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.

Central versus peripheral mechanisms of cold-induced vasodilation: a study in the fingers and toes of people with paraplegia.

PURPOSE: This study examined physiological and perceptual parameters related to cold-induced vasodilation (CIVD) in the fingers and toes of people with paraplegia and compared them with responses observed in able-bodied individuals. METHODS: Seven participants with paraplegia and seven able-bodied individuals participated in a randomized matched-controlled study involving left-hand and -foot immersion in cold water (8 ± 1 °C) for 40 min during exposure to cool (16 ± 1 °C), thermoneutral (23 ± 1 °C), and hot (34 ± 1 °C) ambient conditions. RESULTS: Similar CIVD occurrence was observed in the fingers in the two groups. In toes, three of the seven participants with paraplegia revealed CIVDs: one in cool, two in thermoneutral, and three in hot conditions. No able-bodied participants revealed CIVDs in cool and thermoneutral conditions, while four revealed CIVDs in hot conditions. The toe CIVDs of paraplegic participants were counterintuitive in several respects: they were more frequent in cool and thermoneutral conditions (compared to the able-bodied participants), emerged in these conditions despite lower core and skin temperatures of these participants, and were evident only in cases of thoracic level lesions (instead of lesions at lower spinal levels). CONCLUSION: Our findings demonstrated considerable inter-individual variability in CIVD responses in both the paraplegic and able-bodied groups. While we observed vasodilatory responses in the toes of participants with paraplegia that technically fulfilled the criteria for CIVD, it is unlikely that they reflect the CIVD phenomenon observed in able-bodied individuals. Taken together, our findings favor the contribution of central over peripheral factors in relation to the origin and/or control of CIVD.

Efficacy of cooling vests based on different heat-extraction concepts: The HEAT-SHIELD project.

INTRODUCTION: A wide range of cooling vests for heat-strain mitigation purposes during physical work are available on the market. The decision regarding the optimal cooling vest/concept for a specific environment can be challenging by relying solely on the information provided by the manufacturers. The aim of this study was to investigate how different types of cooling vests would manifest/perform in a simulated industrial setting, in a warm and moderately humid environment with low air velocity. METHODS: Ten young males completed six experimental trials, including a control trial (no vest) and five trials with vests of different cooling concepts. Once entering the climatic chamber (ambient temperature: 35 °C, relative humidity: 50 %), participants remained seated for 30 min to induce passive heating, after which they donned a cooling vest and started a 2.5-h of walk at 4.5 km·h-1. During the trial, torso skin temperature (Tsk), microclimate temperature (Tmicro) and relative humidity (RHmicro), as well as core temperature (rectal and gastrointestinal; Tc) and heart rate (HR) were measured. Before and after the walk, participants conducted different cognitive tests and provided subjective ratings throughout the walk. RESULTS: The use of the vests attenuated the increase in HR (103 ± 12 bpm) when compared to control trial (116 ± 17 bpm, p < 0.05). Four vests maintained a lower torso Tsk (31.7 ± 1.5 °C) compared to control trial (36.1 ± 0.5 °C, p < 0.05). Two vests using PCM inserts attenuated the increase in Tc between 0.2 and 0.5 °C in relation to control trial (p < 0.05). Cognitive performance remained unchanged between the trials. Physiological responses were also well reflected in subjective reports. CONCLUSION: Most vests could be considered as an adequate mitigation strategy for workers in industry under the conditions simulated in the present study.

Cross-adaptation between heat and hypoxia: mechanistic insights into aerobic exercise performance.

Acclima(tiza)tion to heat or hypoxia enhances work capacity in hot and hypoxic environmental conditions, respectively; an acclimation response is considered to be mediated by stimuli-specific molecular/systemic adaptations and potentially facilitated by the addition of exercise sessions. Promising findings at the cellular level provided the impetus for recent studies investigating whether acclimation to one stressor will ultimately facilitate whole body performance when exercise is undertaken in a different environmental condition. The present critical Mini-Review examines the theory of cross-adaptation between heat and hypoxia with particular reference to the determinants of aerobic performance. Indeed, early functional adaptations (improved exercise economy and enhanced oxyhemoglobin saturation) succeeded by later morphological adaptations (increased hemoglobin mass) might aid acclimatized humans perform aerobic work in an alternative environmental setting. Longer-term acclimation protocols that focus on the specific adaptation kinetics (and further allow for the adaptation reversal) will elucidate the exact physiological mechanisms that might mediate gains in aerobic performance or explain the lack thereof.

