Differences in conductive foot cooling: a comparison between males and females

Purpose

This study investigated possible sex-related and intra-menstrual differences in local vascular and skin temperature responses when conductive cooling was applied to the soles of the feet.

Method

Twelve females and twelve males exposed the soles of their feet to a cooling plate (which cooled from 35 to 15 °C) on two occasions 12–15 days apart. For females, sessions took place during their inactive and active contraceptive pill phases. Tip of Great toe temperature and Great toe skin blood flow were recorded throughout.

Results

Females’ feet cooled to a greater extent than males’ (P = 0.001). Sensitivity of toe skin blood flow to changes in skin temperature (1 or 2 °C) was not different between males and females. Dimensions of males’ feet were larger than females’ (P < 0.05) and correlations between foot dimensions and toe skin cooling were found (r = 0.728, P < 0.001). Analysis of the residual variance showed that foot volume, contact area and skin blood flow correlated with the rate of toe skin cooling (r = 0.812, r ² = 0.659, P < 0.001). No intra-menstrual differences were found.

Conclusion

The feet of females cooled at a faster rate than those of males in response to the same conductive cooling stimulus to the soles of the feet. However, similar reductions in skin blood flow were found for the same change in toe skin temperature. Therefore, sex related differences may be due to the differing dimensions of the feet, but further research including males and females matched for foot dimensions are required to confirm this mechanism.     

Thermoregulatory effects of three different types of head cooling in humans during a mild hyperthermia

Seven healthy young men participated in six trials with three different types of local cooling [cool air breathing (CAB), face skin cooling (FaC), and combined cooling (CoC)] in a warm environment for 90 min while either resting (operative temperature: T ₀ = 40°C, dew point temperature: T _dp = 15°C, air velocity: v _a = 0.3 m·s⁻¹) or exercising on a cycle ergometer with an external work load of 90 W (T ₀ = 36°C, T _dp = 15°C, v _a = 0.3 m·s⁻¹). Cool air (10°C) arrived at the entry point of the hood and/or the mask at a ventilation rate of 12 m · s⁻¹. Oesophageal temperature was not affected by any kind of cooling, while tympanic temperature was decreased at rest by both FaC and CoC [respectively −0.15 (0.06) and −0.09 (0.03)°C, P ≤ 0.05]. Mean skin temperature was decreased by FaC and CoC at rest [respectively −0.31 (0.07) and −0.27 (0.09)°C, P ≤ 0.05] and during exercise [respectively −0.64 (0.15) and −1.04 (0.22)°C, P ≤ 0.01]. CAB had no effect on skin temperatures. CoC and FaC reduced head skin temperature during both rest and work (P < 0.001) with no effect on the skin temperature of the rest of the body, except under CoC with exercise (P < 0.05). CAB did not influence local sweating. FaC, however, decreased the more profuse sweat rates (P ≤ 0.05) at rest, while CoC decreased all sweating rates at rest (P ≤ 0.05) and only the back, head and leg sweating rates during exercise (P ≤ 0.05). These results suggest that head skin cooling causes a reduction in heat strain, while CAB does not. This beneficial influence does not, however, appear to be the result of selective brain cooling. Tympanic temperature seems to be a good index of the core thermal inputs to the hypothalamic regulatory system, since variations in that parameter were associated with similarly directed variations in the sweating outputs. Accepted: 12 April 1999     

The effect of ethnicity on the vascular responses to cold exposure of the extremities

Purpose

Cold injuries are more prevalent in individuals of African descent (AFD). Therefore, we investigated the effect of extremity cooling on skin blood flow (SkBF) and temperature (T _sk) between ethnic groups.

Methods

Thirty males [10 Caucasian (CAU), 10 Asian (ASN), 10 AFD] undertook three tests in 30 °C air whilst digit T _sk and SkBF were measured: (i) vasomotor threshold (VT) test—arm immersed in 35 °C water progressively cooled to 10 °C and rewarmed to 35 °C to identify vasoconstriction and vasodilatation; (ii) cold-induced vasodilatation (CIVD) test—hand immersed in 8 °C water for 30 min followed by spontaneous warming; (iii) cold sensitivity (CS) test—foot immersed in 15 °C water for 2 min followed by spontaneous warming. Cold sensory thresholds of the forearm and finger were also assessed.

