 0.04). CWI seems to benefit the acute recovery of endurance performance, and longer-term recovery of muscle strength and power, coinciding with changes in muscle damage markers. This, however, depends on the nature of the preceding exercise.

Prior heat exposure diminishes upper-body endurance work capacity and maximal arm and leg strength in young men

Workers often experience heat exposure before manual labour. This study investigated whether prior heat exposure diminished upper-body endurance work capacity and maximal isokinetic arm and leg strength in young men. Ten male participants completed two trials in a climatic chamber maintained at 25°C with 50% relative humidity. The two trials required them to complete a 30-min pre-exercise water-immersion at either 40°C (HOT) to provoke an approximately 1°C rise in rectal temperature (T_re) or 36°C (CON) to maintain a normal T_re. Pre- and post-immersion and following arm-cranking, isokinetic maximal voluntary contraction (MVC) torque was assessed for the elbow flexors and knee extensors. During arm-cranking, time to exhaustion was measured using arm crank ergometry at 60% peak oxygen uptake. Electromyography was recorded from the elbow flexors and knee extensors to calculate the integrated electromyography. T_re at post-immersion was higher in HOT (Mean ± SD, 38.1 ± 0.3°C) than CON (37.1 ± 0.3°C; P < 0.01). Time to exhaustion was less in HOT (41 ± 13 min) than CON (52 ± 12 min; P < 0.01). Isokinetic MVC torque in the elbow flexors and knee extensors was lower in HOT than CON (both P < 0.05). The integrated electromyography of the elbow flexors and knee extensors were lower in HOT than CON (both P < 0.05). This study indicates that a 1°C rise in T_re by prior heat exposure reduces time to exhaustion during arm-cranking and isokinetic MVC torque of the elbow flexors and knee extensors. Workers should be careful about reductions in upper-body endurance work capacity and maximal arm and leg strength when exposed to heat before manual labour.

Is the Cooling Vest an Ergogenic Tool for Physically Active Individuals? Assessment of Perceptual Response, Thermo-Physiological Behavior, and Sports Performance: A Systematic Review and Meta-Analysis

Exercise capacity is limited by environmental heat stress because thermoregulatory systems are altered and cannot prevent the elevation of body temperature due to a complex interplay of physiological, physical, and perceptual alterations. Cooling is an effective strategy to attenuate the temperature rise. Based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and the PEDro scale for assessing methodological quality, we systematically reviewed studies indexed in Medline, Web of Science, EMBASE, Science Direct, Sportdiscus, and Scopus, to evaluate the effects of the cooling vest (CVs) on perceptual response, physiological behavior, and sports performance in adult physical activity practitioners under heat stress conditions. Among the 711 studies identified in the search, 10 studies for the systematic review and eight for the meta-analysis met the inclusion and exclusion criteria. Overall, the use of CVs showed improvements in certain sports performance indicators, being significant (p < 0.05) in test time and substantial in peak power that could be influenced directly by the significant reduction (p < 0.05) in skin temperature and indirectly by the significant improvement (p < 0.05) in thermal and exertional perceptual responses, without the involvement of core temperature. In conclusion, the use of CVs is a cooling technique that influences perceptual response, thermo-physiological behavior, and sports performance. However, further studies are needed to elucidate the relevance of its application to CVs.

Heat acclimation reduces the effects of whole-body hyperthermia on knee-extensor relaxation rate, but does not affect voluntary torque production

PURPOSE

This study investigated the effects of acute hyperthermia and heat acclimation (HA) on maximal and rapid voluntary torque production, and their neuromuscular determinants.

METHODS

Ten participants completed 10 days of isothermic HA (50 °C, 50% rh) and had their knee-extensor neuromuscular function assessed in normothermic and hyperthermic conditions, pre-, after 5 and after 10 days of HA. Electrically evoked twitch and octet (300 Hz) contractions were delivered at rest. Maximum voluntary torque (MVT), surface electromyography (EMG) normalised to maximal M-wave, and voluntary activation (VA) were assessed during brief maximal isometric voluntary contractions. Rate of torque development (RTD) and normalised EMG were measured during rapid voluntary contractions.

