James R. House

Influence of repeated daily menthol exposure on human temperature regulation and perception

A single exposure to menthol can, depending on concentration, enhance both cool sensations and encourage body heat storage. This study tested whether there is an habituation in either response after repeated-daily exposures. Twenty-two participants were assigned to one of three spray groups: Control (CON; n=6), 0.05% L-menthol (M(0.05%); n=8), and 0.2% L-menthol (M(0.2%); n=8). On Monday (20°C, 50% rh) participants were sprayed with 100 mL of solution and undertook 40 min of cycling at 45% of their peak power (Ex1), from Tuesday to Thursday (30°C, 50% rh) they were sprayed twice daily whilst resting (R1 to R6), Friday was a repeat of Monday (Ex2). Thermal sensation (TS), thermal comfort, perceived exertion, irritation, rectal and skin temperature (Tsk), skin blood flow (SkBF) and sweat rate were monitored. A two-way ANOVA (alpha=0.05) compared responses from the beginning (Ex1, R1) and end (Ex2, R5) of the testing week. M(0.2%) induced significantly (P<0.05) cooler TS at the beginning of the week (Ex1, R1) compared to the end (Ex2, R5), indicating habituation of TS; this was not observed in M(0.05%). No other perceptual or physiological responses habituated. 0.2% Menthol caused a heat storage response, mediated by vasoconstriction, at the beginning and end of the week, suggesting the habituation of TS occurred in a pathway specific to sensation. In summary, the cooling influence of 0.2% menthol habituates after repeated-daily exposures, but with no habituation in heat storage.

The influence of a menthol and ethanol soaked garment on human temperature regulation and perception during exercise and rest in warm, humid conditions

UNLABELLED This study assessed whether donning a garment saturated with menthol and ethanol (M/E) can improve evaporative cooling and thermal perceptions versus water (W) or nothing (CON) during low intensity exercise and rest in warm, humid conditions often encountered in recreational/occupational settings. It was hypothesised there would be no difference in rectal (Tre) and skin (Tsk) temperature, infra-red thermal imagery of the chest/back, thermal comfort (TC) and rating of perceived exertion (RPE) between M/E, W and CON, but participants would feel cooler in M/E versus W or CON. METHODS Six volunteers (mean [SD] 22 [4] years, 72.4 [7.4] kg and 173.6 [3.7] cm) completed (separate days) three, 60-min tests in 30°C, 70%rh, in a balanced order. After 15-min of seated rest participants donned a dry (CON) or 80mL soaked (M/E, W) long sleeve shirt appropriate to their intervention. They then undertook 30-min of low intensity stepping at a rate of 12steps/min on a 22.5cm box, followed by 15-min of seated rest. Measurements included heart rate (HR), Tre, Tsk (chest/back/forearm), thermal imaging (back/chest), thermal sensation (TS), TC and RPE. Data were reported every fifth minute as they changed from baseline and the area under the curves were compared by condition using one-way repeated measures ANOVA, with an alpha level of 0.05. RESULTS Tre differed by condition, with the largest heat storage response observed in M/E (p<0.05). Skin temperature at the chest/back/forearm, and thermal imaging of the chest all differed by condition, with the greatest rate of heat loss observed in W and M/E respectively (p<0.01). Thermal sensation differed by condition, with the coolest sensations observed in M/E (p<0.001). No other differences were observed. CONCLUSIONS Both M/E and W enhanced evaporative cooling compared CON, but M/E causes cooler sensations and a heat storage response, both of which are likely mediated by menthol.

The Effects of Direct Current Stimulation on Exercise Performance, Pacing and Perception in Temperate and Hot Environments

