The effect of seasonal heat acclimatization on cool-seeking behaviour during passive heat stress in young adults.

Seasonal heat acclimatization is known to enhance autonomic thermoeffector responses, whereas the behavioural response following seasonal heat acclimatization remains unknown. We investigated whether seasonal heat acclimatization would alter autonomic and behavioural thermoregulatory responses. Sixteen healthy participants (eight males and eight females) underwent two trials involving 50 min of lower-leg passive heating (lower-leg submersion in 42°C water) with (Fan trial) and without (No fan trial) the voluntary use of a fan in a moderate thermal environment (27°C, 50% relative humidity) across winter and summer months. In Fan trials, participants were allowed to use a fan to maintain thermal comfort, but this was not allowed in the No fan trials. Cool-seeking behaviour was initiated at a lower change in rectal temperature [mean (SD): 0.21 (0.18)°C vs. 0.11 (0.13)°C, P = 0.0327] and change in mean skin temperature [2.34 (0.56)°C vs. 1.81 (0.32)°C, P < 0.0001], and cooling time was longer [16.46 (5.62) vs. 20.40 (4.87) min, P = 0.0224] in summer compared with winter. However, thermal perception was not modified by season during lower-leg passive heating (all P > 0.0864). Furthermore, rectal temperature was higher in summer (P = 0.0433), whereas mean body temperature and skin temperature were not different (all P > 0.0631) between the two seasons in Fan trials. In conclusion, seasonal heat acclimatization enhanced the cool-seeking behaviour from winter to summer.

Hot water immersion is associated with higher thermal comfort than dry passive heating for a similar rise in rectal temperature and plasma interleukin-6 concentration.

PURPOSE: To compare the perceptual responses and interleukin-6 (IL-6) concentration following rectal temperature-matched dry heat exposure (DH) and hot water immersion (HWI). METHODS: Twelve healthy young adults (BMI 23.5 ± 3.6 kg/m2; age: 25.8 ± 5.7 years) underwent 3 trials in randomised order: DH (air temperature 68.9 °C), HWI (water temperature 37.5 °C), and thermoneutral dry exposure (CON, air temperature 27.3 °C). Blood samples to determine IL-6 plasma concentration were collected; basic affect and thermal comfort, rectal and skin temperature (Tskin) were assessed throughout the intervention. RESULTS: Rectal temperature (Trec) did not differ between DH (end temperature 38.0 ± 0.4 °C) and HWI (37.9 ± 0.2 °C, P = 0.16), but was higher compared with CON (37.0 ± 0.3 °C; P ≤ 0.004). Plasma IL-6 concentration was similar after DH (pre to post: 0.8 ± 0.5 to 1.4 ± 1.5 pg·ml-1) and HWI (0.5 ± 0.2 to 0.9 ± 0.6 pg·ml-1; P = 0.46), but higher compared with CON (0.6 ± 0.5 to 0.6 ± 0.4 pg·ml-1; P = 0.01). At the end of the intervention, basic affect and thermal comfort were most unfavourable during DH (Basic affect; DH: - 0.7 ± 2.9, HWI: 0.8 ± 1.9, CON 1.9 ± 1.9, P ≤ 0.004; Thermal comfort; 2.6 ± 0.8, HWI: 1.4 ± 0.9 and CON: 0.2 ± 0.4; P ≤ 0.004). Mean Tskin was highest for DH, followed by HWI, and lowest for CON (DH: 38.5 ± 1.3 °C, HWI: 36.2 ± 0.5 °C, CON: 31.6 ± 0.7 °C, P < 0.001). CONCLUSION: The IL-6 response did not differ between DH and HWI when matched for the elevation in Trec. However, thermal comfort was lower during DH compared to HWI, which may be related to the higher Tskin during DH.

Exposure to total 36-hr sleep deprivation reduces physiological and psychological thermal strain to whole-body uncompensable passive heat stress in young adult men.