Results

In the VT test, vasoconstriction and vasodilatation occurred at a warmer finger T _sk in AFD during cooling [21.2 (4.4) vs. 17.0 (3.1) °C, P = 0.034] and warming [22.0 (7.9) vs. 12.1 (4.1) °C, P = 0.002] compared with CAU. In the CIVD test, average SkBF during immersion was greater in CAU [42 (24) %] than ASN [25 (8) %, P = 0.036] and AFD [24 (13) %, P = 0.023]. Following immersion, SkBF was higher and rewarming faster in CAU [3.2 (0.4) °C min^?1] compared with AFD [2.5 (0.7) °C min^?1, P = 0.037], but neither group differed from ASN [3.0 (0.6) °C min^?1]. Responses to the CS test and cold sensory thresholds were similar between groups.

Conclusion

AFD experienced a more intense protracted finger vasoconstriction than CAU during hand immersion, whilst ASN experienced an intermediate response. This greater sensitivity to cold may explain why AFD are more susceptible to cold injuries.     

The impact of different cooling modalities on the physiological responses in firefighters during strenuous work performed in high environmental temperatures

This study investigated the impact of ice vests and hand/forearm immersion on accelerating the physiological recovery between two bouts of strenuous exercise in the heat [mean (SD), 49.1(1.3)°C, RH 12 (1)]. On four occasions, eight firefighters completed two 20-min bouts of treadmill walking (5 km h, 7.5% gradient) while wearing standard firefighter protective clothing. Each bout was separated by a 15-min recovery period, during which one of four conditions were administered: ice vest (VEST), hand/forearm immersion (W), ice vest combined with hand/forearm immersion (VEST + W) and control (CON). Core temperature was significantly lower at the end of the recovery period in the VEST + W (37.97 ± 0.23°C) and W (37.96 ± 0.19°C) compared with the VEST (38.21 ± 0.12°C) and CON (38.29 ± 0.25°C) conditions and remained consistently lower throughout the second bout of exercise. Heart rate responses during the recovery period and bout 2 were similar between the VEST + W and W conditions which were significantly lower compared with the VEST and CON which did not differ from each other. Mean skin temperature was significantly lower at the start of bout 2 in the cooling conditions compared with CON; these differences reduced as exercise progressed. These findings demonstrate that hand/forearm immersion (~19°C) is more effective than ice vests in reducing the physiological strain when firefighters re-enter structural fires after short rest periods. Combining ice vests with hand/forearm immersion provides no additional benefit.     

Neck cooling and cognitive performance following exercise-induced hyperthermia

Purpose

To assess the efficacy of neck cooling on cognitive performance following exertional hyperthermia.

Methods

Twelve healthy men completed two experimental trials [control (CON) and neck cooling collar (NCC)] in a counter-balanced design. They ran on a treadmill at 70 % VO_2peak under warm and humid conditions (dry bulb temperature: 30.2 ± 0.3 °C, relative humidity: 71 ± 2 %) for 75 min or until volitional exhaustion. Gastrointestinal, neck and skin temperatures, heart rate and subjective ratings were assessed. Serum brain-derived neurotrophic factor (BDNF) levels were measured before and after each run. Cognitive performance comprising symbol digit matching, search and memory, digit span, choice reaction time and psychomotor vigilance test (PVT) were assessed before and after exercise.

Results

Mean gastrointestinal temperature was similar after exercise between trials (CON: 39.5 ± 0.4 °C vs. NCC: 39.6 ± 0.3 °C; p = 0.15). Mean neck temperature was lowered in NCC compared to CON after the run (36.4 ± 1.6 °C vs. NCC: 26.0 ± 0.3 °C; p < 0.001). Exercise-induced hyperthermia improved mean reaction time in the symbol digit matching test (?134 ± 154 ms; p < 0.05) and the PVT (?18 ± 30 ms; p < 0.05). Maximum span was increased in the digit span test (1 ± 2; p < 0.05). Application of NCC reduced the number of search errors made in level 3 of the search and memory test (p < 0.05). Mean serum BDNF levels were increased following exercise-induced hyperthermia in both trials (p < 0.05).