RESULTS

Acute hyperthermia reduced neural drive (EMG at MVT and during rapid voluntary contractions; P < 0.05), increased evoked torques (P < 0.05), and shortened contraction and relaxation rates (P < 0.05). HA lowered resting rectal temperature and heart rate after 10 days (P < 0.05), and increased sweating rate after 5 and 10 days (P < 0.05), no differences were observed between 5 and 10 days. The hyperthermia-induced reduction in twitch half-relaxation was attenuated after 5 and 10 days of HA, but there were no other effects on neuromuscular function either in normothermic or hyperthermic conditions.

CONCLUSION

HA-induced favourable adaptations to the heat after 5 and 10 days of exposure, but there was no measurable benefit on voluntary neuromuscular function in normothermic or hyperthermic conditions. HA did reduce the hyperthermic-induced reduction in twitch half-relaxation time, which may benefit twitch force summation and thus help preserve voluntary torque in hot environmental conditions.

Thermal strain is greater in the late afternoon than morning during exercise in the gym without airflow and air conditioning on a clear summer day

Introduction

There are no reports examining the time-of-day effect on team training sessions in the gym without airflow and air conditioning on thermal strain in the summer heat. We investigated this effect during badminton training sessions on a clear summer day.

Methods

Nine male high school badminton players (Mean ± SD; age 17.1 ± 0.6 y, height 171 ± 4 cm, body mass 59 ± 7 kg) completed two 2.5-h badminton training sessions in the gym without airflow and air conditioning. The training sessions were started at 0900 h (AM) and 1600 h (PM) on separate days in August. Skin temperatures (chest, triceps, thigh, calf), infrared tympanic temperature, heart rate, thermal sensation and rating of perceived exertion were recorded at rest and at regular intervals during the sessions.

Results

Indoor and outdoor environmental heat stress progressively increased in AM and decreased in PM during the sessions. Ambient temperature (AM 30.1 ± 0.9°C; PM 33.2 ± 1.0°C: P < 0.001) and wet-bulb globe temperature (AM 28.1 ± 0.5°C; PM 30.0 ± 0.9°C: P = 0.001) during the sessions in the gym were higher in PM than AM. Mean skin temperature (AM 34.2 ± 1.0°C; PM 34.7 ± 0.7°C: P < 0.001), infrared tympanic temperature (AM 37.8 ± 0.5°C; PM 38.1 ± 0.4°C: P = 0.001) and thermal sensation (AM 2.7 ± 1.4; PM 3.3 ± 1.0: P < 0.001) during the sessions were higher in PM than AM. Body heat storage (AM 159 ± 30 W·m^-2; PM 193 ± 30 W·m^-2: P < 0.05) was greater in PM than AM. There were no time-of-day differences in the average heart rate (AM 75 ± 4% age-predicted maximal heart rate; PM 76 ± 5 age-predicted maximal heart rate: P = 0.534), body mass loss (AM 0.6 ± 0.3 kg; PM 0.8 ± 0.2°C: P = 0.079), the volume of water ingested (AM 1.5 ± 0.1 L; PM 1.6 ± 0.3 L: P = 0.447) and rating of perceived exertion (AM 16 ± 2; PM 16 ± 3: P = 0.281).

Conclusions

This study indicates greater thermal strain in PM trial than in AM trial during team training sessions in the gym without airflow and air conditioning on a clear summer day. Therefore, athletes and coaches of indoor sports should perceive that athletes may be exposed to a greater risk for thermal strain in the late afternoon from 1600 h than in the morning from 0900 h during the sessions in the gym under these conditions.

Active warm-up and time-of-day effects on repeated-sprint performance and post-exercise recovery

PURPOSE

This study investigated the effects of both an active warm-up and the time-of-day variation on repeated-sprint performance. A second objective was to compare the post-exercise recovery between the experimental conditions.

METHODS

Eleven male participants performed ten maximal cycling sprints (6 s each, with a 30-s interval between them) in the morning and late afternoon, either after a warm-up or control condition. The warm-up consisted of cycling for 10 min at 50% of the peak aerobic power.

RESULTS

Rest measurements of rectal, muscle, and skin temperatures were higher in the afternoon compared to the morning (p < 0.05), with no significant differences in heart rate (p = 0.079) and blood lactate concentration (p = 0.300). Warm-up increased muscle temperature, heart rate, and lactate, and reduced skin temperature (all p < 0.001), though no significant differences were observed for rectal temperature (p = 0.410). The number of revolutions (p = 0.034, η_p² = 0.375), peak (p = 0.034, η_p² = 0.375), and mean (p = 0.037, η_p² = 0.365) power of the first sprint (not the average of ten sprints) were higher in the afternoon compared to the morning, regardless of warm-up. However, beneficial performance effects of warming up were evident for the first (p < 0.001) and the average of ten sprints (p < 0.05), regardless of time of day. More remarkable changes during the 60-min post-exercise were observed for rectal temperature (p = 0.005) and heart rate (p = 0.010) in the afternoon than in the morning.