BACKGROUND Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulatory technique and has previously been shown to enhance submaximal exercise by reducing rating of perceived exertion (RPE). The present study examined the effects of tDCS on high-intensity self-paced exercise in temperate conditions and fixed followed by maximal exercise in the heat; it was hypothesised that performance and RPE would be altered. METHODS Two separate studies were undertaken in which exercise was preceded by 20-minutes of sham tDCS (SHAM), or anodal tDCS (TDCS). In study 1, six males completed a 20-km cycling time trial, on two occasions. Power output (PO), RPE, O2 pulse, and heart rate (HR) were measured throughout. In study 2, eight males completed fixed intensity cycling exercise at 55% of a pre-determined maximal power output (PMax) for 25-minutes before undertaking a time to exhaustion test (TTE; 75% PMax) in hot conditions (33 °C), on two occasions. Test duration, heart rate, thermal and perceptual responses were measured. Study specific and combined statistical analyses were undertaken and effect sizes established. RESULTS In study 1, mean PO was not improved with the tDCS (197 ± 20 W) compared to SHAM (197 ± 12 W) and there were no differences in pacing profile HR, O2 pulse or RPE (p > .05). In study 2, TTE duration (SHAM 314 ± 334 s cf 237 ± 362 s tDCS), thermal, heart rate and perceptual responses were unchanged by tDCS compared to SHAM (p > .05). When combined, performance in the SHAM trial tended to better than the tDCS. CONCLUSION tDCS did not influence cycling performance (study 1) exercise tolerance (study 2) or perception (studies 1 and 2). tDCS does not appear to facilitate high intensity exercise performance or exercise performance in the heat.

The determinants of thermal comfort in cool water

Water‐based activities may result in the loss of thermal comfort (TC). We hypothesized that in cooling water, the hands and feet would be responsible. Supine immersions were conducted in up to five clothing conditions (exposing various regions), as well as investigations to determine if a “reference” skin temperature (Tsk) distribution in thermoneutral air would help interpret our findings. After 10 min in 34.5 °C water, the temperature was decreased to 19.5 °C over 20 min; eight resting or exercising volunteers reported when they no longer felt comfortable and which region was responsible. TC, rectal temperature, and Tsk were measured. Rather than the extremities, the lower back and chest caused the loss of overall TC. At this point, mean (SD) chest Tsk was 3.3 (1.7) °C lower than the reference temperature (P = 0.005), and 3.8 (1.5) °C lower for the back (P = 0.002). Finger Tsk was 3.1 (2.7) °C higher than the reference temperature (P = 0.037). In cool and cooling water, hands and feet, already adapted to colder air temperatures, will not cause discomfort. Contrarily, more discomfort may arise from the chest and lower back, as these regions cool by more than normal. Thus, Tsk distribution in thermoneutral air may help understand variations in TC responses across the body.

Thermal comfort following immersion

Unlike thermal comfort in air, little research has been undertaken exploring thermal comfort around water sports. We investigated the impact of swimming and cooling in air after swimming on thermal comfort. After 10 min of swimming-and-resting cycles in 28°C water, volunteers wearing two types of garments or in swim briefs, faced winds in 24°C air, at rest or when stepping. Thermal comfort was significantly higher during swimming than resting. Post-immersion, following maximum discomfort, in 45 of 65 tests thermal comfort improved although mean skin temperature was still cooling (0.26 [SD 0.19] °C·min(-1) - max was 0.89°C·min(-1)). When thermal comfort was re-established mean skin temperature was lower than at maximal discomfort in 39 of 54 tests (0.81 [SD 0.58] °C - max difference was 2.68°C). The reduction in thermal discomfort in this scenario could be due to the adaptation of thermoreceptors, or to reductions in cooling rates to levels where discomfort was less stimulated. The relief from the recent discomfort may explain why, later, thermal comfort returned to initial levels in spite of poorer thermal profiles.

Role of cyclooxygenase in the vascular response to locally delivered acetylcholine in Caucasian and African descent individuals

INTRODUCTION Individuals of African descent (AFD) are more susceptible to non-freezing cold injury (NFCI) compared with Caucasian individuals (CAU). Vasodilatation to acetylcholine (ACh) is lower in AFD compared with CAU in the non-glabrous foot and finger skin sites; the reason for this is unknown. Prostanoids are responsible, in part, for the vasodilator response to ACh, however it is not known whether the contribution differs between ethnicities. METHODS 12 CAU and 12 AFD males received iontophoresis of ACh (1 w/v%) on non-glabrous foot and finger skin sites following placebo and then aspirin (600mg, single blinded). Aspirin was utilised to inhibit prostanoid production by inhibiting the cyclooxygenase (COX) enzyme. Laser Doppler flowmetry was utilised to measure changes in skin blood flow. RESULTS Not all participants could receive iontophoresis charge due to high skin resistance; these participants were therefore excluded from the analyses. Foot: ACh elicited greater maximal vasodilatation in CAU than AFD following placebo (P=0.003) and COX inhibition (COXib) (P<0.001). COXib did not affect blood flow responses in AFD, but caused a reduction in the area under the curve for CAU (P=0.031). Finger: ACh elicited a greater maximal vasodilatation in CAU than AFD following placebo (P=0.013) and COXib (P=0.001). COXib tended to reduce the area under the curve in AFD (P=0.053), but did not affect CAU. CONCLUSIONS CAU have a greater endothelial reactivity than AFD in both foot and finger skin sites irrespective of COXib. It is concluded that the lower ACh-induced vasodilatation in AFD is not due to a compromised COX pathway.