Conclusion

Exercise-induced hyperthermia improves working memory and alertness. Neck cooling may only enhance performance in tasks of higher complexity.     

Safe cooling limits from exercise-induced hyperthermia

We evaluated the cooling rate of hyperthermic subjects, as measured by three estimates of deep core temperatures (esophageal, rectal and aural canal temperatures), during immersion in a range of water temperatures. The objective of the study was to compare the three indices of core temperature and define safe cooling limits when using rectal temperature to avoid the development of hypothermia. On 4 separate days, seven subjects (four males, three females) exercised for 45.4±4.1 min at 65% at an ambient temperature of 39°C, RH: 36.5%, until rectal temperature (T _re) increased to 40.0°C (39.5°C for two subjects). Following exercise, the subjects were immersed in a circulated water bath controlled at 2, 8, 14 and 20°C until T _re returned to 37.5°C. When T _re reached normothermia during the cooling period (37.5±0.05°C), both esophageal (T _es) (35.6±1.3°C) and aural canal (T _ac) (35.9±0.9°C) temperatures were approaching or reaching hypothermia, particularly during immersion in 2°C water (T _es=34.5±1.2°C). On the basis of the heat loss data, the heat gained during the exercise was fully eliminated after 5.4±1.5, 7.9±2.9, 10.4±3.8 and 13.1±2.8 min of immersion in 2, 8, 14 and 20°C water, respectively, with the coldest water showing a significantly faster cooling rate. During the immersion in 2°C water, a decrease of only 1.5°C in T _re resulted in the elimination of 100% of the heat gained during exercise without causing hypothermia. This study would therefore support cooling the core temperature of hyperthermic subjects to a rectal temperature between 37.8°C (during immersion in water >10°C) and 38.6°C (during immersion in water <10°C) to eliminate the heat gained during exercise without causing hypothermia.     

Thermal perceptions and skin temperatures during continuous and intermittent ventilation of the torso throughout and after exercise in the heat

Objective

This study tested the hypothesis that intermittent cooling in air-perfused vests (APV) will not only maintain thermal balance but, due to cyclical activations of cutaneous thermoreceptors, also enhance thermal perceptions.

Method

Ten physically active males completed four conditions where they exercised (walking: 5 km h^?1, 2 % gradient) in a hot environment (~34.0 °C, 50 % RH) for 72 min, followed by a 33-min period of rest. They wore an APV throughout. The four conditions differed in respect to the profile of ambient air that was perfused through the APV: continuous perfusion (CP); intermittent perfusion of 6 min ON/OFF periods (IP_onoff); a steady increase and decrease in flow rate in equal increments (IP_ramp); and an initial step-increase in the flow rate followed by an incremental decrease to zero flow rate (IP_triang). Whole body and torso thermal comfort (TC, TTC), whole body and torso temperature sensation (TS, TTS), whole body and torso skin temperature ( $\bar{T}_{\text{sk}}$ , $\bar{T}_{\text{sktorso}}$ ), local relative humidity ( $\overline{RH}_{\text{torso}}$ ) and rectal temperature (T _re) were measured.

Results

There were no significant differences in T _re, absolute whole body and local $\bar{T}_{\text{sk}}$ , TC, TTC and TS between the cooling profiles. However, TTS was cooler in CP and IP_ramp than IP_onoff and IP_triang. Even though intermittent cooling did not significantly enhance thermal perceptions in CP, a trend existed for TC (P = 0.063) to become less favourable over time.

Conclusion

To reduce the power consumption and extend the battery life of an APV, it is recommended that an intermittent cooling profile should be adopted.     

