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Rationale: Monitoring physiological strain is recommended to safeguard workers during heat exposure, but is logistically challenging. The perceptual strain index (PeSI) is a subjective estimate thought to reflect the physiological strain index (PSI) that requires no physiological monitoring. However, sex is known to influence perceptions of heat stress, potentially limiting the utility of the PeSI., Objectives: The objective of this study was to assess whether sex modifies the relationship between PeSI and PSI., Methods: Thirty-four adults (15 females) walked on a treadmill (moderate intensity; ~200 W/m 2 ) for 180 min or until termination (volitional fatigue, rectal temperature ≥39.5°C) in 16°C, 24°C, 28°C, and 32°C wet-bulb globe temperatures. Rectal temperature and heart rate were recorded to calculate PSI (0-10 scale). Rating of perceived exertion and thermal sensation were recorded to calculate PeSI (0-10 scale). Relationships between PSI and PeSI were evaluated via linear mixed models. Mean bias (95% limits of agreement [LoA]) between PSI and PeSI was assessed via Bland-Altman analysis. Mean absolute error between measures was calculated by summing absolute errors between the PeSI and the PSI and dividing by the sample size., Findings: PSI increased with PeSI (p < 0.01) but the slope of this relation was not different between males and females (p = 0.83). Mean bias between PSI and PeSI was small (-0.4 points), but the 95% LoA (-3.5 to 2.7 points) and mean absolute error were wide (1.3 points)., Impact: Our findings indicate that sex does not appreciably impact the agreement between the PeSI and PSI during simulated occupational heat stress. The PeSI is not a suitable surrogate for the PSI in either male or female workers., (© 2024 The Authors. American Journal of Industrial Medicine published by Wiley Periodicals LLC.)

New Findings: What is the central question of this study? Is the impairment in heat dissipation during exercise observed in men with type 2 diabetes related to glycaemic control (indexed by glycated haemoglobin; haemoglobin A 1c )? What is the main finding and its importance? No association was found between haemoglobin A 1c (range: 5.1-9.1%) and whole-body heat loss in men with type 2 diabetes during exercise in the heat. However, individuals with elevated haemoglobin A 1c exhibited higher body core temperature and heart rate responses. Thus, while haemoglobin A 1c is not associated with heat loss per se, it may still have important implications for physiological strain during exercise., Abstract: Type 2 diabetes is associated with a reduced capacity to dissipate heat. It is unknown whether this impairment is related to glycaemic control (indexed by glycated haemoglobin; haemoglobin A 1c ) is unknown. We evaluated the association between haemoglobin A 1c and whole-body heat loss (via direct calorimetry), body core temperature, and heart rate in 26 physically active men with type 2 diabetes (43-73 years; HbA 1c 5.1-9.1%) during exercise at increasing rates of metabolic heat production (∼150, 200, 250 W m -2 ) in the heat (40°C, ∼17% relative humidity). Haemoglobin A 1c was not associated with whole-body heat loss (P = 0.617), nor the increase in core temperature from pre-exercise (P = 0.347). However, absolute core temperature and heart rate were elevated ∼0.2°C (P = 0.014) and ∼6 beats min -1 (P = 0.049), respectively, with every percentage point increase in haemoglobin A 1c . Thus, while haemoglobin A 1c does not appear to modify diabetes-related reductions in capacity for heat dissipation, it may still have important implications for physiological strain during exercise-heat stress., (© 2022 The Authors. Experimental Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society.)