CONCLUSION

Warming-up and time-of-day effects in enhancing muscular power are independent. Although warm-up ensured further beneficial effects on performance than the time-of-day variation, a faster post-exercise recovery was observed in the late afternoon.

The Effects of an External Equine Nasal Strip on Thermoregulation During Exercise

The purpose of this study was to examine the effects of an external nasal strip on thermoregulation during submaximal exercise in Standardbred horses. While several studies have been conducted to determine the effects of the external nasal strip on airway resistance, exercise induced pulmonary hemorrhage, gas exchange, and time to fatigue in maximally exercising horses, the effects of the nasal strip on equine thermoregulation have not yet been examined. It was hypothesized that the application of an external nasal strip would alter central venous temperature (T_core), skin temperature (T_skin), and exercise time to reach a central venous temperature of 40 °C. Eight mature Standardbred horses each performed two submaximal exercise trials, one with a nasal strip (NS), and one without (control), on a high-speed equine treadmill with exercise concluding upon T_core reaching 40 °C. There were no significant differences in T_core or T_skin between the NS and control groups during pre-exercise, exercise or recovery (P > .05), nor were there differences (P > .05) in exercise time to reach a T_core of 40 °C (NS: 11.8 ± 1.5 minutes; Control: 11.5 ± 1.1 min). We conclude that the application of an external nasal strip does not affect the equine thermoregulatory response during submaximal exercise.

Performance effects of internal pre- and per-cooling across different exercise and environmental conditions: A systematic review

Exercise in a hot and humid environment may endanger athlete's health and affect physical performance. This systematic review aimed to examine whether internal administration of ice, cold beverages or menthol solutions may be beneficial for physical performance when exercising in different environmental conditions and sports backgrounds. A systematic search was performed in PubMed, Web of Science, Scopus and SPORTDiscus databases, from inception to April 2022, to identify studies meeting the following inclusion criteria: healthy male and female physically active individuals or athletes (aged ≥18 years); an intervention consisting in the internal administration (i.e., ingestion or mouth rinse) of ice slush, ice slurry or crushed ice and/or cold beverages and/or menthol solutions before and/or during exercise; a randomized crossover design with a control or placebo condition; the report of at least one physical performance outcome; and to be written in English. Our search retrieved 2,714 articles in total; after selection, 43 studies were considered, including 472 participants, 408 men and 64 women, aged 18-42 years, with a VO2max ranging from 46.2 to 67.2 mL⋅kg-1⋅min-1. Average ambient temperature and relative humidity during the exercise tasks were 32.4 ± 3.5°C (ranging from 22°C to 38°C) and 50.8 ± 13.4% (varying from 20.0% to 80.0%), respectively. Across the 43 studies, 7 exclusively included a menthol solution mouth rinse, 30 exclusively involved ice slurry/ice slush/crushed ice/cold beverages intake, and 6 examined both the effect of thermal and non-thermal internal techniques in the same protocol. Rinsing a menthol solution (0.01%) improved physical performance during continuous endurance exercise in the heat. Conversely, the ingestion of ice or cold beverages did not seem to consistently increase performance, being more likely to improve performance in continuous endurance trials, especially when consumed during exercises. Co-administration of menthol with or within ice beverages seems to exert a synergistic effect by improving physical performance. Even in environmental conditions that are not extreme, internal cooling strategies may have an ergogenic effect. Further studies exploring both intermittent and outdoor exercise protocols, involving elite male and female athletes and performed under not extreme environmental conditions are warranted. Systematic review registration: [https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42021268197], identifier [CRD42021268197].