Age-Related Declines in Lower Limb Muscle Function are Similar in Power and Endurance Athletes of Both Sexes: A Longitudinal Study of Master Athletes.

The age-related decline in muscle function, particularly muscle power, is associated with increased risk of important clinical outcomes. Physical activity is an important determinant of muscle function, and different types of physical activity e.g. power-based versus endurance-based exercise appear to have differential effects on muscle power. Cross-sectional studies suggest that participation in power-based exercise is associated with greater muscle power across adulthood but this has not been investigated longitudinally. We recruited eighty-nine male and female power and endurance master athletes (sprint and distance runners respectively, baseline age 35-90y). Using jumping mechanography, we measured lower limb muscle function during a vertical jump including at least two testing sessions longitudinally over 4.5 ± 2.4y. We examined effects of time, discipline (power/endurance) and sex in addition to two- and three-way interactions using linear mixed-effects models. Peak relative power, relative force and jump height, but not Esslingen Fitness Index (indicating peak power relative to sex and age-matched reference data) declined with time. Peak power, force, height and EFI were greater in power than endurance athletes. There were no sex, discipline or sex*discipline interactions with time for any variable, suggesting that changes were similar over time for athletes of both sexes and disciplines. Advantages in lower limb muscle function in power athletes were maintained with time, in line with previous cross-sectional studies. These results suggest that improvements in lower limb function in less active older individuals following power-based training persist with continued adherence, although this requires further investigation in interventional studies.

Myths and methodologies: Degrees of freedom - limitations of infrared thermographic screening for Covid-19 and other infections.

Around the planet, in many different scenarios, skin temperature is being used as a surrogate measure of deep body (core) temperature in the assessment of whether an individual is infected with Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), which causes the coronavirus disease (Covid-19), as indicated by the presence of fever. The key question is whether this is a valid methodology. If it is not, we run the risk of falsely excluding individuals from places they may want, or need, to go. We also run the risk of falsely allowing people into places where they can spread the undetected infection they have. In this review, we explore these and associated questions. We establish the limited utility of the current methodology for the mass screening of individuals for Covid-19 related fever using infrared thermography. We propose the development of an alternative method that may prove to be more sensitive.

Effects of vision on energy expenditure and kinematics during level walking.

PURPOSE: We have previously observed substantially higher oxygen uptake in soldiers walking on terrain at night than when performing the same walk in bright daylight. The aims of the present study were to investigate the influence of vision on mechanical efficiency during slow, horizontal, constant-speed walking, and to determine whether any vision influence is modified by load carriage. METHODS: Each subject (n = 15) walked (3.3 km/h) for 10 min on a treadmill in four different conditions: (1) full vision, no carried load, (2) no vision, no carried load, (3) full vision with a 25.5-kg rucksack, (4) no vision with a 25.5-kg rucksack. RESULTS: Oxygen uptake was 0.94 ± 0.12 l/min in condition (1), 1.15 ± 0.20 l/min in (2), 1.15 ± 0.12 l/min in (3) and 1.35 ± 0.19 l/min in (4). Thus, lack of vision increased oxygen uptake by about 19%. Analyses of movement pattern, by use of optical markers attached to the limbs and torso, revealed considerably shorter step length (12 and 10%) in the no vision (2 and 4) than full vision conditions (1 and 3). No vision conditions (2 and 4) increased step width by 6 and 6%, and increased vertical foot clearance by 20 and 16% compared to full vision conditions (1 and 3). CONCLUSION: The results suggest that vision has a marked influence on mechanical efficiency even during entrained, repetitive movements performed on an obstacle-free horizontal surface under highly predictable conditions.

Predicting Deep Body Temperature (Tb) from Forehead Skin Temperature: Tb or Not Tb?