New Findings: What is the central question of this study? Does acute intradermal administration of the antioxidant ascorbate augment local forearm cutaneous vasodilatation and sweating via nitric oxide synthase (NOS)-dependent mechanisms during exercise-heat stress in older adults with uncomplicated controlled hypertension? What is the main finding and its importance? Relative to the control site, ascorbate had no effect on forearm cutaneous vascular conductance (CVC) and sweat rate, although CVC was reduced with NOS inhibition in older adults with hypertension. Acute local administration of ascorbate to forearm skin does not modulate heat loss responses during exercise-heat stress in older adults with hypertension., Abstract: Nitric oxide synthase (NOS) contributes to the heat loss responses of cutaneous vasodilatation and sweating during exercise. However, the contribution of NOS may be attenuated in individuals with uncomplicated, controlled hypertension due to elevated oxidative stress, which can reduce NO bioavailability. We evaluated the hypothesis that the acute local intradermal administration of the antioxidant ascorbate would enhance cutaneous vasodilatation and sweating via NOS-dependent mechanisms during an exercise-heat stress in adults with hypertension. Habitually active adults who were normotensive (n = 14, 7 females, 62 ± 4 years) or had uncomplicated, controlled hypertension (n = 13, 6 females, 62 ± 5 years) performed 30 min of moderate-intensity (50% of their pre-determined peak oxygen uptake) semi-recumbent cycling in the heat (35°C, 20% relative humidity). Cutaneous vascular conductance (CVC) and sweat rate were assessed at four forearm skin sites continuously perfused with (1) lactated Ringer solution (Control), (2) 10 mM antioxidant ascorbate, (3) 10 mM N G -nitro-l-arginine methyl ester (l-NAME), a non-selective NOS inhibitor, or (4) a combination of ascorbate and l-NAME. Relative to Control, no effect of ascorbate was observed on CVC or sweating in either group (P = 0.619). However, l-NAME reduced CVC relative to Control in both groups (P ≤ 0.038). No effect of any treatment on sweating was observed (P ≥ 0.306). Thus, acute local administration of ascorbate to forearm skin does not enhance the activation of heat loss responses of cutaneous vasodilatation and sweating in older adults, and those with hypertension during an exercise-heat stress., (© 2022 The Authors. Experimental Physiology © 2022 The Physiological Society.)

Observed individual variability in cardiac baroreflex sensitivity (cBRS) and heart rate variability (HRV) is extensive, especially during exposure to stressors such as heat. A large part of the observed variation may be related to the reliability (consistency) of the measurement. We therefore examined the test-retest reliability of cBRS and HRV measurements on three separate occasions in 14 young men (age: 24 (SD 5) years), at rest and during whole-body heating (water-perfused suit) to raise and clamp oesophageal temperature 0.6°C, 1.2°C and 1.8°C above baseline. Beat-to-beat measurements of RR interval and systolic blood pressure (BP) were obtained for deriving HRV (from RR), and cBRS calculated via (i) the spontaneous method, α coefficients and transfer function analysis at each level of heat strain, and (ii) during forced oscillations via squat-stand manoeuvres (0.1 Hz) before and after heating. Absolute values and changes in all cBRS estimates were variable but generally consistent with reductions in parasympathetic activity. cBRS estimates demonstrated poor absolute reliability (coefficient of variation ≥25%), but relative reliability (intraclass correlation coefficient; ICC) of some frequency estimates was acceptable (ICC ≥0.70) during low-heat strain (ICC: 0.56-0.74). After heating, forced oscillations in BP demonstrated more favourable responses than spontaneous oscillations (better reliability, lower minimum detectable change). Absolute reliability of HRV estimates were poor, but relative reliability estimates were often acceptable (≥0.70). Our findings illustrate how measurement consistency of cardiac autonomic modulation estimates are altered during heat stress, and we demonstrate the possible implications on research design and data interpretation., (© 2020 The Authors. Experimental Physiology © 2020 The Physiological Society.)