The use of thermal imaging for monitoring the training progress of professional male sweep rowers

This study assesses the thermal profile of the skin in highly trained rowers and investigates the relationship between resting skin temperature (Ts) and the muscle peak torque (PT) measured in statics at the beginning (autumn) and the end (spring) of the preparatory period. Ten professional male sweep rowers, members of the Polish national rowing team, were investigated. A thermal imaging camera was used to analyze the Ts. The PT of the muscles involved in the rowing cycle were measured isometrically. No significant temperature asymmetries were found, except in front of arms after exercise in the spring (p = 0.0228). In contrast, the PT test in the autumn confirmed the significant asymmetry of the knee joint extensors (p = 0.0192). In spring compared to autumn, Ts in many areas of the body were slightly higher, as was PT of underlying muscles. Significant correlations between resting Ts and PT of the underlying muscles were found. Thermal imaging makes it possible to observe changes in skin temperature and symmetry before and after exercise. At this stage, it does not appear to be a method that, without supporting of other methods such as those assessing muscle function, will allow monitoring of training progress.

Palm cooling temperatures on thermal, physiological, perceptual, and ergogenic indices from rowing workouts in a thermoneutral environment

Gel pack temperatures were compared for responses to thermal, physiological, perceptual, and ergogenic indices from healthy women (n = 12) and men (n = 8). They did three rowing workouts to identify an ideal temperature. In a randomised sequence, and as subjects wore gloves equipped with mesh pouches during workouts, gel packs at one of the three average temperatures (10.6, 12.6, or 14.9°C) were inserted into the pouches. Data were collected before, during and after multi-stage workouts. Thermal, physiological, and perceptual data were each compared with three-factor (condition, gender, time) mixed effect model ANCOVAs, with repeated measures for condition and time, and gender as a between subjects' factor. Distance rowed was assessed with two-factor (condition, gender) mixed effect model ANCOVAs, with repeated measures for condition, and gender as a between subjects' factor. Within-subject contrasts was the post-hoc, and α = 0.05 denoted significance. Despite small differences for distance rowed, many dependent variables had significant inter-condition effects, whereby 10.6°C gel packs had the best thermal and physiological responses. The 10.6°C temperature 1): likely removed the most body heat, perhaps through cold-induced vasodilation and, 2): may be optimal, as it evoked the best thermal and physiological responses.

Do the National Institute for Occupational Safety and Health recommendations for working in the heat prevent excessive hyperthermia and body mass loss in unacclimatized males?

The National Institute for Occupational Safety and Health recommendations for work in the heat suggest workers consume 237 mL of water every 15-20 min and allow for continuous work at heavy intensities in hot environments up to 34 °C and 30% relative humidity. The goal was to determine whether the National Institute for Occupational Safety and Health recommendations prevented core temperature from exceeding 38.0 °C and greater than 2% body mass loss during heavy-intensity work in the heat. Eight males consumed 237 mL of water every 20 min during 2 hr of continuous heavy-intensity walking (6.4 kph, 1% grade) in a 34 °C/30% relative humidity environment, in accordance with the National Institute for Occupational Safety and Health recommendations. Projected core temperature and percent body mass loss were calculated for 4 and 8 hr of continuous work. Core temperature rose from baseline (36.8 ± 0.3 °C) to completion of 2 hr of work (38.1 ± 0.6 °C, p < 0.01), with two participants reaching the 38.0 °C threshold. Projected core temperatures remained elevated from baseline (p < 0.01), did not change from 2 to 4 hr (38.1 ± 0.7 °C, p > 0.99) and 4 to 8 hr (38.1 ± 0.8 °C, p > 0.99), respectively, and one participant exceeded 38.0 °C at 4 to 8 hr. There was no change in body mass loss over time (p > 0.99). During 2 hr of continuous heavy-intensity work in the heat, 75% of participants did not reach 38 °C core temperature and 88% did not reach 2% body mass loss when working to National Institute for Occupational Safety and Health recommendations.

Core body temperatures of rats subjected to treadmill exercise to fatigue or exhaustion: The journal Temperature toolbox

