Sex differences in the physiological responses to exercise-induced dehydration: consequences and mechanisms.

Physiological strain during exercise is increased by mild dehydration (∼1%-3% body mass loss). This response may be sex-dependent, but there are no direct comparative data in this regard. This review aimed to develop a framework for future research by exploring the potential impact of sex on thermoregulatory and cardiac strain associated with exercise-induced dehydration. Sex-based comparisons were achieved by comparing trends from studies that implemented similar experimental protocols but recruited males and females separately. This revealed a higher core temperature (Tc) in response to exercise-induced dehydration in both sexes; however, it seemingly occurred at a lower percent body mass loss in females. Although less clear, similar trends existed for cardiac strain. The average female may have a lower body water volume per body mass compared with males, and therefore the same percent body mass loss between the sexes may represent a larger portion of total body water in females potentially posing a greater physiological strain. In addition, the rate at which Tc increases at exercise onset might be faster in females and induce a greater thermoregulatory challenge earlier into exercise. The Tc response at exercise onset is associated with lower sweating rates in females, which is commonly attributed to sex differences in metabolic heat production. However, a reduced sweat gland sensitivity to stimuli, lower fluid output per sweat gland, and sex hormones promoting fluid retention in females may also contribute. In conclusion, the limited evidence suggests that sex-based differences exist in thermoregulatory and cardiac strain associated with exercise-induced dehydration, and this warrants future investigations.

Effects of acute dietary nitrate supplementation on cold-induced vasodilation in healthy males.

PURPOSE: Cold-induced vasodilation (CIVD) is a paradoxical rise in blood flow to the digits that occur during prolonged cold exposure. CIVD is thought to occur through active vasodilation and/or sympathetic withdrawal, where nitric oxide (NO) may play a key role in mediating these mechanisms. Beetroot juice (BRJ) is high in dietary nitrate (NO3-) which undergoes sequential reduction to nitrite (NO2-) and subsequently NO. Using a double-blind, randomized, crossover design, we examined the effect of acute BRJ supplementation on the CIVD response in 10 healthy males. METHODS: Participants had a resting blood pressure measurement taken prior to ingesting 140 mL of nitrate-rich BRJ (13 mmol NO3-) or a NO3--free placebo (PLA). After 2 h, participants immersed their hand in neutral water (~ 35 °C) for 10 min of baseline before cold water immersion (~ 8 °C) for 30 min. Laser-Doppler fluxmetry and skin temperature were measured continuously on the digits. RESULTS: Compared to PLA (100 ± 3 mmHg), acute BRJ supplementation significantly reduced mean arterial pressure at -30 min (96 ± 2 mmHg; p = 0.007) and 0 min (94 ± 2 mmHg; p = 0.008). Acute BRJ supplementation had no effect on Laser-Doppler fluxmetry during CIVD (expressed as cutaneous vascular conductance) measured as area under the curve (BRJ: 843 ± 148 PU mmHg-1 s; PLA: 1086 ± 333 PU mmHg-1 s), amplitude (BRJ: 0.60 ± 0.12 PU mmHg-1; PLA: 0.69 ± 0.14 PU mmHg-1), and duration (BRJ: 895 ± 60 s; PLA: 894 ± 46 s). CONCLUSION: Acute BRJ supplementation does not augment the CIVD response in healthy males.

Separate and combined effects of dehydration and thirst sensation on exercise performance in the heat.