Laser-Doppler flowmetry (LDF) is commonly used to assess cutaneous vasodilatation responses, but its reliability (i.e. consistency) during whole-body passive heating is unknown. We therefore assessed the reliability of LDF-derived indices of cutaneous vasodilatation during incremental whole-body heating. Fourteen young men (age: 24 (SD 5) years) completed three identical trials, each separated by 1 week. During each trial, a water-perfused suit was used to raise and clamp oesophageal temperature at 0.6°C (low-heat strain; LHS) and 1.2°C (moderate-heat strain; MHS) above baseline. LDF-derived skin blood flow (SkBF) was measured at three dorsal mid-forearm sites, with local skin temperature clamped at 34°C. Data were expressed as absolute cutaneous vascular conductance (CVC abs ; SkBF/mean arterial pressure) and normalised to maximal conductance (%CVC max ) achieved via simultaneous local skin heating to 44°C and increasing oesophageal temperature to 1.8°C above baseline. Between-day reliability was characterised as measurement consistency across trials, while within-day reliability was characterised as measurement consistency across adjacent skin sites during each trial. Between- and within-day absolute reliability (coefficient of variation) generally improved with increasing heat strain, changing from poor (>25%) at baseline, poor-to-moderate (15-34%) at LHS, and moderate (10-25%) at MHS. Generally, these estimates were more consistent when expressed as %CVC max . Conversely, relative reliability was mostly acceptable (intraclass correlation coefficient ≥0.70) during LHS and when data were expressed as CVC abs . These findings indicate that the consistency of LDF-derived CVC estimates during heat stress depends on the level of heat strain and method of data expression, which should be considered when designing and interpreting experiments., (© 2020 The Authors. Experimental Physiology © 2020 The Physiological Society.)

The ventilated capsule technique is widely used to measure time-dependent changes in sweating in humans. However, evaluations of its reliability (consistency) have been restricted to the forearm, despite extensive regional heterogeneity in the sweating response. Given the importance of such information for experimental design, statistical analysis and interpretation, we determined the reliability of local sweat rate at nine sites during whole-body passive (resting) heating. On three separate occasions, a water-perfused suit was used to increase and clamp oesophageal temperature 0.6, 1.2 and 1.8°C above baseline in 14 young men [24 (SD 5) years of age], while sweat rate was measured at the forehead, chest, abdomen, biceps, forearm, hand, quadriceps, calf and foot using ventilated capsules (3.8 cm 2 ). Absolute and relative reliability were determined via the coefficient of variation (CV) and intraclass correlation coefficient (ICC), respectively. At low heat strain (0.6°C), almost all sites had acceptable relative reliability (ICC ≥ 0.70) and moderate absolute reliability (CV < 25%). At moderate heat strain (1.2°C), only the abdomen, hand, quadriceps and foot had acceptable relative reliability, whereas the forehead, abdomen, forearm, hand and quadriceps had moderate absolute reliability. At high heat strain (1.8°C), relative reliability was acceptable at the abdomen, quadriceps, calf and foot, whereas the chest, abdomen, forearm, hand, quadriceps, calf and foot had moderate absolute reliability. Our findings indicate that the measurement site and level of heat strain impact the consistency of local sweat rate measured via the ventilated capsule technique, and we demonstrate the possible implications for research design and data interpretation., (© 2020 The Authors. Experimental Physiology © 2020 The Physiological Society.)