Total sleep deprivation (TSD) is associated with endothelial dysfunction and a consequent decrease in vascular reactivity and increase in peripheral vascular resistance. These effectors compromise the body's ability to thermoregulate in hot and cold stress conditions. We investigated heat-unacclimated young adult men (26 ± 2 years) to determine whether 36 hr of TSD compared to an 8 or 4-hr sleep condition, would suppress the responses of the autonomic system (body rectal temperature [Tre ], heart rate [HR], root mean square of successive interbeat intervals, physiological strain, blood pressure [BP], circulating blood catecholamines, sweating rate and subjective sensations) to whole-body uncompensable passive heat stress in traditional Finnish sauna heat (Tair  = 80-90°C, rh = 30%). Sauna bathing that induced whole-body hyperthermia had a residual effect on reducing BP in the 8-hr and 4-hr sleep per night conditions according to BP measurements. By contrast, 36 hr of total wakefulness led to an increase in BP. These observed sleep deprivation-dependent differences in BP modifications were not accompanied by changes in the blood plasma epinephrine and norepinephrine concentrations. However, during sauna bathing, an increase in BP following 36 hr of TSD was accompanied by significant decreases in body Tre , HR and physiological strain, together with a diminished sweating rate, enhanced vagus-mediated autonomic control of HR variability, and improved thermal perception by the subjects. Our results suggest the impaired ability of the body to accumulate external heat in the body's core under uncompensable passive heat conditions following 36 hr of TSD, because of the TSD-attenuated autonomic system response to acute heat stress.

Three weeks of passive and intervallic heat at high temperatures (100±2 °C) in a sauna improve acclimation to external heat (42±2 °C) in untrained males.

Currently, the effect of passive heat acclimation on aerobic performance is still controversial. Therefore, this study aimed to observe the effect of passive and intervallic exposure to high temperatures (100 ± 2 °C) in untrained males. Forty healthy untrained men participated in this investigation. They were randomised into a Control Group (CG; n = 18) and an Experimental Group (EG; n = 22). Both groups performed maximum incremental tests until exhaustion in normothermia (GXT1; 22 ± 2 °C), and 48h afterwards, in hyperthermia (GXT2; 42 ± 2 °C). The EG performed 9 sessions of intervallic exposure to heat (100 ± 2 °C) over 3 weeks. Subsequently, both groups performed two maximal incremental trials in normothermia (GXT3; 22 ± 2 °C) and 48h later, in hyperthermia (GXT4; 42 ± 2 °C). In each test, the maximal ergospirometric parameters and the aerobic (VT1), anaerobic (VT2) and recovery ventilatory thresholds were recorded. The Wilcoxon Test was used for intra-group comparisons and the Mann-Whitney U for inter-group comparisons. There were improvements in absolute VO2max (p = 0.049), W (p = 0.005) and O2pulse (p = 0.006) in hyperthermia. In VT1 there was an increase in W (p = 0.046), in VO2 in absolute (p = 0.025) and relative (p = 0.013) values, O2pulse (p = 0.006) and VE (p = 0.028) in hyperthermia. While W increased in hyperthermia (p = 0.022) at VT2. The results suggest that passive and intervallic acclimation at high temperatures improves performance in hyperthermia. This protocol could be implemented in athletes when they have to compete in hot environments.

Increased skin wetness independently augments cool-seeking behaviour during passive heat stress.

KEY POINTS: Skin wetness occurring secondary to the build-up of sweat on the skin provokes thermal discomfort, the precursor to engaging in cool-seeking behaviour. Associative evidence indicates that skin wetness stimulates cool-seeking behaviour to a greater extent than increases in core and mean skin temperatures. The independent contribution of skin wetness to cool-seeking behaviour during heat stress has never been established. We demonstrate that skin wetness augments cool-seeking behaviour during passive heat stress independently of differential increases in skin temperature and core temperature. We also identify that perceptions of skin wetness were not elevated despite increases in actual skin wetness. These data support the proposition that afferent signalling from skin wetness enhances the desire to engage in cool-seeking behaviour during passive heat stress. ABSTRACT: This study tested the hypothesis that elevations in skin wetness augments cool-seeking behaviour during passive heat stress. Twelve subjects (6 females, age: 24 ± 2 y) donned a water-perfused suit circulating 34 °C water and completed two trials resting supine in a 28.5 ± 0.4 °C environment. The trials involved a 20 min baseline period (26 ± 3% relative humidity (RH)), 60 min while ambient humidity was maintained at 26±3% RH (LOW) or increased to 67 ± 5% RH (HIGH), followed by 60 min passive heat stress (HS) where the water temperature in the suit was incrementally increased to 50 °C. Subjects were able to seek cooling when their neck was thermally uncomfortable by pressing a button. Each button press initiated 30 s of -20 °C fluid perfusing through a custom-made device secured against the skin on the dorsal neck. Mean skin (Tskin ) and core (Tcore ) temperatures, mean skin wetness (Wskin ) and neck device temperature (Tdevice ) were measured continuously. Cool-seeking behaviour was determined from total time receiving cooling (TTcool ) and cumulative button presses. Tskin and Tcore increased during HS (P < 0.01) but were not different between conditions (P ≥ 0.11). Wskin was elevated in HIGH vs. LOW during HS (60 min: by + 0.06 ± 0.07 a.u., P ≤ 0.04). Tdevice was lower in HIGH vs. LOW at 40-50 min of HS (P ≤ 0.01). TTcool was greater for HIGH (330 ± 172 s) vs. LOW (225 ± 167 s, P < 0.01), while the number of cumulative button presses was greater from 40-60 min in HS for HIGH vs. LOW (P ≤ 0.04). Increased skin wetness amplifies the engagement in cool-seeking behaviour during passive heat stress.