Using intravenous infusion, we separated the physiologic consequences of 3% body mass dehydration from the conscious awareness of fluid replacement on time trial (TT) performance in the heat. Eleven trained cyclists performed 90 min of steady-state (50% V ˙ O 2 peak ) cycling followed by a self-paced 20-km TT in a hot-dry (35 °C, 10% relative humidity, wind speed 3.0 m/s) environment while euhydrated-not thirsty (EU-NT), euhydrated-thirsty (EU-T), dehydrated-not thirsty (DH-NT), or dehydrated-thirsty (DH-T). Thirst was manipulated by providing (NT) or withholding (T) ad libitum 35 °C water oral rinse. Distinct hydration states existed, with 0.4 ± 0.5% dehydration following the 20-km TT (EU) compared with 3.2 ± 0.6% in DH (P < 0.001). Greater perceived thirst existed in T (7 ± 2 on a 1-9 scale) than NT (4 ± 2, P < 0.001) after the TT. No significant differences in power output existed during the TT between hydration (EU 202.9 ± 36.5 W vs DH 207.0 ± 35.9 W, P = 0.362) and thirst conditions (NT 203.3 ± 35.6 W vs T 206.6 ± 36.8 W, P = 0.548), nor were there differences in completion time (P = 0.832) or pacing profile (P = 0.690). Within the range of up to 3% body mass loss, neither the physiologic effects from lowered hydration status nor the perception of thirst, separately or combined, affected sustained submaximal exercise performance in the heat for a healthy and fit population.

Antiproliferation activity of Devil's club (Oplopanax horridus) and anticancer agents on human pancreatic cancer multicellular spheroids.

Devil's club (DC, Oplopanax horridus) is an important medicinal herb of the Pacific Northwest which has significant antiproliferation activity against a variety of human tumor cell lines in vitro. This study compared the antiproliferation activity of DC extract alone, and in combination with chemotherapeutic agents gemcitabine (GEM), cisplatin (CDDP), and paclitaxel (PTX) on human pancreatic cancer PANC-1 3D spheroids and 2D monolayer cells. 3D tumor spheroids were prepared with a rotary cell culture system. PANC-1 3D spheroids were significantly more resistant to killing by DC extract, GEM and PTX compared to 2D cells, with IC50 levels closer to that observed in vivo. DC extract significantly enhanced the antiproliferation activity of CDDP and GEM at some concentrations. The bioactive compound identified as a polyacetylene showed strong antiproliferation activity against PANC-1 2D cells and 3D spheroids with IC50 at 0.73±0.04 and 3.15±0.16µM, respectively. 3D spheroids and 2D cells differentially expressed a number of apoptosis related genes. Cell cycle analysis showed that the proportion of cells in S phase was increased and in G2/M phase reduced in 3D spheroids compared to 2D cells. DC extract can potentially be used to enhance the activity of chemotherapeutic agents against pancreatic cancer cells. Use of 3D spheroid model for screening of natural products can potentially increase the efficiency in discovering in vivo bioactive compounds.

Compression garments do not alter cerebrovascular responses to orthostatic stress after mild passive heating.

Whole-body heating increases the likelihood of syncope, whereas utilizing lower-body compression garments may reduce syncope risk. We hypothesized that graded compression garments would reduce the typically observed large postural reductions in arterial blood pressure and middle cerebral artery velocity, in normothermia and especially once passively heat stressed. Fifteen men (age: 27 ± 4 years, aerobic fitness range: 30-75 mL/kg(/) min) completed a supine-to-stand orthostatic challenge for 3 min at normothermia and after passive heating (esophageal temperature, +0.5 °C from baseline) on two occasions (> 7 days): once wearing commercially available compression trousers and once wearing low-compression placebo trousers (randomized order). Blood flow velocity in the middle cerebral artery (transcranial Doppler), mean arterial blood pressure (mean BP: Finometer) and end-tidal carbon dioxide pressure were measured continuously. During normothermia, compression, garments did not alter the magnitude of the postural changes in mean BP or middle cerebral artery velocity. After passive heating, although the magnitudes of these changes were exaggerated, they were not significantly affected by compression garments. Compression garments did not attenuate the initial or sustained orthostatic hypotension associated with posture change, either during normothermia or following passive heat stress.

The effect of lower body cooling on the changes in three core temperature indices.