Role of cyclooxygenase in the vascular responses to extremity cooling in Caucasian and African males

What is the central question of this study? Compared with Caucasians, African individuals are more susceptible to non‐freezing cold injury and experience greater cutaneous vasoconstriction and cooler finger skin temperatures upon hand cooling. We investigated whether the enzyme cyclooxygenase is, in part, responsible for the exaggerated response to local cooling. What is the main finding and its importance? During local hand cooling, individuals of African descent experienced significantly lower finger skin blood flow and skin temperature compared with Caucasians irrespective of cyclooxygenase inhibition. These data suggest that in young African males the cyclooxygenase pathway appears not to be the primary reason for the increased susceptibility to non‐freezing cold injury.

Effects of ice-slurry and carbohydrate on exercise in the heat

There is considerable interest in reducing the ergolytic effect of heat. One approach is to reduce body-heat content through ingesting ice-slurry (IS), which provides a substantially greater heat sink benefit than cool liquids because of the enthalpy of fusion absorbed in the phase change from ice to water [1]. Many studies using IS have employed a formulation containing carbohydrate [2], which is itself ergogenic during prolonged exercise in the heat [3]. Although the separate effects of IS and carbohydrate on performance in the heat are established, it is unclear if there is an interaction when co-ingested. For example, exogenous carbohydrate oxidation is impaired with hyperthermia [4], whereas IS reduces heat stress. This study examined the separate and combined effects of IS and carbohydrate on performance, thermoregulation, substrate utilisation and thermal perception during prolonged cycling exercise in the heat.

Individual and cumulative benefits of making body armour and chemical & biological protective gloves, respirator and overboots from moisture vapour permeable materials

Some chemical & biological (CB) personal protective equipment, such as gloves, overboots and respirators, and other equipment such as body armour (BA) are made from moisture vapour impermeable (MVIP) materials, which increase insulation and impede evaporative cooling, thereby increasing the thermal burden. The aim of this study was to quantify the thermal burden imposed by each individual MVIP CB protective item and that imposed by BA.

Cutaneous Vascular Responses of the Hands and Feet to Cooling, Rewarming, and Hypoxia in Humans

INTRODUCTION This study investigated skin vasomotor responses in the fingers and toes during cooling and rewarming with and without normobaric hypoxia. METHODS Fourteen volunteers (8 males and 6 females) were exposed to gradual air cooling (mean±SD: -0.4±0.1oC·min-1) followed by rewarming (+0.5±0.1oC·min-1) while breathing normoxic air (FIO2 0.21 at 761±3 mm Hg) or hypoxic gas (FIO2 0.12, at 761±3 mm Hg, equivalent to ~5000 m above sea level). Throughout the gradual cooling and rewarming phases, rectal temperature was measured, and skin temperatures and laser Doppler skin blood flow were measured on the thumb, little finger, and great and little toe pads. RESULTS During gradual cooling, skin temperature but not deep body temperature decreased. No differences in cutaneous vascular conductance were found for the toes or thumb (P=0.169 great toe; P=0.289 little toe; P=0.422 thumb). Cutaneous vascular conductance was reduced in the little finger to a greater extent at the same mean skin temperatures (34.5-33.5oC) in the hypoxic compared with normoxic conditions (P=0.047). The onset of vasoconstriction and release of vasoconstriction in the thumb and little finger occurred at higher mean skin temperatures in hypoxia compared with normoxia (P<0.05). The onset of vasoconstriction and release of vasoconstriction in the toes occurred at similar skin temperatures (P=0.181 and P=0.132, respectively). CONCLUSION The earlier vasoconstrictor response and later release of vasoconstriction in the finger during hypoxic conditions may result in a greater dose of cold to that digit, taking longer to rewarm following the release of vasoconstriction.