New Findings: What is the central question of this study? Does higher aerobic fitness, indexed by peak oxygen uptake ( V ̇ O 2 peak ), attenuate the age-related decline in thermoregulatory function during exercise in the heat? What is the main finding and its importance? When assessed in aerobically fit and less fit adults ( V ̇ O 2 peak : ∼30 vs. ∼50 ml kg -1  min -1 ) aged 18-66 years, a steeper decline in whole-body total heat loss (evaporative + dry heat exchange) was observed with increasing age in less aerobically fit adults. These outcomes indicate that increased aerobic fitness may attenuate the age-related decline in thermoregulatory function during exercise-heat stress., Abstract: Ageing is associated with decrements in cutaneous vasodilatation and sweating that attenuate whole-body total heat loss (evaporative + dry heat exchange) during exercise-heat stress. However, it remains uncertain whether increased aerobic fitness, as indexed by peak oxygen uptake ( V ̇ O 2 peak ), slows that age-related decline. To evaluate this possibility, we conducted a retrospective analysis of data from aerobically fit (n = 38; V ̇ O 2 peak : (mean (SD)) 49 (4) ml kg -1  min -1 ) and less fit (n = 35; V ̇ O 2 peak : 32 (3) ml kg -1  min -1 ) adults spanning a broad age range (18-65 vs. 18-66 years). Participants performed three, 30 min bouts of cycling at metabolic heat productions of 150, 200 and 250 W m -2 , each separated by 15 min recovery, in dry heat (40˚C, ∼15% relative humidity). Metabolic heat production and whole-body total heat loss were measured using indirect and direct calorimetry, respectively. Total heat loss (mean (95% CI)) declined at a rate of 5 (2, 8), 6 (3, 8) and 5 (3, 10) W m -2 per decade during exercise at metabolic heat productions of 150, 200 and 250 W m -2 , respectively, in less aerobically fit individuals (all P ≤ 0.002), due primarily to reductions in evaporative heat loss. In contrast, no significant associations between age and total heat loss were observed in aerobically fit individuals (all P ≥ 0.146). As such, the slope of the age-related reduction in total heat loss was steeper in less fit compared to fit individuals across all three exercise bouts (all P ≤ 0.029). These outcomes indicate that increased aerobic fitness attenuates the age-related decline in exercise thermoregulation., (© 2020 The Authors. Experimental Physiology © 2020 The Physiological Society.)

New Findings: What is the central question of this study? Repeated heat exposure during the summer months can enhance heat loss in humans (seasonal heat acclimatisation), but does the magnitude of that enhancement differ between young and older adults when assessed during passive heat exposure? What is the main finding and its importance? While seasonal heat acclimatisation enhanced evaporative heat loss (i.e. sweating) in both young and older adults, those improvements led to a greater reduction in body heat storage in older adults. These outcomes indicate that heat acclimatisation may confer greater thermoregulatory benefits with increasing age., Abstract: Repeated heat exposure throughout summer can enhance heat loss in humans (seasonal heat acclimatisation), although the effect of ageing on those improvements remains unclear. We therefore sought to assess thermoregulatory function in young and older adults during environmental heat exposure prior to and following seasonal heat acclimatisation, hypothesizing that the magnitude of adaptation would be greater in older relative to young adults. To achieve this, 14 young (19-27 years) and 10 older adults (55-72 years), who resided in a temperate humid-continental climate, completed a 3 h resting heat exposure (44°C, ∼30% relative humidity) in the winter-spring months as part of a larger investigation (pre-acclimatisation), before being re-evaluated using the same heat stress test following the summer months (post-acclimatisation). Whole-body dry and evaporative heat exchange, and metabolic rate were measured throughout using direct and indirect calorimetry (respectively), and used to quantify body heat storage (metabolic rate + dry heat gain - evaporative heat loss). Evaporative heat loss increased in both groups following acclimatisation, but those improvements led to a decrease in body heat storage in older (mean difference (95% CI); 213 (295, 131) kJ; P < 0.001), but not young adults (-25 (-94, 44) kJ; P = 0.458). Thus, body heat storage was greater in older compared to young adults before (222 (123, 314) kJ; P < 0.001), but not following acclimatisation (34 (-55, 123) kJ; P = 0.433). Although there is a need for larger and more controlled confirmatory studies, our findings indicate that seasonal heat acclimatisation may induce greater thermoregulatory adaptation in older compared to young adults., (© 2020 The Authors. Experimental Physiology © 2020 The Physiological Society.)