Skin cooling aids cerebrovascular function more effectively under severe than moderate heat stress

Skin surface cooling has been shown to improve orthostatic tolerance; however, the influence of severe heat stress on cardiovascular and cerebrovascular responses to skin cooling remains unknown. Nine healthy males, resting supine in a water-perfusion suit, were heated to +1.0 and +2.0°C elevation in body core temperature (T _c). Blood flow velocity in the middle cerebral artery (transcranial Doppler ultrasound), mean arterial pressure (MAP; photoplethysmography), stroke volume (SV; Modelflow), total peripheral resistance (TPR; Modelflow), heart rate (HR; ECG) and the partial pressure of end-tidal carbon dioxide (P_ETCO₂) were measured continuously during 1-min baseline and 3-min lower body negative pressure (LBNP, −15 mm Hg) when heated without and again with skin surface cooling. Nine participants tolerated +1°C and six participants reached +2°C. Skin cooling elevated (P = 0.004) MAP ~4% during baseline and LBNP at +1°C T _c. During LBNP, skin cooling increased SV (9%; P = 0.010) and TPR (0.9 mm Hg L⁻¹ min, P = 0.013) and lowered HR (13 b min⁻¹, P = 0.012) at +1°C T _c and +2°C T _c collectively. At +2°C T _c, skin cooling elevated P_ETCO₂ ~4.3 mm Hg (P = 0.011) and therefore reduced cerebral vascular resistance ~0.1 mm Hg cm⁻¹ s at baseline and LBNP (P = 0.012). In conclusion, skin cooling under severe heating and mild orthostatic stress maintained cerebral blood flow more effectively than it did under moderate heating, in conjunction with elevated carbon dioxide pressure, SV and arterial resistance.     

Post-exercise cooling techniques in hot,humid conditions

Major sporting events are often held in hot and humid environmental conditions. Cooling techniques have been used to reduce the risk of heat illness following exercise. This study compared the efficacy of five cooling techniques, hand immersion (HI), whole body fanning (WBF), an air cooled garment (ACG), a liquid cooled garment (LCG) and a phase change garment (PCG), against a natural cooling control condition (CON) over two periods between and following exercise bouts in 31°C, 70%RH air. Nine males [age 22 (3) years; height 1.80 (0.04) m; mass 69.80 (7.10) kg] exercised on a treadmill at a maximal sustainable work intensity until rectal temperature (T _re) reached 38.5°C following which they underwent a resting recovery (0–15 min; COOL 1). They then recommenced exercise until T _re again reached 38.5°C and then undertook 30 min of cooling with (0–15 min; COOL 2A), and without face fanning (15–30 min; COOL 2B). Based on mean body temperature changes (COOL 1), WBF was most effective in extracting heat: CON 99 W; WBF: 235 W; PCG: 141 W; HI: 162 W; ACG: 101 W; LCG: 49 W) as a consequence of evaporating more sweat. Therefore, WBF represents a cheap and practical means of post-exercise cooling in hot, humid conditions in a sporting setting.     

Skin blood flow influences cerebral oxygenation measured by near-infrared spectroscopy during dynamic exercise

Purpose

Near-infrared spectroscopy (NIRS) is widely used to investigate cerebral oxygenation and/or neural activation during physiological conditions such as exercise. However, NIRS-determined cerebral oxygenated hemoglobin (O₂Hb) may not necessarily correspond to intracranial blood flow during dynamic exercise. To determine the selectivity of NIRS to assess cerebral oxygenation and neural activation during exercise, we examined the influence of changes in forehead skin blood flow (SkBF_head) on NIRS signals during dynamic exercise.

Methods

In ten healthy men (age: 20 ± 1 years), middle cerebral artery blood flow velocity (MCA V _mean, via transcranial Doppler ultrasonography), SkBF_head (via laser Doppler flowmetry), and cerebral O₂Hb (via NIRS) were continuously measured. Each subject performed 60 % maximum heart rate moderate-intensity steady-state cycling exercise. To manipulate SkBF_head, facial cooling using a mist of cold water (~4 °C) was applied for 3 min during steady-state cycling.

Results

MCA V _mean significantly increased during exercise and remained unchanged with facial cooling. O₂Hb and SkBF_head were also significantly increased during exercise; however, both of these signals were lowered with facial cooling and returned to pre-cooling values with the removal of facial cooling. The changes in O₂Hb correlated significantly with the relative percent changes in SkBF_head in each individual (r = 0.71–0.99).