Rectal (T(re)), ear canal (T(ear)) and esophageal (T(es)) temperatures have been used in the literature as core temperature indices in humans. The aim of the study was to investigate if localized lower body cooling would have a different effect on each of these measurements. We hypothesized that prolonged lower body surface cooling will result in a localized cooling effect for the rectal temperature not reflected in the other core measurement sites. Twelve participants (mean ± SD; 26.8 ± 6.0 years; 82.6 ± 13.9 kg; 179 ± 10 cm, BSA = 2.00 ± 0.21 m(2)) attended one experimental session consisting of sitting on a rubberized raft floor surface suspended in 5 °C water in a thermoneutral air environment (approximately 21.5 ± 0.5 °C). Experimental conditions were (a) a baseline phase during which participants were seated for 15 min in an upright position on an insulated pad (1.408 K ⋅ m(2) ⋅ W(-1)); (b) a cooling phase during which participants were exposed to the cooling surface for 2 h, and (c) an insulation phase during which the baseline condition was repeated for 1 h. Temperature data were collected at 1 Hz, reduced to 1 min averages, and transformed from absolute values to a change in temperature from baseline (15 min average). Metabolic data were collected breath-by-breath and integrated over the same temperature epoch. Within the baseline phase no significant change was found between the three indices of core temperature. By the end of the cooling phase, T(re) was significantly lower (Δ = -1.0 ± 0.4 °C) from baseline values than from T(ear) (Δ = -0.3 ± 0.3 °C) and T(es) (Δ = -0.1 ± 0.3 °C). T(re) continued to decrease during the insulation phase from Δ -1.0 ± 0.4 °C to as low as Δ -1.4 ± 0.5 °C. By the end of the insulation phase T(re) had slightly risen back to Δ -1.3 ± 0.4 °C but remained significantly different from baseline values and from the other two core measures. Metabolic data showed no variation throughout the experiment. In conclusion, the local cooling of the buttock area results in a drop in rectal temperature compromising the validity of the rectal temperature as a core temperature index under these conditions.

Physiological strain and countermeasures with firefighting.

Protective clothing is integral to the task of firefighting, but at the same time can increase physiological strain and impair work capacity. Encapsulation of the head and the high thermal resistance and/or low water vapor permeability of the clothing ensemble impede evaporative heat dissipation, thus elevating the rate of heat storage and creating a state of uncompensable heat stress (UHS). In addition, the additional weight from carrying a supplemental air supply and the greater respiratory work of breathing through a regulator can create a negative spiral of thermal hyperpnea from greater respiratory demands and metabolic heat production. The elevated respiratory demands also increase cardiac strain and potentially the risk for myocardial events. Tolerance time during UHS is determined by three factors: the core temperature at the beginning of the heat stress exposure, the core temperature that can be tolerated before exhaustion or collapse ensues, and the rate of increase in core temperature from the beginning to end of the heat stress exposure. Protective clothing is often employed in highly dynamic environments, making portability, longevity and integration with the task requirements and clothing critical design characteristics for countermeasures. To date, most countermeasures have been relatively indirect in nature, primarily with alterations in work scheduling along with physiological manipulations such as cooling manipulations during recovery periods. Advances are required in materials science to develop lighter and less restrictive protective equipment, concurrent with cooling strategies that target specific regions or which can be effectively implemented during exercise.

Human conscious response to thermal input is adjusted to changes in mean body temperature.

OBJECTIVE AND DESIGN: To detect the dependable criteria of behavioural thermoregulation through modelling temperature fluctuations of individuals allowed to freely manipulate inlet water temperature of a liquid conditioning garment (LCG) during 130 min of passive exposure to -20 degrees C interspersed with a 10 min period of moderate exercise at the 65th minute using a double-blind experiment. PARTICIPANTS: Eleven volunteers (5 women; 23.40 (SD 2.09) years; BMI: 23.24 (SD 2.19)) who lacked previous experience with LCG and cold exposure experiments. RESULTS: Despite variations in core and skin temperatures, thermal comfort, thermal sensation, and mean body temperature did not fluctuate significantly over time. Participants were able to find a desired level of LCG inlet temperature within 25 minutes which was maintained at similar levels until the 65th minute of the cold exposure. During exercise, LCG inlet water temperature decreased significantly. Regression models demonstrated that mean skin temperature and change in mean body temperature were significantly associated with thermal comfort and thermal sensation. Subsequent models revealed that, although all temperature variables were associated with LCG inlet water temperature, the coefficient of determination mainly depended on mean skin temperature and change in mean body temperature. The involvement of skin temperature was anticipated as the liquid conditioning garment was in contact with the skin. CONCLUSIONS: Humans generate conscious thermoregulatory responses in resting and exercise conditions during exposures to cold environments that are aimed towards maintaining a threshold mean body temperature, rather than temperature changes in individual body regions.