Effectiveness of skin-heating using a water-perfused suit as passive and post-exercise heat acclimation strategies.

The purpose of the present study was to evaluate the effectiveness of passive and post-exercise heat acclimation strategies through directly heating the skin with a water-perfused suit. Nineteen young males participated in the heat acclimation (HA) protocols for 10 days, which were conducted at an air temperature of 33oC with 60%RH. The exercise-only condition (N = 6) conducted 1-h treadmill walking (6 km·h-1) followed by 1-h rest. The post-exercise passive-heating condition (N = 6) wore the suit (inflow water temperature 44.2oC) for 1-h after 1-h walking. The passive-heating condition (N = 7) donned the suit for 2 h. Heat tolerance tests (leg immersion in 42oC water for 60 min) were conducted before and after the training to evaluate changes due to the 10-day intervention. Reflecting that suit-wearing for 10 days as both passive and post-exercise HA strategies can effectively induce adaptive changes, significant interaction effects appeared in: increase or decrease in mean skin temperature (P < 0.05) and elevation in whole-body sweat rate (P < 0.05). Reduction in rectal temperature (P < 0.05) and blood pressure (P < 0.05) were found most prominently in the passive-heating condition. These results indicate that this new method of heat acclimation training, donning a skin-heating water-perfused suit, can generate thermoregulatory benefits. The passive HA intervention could be applied to individuals for whom doing exercise regularly are not feasible.

Heat therapy vs. supervised exercise therapy for peripheral arterial disease: a 12-wk randomized, controlled trial.

Peripheral arterial disease (PAD) is characterized by lower limb atherosclerosis impairing blood supply and causing walking-induced leg pain or claudication. Adherence to traditional exercise training programs is poor due to these symptoms despite exercise being a mainstay of conservative treatment. Heat therapy improves many cardiovascular health outcomes, so this study tested if this was a viable alternative cardiovascular therapy for PAD patients. Volunteers with PAD were randomized to 12 wk of heat (n = 11; mean age 76 ± 8 yr, BMI 28.7 ± 3.5 kg/m2, 4 females) or exercise (n = 11; 74 ± 10 yr, 28.5 ± 6.8 kg/m2, 3 females). Heat involved spa bathing at ∼39°C, 3-5 days/wk for ≤30 min, followed by ≤30 min of callisthenics. Exercise involved ≤90 min of supervised walking and gym-based exercise, 1-2 days/wk. Following the interventions, total walking distance during a 6-min walk test increased (from ∼350 m) by 41 m (95% CI: [13, 69], P = 0.006) regardless of group, and pain-free walking distance increased (from ∼170 m) by 43 m ([22, 63], P < 0.001). Systolic blood pressure was reduced more following heat (-7 mmHg, [-4, -10], P < 0.001) than following exercise (-3 mmHg, [0, -6], P = 0.078), and diastolic and mean arterial pressure decreased by 4 mmHg in both groups (P = 0.002). There were no significant changes in blood volume, ankle-brachial index, or measures of vascular health. There were no differences in the improvement in functional or blood pressure outcomes between heat and exercise in individuals with PAD. NEW & NOTEWORTHY Heat therapy via hot-water immersion and supervised exercise both improved walking distance and resting blood pressure in peripheral arterial disease (PAD) patients over 12 wk. Adherence to heat therapy was excellent, and the heat intervention was well tolerated. The results of the current study indicate that heat therapy can improve functional ability and has potential as an effective cardiovascular conditioning tool for individuals with PAD.