This study systematically reviewed the literature reporting the changes in rats' core body temperature (TCORE) induced by either incremental- or constant-speed running to fatigue or exhaustion. In addition, multiple linear regression analyses were used to determine the factors contributing to the TCORE values attained when exercise was interrupted. Four databases (EMBASE, PubMed, SPORTDiscus, and Web of Science) were searched in October 2021, and this search was updated in August 2022. Seventy-two studies (n = 1,538 rats) were included in the systematic review. These studies described heterogeneous experimental conditions; for example, the ambient temperature ranged from 5 to 40°C. The rats quit exercising with TCORE values varying more than 8°C among studies, with the lowest and highest values corresponding to 34.9°C and 43.4°C, respectively. Multiple linear regression analyses indicated that the ambient temperature (p < 0.001), initial TCORE (p < 0.001), distance traveled (p < 0.001; only incremental exercises), and running speed and duration (p < 0.001; only constant exercises) contributed significantly to explaining the variance in the TCORE at the end of the exercise. In conclusion, rats subjected to treadmill running exhibit heterogeneous TCORE when fatigued or exhausted. Moreover, it is not possible to determine a narrow range of TCORE associated with exercise cessation in hyperthermic rats. Ambient temperature, initial TCORE, and physical performance-related variables are the best predictors of TCORE at fatigue or exhaustion. From a broader perspective, this systematic review provides relevant information for selecting appropriate methods in future studies designed to investigate exercise thermoregulation in rats.

Cold Water Immersion Improves the Recovery of Both Central and Peripheral Fatigue Following Simulated Soccer Match-Play

The present study aimed to investigate the effect of cold water immersion (CWI) on the recovery of neuromuscular fatigue following simulated soccer match-play. In a randomized design, twelve soccer players completed a 90-min simulated soccer match followed by either CWI or thermoneutral water immersion (TWI, sham condition). Before and after match (immediately after CWI/TWI through 72 h recovery), neuromuscular and performance assessments were performed. Maximal voluntary contraction (MVC) and twitch responses, delivered through electrical femoral nerve stimulation, were used to assess peripheral fatigue (quadriceps resting twitch force, Q_tw,pot) and central fatigue (voluntary activation, VA). Performance was assessed via squat jump (SJ), countermovement jump (CMJ), and 20 m sprint tests. Biomarkers of muscle damages (creatine kinase, CK; Lactate dehydrogenase, LDH) were also collected. Smaller reductions in CWI than TWI were found in MVC (-9.9 ± 3%vs-23.7 ± 14.7%), VA (-3.7 ± 4.9%vs-15.4 ± 5.6%) and Q_tw,pot (-15.7 ± 5.9% vs. -24.8 ± 9.5%) following post-match intervention (p < 0.05). On the other hand, smaller reductions in CWI than TWI were found only in Q_tw,pot (-0.2 ± 7.7% vs. -8.8 ± 9.6%) at 72 h post-match. Afterwards, these parameters remained lower compared to baseline up to 48–72 h in TWI while they all recovered within 24 h in CWI. The 20 m sprint performance was less impaired in CWI than TWI (+11.1 ± 3.2% vs. +18 ± 3.6%, p < 0.05) while SJ and CMJ were not affected by the recovery strategy. Plasma LDH, yet no CK, were less increased during recovery in CWI compared to TWI. This study showed that CWI reduced both central and peripheral components of fatigue, which in turn led to earlier full recovery of the neuromuscular function and performance indices. Therefore, CWI might be an interesting recovery strategy for soccer players.

Moving in a hotter world: Maintaining adequate childhood fitness as a climate change countermeasure

Children cope with high temperatures differently than adults do, largely because of slight alterations in their body proportions and heat loss mechanisms compared to fully mature humans. Paradoxically, all current tools of assessing thermal strain have been developed on adults. As the Earth's warming continues to accelerate, children are set to bear the health risk brunt of rising global temperatures. Physical fitness has a direct impact on heat tolerance, yet children are less fit and more obese than ever before. Longitudinal research reveals that children have 30% lower aerobic fitness than their parents did at the same age; this deficit is greater than can be recovered by training alone. So, as the planet's climate and weather patterns become more extreme, children may become less capable of tolerating it. This comprehensive review provides an outline of child thermoregulation and assessment of thermal strain, before moving to summarize how aerobic fitness can modulate hyperthermia, heat tolerance, and behavioral thermoregulation in this under-researched population. The nature of child physical activity, physical fitness, and one's physical literacy journey as an interconnected paradigm for promoting climate change resilience is explored. Finally, future research foci are suggested to encourage continued exploration of this dynamic field, notable since more extreme, multifactorial environmental stressors are expected to continue challenging the physiological strain of the human population for the foreseeable future.