There is a need to rapidly screen individuals for heat strain and fever using skin temperature (Tsk) as an index of deep body temperature (Tb). This study's aim was to assess whether Tsk could serve as an accurate and valid index of Tb during a simulated heatwave. Seven participants maintained a continuous schedule over 9-days, in 3-day parts; pre-/post-HW (25.4 °C), simulated-HW (35.4 °C). Contact thermistors measured Tsk (Tforehead, Tfinger); radio pills measured gastrointestinal temperature (Tgi). Proximal-distal temperature gradients (ΔTforehead-finger) were also measured. Measurements were grouped into ambient conditions: 22, 25, and 35 °C. Tgi and Tforehead only displayed a significant relationship in 22 °C (r: 0.591; p < 0.001) and 25 °C (r: 0.408; p < 0.001) conditions. A linear regression of all conditions identified Tforehead and ΔTforehead-finger as significant predictors of Tgi (r2: 0.588; F: 125.771; p < 0.001), producing a root mean square error of 0.26 °C. Additional residual analysis identified Tforehead to be responsible for a plateau in Tgi prediction above 37 °C. Contact Tforehead was shown to be a statistically suitable indicator of Tgi in non-HW conditions; however, an error of ~1 °C makes this physiologically redundant. The measurement of multiple sites may improve Tb prediction, though it is still physiologically unsuitable, especially at higher ambient temperatures.

Heterogeneity of human adaptations to bed rest and hypoxia: a retrospective analysis within the skeletal muscle oxidative function.

This retrospective study was designed to analyze the interindividual variability in the responses of different variables characterizing the skeletal muscle oxidative function to normoxic (N-BR) and hypoxic (H-BR) bed rests and to a hypoxic ambulatory confinement (H-AMB) of 10 and 21 days. We also assessed whether and how the addition of hypoxia to bed rest might influence the heterogeneity of the responses. In vivo measurements of O2 uptake and muscle fractional O2 extraction were carried out during an incremental one-leg knee-extension exercise. Mitochondrial respiration was assessed in permeabilized muscle fibers. A total of 17 subjects were included in this analysis. This analysis revealed a similar variability among subjects in the alterations induced by N-BR and H-BR both in peak O2 uptake (SD: 4.1% and 3.3% after 10 days; 4.5% and 8.1% after 21 days, respectively) and peak muscle fractional O2 extraction (SD: 5.9% and 7.3% after 10 days; 6.5% and 7.3% after 21 days), independently from the duration of the exposure. The individual changes measured in these variables were significantly related (r = 0.66, P = 0.004 after N-BR; r = 0.61, P = 0.009 after H-BR). Mitochondrial respiration showed a large variability of response after both N-BR (SD: 25.0% and 15.7% after 10 and 21 days) and H-BR (SD: 13.0% and 19.8% after 10 and 21 days); no correlation was found between N-BR and H-BR changes. When added to bed rest, hypoxia altered the individual adaptations within the mitochondria but not those intrinsic to the muscle oxidative function in vivo, both after the short- and medium-term exposures.

The eye in extreme environments.

NEW FINDINGS: What is the topic of this review? This review describes the effect of extreme environments on the visual system. What advances does it highlight? The review highlights the way in which environmental stressors affect the eye and vision, both directly and indirectly. ABSTRACT: Much is known about the physiology and anatomy of the eye. Much less is known about the impact of different environments on the eye, and yet it is the pathophysiology that results from this interaction that is often the precursor to disaster. The present review focuses on the effect of different extreme environments on the visual system; in particular, the way in which such environments affect the sensory mechanism of that system.

Exercise temperature regulation following a 35-day horizontal bedrest.