Influence of body heat content on hand function during prolonged cold exposures.

We examined the influence of 1) prior increase [preheating (PHT)], 2) increase throughout [heating (HT)], and 3) no increase [control (Con)] of body heat content (H(b)) on neuromuscular function and manual dexterity of the hands during a 130-min exposure to -20 degrees C (coldEx). Ten volunteers randomly underwent three passive coldEx, incorporating a 10-min moderate-exercise period at the 65th min while wearing a liquid conditioning garment (LCG) and military arctic clothing. In PHT, 50 degrees C water was circulated in the LCG before coldEx until core temperature was increased by 0.5 degrees C. In HT, participants regulated the inlet LCG water temperature throughout coldEx to subjective comfort, while the LCG was not operating in Con. Thermal comfort, rectal temperature, mean skin temperature, mean finger temperature (T(fing)), change in H(b) (DeltaH(b)), rate of body heat storage, Purdue pegboard test, finger tapping, handgrip, maximum voluntary contraction, and evoked twitch force of the first dorsal interosseus muscle were recorded. Results demonstrated that, unlike in HT and PHT, thermal comfort, rectal temperature, mean skin temperature, twitch force, maximum voluntary contraction, and finger tapping declined significantly in Con. In contrast, T(fing) and Purdue pegboard test remained constant only in HT. Generalized estimating equations demonstrated that DeltaH(b) and T(fing) were associated over time with hand function, whereas no significant association was detected for rate of body heat storage. It is concluded that increasing H(b) not only throughout but also before a coldEx is effective in maintaining hand function. In addition, we found that the best indicator of hand function is DeltaH(b) followed by T(fing).

Design and control optimization of microclimate liquid cooling systems underneath protective clothing.

The use of protective clothing, whether in space suits, hazardous waste disposal, or sporting equipment, generally increases the risk of heat stress and hyperthermia by impairing the capacity for evaporative heat exchange from the body to the environment. To date the most efficient method of microclimate cooling underneath protective clothing has been via conductive heat exchange from circulating cooling fluid next to the skin. In order to make the use of liquid microclimate cooling systems ((LQ)MCSs) as portable and practical as possible, the physiological and biomedical engineering design goals should be towards maximizing the efficiency of cooling to maintain thermal comfort/neutrality with the least cooling possible to minimize coolant and power requirements. Meeting these conditions is an extremely complex task that requires designing for a plethora of different factors. The optimal fitting of the (LQ)MCSs, along with placement and design of tubing and control of cooling, appear to be key avenues towards maximizing efficiency of heat exchange. We review the history and major design constraints of (LQ)MCSs, the basic principles of human thermoregulation underneath protective clothing, and explore potential areas of research into tubing/fabric technology, coolant distribution, and control optimization that may enhance the efficiency of (LQ)MCSs.

Local cold acclimation of the hand impairs thermal responses of the finger without improving hand neuromuscular function.

AIM: To investigate the effects of cold acclimation on the thermal response and neuromuscular function of the hand. METHODS: Ten healthy subjects [three female, seven male, age (mean +/- SD): 27.9 +/- 7.9 years] immersed their right hand in 8 degrees C water for 30 min, 5 days a week for 3 weeks. On the first and the last day, neuromuscular function of the first dorsal interosseus (FDI) muscle was tested. RESULTS: There was no significant change in maximal voluntary contraction strength or evoked contractile characteristics of the FDI after cold acclimation. Minimum finger temperature decreased significantly from 10.6 +/- 1.2 to 9.3 +/- 0.8 degrees C after 3 weeks (P < 0.01), with most of the decrease occurring after a single exposure. Mean finger temperature dropped significantly from 14.2 +/- 1.9 to 11.7 +/- 1.4 degrees C following cold acclimation (P < 0.05), with 90% of this adaptation occurring after 5 days. Onset time of cold-induced vasodilatation increased from 446 +/- 171 to 736 +/- 384 s (P < 0.05) and the amplitude decreased from 5.3 +/- 3.2 to 2.5 +/- 2.1 degrees C (P < 0.05). This was significantly different from the control group, who immersed their right hand on the first and last days only. CONCLUSION: These data suggest that cold acclimation does not enhance hand temperature or function but may put the hands at a greater risk of cold injury when exposed to the cold.