Ice Ingestion Maintains Cognitive Performance during a Repeated Sprint Performance in The Heat

This study investigated the effects of precooling via crushed ice ingestion on cognitive performance during repeated-sprint cycling in the heat. Nine males, non-heat acclimatised to heat (mean age: 28.2 ± 2.7 y; height: 175.7 ± 9.7 cm; body-mass: 76.9 ± 10.6 kg) completed a 30 min bout of repeated-sprint (36 × 4 s sprints, interspersed with 56 s rest-breaks) on a cycle ergometer in a climate chamber (35°C, 70% relative humidity). Crushed ice ingestion (7g·kg^-1, -0.4°C, ICE) or no cooling (CON) interventions were completed at rest, in the climate chamber, 30 min prior to exercise. Working memory was assessed via the serial seven test (S7) and the automated operation span task (OSPAN) at various time points before, during, and post-exercise. Core body temperature (T_c), forehead temperature (T_h), and thermal sensation (TS) were assessed throughout the protocol. Working memory significantly declined during exercise in CON as measured by S7 (p = 0.01) and OSPAN (p = 0.03); however, it was preserved in ICE with no change at the end of exercise in either S7 or OSPAN scores compared to baseline (p = 0.50, p = 0.09, respectively). Following precooling, T_h (-0.59°C, p < 0.001) and T_c (-0.67°C, p = 0.005) were significantly decreased in ICE compared to CON. At the end of the exercise, ICE significantly reduced T_c compared to CON (p = 0.03), but no significant differences were recorded for T_h. Further, TS was lower following precooling in ICE (p = 0.008) but not during exercise. In conclusion, ice ingestion significantly reduced T_h and T_c and facilitated maintenance of cognitive performance during repeated-sprint exercise in the heat, which may lead to better decision making.

Occupational heat strain in outdoor workers: A comprehensive review and meta-analysis

The present comprehensive review (i) summarizes the current knowledge on the impacts of occupational heat stress on outdoor workers, (ii) provides a historical background on this issue, (iii) presents a meta-analysis of published data, (iv) explores inter-individual and intra-individual factors, (v) discusses the available heat mitigation strategies, (vi) estimates physical work capacity, labour productivity, and metabolic rate for the year 2030, and (vii) provides an overview of existing policy and legal frameworks on occupational heat exposure. Meta-analytic findings from 38 field studies that involved monitoring 2,409 outdoor workers across 41 jobs in 21 countries suggest that occupational heat stress increases the core (r = 0.44) and skin (r = 0.44) temperatures, as well as the heart rate (r = 0.38) and urine specific gravity (r = 0.13) of outdoor workers (all p < 0.05). Moreover, it diminishes the capacity of outdoor workers for manual labour (r = -0.82; p < 0.001) and is responsible for more than two thirds of the reduction in their metabolic rate. Importantly, our analysis shows that physical work capacity is projected to be highly affected by the ongoing anthropogenic global warming. Nevertheless, the metabolic rate and, therefore, labour productivity are projected to remain at levels higher than the workers' physical work capacity, indicating that people will continue to work more intensely than they should to meet their financial obligations for food and shelter. In this respect, complementary measures targeting self-pacing, hydration, work-rest regimes, ventilated garments, and mechanization can be adopted to protect outdoor workers.

Transcranial Direct Current Stimulation Enhances Exercise Performance: A Mini Review of the Underlying Mechanisms

Exercise performance (EP) is affected by a combination of factors including physical, physiological, and psychological factors. This includes factors such as peripheral, central, and mental fatigue, external peripheral factors such as pain and temperature, and psychological factors such as motivation and self-confidence. During the last century, numerous studies from different fields of research were carried out to improve EP by modifying these factors. During the last two decades, the focus of research has been mainly moved toward the brain as a dynamic ever-changing organ and the ways changes in this organ may lead to improvements in physical performance. Development of centrally-acting performance modifiers such as level of motivation or sleep deprivation and the emergence of novel non-invasive brain stimulation (NIBS) techniques such as transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) are the key motives behind this move. This article includes three sections. Section Introduction provides an overview of the mechanisms behind the reduction of EP. The main focus of the Effects of tDCS on EP section is to provide a brief description of the effects of tDCS on maximal and submaximal types of exercise and finally, the section Mechanisms Behind the Effects of tDCS on EP provides description of the mechanisms behind the effects of tDCS on EP.