Effect of an Auxiliary Plate on Passive Heat Dissipation of Carbon Nanotube-Based Materials.

Carbon nanotubes (CNTs) and other related CNT-based materials with a high thermal conductivity can be used as promising heat dissipation materials. Meanwhile, the miniaturization and high functionality of portable electronics, such as laptops and mobile phones, are achieved at the cost of overheating the high power-density components. The heat removal for hot spots occurring in a relatively narrow space requires simple and effective cooling methods. Here, an auxiliary passive cooling approach by the aid of a flat plate (aluminum-magnesium alloy) is investigated to accommodate heat dissipation in a narrow space. The cooling efficiency can be raised to 43.5%. The cooling performance of several CNT-based samples is compared under such circumstances. Heat dissipation analyses show that, when there is a nearby plate for cooling assistance, the heat radiation is weakened and natural convection is largely improved. Thus, improving heat radiation by increasing emissivity without reducing natural convection can effectively enhance the cooling performance. Moreover, the decoration of an auxiliary cooling plate with sprayed CNTs can further improve the cooling performance of the entire setup.

Lower-limb hot-water immersion acutely induces beneficial hemodynamic and cardiovascular responses in peripheral arterial disease and healthy, elderly controls.

Passive heat induces beneficial perfusion profiles, provides substantive cardiovascular strain, and reduces blood pressure, thereby holding potential for healthy and cardiovascular disease populations. The aim of this study was to assess acute responses to passive heat via lower-limb, hot-water immersion in patients with peripheral arterial disease (PAD) and healthy, elderly controls. Eleven patients with PAD (age 71 ± 6 yr, 7 male, 4 female) and 10 controls (age 72 ± 7 yr, 8 male, 2 female) underwent hot-water immersion (30-min waist-level immersion in 42.1 ± 0.6°C water). Before, during, and following immersion, brachial and popliteal artery diameter, blood flow, and shear stress were assessed using duplex ultrasound. Lower-limb perfusion was measured also using venous occlusion plethysmography and near-infrared spectroscopy. During immersion, shear rate increased (P < 0.0001) comparably between groups in the popliteal artery (controls: +183 ± 26%; PAD: +258 ± 54%) and brachial artery (controls: +117 ± 24%; PAD: +107 ± 32%). Lower-limb blood flow increased significantly in both groups, as measured from duplex ultrasound (>200%), plethysmography (>100%), and spectroscopy, while central and peripheral pulse-wave velocity decreased in both groups. Mean arterial blood pressure was reduced by 22 ± 9 mmHg (main effect P < 0.0001, interaction P = 0.60) during immersion, and remained 7 ± 7 mmHg lower 3 h afterward. In PAD, popliteal shear profiles and claudication both compared favorably with those measured immediately following symptom-limited walking. A 30-min hot-water immersion is a practical means of delivering heat therapy to PAD patients and healthy, elderly individuals to induce appreciable systemic (chronotropic and blood pressure lowering) and hemodynamic (upper and lower-limb perfusion and shear rate increases) responses.

Elite sprint swimming performance is enhanced by completion of additional warm-up activities.

This study investigated the effect of completing additional warm-up strategies in the transition phase between the pool warm up and the start of a race on elite sprint swimming performance. Twenty-five elite swimmers (12 men, 20 ± 3 years; 13 women, 20 ± 2 years, performance standard ~807 FINA2014 points) completed a standardised pool warm up followed by a 30-min transition phase and a 100-m freestyle time trial. During the transition phase, swimmers wore a tracksuit jacket with integrated heating elements and performed a dry land-based exercise routine (Combo), or a conventional tracksuit and remained seated (Control). Start (1.5% ± 1.0%, P = 0.02; mean ± 90% confidence limits) and 100-m time trial (0.8% ± 0.4%, P < 0.01) performances were improved in Combo. Core temperature declined less (-0.2°C ± 0.1°C versus -0.5°C ± 0.1°C, P = 0.02) during the transition phase and total local (trapezius) haemoglobin concentration was greater before the time trial in Combo (81 µM ± 25 µM versus 30 µM ± 18 µM, P < 0.01; mean ± standard deviation) than in Control. Combining swimmers traditional pool warm up with passive heating via heated jackets and completion of dry land-based exercises in the transition phase improves elite sprint swimming performance by ~0.8%.