NEW FINDINGS: What is the central question of this study? Does a 35-day horizontal bedrest impair thermoeffector responses during whole-body submaximal exercise performed in temperate conditions? What is the main finding and its importance? Cardiovascular and muscular deconditioning ensuing from prolonged recumbency seems to augment, at least to a degree, exercise-induced increase in body core temperature, most likely due to an impairment in non-evaporative heat loss. The response is a function of the absolute exercise intensity imposed. ABSTRACT: We examined the effects of a 35-day horizontal bedrest on thermoregulation during whole-body exercise. Fifteen healthy men were randomly assigned to either a bedrest (BR; n = 10) or a control (CON; n = 5) group. Prior to bedrest, both groups performed 40-min constant-load upright cycling at 30% of their peak workload (Wpeak ; PRE). One and 2 days after bedrest, the BR group performed, in a randomised counterbalanced order, two 40-min trials at 30% of (i) the pre-bedrest Wpeak (i.e., at a fixed absolute intensity; POST-A) and (ii) the post-bedrest Wpeak (i.e., at a fixed relative intensity; POST-R). The CON group conducted only the POST-A trial, at the same time intervals. During the trials, rectal (Trec ) and skin ( T¯sk ) temperatures, and the forehead sweating rate (SwR) were monitored. In the CON group, no differences were observed between the trials. Bedrest potentiated moderately the Trec elevation during the latter part of POST-A (∼0.10°C; P ≤ 0.05), but not of POST-R (∼0.04°C; P = 0.11). In both post-bedrest trials, T¯sk was attenuated by ∼1.5-2.0°C throughout (P < 0.01), whereas the forehead SwR was not modulated. Trec and T¯sk were similar in POST-A and POST-R, yet the forehead SwR was more dependent on the relative workload imposed (P = 0.04). The present findings therefore suggest that the cardiovascular and muscular deconditioning ensuing from a 35-day bedrest may aggravate the exercise-induced increase in body core temperature when working at a given absolute intensity, most likely due to an impairment in non-evaporative heat loss.

Do females and males exhibit a similar sarcopenic response as a consequence of normoxic and hypoxic bed rest?

NEW FINDINGS: What is the central question of this study? Do females and males exhibit a similar sarcopenic response as a consequence of normoxic and hypoxic bed rest? What is the main finding and its importance? During 10-day bed rest, exposure to a simulated (normobaric hypoxia) altitude of ∼4000 m does not exert additional significant structural or functional effect on the weight-bearing muscles in females compared to those noted under normoxic conditions. Whereas males and females exhibit decrements in muscle cross-sectional area and mass during normoxic and hypoxic bed rest, a concomitant strength decrement was only observed in males. ABSTRACT: This study investigated the effects of hypoxia on the known processes of adaptation of body composition and muscle function to normoxic inactivity. Females (n = 12) and males (n = 11) took part in the following interventions: hypoxic ambulation (HAMB; ∼4000 m); hypoxic bed rest (HBR; ∼4000 m) and normoxic bed rest (NBR). Prior to and immediately following each intervention, body composition, thigh and lower leg cross-sectional area (CSA) and isometric muscular strength were recorded. Participants lost body mass (HAMB: male -1.5 ± 0.9 kg, female -1.9 ± 0.7 kg; HBR: male -2.0 ± 1.8 kg, female -2.4 ± 0.8 kg; NBR: male -1.4 ± 1.3 kg, female -1.4 ± 0.9 kg) and lean mass (HAMB: male -3.9 ± 3.0%, female -3.4 ± 2.0%; HBR: male -4.0 ± 4.4%, female -4.1 ± 2.0%; NBR: male -4.0 ± 3.4%, female -2.2 ± 2.7%) with no between-condition or sex differences. Knee extension decreased for males in NBR compared to HAMB (HAMB: male -0.2 ± 9.1%, female 1.3 ± 4.9%; HBR: male -7.8 ± 10.3%, female -3.3 ± 10.9%; NBR: male -14.5 ± 11%, female -3.4 ± 6.9%). Loss of force during maximal voluntary contraction (MVC) in the knee extensors was significantly different between males and females following NBR. There were no other significant changes noted following the experimental interventions. There were no differences recorded between sexes in maximal MVC for elbow or ankle joints. Female lower leg CSA decreased following bed rest (HAMB: -4.5 ± 2.0%; HBR: -9.9 ± 2.6%; NBR: -8.0 ± 1.6%). These findings indicate that a 10-day hypoxic bed rest does not exert any significant additional effect on muscle atrophy when compared to NBR, except for female thigh CSA. In contrast to males, who exhibited a significant loss of muscle strength, no such decrement in strength was observed in the female participants.