Conclusions

These findings suggest that during dynamic exercise NIRS-derived O₂Hb signal can be influenced by thermoregulatory changes in SkBF_head and therefore, may not be completely reflective of cerebral oxygenation or neural activation.     

Relationship between core temperature, skin temperature, and heat flux during exercise in heat

Purpose

This paper investigates the relationship between core temperature (T _c), skin temperature (T _s) and heat flux (HF) during exercise in hot conditions.

Method

Nine test volunteers, wearing an Army Combat Uniform and body armor, participated in three sessions at 25 °C/50 % relative humidity (RH); 35 °C/70 % RH; and 42 °C/20 % RH. Each session consisted of two 1-h treadmill walks at ~350 W and ~540 W intensity. T _s and HF from six sites on the forehead, sternum, pectoralis, left rib cage, left scapula, and left thigh, and T _c (i.e., core temperature pill used as a suppository) were measured. Multiple linear regressions were conducted to derive algorithms that estimate T _c from T _s and HF at each site. A simple model was developed to simulate influences of thermal conductivity and thickness of the local body tissues on the relationship between T _c, T _s, and HF.

Results

Coefficient of determination (R ²) ranged from 0.30 to 0.88, varying with locations and conditions. Good sites for T _c measurement at surface were the sternum, and a combination of the sternum, scapula, and rib sites. The combination of T _s and HF measured at the sternum explained ~75 % or more of variance in observed T _c in hot environments. The forehead was found unsuitable for exercise in heat due to sweating and evaporative heat loss. The derived algorithms are likely applicable only for the same ensemble or ensembles with similar thermal and vapor resistances.

Conclusion

Algorithms for T _c measurement are location-specific and their accuracy is dependent, to a large degree, on sensor placement.     

Effects of cooling on human skin and skeletal muscle

To investigate the effects of cooling on local temperature and circulation in the skin and skeletal muscle at different cooling temperatures. Ten male subjects (mean age 24.9 years) participated in this study. Intramuscular temperatures were measured by inserting two 22-gauge temperature probes (needle length; 8 and 18 mm) into the ankle dorsiflexors, while skin temperature was measured using a thermocouple attached to the leg skin anteriorly. Near-infrared spectroscopy was also used to evaluate the concentration changes in oxygenated, deoxygenated, and total hemoglobin/myoglobin in local skin and skeletal muscle. These measurements were simultaneously performed during the 10-min noncooling, 30-min cooling (cooling pad temperature; 0, 10, or 20°C), and 60-min recovery periods. Under all cooling conditions, skin and intramuscular temperatures decreased during cooling (P < 0.01) and began to increase after the cooling pad was removed. However, these values did not return to baseline values during the recovery period (P < 0.01). Moreover, tissue temperatures tended to show lower values during cooling at lower cooling temperatures. All hemoglobin/myoglobin concentrations also showed a concomitant significant decrease during cooling under three cooling conditions (P < 0.01); the oxygenated and total hemoglobin/myoglobin concentrations did not return to the exact values before cooling during the recovery period. This study suggested that the rate of decrease in tissue temperature depends on the cooling temperature and the effects of cooling on tissue temperatures and circulation tend to be maintained during 60 min post-cooling period despite the cooling temperature.     

Palm cooling to reduce heat strain in subjects during simulated armoured vehicle transport

This study examined whether palm cooling (PC) could reduce heat strain, measured through changes in core, mean skin, mean body temperatures, and thermal sensation in resting hyperthermic subjects wearing chemical protective garments. Ten male subjects performed three exercise bouts (6.1 km h⁻¹, 2–4% grade) in a hot, dry environment [mean (SD) air temperature 42.2 (0.5°C), relative humidity 36.5 (1%)] until core temperature reached 38.8°C. Subjects then simulated transport in an armoured vehicle by resting in a seated position for 50 min with either no cooling (NC), (PC at 10°C) or palm cooling with vacuum application around the hand (PCVAC, 10°C, 7.47 kPa negative pressure). Core, skin, and mean body temperatures with PC and PCVAC were lower (P < 0.05) than NC from 15 to 50 min of cooling, and thermal sensation was lower (P < 0.05) from 30 to 50 min, with no differences in any variables between PC and PCVAC. Maximal heat extraction averaged 42 (12 W), and core temperature was reduced by 0.38 (0.21°C) after 50 min of PC. Heat extraction with PC was modest compared to other cooling approaches in the literature.     