Post-warmup strategies to maintain body temperature and physical performance in professional rugby union players.

We compared the effects of using passive-heat maintenance, explosive activity or a combination of both strategies during the post-warmup recovery time on physical performance. After a standardised warmup, 16 professional rugby union players, in a randomised design, completed a counter-movement jump (peak power output) before resting for 20 min and wearing normal-training attire (CON), wearing a passive heat maintenance (PHM) jacket, wearing normal attire and performing 3 × 5 CMJ (with a 20% body mass load) after 12 min of recovery (neuromuscular function, NMF), or combining PHM and NMF (COMB). After 20 min, participants completed further counter-movement jump and a repeated sprint protocol. Core temperature (Tcore) was measured at baseline, post-warmup and post-20 min. After 20 min of recovery, Tcore was significantly lower under CON and NMF, when compared with both PHM and COMB (P < 0.05); PHM and COMB were similar. Peak power output had declined from post-warmup under all conditions (P < 0.001); however, the drop was less in COMB versus all other conditions (P < 0.05). Repeated sprint performance was significantly better under COMB when compared to all other conditions. Combining PHM with NMF priming attenuates the post-warmup decline in Tcore and can positively influence physical performance in professional rugby union players.

Combined facial heating and inhalation of hot air do not alter thermoeffector responses in humans.

The influence of thermoreceptors in human facial skin on thermoeffector responses is equivocal; furthermore, the presence of thermoreceptors in the respiratory tract and their involvement in thermal homeostasis has not been elucidated. This study tested the hypothesis that hot air directed on the face and inhaled during whole body passive heat stress elicits an earlier onset and greater sensitivity of cutaneous vasodilation and sweating than that directed on an equal skin surface area away from the face. Six men and two women completed two trials separated by ∼1 wk. Participants were passively heated (water-perfused suit; core temperature increase ∼0.9°C) while hot air was directed on either the face or on the lower leg (counterbalanced). Skin blood flux (laser-Doppler flowmetry) and local sweat rate (capacitance hygrometry) were measured at the chest and one forearm. During hot-air heating, local temperatures of the cheek and leg were 38.4 ± 0.8°C and 38.8 ± 0.6°C, respectively (P = 0.18). Breathing hot air combined with facial heating did not affect mean body temperature onsets (P = 0.97 and 0.27 for arm and chest sites, respectively) or slopes of cutaneous vasodilation (P = 0.49 and 0.43 for arm and chest sites, respectively), or the onsets (P = 0.89 and 0.94 for arm and chest sites, respectively), or slopes of sweating (P = 0.48 and 0.65 for arm and chest sites, respectively). Based on these findings, respiratory tract thermoreceptors, if present in humans, and selective facial skin heating do not modulate thermoeffector responses during passive heat stress.

Passive heat loading links lipolysis and regulation of fibroblast growth factor-21 in humans.

There is relativley little information on the serum biomarkers of heat stress. Therefore, the goal of this study was to verify the effect of passive heat loading (PHL) on the expression of fibroblast growth factor-21 (FGF21) and free fatty acids (FFAs). Four PHL protocols based on intensity (39 °C vs. 43 °C, leg immersion, 30 min) and type (leg vs. half immersion, 42 °C, 30 min) were used. Each protocol was applied on a 2 day cycle to 12 healthy adult males (age, 22.4 ± 2.9 years; height, 174.1 ± 4.6 cm; weight, 71.3 ± 5.6 kg; body mass index, 23.1 ± 3.0). The subjects were categorized into two groups according to the study design (randomized, with a parallel trial). Body temperature, FGF21 and FFAs were determined prior to PH L, immediately and 60 min after PHL. Body temperature was significant higher (43 °C) than the 39 °C measured under identical PHL type (leg immersion). PHL was effective for the expression of FGF21 and for lipolysis. The quantitative levels of FGF21 and FFA increased with increasing temperature (39 °C<42 °C<43 °C). A significant difference in the quantitative levels of FGF21 and FFAs was also evident based on the type of PHL (leg

The multifaceted benefits of passive heat therapies for extending the healthspan: A comprehensive review with a focus on Finnish sauna.