Breath-holding ability of offshore workers inadequate to ensure escape from ditched helicopters.

BACKGROUND: Following a helicopter ditching in water, the survival rate of individuals not mortally injured by the impact ranges from 50-85%. One possible cause for this low survival rate is that the crew and passengers cannot hold their breath underwater long enough to make the often difficult escape from an inverted and submerged helicopter. METHODS: We investigated pulmonary function, breath-holding times in air (BHTa) and water (BHTw) of 228 students enrolled in offshore survival courses required to work in either the offshore petroleum industry or in military marine aviation. Comparisons were performed based on occupation, SCUBA experience, and smoking. RESULTS: In 25 degrees C pool water, the overall BHTw ranged from 5.4 to 120 s with a median of 37 s. Of the 228 subjects, 34% had a BHTw less than the 28 s required for the complete evacuation of a Super Puma helicopter under ideal conditions. No significant differences in BHTw were observed based on either smoking history (Non-Smoker, 41.5 +/- 21.6 s; Smoker, 37.2 +/- 20.2 s) or occupation (Novice, 37.5 +/- 21.1 s; Offshore, 40.5 +/- 21.1 s; Military, 45.2 +/- 20.9 s). However, SCUBA-trained individuals had a significantly longer BHT, (47.4 +/- 21.6 s) than non-SCUBA (37.6 +/- 20.6 s), as well as a greater force vital capacity (FVC), BHTa, and subjective comfort in water. CONCLUSIONS: It is concluded that the inability to breath-hold in emergency situations is a major contributor to the low survival rates of marine helicopter ditchings. Therefore, efforts must be made to both decrease escape times and to increase survival time underwater.

Evaluation of an oral care protocol intervention in the prevention of chemotherapy-induced oral mucositis in paediatric cancer patients.

Oral mucositis is the most frequent and severe complication of chemotherapy in children with cancer that can aggravate the child's clinical condition and increase the risk of infection. This prospective comparative study was designed to determine the effectiveness of a preventive oral care protocol in reducing chemotherapy-induced oral mucositis in children with cancer. During an 8-month period, 42 children aged 6 to 17 years with haematological malignancies or solid tumours were evaluated. The 21 children who were included in the first 4-month period of the study constituted the control group. Another 21 children were enrolled in the subsequent 4 months and were assigned to the experimental group, in which they were given an oral care protocol intervention. The oral care protocol consisted of tooth brushing, 0.2% chlorhexidine mouth rinse and 0.9% saline rinse. Children in both groups were evaluated twice a week for 3 weeks. The incidence of ulcerative lesions, severity of oral mucositis and the related pain intensity were used as the main outcome variables. A 38% reduction in the incidence of ulcerative mucositis was found in children using the oral care protocol compared with children in the control group. The severity of oral mucositis (P=0.000002) and the related pain (P=0.0001) were significantly reduced with the intervention. These results support the preventive use of the oral care protocol in paediatric cancer patients who undergo chemotherapy for cancer treatment.

Differential gene expression of rat neonatal heart analyzed by suppression subtractive hybridization and expressed sequence tag sequencing.