Heart rate variability during exertional heat stress: effects of heat production and treatment

Purpose

We assessed the efficacy of different treatments (i.e., treatment with ice water immersion vs. natural recovery) and the effect of exercise intensities (i.e., low vs. high) for restoring heart rate variability (HRV) indices during recovery from exertional heat stress (EHS).

Methods

Nine healthy adults (26 ± 3 years, 174.2 ± 3.8 cm, 74.6 ± 4.3 kg, 17.9 ± 2.8 % body fat, 57 ± 2 mL·kg·^?1 min^?1 peak oxygen uptake) completed four EHS sessions incorporating either walking (4.0–4.5 km·h^?1, 2 % incline) or jogging (~7.0 km·h^?1, 2 % incline) on a treadmill in a hot-dry environment (40 °C, 20–30 % relative humidity) while wearing a non-permeable rain poncho for a slow or fast rate of rectal temperature (T _re) increase, respectively. Upon reaching a T _re of 39.5 °C, participants recovered until T _re returned to 38 °C either passively or with whole-body immersion in 2 °C water. A comprehensive panel of 93 HRV measures were computed from the time, frequency, time–frequency, scale-invariant, entropy and non-linear domains.

Results

Exertional heat stress significantly affected 60/93 HRV measures analysed. Analyses during recovery demonstrated that there were no significant differences between HRV measures that had been influenced by EHS at the end of passive recovery vs. whole-body cooling treatment (p > 0.05). Nevertheless, the cooling treatment required statistically significantly less time to reduce T _re (p < 0.001).

Conclusions

While EHS has a marked effect on autonomic nervous system modulation and whole-body immersion in 2 °C water results in faster cooling, there were no observed differences in restoration of autonomic heart rate modulation as measured by HRV indices with whole-body cold-water immersion compared to passive recovery in thermoneutral conditions.     

Assessing neonatal heat balance and physiological strain in newborn infants nursed under radiant warmers in intensive care with fentanyl sedation

Purpose

To assess heat balance status of newborn infants nursed under radiant warmers (RWs) during intensive care.

Methods

Heat balance, thermal status and primary indicators of physiological strain were concurrently measured in 14 newborns nursed under RWs for 105 min. Metabolic heat production (M), evaporative heat loss (E), convective (C) and conductive heat flow (K), rectal temperature (T _re) and mean skin temperatures (T _sk) were measured continuously. The rate of radiant heat required for heat balance (R _req) and the rate of radiant heat provided (R _prov) were derived. The rate of body heat storage (S) was calculated using a two-compartment model of ‘core’ (T _re) and ‘shell’ (T _sk) temperatures.

Results

Mean M, E, C and K were 10.5 ± 2.7 W, 5.8 ± 1.1 W, 6.2 ± 0.8 W and 0.1 ± 0.1 W, respectively. Mean R _prov (1.7 ± 2.6 W) and R _req (1.7 ± 2.7 W) were similar (p > 0.05). However, while the resultant mean change in body heat content after 105 min was negligible (–0.1 ± 3.7 kJ), acute time-dependent changes in S were evidenced by a mean positive heat storage component of +6.4 ± 2.6 kJ and a mean negative heat storage component of –6.5 ± 3.7 kJ. Accordingly, large fluctuations in both T _re and T _sk occurred that were actively induced by changes in RW output. Nonetheless, no active physiological responses (heart rate, breathing frequency and mean arterial pressure) to these bouts of heating and cooling were observed.

Conclusions

RWs maintain net heat balance over a prolonged period, but actively induce acute bouts of heat imbalance that cause rapid changes in T _re and T _sk. Transient bouts of heat storage do not exacerbate physiological strain, but could in the longer term.     