New Findings: What is the central question of this study? The aim was to identify the greatest contributor(s) to the variation in whole-body heat exchange, as assessed using direct calorimetry, among young men and women with heterogeneous characteristics during exercise at increasing metabolic heat production rates in dry heat. What is the main finding and its importance? The evaporative heat loss requirement, body morphology and aerobic fitness made the greatest contributions to the individual variation in evaporative and dry heat exchange, with the variance explained being exercise intensity dependent. These findings provide a foundation on which to build our ability to explain the individual variation in heat exchange during exercise-induced heat stress., Abstract: Numerous individual factors (e.g. fitness, sex, body morphology) are known to independently modulate heat exchange during exercise in the heat. However, in our view, the individual factor(s) making the greatest contribution to the variation in heat exchange among men and women remains poorly understood, despite several studies. We therefore sought to revisit this question by assessing whole-body dry and evaporative heat exchange using direct calorimetry in a heterogeneous sample of 100 young men (n = 57) and women (n = 43). Participants performed three 30 min bouts of cycling at very light (men/women; 300/250 W), light (400/325 W) and moderate (500/400 W) metabolic heat production rates, separated by a 15 min recovery, in dry heat (40°C, ∼12% relative humidity). Positive associations were observed between the evaporative heat loss requirement (metabolic heat production ± dry heat exchange) and evaporative heat loss (all P < 0.01), especially during moderate exercise (men, r = 0.62; women, r = 0.82), which explained 19-67% of individual variation. Peak aerobic power (in millilitres per kilogram per minute) was also positively related to evaporative heat loss in both sexes, albeit only during light and moderate exercise (r = 0.33-0.43; all P < 0.05), explaining a further 5-9% of individual variation. Dry heat exchange shared negative associations with body mass and surface area during all exercise bouts in both sexes (r = -0.29 to -0.55; all P < 0.05), explaining 9-30% of individual variation. We therefore demonstrate that the evaporative heat loss requirement, peak aerobic power and body morphology are the greatest contributors to the variation in whole-body heat exchange among young men and women exercising in dry heat, with the strength of those relationships being heat-load dependent., (© 2019 The Authors. Experimental Physiology © 2019 The Physiological Society.)

Aim: Ageing and hypohydration independently attenuate heat dissipation during exercise; however, the interactive effects of these factors remain unclear. We assessed the hypothesis that ageing suppresses hypohydration-induced reductions in whole-body heat loss during exercise in the heat., Methods: On two occasions, eight young (mean [SD]: 24 [4] years) and eight middle-aged (59 [5] years) men performed 30-minute bouts of light (heat production of 175 W m -2 ) and moderate (275 W m -2 ) cycling (separated by 15-minute rest) in the heat (40°C, 15% relative humidity) when euhydrated and hypohydrated (~4% reduction in body mass). Heat production and whole-body net heat exchange (evaporative heat loss + dry heat gain) were measured via indirect and direct calorimetry (respectively) and heat storage was calculated via their temporal summation., Results: Net heat exchange was reduced, while heat storage was elevated, in the middle-aged men during moderate exercise when euhydrated (both P ≤ 0.01). In the young, evaporative heat loss was attenuated in the hypohydrated vs euhydrated condition during light (199 ± 6 vs 211 ± 10 W m -2 ; P ≤ 0.01) and moderate (287 ± 15 vs 307 ± 13 W m -2 ; P ≤ 0.01) exercise, but was similar in the middle-aged men, averaging 223 ± 6 and 299 ± 15 W m -2 , respectively, across conditions (both P ≥ 0.32). Heat storage was thereby exacerbated by hypohydration in the young (both P < 0.01) but not the middle-aged (both P ≥ 0.32) during both exercise bouts and, as a result, was similar between groups when hypohydrated (both P ≥ 0.50)., Conclusion: Hypohydration attenuates heat loss via sweating in young but not middle-aged men, indicating that ageing impairs one's ability to mitigate further sweat-induced fluid loss during hypohydration., (© 2018 Scandinavian Physiological Society. Published by John Wiley & Sons Ltd.)