Passive heat therapy is characterized by exposure to a high environmental temperature for a brief period. There are several types of passive heat therapy which include hot tubs, Waon therapy, hydrotherapy, sanarium, steam baths, infrared saunas and Finnish saunas. The most commonly used and widely studied till date are the Finnish saunas, which are characterized by high temperatures (ranging from 80-100°C) and dry air with relative humidity varying from 10-20%. The goal of this review is to provide a summary of the current evidence on the impact of passive heat therapies particularly Finnish saunas on various health outcomes, while acknowledging the potential of these therapies to contribute to the extension of healthspan, based on their demonstrated health benefits and disease prevention capabilities. The Finnish saunas have the most consistent and robust evidence regarding health benefits and they have been shown to decrease the risk of health outcomes such as hypertension, cardiovascular disease, thromboembolism, dementia, and respiratory conditions; may improve the severity of musculoskeletal disorders, COVID-19, headache and flu, while also improving mental well-being, sleep, and longevity. Finnish saunas may also augment the beneficial effects of other protective lifestyle factors such as physical activity. The beneficial effects of passive heat therapies may be linked to their anti-inflammatory, cytoprotective and anti-oxidant properties and synergistic effects on neuroendocrine, circulatory, cardiovascular and immune function. Passive heat therapies, notably Finnish saunas, are emerging as potentially powerful and holistic strategies to promoting health and extending the healthspan in all populations.

Elucidating mechanisms of attenuated skin vasodilation during passive heat stress in persons with spinal cord injury.

OBJECTIVE: Persons with spinal cord injury (SCI) are unable to efficiently dissipate heat via thermoregulatory vasodilation as efficiently as able-bodied persons during whole body passive heat stress (PHS). Skin blood flow (SkBF) is controlled by dual sympathetic vasomotor systems: noradrenergic vasoconstrictor (VC) nerves and cholinergic vasodilator (VD) nerves. Thus, impaired vasodilation could result from inappropriate increases in noradrenergic VC tone that compete with cholinergic vasodilation or diminished cholinergic tone. To address this issue, we used bretylium (BR) which selectively blocks neural release of norepinephrine, thereby reducing noradrenergic VC tone. If impaired vasodilation during PHS is due to inappropriate increase in VC tone, BR treatment will improve SkBF responses during PHS. DESIGN: Prospective interventional trial. SETTING: laboratory. PARTICIPANTS: 22 veterans with SCI. INTERVENTIONS: Skin surface areas with previously defined intact vs. impaired thermoregulatory vasodilation were treated with BR iontophoresis with a nearby untreated site serving as control/CON. Participants underwent PHS until core temperature rose 1°C. OUTCOME MEASURES: Laser doppler flowmeters measured SkBF over BR and CON sites in areas with impaired and intact thermoregulatory vasodilation. Cutaneous vascular conductance (CVC) was calculated for all sites. Peak-PHS CVC was normalized to baseline (BL): (CVC peak-PHS/CVC BL) to quantify SkBF change. RESULTS: CVC rise in BR sites was significantly less than CON sites in areas with intact (P = 0.03) and impaired (P = 0.04) thermoregulatory vasodilation. CONCLUSION: Cutaneous blockade of neural release of noradrenergic neurotransmitters affecting vasoconstriction did not enhance thermoregulatory vasodilation during PHS in persons with SCI; rather BR attenuated the response. Cutaneous blockade of neural release of noradrenergic neurotransmitters affecting vasoconstriction did not restore cutaneous active vasodilation during PHS in persons with SCI.

Investigating sensor location on the effectiveness of continuous glucose monitoring during exercise in a non-diabetic population.