Hypohydration and acute thermal stress affect mood state but not cognition or dynamic postural balance

Equivocal findings have been reported in the few studies that examined the impact of ambient temperature (T _a) and hypohydration on cognition and dynamic balance. The purpose of this study was to determine the impact of acute exposure to a range of ambient temperatures (T _a 10–40 °C) in euhydration (EUH) and hypohydration (HYP) states on cognition, mood and dynamic balance. Thirty-two men (age 22 ± 4 years, height 1.80 ± 0.05 m, body mass 85.4 ± 10.8 kg) were grouped into four matched cohorts (n = 8), and tested in one of the four T _a (10, 20, 30, 40 °C) when EUH and HYP (?4 % body mass via exercise–heat exposure). Cognition was assessed using psychomotor vigilance, 4-choice reaction time, matching to sample, and grammatical reasoning. Mood was evaluated by profile of mood states and dynamic postural balance was tested using a Biodex Balance System. Thermal sensation (TS), core (T _core) and skin temperature (T _sk) were obtained throughout testing. Volunteers lost ?4.1 ± 0.4 % body mass during HYP. T _sk and TS increased with increasing T _a, with no effect of hydration. Cognitive performance was not altered by HYP or thermal stress. Total mood disturbance (TMD), fatigue, confusion, anger, and depression increased during HYP at all T _a. Dynamic balance was unaffected by HYP, but 10 °C exposure impaired balance compared to all other T _a. Despite an increase in TMD during HYP, cognitive function was maintained in all testing environments, demonstrating cognitive resiliency in response to body fluid deficits. Dynamic postural stability at 10 °C appeared to be hampered by low-grade shivering, but was otherwise maintained during HYP and thermal stress.     

Foot cooling does not improve vigilance but may transiently reduce sleepiness

Temperature of the skin (T_Sk) and core (T_C) play key roles in sleep–wake regulation. The diurnal combination of low T_Sk and high T_C facilitates alertness, whereas the transition to high T_Sk and low T_C correlates with sleepiness. Sleepiness and deteriorating vigilance are induced with peripheral warming, whereas peripheral cooling appears to transiently improve vigilance in narcolepsy. This study aimed to test the hypothesis that foot cooling would maintain vigilance during extended wakefulness in healthy adults. Nine healthy young adult participants with habitually normal sleep completed three constant‐routine trials in randomized crossover order. Trials began at 22:30 hours, and involved continuous mild foot cooling (30°C), moderate foot cooling (25°C) or no foot cooling, while undertaking six × 10‐min Psychomotor Vigilance Tasks and seven × 7‐min Karolinska Drowsiness Tasks, interspersed with questionnaires of sleepiness and thermal perceptions. Foot temperatures in control, mild and moderate cooling averaged 34.5 ± 0.5°C, 30.8 ± 0.2°C and 26.4 ± 0.1°C (all p < .01), while upper‐limb temperatures remained stable (34–35°C) and T_C declined (approximately ?0.12°C per hr) regardless of trial (p = .84). Foot cooling did not improve vigilance (repeated‐measures‐ANOVA interaction for response speed: p = .45), but transiently reduced subjective sleepiness (?0.8 ± 0.8; p = .004). Participants felt cooler throughout cooling trials, but thermal comfort was unaffected (p = .43), as were almost all Karolinska Drowsiness Tasks’ encephalographic parameters. In conclusion, mild or moderate cooling of the feet did not attenuate declines in vigilance or core temperature of healthy young adults during the period of normal sleep onset and early sleep, and any effect on sleepiness was small and transient.     

External muscle heating during warm-up does not provide added performance benefit above external heating in the recovery period alone

Purpose

Having previously shown the use of passive external heating between warm-up completion and sprint cycling to have had a positive effect on muscle temperature (T _m) and maximal sprint performance, we sought to determine whether adding passive heating during active warm up was of further benefit.

Methods

Ten trained male cyclists completed a standardised 15 min sprint based warm-up on a cycle ergometer, followed by 30 min passive recovery before completing a 30 s maximal sprint test. Warm up was completed either with or without additional external passive heating. During recovery, external passive leg heating was used in both standard warm-up (CONHOT) and heated warm-up (HOTHOT) conditions, for control, a standard tracksuit was worn (CON).