Sterilization of polypropylene membranes of facepiece respirators by ionizing radiation.

Ionizing radiation has been identified as an option for sterilization of disposable filtering facepiece respirators in situations where the production of the respirators cannot keep up with demand. Gamma radiation and high energy electrons penetrate deeply into the material and can be used to sterilize large batches of masks within a short time period. In relation to reports that sterilization by ionizing radiation reduces filtration efficiency of polypropylene membrane filters on account of static charge loss, we have demonstrated that both gamma and electron beam irradiation can be used for sterilization, provided that the respirators are recharged afterwards.

Heat acclimation enhances the cold-induced vasodilation response.

PURPOSE: It has been reported that the cold-induced vasodilation (CIVD) response can be trained using either regular local cold stimulation or exercise training. The present study investigated whether repeated exposure to environmental stressors, known to improve aerobic performance (heat and/or hypoxia), could also provide benefit to the CIVD response. METHODS: Forty male participants undertook three 10-day acclimation protocols including daily exercise training: heat acclimation (HeA; daily exercise training at an ambient temperature, Ta = 35 °C), combined heat and hypoxic acclimation (HeA/HypA; daily exercise training at Ta = 35 °C, while confined to a simulated altitude of ~ 4000 m) and exercise training in normoxic thermoneutral conditions (NorEx; no environmental stressors). To observe potential effects of the local acclimation on the CIVD response, participants additionally immersed their hand in warm water (35 °C) daily during the HeA/HypA and NorEx. Before and after the acclimation protocols, participants completed hand immersions in cold water (8 °C) for 30 min, followed by 15-min recovery phases. The temperature was measured in each finger. RESULTS: Following the HeA protocol, the average temperature of all five fingers was higher during immersion (from 13.9 ± 2.4 to 15.5 ± 2.5 °C; p = 0.04) and recovery (from 22.2 ± 4.0 to 25.9 ± 4.9 °C; p = 0.02). The HeA/HypA and NorEx protocols did not enhance the CIVD response. CONCLUSION: Whole-body heat acclimation increased the finger vasodilatory response during cold-water immersion, and enhanced the rewarming rate of the hand, thus potentially contributing to improved local cold tolerance. Daily hand immersion in warm water for 10 days during HeA/Hyp and NorEx, did not contribute to any changes in the CIVD response.

Size- and Time-Dependent Particle Removal Efficiency of Face Masks and Improvised Respiratory Protection Equipment Used during the COVID-19 Pandemic.

Size- and time-dependent particle removal efficiency (PRE) of different protective respiratory masks were determined using a standard aerosol powder with the size of particles in the range of an uncoated SARS-CoV-2 virus and small respiratory droplets. Number concentration of particles was measured by a scanning mobility particle sizer. Respiratory protective half-masks, surgical masks, and cotton washable masks were tested. The results show high filtration efficiency of FFP2, FFP3, and certified surgical masks for all sizes of tested particles, while protection efficiency of washable masks depends on their constituent fabrics. Measurements showed decreasing PRE of all masks over time due to transmission of nanoparticles through the mask-face interface. On the other hand, the PRE of the fabric is governed by deposition of the aerosols, consequently increasing the PRE.

Cooling efficiency of vests with different cooling concepts over 8-hour trials.