The purpose of this investigation was to evaluate whether continuous glucose monitoring (CGM) sensors worn on the active muscle may provide enhanced insight into glucose control in non-diabetic participants during cycling exercise compared to traditional sensor placement on the arm. Data from 9 healthy participants (F:3) was recorded using CGM sensors on the arm (triceps brachii) and leg (vastus medialis) following 100 g glucose ingestion during 30 min experimental visits of: resting control, graded cycling, electrically stimulated quadriceps contractions, and passive whole-body heating. Finger capillary glucose was used to assess sensor accuracy. Under control conditions, the traditional arm sensor better reflected capillary glucose, with a mean absolute relative difference (MARD) of 12.4 ± 9.3% versus 18.3 ± 11.4% in the leg (P = 0.02). For the intended use during exercise, the sensor-site difference was attenuated, with similar MARDs during cycling (arm:15.5 ± 12% versus leg:16.7 ± 10.8%, P = 0.96) and quadriceps stimulation (arm:15.5 ± 14.8% versus leg:13.9 ± 9.5%, P = 0.9). At rest, glucose at the leg was consistently lower than the arm (P = 0.01); whereas, during graded cycling, the leg-glucose was lower only after maximal intensity exercise (P = 0.02). There was no difference between sensors during quadriceps stimulation (P = 0.8). Passive heating caused leg-skin temperature to increase by 3.1 ± 1.8°C versus 1.1 ± 0.72°C at the arm (P = 0.002), elevating MARD in the leg (23.5 ± 16.2%) and lowering glucose in the leg (P < 0.001). At rest, traditional placement of CGM sensors on the arm may best reflect blood glucose; however, during cycling, placement on the leg may offer greater insight to working muscle glucose concentrations, and this is likely due to greater blood-flow rather than muscle contractions.HighlightsWearing a continuous glucose monitoring (CGM) sensor on the arm may better reflect capillary glucose concentrations compared to wearing a sensor on the inner thigh at rest.With passive or active leg-muscle contractions, site-specific differences compared to capillary samples are attenuated; therefore, wearing a CGM sensor on the active-muscle during exercise may provide greater information to non-diabetic athletes regarding glucose flux at the active muscle.Discrepancies in CGM sensors worn at different sites likely primarily reflects differences in blood flow, as passive skin heating caused the largest magnitude difference between arm and leg sensor readings compared to the other experimental conditions (control, electric muscle stimulation, and cycling exercise).

Acute and adaptive cardiovascular and metabolic effects of passive heat therapy or high-intensity interval training in patients with severe lower-limb osteoarthritis.

Exercise is painful and difficult to perform for patients with severe lower-limb osteoarthritis; consequently, reduced physical activity contributes to increased cardiometabolic disease risk. The aim of this study was to characterize the acute and adaptive cardiovascular and metabolic effects of two low or no impact therapies in patients with severe lower-limb osteoarthritis: passive heat therapy (Heat) and high-intensity interval training (HIIT) utilizing primarily the unaffected limbs, compared to a control intervention of home-based exercise (Home). Participants completed up to 12 weeks of either Heat (20-30 min immersed in 40°C water followed by ~15-min light resistance exercise), HIIT (6-8 × 60-s intervals on a cross-trainer or arm ergometer at ~90-100% peak V ̇ $$ \dot{V} $$ O2 ) or Home (~15-min light resistance exercise); all 3 sessions/week. Reductions in systolic (12 & 10 mm Hg), diastolic (7 & 4 mm Hg), and mean arterial (8 & 6 mm Hg) blood pressure (BP) were observed following one bout of Heat or HIIT exposure, lasting for the duration of the 20-min monitoring period. Across the interventions (i.e., 12 weeks), resting systolic BP and diastolic BP decreased with Heat (-9 & -4 mm Hg; p < 0.001) and HIIT (-7 & -3 mm Hg; p ≤ 0.011), but not Home (0 & 0 mm Hg; p ≥ 0.785). The systolic and diastolic BP responses to an acute exposure of Heat or HIIT in the first intervention session were moderately correlated with adaptive responses across the intervention (r ≥ 0.54, p ≤ 0.005). Neither intervention improved indices of glycemic control (p = 0.310). In summary, both Heat and HIIT induced potent immediate and adaptive hypotensive effects, and the acute response was moderately predictive of the long-term response.

Are Thermoregulatory Sweating and Active Vasodilation in Skin Controlled by Separate Nerves During Passive Heat Stress in Persons With Spinal Cord Injury?

Background: Sudomotor responses (SR) and active vasodilation (AVD) are the primary means of heat dissipation during passive heat stress (PHS). It is unknown if they are controlled by a single or separate set of nerves. Older qualitative studies suggest that persons with spinal cord injury (SCI) have discordant areas of sweating and vasodilation. Objectives: To test the hypothesis that neural control of SR and AVD is through separate nerves by measuring SR and vasodilation in persons with SCI to determine if these areas are concordant or discordant. Methods: Nine persons with tetraplegia, 13 with paraplegia, and nine able-bodied controls underwent PHS (core temperature rise 1°C) twice. Initially, the starch iodine test measured SR post-PHS in skin surface areas surrounding the level of injury. Subsequently, laser Doppler imagery scans measured vasodilation pre- and post-PHS in areas with and without SR. Percent change in red blood cell (RBC) flux was compared in areas with and without SR. Results: Persons with tetraplegia were anhidrotic on all areas; however, the same areas demonstrated minimal RBC flux change significantly less than equivalent able-bodied skin surface areas. In persons with paraplegia, areas of intact SR correlated with areas of RBC flux change quantitatively comparable to able-bodied persons. In anhidrotic areas, RBC flux change was significantly less than areas with SR and likely resulted from non-AVD mechanisms. Conclusion: In persons with SCI under PHS, areas with intact SR and AVD are concordant, suggesting these two aspects of thermoregulation are controlled by a single set of nerves.