Thermal dysregulation in patients with multiple sclerosis during SARS-CoV-2 infection. The potential therapeutic role of exercise

Thermoregulation is a homeostatic mechanism that is disrupted in some neurological diseases. Patients with multiple sclerosis (MS) are susceptible to increases in body temperature, especially with more severe neurological signs. This condition can become intolerable when these patients suffer febrile infections such as coronavirus disease-2019 (COVID-19). We review the mechanisms of hyperthermia in patients with MS, and they may encounter when infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Finally, the thermoregulatory role and relevant adaptation to regular physical exercise are summarized.

Sodium bicarbonate ingestion mitigates the heat-induced hyperventilation and reduction in cerebral blood velocity during exercise in the heat

Hyperthermia during exercise in the heat causes minute ventilation ([Formula: see text]) to increase, which leads to reductions in arterial CO₂ partial pressure ([Formula: see text]) and cerebral blood flow. On the other hand, sodium bicarbonate ingestion reportedly results in metabolic alkalosis, leading to decreased [Formula: see text] and increased [Formula: see text] during prolonged exercise in a thermoneutral environment. Here, we investigated whether sodium bicarbonate ingestion suppresses heat-induced hyperventilation and the resultant hypocapnia and cerebral hypoperfusion during prolonged exercise in the heat. Eleven healthy men ingested a solution of sodium bicarbonate (0.3 g/kg body wt) (NaHCO₃ trial) or sodium chloride (0.208 g/kg) (NaCl trial). Ninety minutes after the ingestion, the subjects performed a cycle exercise for 60 min at 50% of peak oxygen uptake in the heat (35°C and 40% relative humidity). Esophageal temperature did not differ between the trials throughout (P = 0.56, main effect of trial). [Formula: see text] gradually increased with exercise duration in the NaCl trial, but the increases in [Formula: see text] were attenuated in the NaHCO₃ trial (P = 0.01, main effect of trial). Correspondingly, estimated [Formula: see text] and middle cerebral artery blood velocity (an index of anterior cerebral blood flow) were higher in the NaHCO₃ than the NaCl trial (P = 0.002 and 0.04, main effects of trial). Ratings of perceived exertion were lower in the NaHCO₃ than the NaCl trial (P = 0.02, main effect of trial). These results indicate that sodium bicarbonate ingestion mitigates heat-induced hyperventilation and reductions in [Formula: see text] and cerebral blood velocity during prolonged exercise in the heat.NEW & NOTEWORTHY Hyperthermia causes hyperventilation and concomitant hypocapnia and cerebral hypoperfusion. The cerebral hypoperfusion may underlie central fatigue. We demonstrate that sodium bicarbonate ingestion reduces heat-induced hyperventilation and attenuates hypocapnia-related cerebral hypoperfusion during prolonged exercise in the heat. In addition, we show that sodium bicarbonate ingestion reduces ratings of perceived exertion during the exercise. This study provides new insight into the development of effective strategies for preventing central fatigue during exercise in the heat.

The impact of elevated body core temperature on critical power as determined by a 3-min all-out test

The parameters of the power-duration relationship (critical power and W′) estimated by a 3-min all-out test were not altered by elevated body core temperature as compared with a thermoneutral condition.

Neuromuscular Activity during Cycling Performance in Hot/Dry and Hot/Humid Conditions

To determine the relationships between limiting factors and neuromuscular activity during a self-paced 20-km cycling time trial and evaluate the effect of environmental conditions on fatigue indices. Methods: Ten endurance-trained and heat-acclimated athletes performed in three conditions (ambient temperature, relative humidity): HUMID (30 °C, 90%), DRY (35 °C, 46%) and NEUTRAL (22 °C, 55%). Voluntary muscular contractions and electromagnetic stimulations were recorded before and after the time trials to assess fatigue. The data on performance, temperature, heat storage, electromyogram, heart rate and rating of perceived exertion data were analyzed. Results: Performance was impaired in DRY and HUMID compared with NEUTRAL environment (p < 0.05). The force developed by the vastus lateral muscle during stimulation of the femoral nerve remained unchanged across conditions. The percentage of integrated electromyogram activity, normalized by the value attained during the pre-trial maximal voluntary contraction, decreased significantly throughout the trial only in HUMID condition (p < 0.01). Neuromuscular activity in peripheral skeletal muscle started to fall from the 11th km in HUMID and the 15th km in DRY condition, although core temperature did not reach critical values. Conclusions: These alterations suggest that afferences from core/skin temperature regulate the central neural motor drive, reducing the active muscle recruited during prolonged exercise in the heat in order to prevent the system from hyperthermia.