Heart diseases have been one of the major killers among the human population worldwide. Because the vast majority of cardiomyocytes cannot regenerate once they cease to proliferate shortly after birth, functionally significant myocardial regeneration is not observed clinically. Whether these cells are terminally differentiated and permanently withdrawn from the cell cycle is controversial, but broadening our understanding of the rapid switch from hyperplastic to hypertrophic growth of cardiomyocytes during neonatal myocardial development may shed light on novel cardiovascular therapies. By suppression subtractive hybridization (SSH) and expressed sequence tag (EST) sequencing, we analyzed the differential gene expression of rat neonatal heart. SSH yielded subtracted and normalized cDNA libraries and enhanced the probability of detecting ESTs, which represent genes pertinent to signal transduction/cell regulation and replication/transcription/translation machinery, as compared to the traditional EST sequencing of heart cDNA libraries.

The thermophysiology of uncompensable heat stress. Physiological manipulations and individual characteristics.

In many athletic and occupational settings, the wearing of protective clothing in warm or hot environments creates conditions of uncompensable heat stress where the body is unable to maintain a thermal steady state. Therefore, special precautions must be taken to minimise the threat of thermal injury. Assuming that manipulations known to reduce thermoregulatory strain during compensable heat stress would be equally effective in an uncompensable heat stress environment is not valid. In this review, we discuss the impact of hydration status, aerobic fitness, endurance training, heat acclimation, gender, menstrual cycle, oral contraceptive use, body composition and circadian rhythm on heat tolerance while wearing protective clothing in hot environments. The most effective countermeasure is ensuring that the individual is adequately hydrated both before and throughout the exercise or work session. In contrast, neither short term aerobic training or heat acclimation significantly improve exercise-heat tolerance during uncompensable heat stress. While short term aerobic training is relatively ineffective, long term improvements in physical fitness appear to provide some degree of protection. Individuals with higher proportions of body fat have a lower heat tolerance because of a reduced capacity to store heat. Women not using oral contraceptives are at a thermoregulatory disadvantage during the luteal phase of the menstrual cycle. The use of oral contraceptives eliminates any differences in heat tolerance throughout the menstrual cycle but tolerance is reduced during the quasi-follicular phase compared with non-users. Diurnal variations in resting core temperature do not appear to influence tolerance to uncompensable heat stress.

Impact of fluid replacement on heat storage while wearing protective clothing.

This study used partitional calorimetry to determine the influence of fluid replacement on heat storage during uncompensable heat stress. Eight males performed either light (L; level treadmill walking at 0.97 m x s(-1) (3.5 km x h(-1)) or heavy (H; 1.33 m x s(-1) (4.8 km x h(-1)) at a 4% grade) exercise at 40 degrees C and 30% relative humidity while wearing nuclear, biological and chemical (NBC) protective clothing. Subjects received either no fluid (NF), or 200 or 250 ml of fluid (F) as warm water at approximately 35 degrees C immediately before and every 15 min during the L and H trials respectively. Similar reductions in heart rate were observed at both metabolic rates with F but rectal temperature responses were not different between F and NF. Tolerance time was extended during L/F (106.5 +/- 22.1 min) compared with L/NF (93.1 +/- 20.8 min) but fluid replacement had no influence during H (59.8 +/- 9.5 min and 58.3 +/- 11.1 min for F and NF respectively). Fluid replacement also had no effect on the rate of heat storage during L (108.2 +/- 20.6 W x m(-2) and 111.0 +/- 22.6 W x m(-2) for F and NF respectively) and H (172.5 +/- 11.5 W x m(-2) and 182.1 +/- 15.8 W x m(-2) for F and NF respectively). However, heat storage expressed per unit of mass was significantly increased during L/F (18.5 +/- 4.0 kJ x kg(-1) ) compared with the other trials (16.3 +/- 4.8 kJ x kg(-1), 16.6 +/- 3.0 kJ x kg(-1) and 16.7 +/- 4.0 kJ x kg(-1) for L/NF, H/F and H/NF respectively). It was concluded that fluid replacement does not alter the rate of heat storage during uncompensable heat stress but does increase the heat storage capacity during light exercise when tolerance times are > 60 min.

Effects of dehydration, hypohydration, and hyperhydration on tolerance during uncompensable heat stress.