Competing in Hot Conditions at the Tokyo Olympic Games: Preparation Strategies Used by Australian Race Walkers.

Introduction: The Tokyo 2021 Olympic Games was anticipated to expose athletes to the most challenging climatic conditions experienced in the history of the modern Olympic Games. This study documents strategies executed by Australian endurance athletes during the team holding camp and Olympic Games experiences, including (1) baseline physiological data, training data, and heat acclimation/acclimatization practices; (2) pre- and in-race cooling and nutritional strategies, and (3) Olympic Games race performance data. Methods: Six athletes (three males, three females; age 24 ± 4 years; VO2max 63.2 ± 8.7 mL⋅kg-1⋅min-1; sum of 7 skinfolds 53.1 ± 23.4 mm) were observed prior to and during the team holding camp held in Cairns, QLD, Australia. Athletes completed 6-7 weeks of intermittent heat acclimation training, utilizing a combination of 2-4 passive and active acclimation sessions per week. Active acclimation was systematically increased via exposure time, exercise intensity, temperature, and humidity. In the team holding camp, athletes undertook a further 23 heat acclimatization training sessions over 18 days in a continuous fashion. Hyperhydration (using sodium and glycerol osmolytes), and internal and external pre-and in-race cooling methods were also utilized. A low energy availability intervention was implemented with two athletes, as a strategy to periodize ideal race body composition. Race performance data and environmental conditions from the 2021 Olympic Games were also documented. Results: The highest values for aerobic capacity were 63.6 mL⋅kg-1⋅min-1 for female race walkers and 73.7 mL⋅kg-1⋅min-1 for males. Training volume for the six athletes was the highest in the second week of the team holding camp, and training intensity was lowest in the first week of the team holding camp. Performance outcomes included 6th place in the women's 20 km event (1:30:39), which was within 2% of her 20 km personal best time, and 8th place in the men's 50 km event (3:52:01), which was a personal best performance time. Conclusion: Periodized training, heat acclimation/acclimatization, cooling and nutritional strategies study may have contributed to the race outcomes in Olympic Games held hot, humid conditions, for the race walkers within this observational study.

Thermal influences on groundwater in urban environments - A multivariate statistical analysis of the subsurface heat island effect in Munich.

Shallow aquifers beneath cities are highly influenced by anthropogenic heat sources, resulting in the formation of extensive subsurface urban heat islands. In addition to anthropogenic factors, natural factors also influence the subsurface temperature. However, the effect of individual factors is difficult to capture due to high temporal dynamics in urban environments. Particularly in the case of shallow aquifers, seasonal temperature fluctuations often override the influence of existing heat sources or sinks. For the city of Munich, we identify the dominant anthropogenic and natural influences on groundwater temperature and analyse how the influences change with increasing depth in the subsurface. For this purpose, we use depth temperature profiles from 752 selected groundwater monitoring wells. Since the measurements were taken at different times, we developed a statistical approach to compensate for the different seasonal temperature influences using passive heat tracing. Further, we propose an indicator to spatially assess the thermal stress on the aquifer. A multiple regression analysis of four natural and nine anthropogenic factors identified surface sealing as the strongest and the district heating grid as a weak but significant warming influence. The natural factors, aquifer thickness, depth-to-water and Darcy velocity show a significant cooling influence on the groundwater temperature. In addition, we show that local drivers, like thermal groundwater uses, surface waters and underground structures do not significantly contribute to the city-wide temperature distribution. The subsequent depth-dependent analysis revealed that the influence of aquifer thickness and depth-to-water increases with depth, whereas the influence of Darcy velocity decreases, and surface sealing and the heating grid remain relatively constant. In conclusion, this study shows that the most critical factor for the mitigation of future groundwater warming in cities is to minimize further sealing of the ground, to restore the permeability of heavily sealed areas and to retain open landscapes.