New Findings: What is the central question of this study? We evaluated whether oxidative stress attenuates the contribution of nitric oxide to sweating during high-intensity exercise. What is the main finding and its importance? In contrast to our previous report of an oxidative stress-mediated reduction in nitric oxide-dependent cutaneous vasodilatation in this cohort during intense exercise, we demonstrated no influence of local ascorbate administration on the sweating response during moderate- (∼51% peak oxygen uptake) or high-intensity exercise (∼72% peak oxygen uptake). These new findings provide important mechanistic insight into how exercise-induced oxidative stress impacts sudomotor activity. Nitric oxide (NO)-dependent sweating is diminished during high- but not moderate-intensity exercise. We evaluated whether this impairment stems from increased oxidative stress during high-intensity exercise. On two separate days, 11 young (24 ± 4 years) men cycled in the heat (35°C) at a moderate [500 W; 52 ± 6% peak oxygen uptake (V̇O2 peak )] or high (700 W; 71 ± 5% V̇O2 peak ) rate of metabolic heat production. Each session included two 30 min exercise bouts separated by a 20 min recovery period. Local sweat rate was monitored at four forearm skin sites continuously perfused via intradermal microdialysis with the following: (i) lactated Ringer solution (Control); (ii) 10 mm ascorbate (Ascorbate; non-selective antioxidant); (iii) 10 mm N G -nitro-l-arginine methyl ester (l-NAME; NO synthase inhibitor); or (iv) 10 mm ascorbate plus 10 mm l-NAME (Ascorbate + l-NAME). During moderate exercise, sweat rate was attenuated at the l-NAME and Ascorbate + l-NAME sites (both ∼1.0 mg min -1  cm -2 ; all P < 0.05) but not at the Ascorbate site (∼1.1 mg min -1  cm -2 ; both P ≥ 0.28) in comparison to the Control site (∼1.1 mg min -1  cm -2 ). However, no differences were observed between treatment sites (∼1.4 mg min -1  cm -2 ; P = 0.75) during high-intensity exercise. We conclude that diminished NO-dependent sweating during intense exercise occurs independent of oxidative stress., (© 2017 The Authors. Experimental Physiology © 2017 The Physiological Society.)

New Findings: What is the central question of this study? It remains to be determined whether type 2 diabetes attenuates muscarinic and nicotinic cutaneous vasodilatation and sweating as well as purinergic cutaneous vasodilatation. What is the main finding and its importance? We show that type 2 diabetes specifically attenuates purinergic cutaneous vasodilatation without influencing muscarinic and nicotinic cutaneous vasodilatation and sweating. Our results provide valuable new information regarding the receptor-specific influence of type 2 diabetes on microvascular and sudomotor function., Abstract: The present study evaluated whether type 2 diabetes (T2D) attenuates muscarinic and/or nicotinic cutaneous vasodilatation and sweating as well as purinergic cutaneous vasodilatation. Cutaneous vascular conductance and sweat rate were evaluated in 12 healthy non-diabetic older adults (Control, 60 ± 8 years) and 13 older adults with T2D (62 ± 10 years) at three intradermal forearm skin sites perfused with the following: (i) methacholine (muscarinic receptor agonist, five doses: 0.0125, 0.25, 5, 100 and 2000 mm); (ii) nicotine (nicotinic receptor agonist, five doses: 1.2, 3.6, 11, 33 and 100 mm); or (iii) ATP (purinergic receptor agonist, five doses: 0.03, 0.3, 3, 30 and 300 mm). Each agonist was administered for 25 min per dose. At the end of the protocol, 50 mm sodium nitroprusside was administered to all skin sites to elicit maximal cutaneous vasodilatation. Cutaneous vascular conductance during methacholine and nicotine administration did not differ between groups (all P > 0.05). In contrast, cutaneous vascular conductance during administration of 30 mm (42 ± 28 versus 63 ± 26% maximum, P ≤ 0.05) and 300 mm ATP (56 ± 24 versus 71 ± 20% maximum, P ≤ 0.05) was attenuated in individuals with T2D in comparison to the Control participants. Furthermore, cutaneous vascular conductance during administration of 50 mm sodium nitroprusside was lower in individuals with T2D relative to Control subjects (P = 0.04). Methacholine- and nicotine-induced sweating was similar between groups (all P > 0.05). Thus, T2D attenuates purinergic cutaneous vasodilatation without affecting muscarinic and nicotinic cutaneous vascular and sweating responses., (© 2017 The Authors. Experimental Physiology © 2017 The Physiological Society.)