As frequency and severity of heat waves are increasing, personal cooling systems are being considered as a tool to mitigate heat strain in workers in various occupational settings. This study assessed cooling capacities (C; W·h·m-2) of various commercially available vests using different cooling concepts. Measurements were conducted over 8 h in a climatic chamber (Ta: 35 °C, RH: 35 %) using a thermal manikin (Ts: 35 °C). Cooling power (P) and duration of efficient cooling (tc) determined the C value of each vest. Among the cooling concepts the active cooling vests were the most efficient, extracting 331 W·h·m-2, followed by the vests with phase change material (PCM) inserts, hybrid and evaporative vests, extracting a maximum of 164 W·h·m-2, 146 W·h·m-2 and 113 W·h·m-2, respectively. While some vests with PCM inserts provided intense but shorter cooling, evaporative vests provided mild but longer cooling throughout. Practitioner summary: The study assessed the cooling capacity of commercially available vests, using a thermal manikin. The vests present an affordable solution in various occupational settings where air-conditioning is not an option. A range of cooling capacities among different cooling concepts and vests of the same category were noted. Abbreviations: ACVs: air-cooled vests; LCVs: liquid-cooled vests; ECVs: evaporative cooling vests; HCVs: hybrid cooling vests; PCVs: phase-change cooling vests; PCM: phase change material; C: cooling capacity; Rt: thermal resistance; Re: evaporative resistance; Re (%): relative evaporative resistance; P: cooling power; Pmax: maximal cooling power; Pavg: average cooling power; tc: cooling duration; AUC: area under the curve; Ta: ambient temperature; RH: relative humidity; va: chamber air flow; Ts: manikin surface temperature.

Hypercapnia augments resistive exercise-induced elevations in intraocular pressure in older individuals.

NEW FINDINGS: What is the central question of this study? Astronauts on-board the International Space Station (ISS) perform daily exercises designed to prevent muscle atrophy and bone demineralization: what is the effect of resistive exercise performed by subjects while exposed to the same level of hypercapnia as on the ISS on intraocular pressure (IOP)? What is the main finding and its importance? The static exercise-induced elevation in IOP during 6° prone head-down tilt (simulating the headward shift of body fluids in microgravity) is augmented by hypercapnia and exceeds the ocular hypertension threshold. ABSTRACT: The present study assessed the effect of 6° head-down (establishing the cephalad fluid displacement noted in astronauts in microgravity) prone (simulating the effect on the eye) tilt during rest and exercise (simulating exercise performed by astronauts to mitigate the sarcopenia induced by unloading of weight-bearing limbs), in normocapnic and hypercapnic conditions (the latter simulating conditions on the International Space Station) on intraocular pressure (IOP). Volunteers (mean age = 57.8 ± 6 years, n = 10) participated in two experimental sessions, each comprising: (i) 10 min rest, (ii) 3 min static handgrip exercise (30% max), and (iii) 2 min recovery, inspiring either room air (NCAP) or a hypercapnic mixture (1% CO2 , HCAP). We measured IOP in the right eye, cardiac output (CO), stroke volume (SV), heart rate (HR) and mean arterial pressure (MAP) at regular intervals. Baseline IOP in the upright seated position while breathing room air was 14.1 ± 2.9 mmHg. Prone 6° head-down tilt significantly (P < 0.01) elevated IOP in all three phases of the NCAP (rest: 27.0 ± 3.7 mmHg; exercise: 32.2 ± 4.8 mmHg; recovery: 27.4 ± 4.0 mmHg) and HCAP (rest: 27.3 ± 4.3 mmHg; exercise: 34.2 ± 6.0 mmHg; recovery: 29.1 ± 5.8 mmHg) trials, with hypercapnia augmenting the exercise-induced elevation in IOP (P < 0.01). CO, SV, HR and MAP were significantly increased during handgrip dynamometry, but there was no effect of hypercapnia. The observed IOP measured during prone 6° HDT in all phases of the NCAP and HCAP trials exceeded the threshold pressure defining ocular hypertension. The exercise-induced increase in IOP is exacerbated by hypercapnia.

Seasonal variation of temperature regulation: do thermoregulatory responses "spring" forward and "fall" back?