Results

T _m declined exponentially during CON, CONHOT and HOTHOT reduced the exponential decline during recovery. Peak (11.1 %, 1561 ± 258 W and 1542 ± 223 W), relative (10.6 % 21.0 ± 2.2 W kg^–1 and 20.9 ± 1.8 W kg^–1) and mean (4.1 %, 734 ± 126 W and 729 ± 125 W) power were all improved with CONHOT and HOTHOT, respectively compared to CON (1,397 ± 239 W; 18.9 ± 3.0 W kg^–1 and 701 ± 109 W). There was no additional benefit of HOTHOT on T _m or sprint performance compared to CONHOT.

Conclusion

External heating during an active warm up does not provide additional physiological or performance benefit. As noted previously, external heating is capable of reducing the rate of decline in T _m after an active warm-up, improving subsequent sprint cycling performance.     

Effect of acute hypoxia on muscle blood flow, VO2p, and [HHb] kinetics during leg extension exercise in older men

The adjustment of pulmonary oxygen uptake (VO_2p), heart rate (HR), limb blood flow (LBF), and muscle deoxygenation [HHb] was examined during the transition to moderate-intensity, knee-extension exercise in six older adults (70 ± 4 years) under two conditions: normoxia (FIO₂ = 20.9 %) and hypoxia (FIO₂ = 15 %). The subjects performed repeated step transitions from an active baseline (3 W) to an absolute work rate (21 W) in both conditions. Phase 2 VO_2p, HR, LBF, and [HHb] data were fit with an exponential model. Under hypoxic conditions, no change was observed in HR kinetics, on the other hand, LBF kinetics was faster (normoxia 34 ± 3 s; hypoxia 28 ± 2), whereas the overall [HHb] adjustment ( $ \tau^{\prime } = {\text{TD}} + \tau $ ) was slower (normoxia 28 ± 2; hypoxia 33 ± 4 s). Phase 2 VO_2p kinetics were unchanged (p < 0.05). The faster LBF kinetics and slower [HHb] kinetics reflect an improved matching between O₂ delivery and O₂ utilization at the microvascular level, preventing the phase 2 VO_2p kinetics from become slower in hypoxia. Moreover, the absolute blood flow values were higher in hypoxia (1.17 ± 0.2 L min^?1) compared to normoxia (0.96 ± 0.2 L min^?1) during the steady-state exercise at 21 W. These findings support the idea that, for older adults exercising at a low work rate, an increase of limb blood flow offsets the drop in arterial oxygen content (CaO₂) caused by breathing an hypoxic mixture.     

The contribution of blood flow to the skin temperature responses during a cold sensitivity test

Purpose

The presumption in a cold sensitivity test (CST) used for cold injuries is that the skin temperature (T _sk) observed reflects the return of blood flow to the extremity following a local cold challenge. We questioned this assumption.

Methods

Six non-cold injured participants undertook two CSTs in 30 °C air. The control (CON) CST involved 12 min gentle exercise prior to immersing the foot into 15 °C water for 2 min followed by 15 min of spontaneous rewarming. The occlusion (OCC) CST was the same except that blood flow to the foot was occluded during the rewarming period. These results were compared to CSTs from six individuals with non-freezing cold injury and moderate–severe cold sensitivity (CS) and a non-perfused human digit model (NPDM).

Results

Before immersion, great toe skin blood flow (SkBF) was similar in CON and OCC conditions [255 (107) laser Doppler units (LDU)] and was higher than CS [59 (52) LDU]. During rewarming, SkBF in CON returned to 104 % of the pre-immersion value and was higher than both OCC and CS. Great toe T _sk before immersion was lower in CS [28.5 (2.1) °C] compared to CON [34.7 (0.4) °C], OCC [34.6 (0.9) °C] and NPDM [35.0 (0.4) °C]. During rewarming skin/surface temperature in OCC, CS and NPDM were similar and all lower than CON.

Conclusions

SkBF does contribute to the skin rewarming profile during a CST as a faster rate of rewarming was observed in CON compared to either OCC or NPDM. The lower T _sk in CS may be due to a reduced basal SkBF.