The present study examined the effects of dehydration from prior exercise on subsequent exercise tolerance time (TT) that involved wearing nuclear, biological, and chemical (NBC) protective clothing. It was hypothesised that TT would be reduced in the dehydrated state. Ten men undertook continuous treadmill walking at 4.8 km.h-1 at 35 degrees C and 50% relative humidity, wearing NBC clothing while euhydrated (EU) or dehydrated (D) by 2.3% of body weight. Hydration status had no impact on thermoregulatory or cardiovascular responses during exercise. Also rectal temperature at exhaustion did not differ between EU (38.52 +/- 0.39 degrees C) and D (38.43 +/- 0.45 degrees C). Exercise TT during this uncompensable heat stress was reduced significantly for D (47.7 +/- 15.3 min) compared with EU (59.0 +/- 13.6 min). It was concluded that prior exercise leading to levels of dehydration to 2.3% of body weight, together with subsequent fluid restriction during exposure to uncompensable heat stress, impaired TT while wearing the NBC protective clothing. The integration of these findings together with other comparable studies that have examined the influence of hypo- and hyperhydration on TT while wearing NBC protective clothing revealed that hydration status has less effect on TT as the severity of uncompensable heat stress increases.

Comparison of short-term aerobic training and high aerobic power on tolerance to uncompensable heat stress.

This study investigated whether, in subjects of moderate aerobic fitness, short-term aerobic training could replicate the improved physiological responses to exercise-heat stress observed in individuals with a high level of aerobic fitness. Males of moderate (MF; <50 ml x kg(-1) min(-1) VO2peak, n = 8) and high (HF; >55 ml x kg(-1) x min(-1) VO2peak, n = 8) aerobic fitness walked at 3.5 km x h(-1) in the heat (40 degrees C, 30% relative humidity) wearing nuclear, biological, and chemical protective clothing. Tests were conducted once on HF subjects and on MF subjects before (MF-Pre) and after (MF-Post) a 2-week program 6 d x week(-1) of daily aerobic training (1 h treadmill exercise at 65% VO2peak for 12 d, 22 degrees C, 40% relative humidity). The training significantly increased VO2peak by 6.5%, while heart rate (fc) and rectal temperature (Tre) rise decreased during exercise in a thermoneutral environment. HF had lower body mass and body fat content than MF, and VO2peak remained lower in MF pre-or post-training. In the heat, MF-Post had a decreased skin temperature (Tsk) and an increased sweat rate compared with MF-Pre, but no changes were observed in fc, Tre, or tolerance time (TT). No significant differences during the first 60 min in Tre and fc were observed between the MF-Post and the HF subjects, though the HF subjects exhibited a lower Tsk. The endpoint Tre, deltaTre, and TT remained significantly higher in HF than in either the MF-Pre or MF-Post subjects. It was concluded that, in preparation for exercise in an uncompensable heat stress environment, short-term aerobic training offers little, if any, benefit and is not an adequate substitute for a high level of aerobic fitness resulting from habitual exercise and training.

Influence of short-term aerobic training and hydration status on tolerance during uncompensable heat stress.

The purpose of the present study was to determine the separate and combined effects of a short-term aerobic training program and hypohydration on tolerance during light exercise while wearing nuclear, biological, and chemical protective clothing in the heat (40 degrees C, 30% relative humidity). Males of moderate fitness [< 50 ml x kg(-1) x min(-1) maximal O2 consumption (VO2max)] were tested while euhydrated or hypohydrated by approximately 2% of body weight through exercise and fluid restriction the day preceding the trials. Tests were conducted before and after either a 2-week program of daily aerobic training (1 h treadmill exercise at 65% VO2max for 12 days; n = 8) or a control period (n = 7), which had no effect on any measured variable. The training increased VO2max by 6.5%, while heart rate (f(c)) and the rectal temperature (T(re)) rise decreased during exercise in a thermoneutral environment. In the heat, training resulted in a decreased skin temperature and increased sweat rate, but did not affect f(c), T(re) or tolerance time (TT). In both training and control groups, hypohydration significantly increased T(re) and f(c) and decreased the TT. It was concluded that the short-term aerobic training program had no benefit on exercise-heat tolerance in this uncompensable heat stress environment.