Seasonal variations in day length and light intensity can affect the circadian rhythm as well as some characteristics of temperature regulation. We investigated characteristics of autonomic (ATR), behavioural (BTR) and nocturnal (NTR) temperature regulation during spring and autumn. Eleven participants underwent experiments in both seasons. To assess ATR, participants performed a 30-min bout of submaximal upright exercise on a cycle ergometer, followed by 100 min of water immersion (28 °C). Thresholds for the onset of shivering and sweating and vasomotor response were measured. BTR was assessed using a water-perfused suit, with participants regulating the water-perfused suit temperature (Twps) within a range, considered as thermally comfortable. The Twps changed in a saw-tooth manner from 10 to 50 °C; by depressing a switch, the direction of the Twps changed, and this limit defined the thermal comfort zone (TCZ) for each participant. A 24-h proximal (calf)-distal (toe) skin temperature gradient (∆Tc-t) was measured to assess NTR. Initiation of vasomotor tone, shivering and sweating was similar between trials. Width of the TCZ was 8.1 °C in spring and 8.6 °C in autumn (p = 0.1), with similar upper and lower regulated temperatures. ∆Tc-t exhibited a typical circadian rhythm with no difference between seasons. Minor changes in skin temperature and oxygen consumption (p Ë‚ 0.05) between the seasons may indicate a degree of seasonal adaptation over the course of winter and summer, which persisted in spring and autumn. Other factors, such as country, race, sex and age could however modify the outcome of the study.

Heat acclimation does not modify autonomic responses to core cooling and the skin thermal comfort zone.

Exercise heat acclimation (HA) is known to magnify the sweating response by virtue of a lower threshold as well as increased gain and maximal capacity of sweating. However, HA has been shown to potentiate the shivering response in a cold-air environment. We investigated whether HA would alter heat loss and heat production responses during water immersion. Twelve healthy male participants underwent a 10-day HA protocol comprising daily 90-min controlled-hyperthermia (target rectal temperature, Tre 38.5 °C) exercise sessions. Preceding and following HA, the participants performed a maximal exercise test in thermoneutral conditions (ambient temperature 23 °C, relative humidity 50%) and were, following exercise, immersed in 28 °C water for 60 min. Thermal comfort zone (TCZ) was also assessed with participants regulating the temperature of a water-perfused suit during heating and cooling. Baseline pre-immersion Tre was similar pre- and post-HA (pre: 38.33 ± 0.33 °C vs post: 38.12 ± 0.36 °C, p = 0.092). The Tre cooling rate was identical pre-to post-HA (-0.03 ± 0.01 °C·min-1, p = 0.31), as was the vasomotor response reflected in the forearm-fingertip temperature difference. Shivering thresholds (p = 0.43) and gains (p = 0.61) were not affected by HA. TCZ was established at similar temperatures, with the magnitude in regulated water temperature being 7.6 (16.3) °C pre-HA and 5.1 (24.7) °C post-HA (p = 0.65). The present findings suggest that heat production and heat loss responses during whole body cooling as well as the skin thermal comfort zone remained unaltered by a controlled-hyperthermia HA protocol.

Interactions of mild hypothermia and hypoxia on finger vasoreactivity to local cold stress.

We examined the interactive effects of mild hypothermia and hypoxia on finger vasoreactivity to local cold stress. Eight male lowlanders performed, in a counterbalanced order, a normoxic and a hypoxic (partial pressure of oxygen: ~12 kPa) hand cold provocation (consisting of a 30-min immersion in 8°C water), while immersed to the chest either in 21°C [cold trials; 0.5°C fall in rectal temperature (Trec) from individual preimmersion values], or in 35.5°C water, or while exposed to 27°C air. The duration of the trials was kept constant in each breathing condition. Physiological (Trec, skin temperature, cutaneous vascular conductance, oxygen uptake) and perceptual (thermal sensation and comfort, local pain, affective valence) reactions were monitored continually. Hypoxia accelerated the drop in Trec by ~14 min (P = 0.06, d = 0.67). In the air-exposure trials, hypoxia did not alter finger perfusion during the local cooling, whereas it impaired the finger rewarming response following the cooling (P < 0.01). During the 35.5°C immersion, the finger vasomotor tone was enhanced, especially in hypoxia (P = 0.01). Mild hypothermia aggravated finger vasoconstriction instigated by local cooling (P < 0.01), but the response did not differ between the two breathing conditions (P > 0.05). Hypoxia tended to attenuate the sensation of coldness (P = 0.10, r = 0.40) and thermal discomfort (P = 0.09, r = 0.46) in the immersed hand. Both in normoxia and hypoxia, the whole body thermal state dictates the cutaneous vasomotor reactivity to localized cold stimulus.