Hydration and vascular fluid shifts during exercise in the heat

This study examined the effects of hypohydration on plasma volume and red cell volume during rest in a comfortable (20 degrees C, 40% relative humidity) and exercise in a hot-dry (49 degrees C, 20% relative humidity) environment. A group of six male and six female volunteers [matched for maximal O2 uptake (VO2 max)] completed two test sessions following a 10-day heat acclimation program. One test session was completed when subjects were euhydrated and the other when subjects were hypohydrated (-5% from base-line body wt). The test sessions consisted of rest for 30 min in a 20 degrees C antechamber, followed by two 25-min bouts of treadmill walking (approximately 30% of VO2 max) in the heat, interspersed by 10 min of rest. No significant differences were found between the genders for the examined variables. At rest, hypohydration elicited a 5% decrease in plasma volume with less than 1% change in red cell volume. During exercise, plasma volume increased by 4% when subjects were euhydrated and decreased by 4% when subjects were hypohydrated. These percent changes in plasma volume values were significantly (P less than 0.01) different between the euhydration and hypohydration tests. Although red cell volume remained fairly constant during the euhydration test, these values were significantly (P less than 0.01) lower when hypohydrated during exercise. We conclude that hydration level alters vascular fluid shifts during exercise in a hot environment; hemodilution occurs when euhydrated and hemoconcentration when hypohydrated during light intensity exercise for this group of fit men and women.     

Gastric emptying during exercise: effects of heat stress and hypohydration

To determine the effects of acute heat stress, heat acclimation and hypohydration on the gastric emptying rate of water (W) during treadmill exercise, ten physically fit men ingested 400 ml of W before each of three 15 min bouts of exercise (treadmill, approximately 50% VO2max) on five separate occasions. Stomach contents were aspirated after each exercise bout. Before heat acclimation (ACC), experiments were performed in a neutral (18 degrees C), hot (49 degrees C) and warm (35 degrees C) environment. Subjects were euhydrated for all experiments before ACC. After ACC, the subjects completed two more experiments in the warm (35 degrees C) environment; one while euhydrated and a final one while hypohydrated (-5% of body weight). The volume of ingested water emptied into the intestines at the completion of each exercise bout was inversely correlated (P less than 0.01) with the rectal temperature (r = -0.76). The following new observations were made: 1) exercise in a hot (49 degrees C) environment impairs gastric emptying rate as compared with a neutral (18 degrees C) environment, 2) exercise in a warm (35 degrees C) environment does not significantly reduce gastric emptying before or after heat acclimation, but 3) exercise in a warm environment (35 degrees C) when hypohydrated reduces gastric emptying rate and stomach secretions. Reductions in gastric emptying appear to be related to the severity of the thermal strain induced by an exercise/heat stress.     

The importance of aerobic fitness in determining tolerance to uncompensable heat stress

When protective clothing is worn that restricts evaporative heat loss, it is not valid to assume that the higher sweat rates associated with improvements in aerobic fitness will increase heat tolerance. An initial study compared thermoregulatory and cardiovascular responses to both compensable and uncompensable heat stress before and after 8 weeks of endurance training in previously sedentary males. Despite a 15% improvement in VO2peak, and lower heart rates and rectal temperature (T(re)) responses while wearing combat clothing, no changes were noted when subjects wore a protective clothing ensemble. Tolerance times were unchanged at approximately 50 min. A subsequent short-term training model that used daily 1-h exercise sessions for 2 weeks also failed to show any benefit when the protective clothing was worn in the heat. Cross-sectional comparisons between groups of high and low aerobic fitness, however, have revealed that a high aerobic fitness is associated with extended tolerance time when the protective clothing is worn. The longer tolerance time is a function of both a lower starting T(re) and a higher T(re) tolerated at exhaustion. Improvements in cardiovascular function with long-term training may allow higher core temperatures to be reached prior to exhaustion. Conversely, elevations in core temperature that occur with normal training sessions may familiarize the more fit subjects to the discomforts of exercise in the heat. Other factors such as differences in body fatness may account for a faster increase in tissue temperature at a given metabolic rate for less fit individuals.     

Effects of varied air velocity on sweating and evaporative rates during exercise.

This study was designed to determine the extent to which changes in the evaporative power of the environment (Emax) affect sweating and evaporative rates. Six male subjects undertook four 60-min bouts of cycle ergometer exercise at 56% maximal O2 uptake (VO2max).Emax was varied by differences in ambient temperature and airflow; two exercise bouts took place at 24 degrees C and two at 35 degrees C, with air velocity at < 0.2 and 3.0 m/s in both. Total sweat production was estimated from body weight loss, whereas whole body evaporative rate was measured continuously from a Potter beam balance. Body core temperature was measured continuously from a thermocouple in the esophagus (T(es)), with mean skin temperature (Tsk) computed each minute from thermocouples at eight sites. Total body sweat loss was significantly greater (P < 0.05) in the 0.2- than in the 3.0-m/s condition at both 24 and 35 degrees C. Tsk was higher (P < 0.05) in the still-air conditions at both temperatures, but final T(es) was significantly higher (P < 0.05) in still air only in the 35 degrees C environment. Thus the reduced Emax in still air caused a greater heat storage, thereby stimulating a greater total sweat loss. However, in part because of reduced skin wettedness, the slope of the sweat rate-to-T(es) relation at 35 degrees C in the 3.0-m/s condition was 118% that at 0.2 m/s (P < 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)     

Thermoregulatory and blood responses during exercise at graded hypohydration levels

We studied the effects of graded hypohydration levels on thermoregulatory and blood responses during exercise in the heat. Eight heat-acclimated male subjects attempted four heat-stress tests (HSTs). One HST was attempted during euhydration, and three HSTs were attempted while the subjects were hypohydrated by 3, 5, and 7% of their body weight. Hypohydration was achieved by an exercise-heat regimen on the day prior to each HST. After 30 min of rest in a 20 degrees C antechamber the HST consisted of a 140-min exposure (4 repeats of 10 min rest and 25 min treadmill walking) in a hot-dry (49 degrees C, 20% relative humidity) environment. The following observations were made: 1) a low-to-moderate hypohydration level primarily reduced plasma volume with little effect on plasma osmolality, whereas a more severe hypohydration level resulted in no further plasma volume reduction but a large increment in plasma osmolality; 2) core temperature and heart rate responses increased with severity of hypohydration; 3) sweating rate responses for a given rectal temperature were systematically decreased with severity of hypohydration; and 4) the reduction in sweating rate was more strongly associated with plasma hyperosmolality than hypovolemia. In conclusion, an individual's thermal strain increases linearly with the severity of hypohydration during exercise in the heat, and plasma hyperosmolality influences the reduction in sweating more profoundly than hypovolemia.     

Influence of heat stress and acclimation on maximal aerobic power

Thirteen male volunteers performed cycle ergometer maximal oxygen uptake (VO2max tests) in moderate (21 degrees C, 30% rh) and hot (49 degrees C, 20% rh) environments, before and after a 9-day heat acclimation program. This program resulted in significantly decreased (P less than 0.01) final heart rate (24 bt X min-1) and rectal temperature (0.4 degrees C) from the first to last day of acclimation. The VO2max was lower (P less than 0.01) in the hot environment relative to the moderate environment both before (8%) and after (7%) acclimation with no significant difference (P greater than 0.05) shown for maximal power output (PO max, watts) between environments either before or after acclimation. The VO2max was higher (P less than 0.01) by 4% after acclimation in both environments. Also, PO max was higher (P less than 0.05) after acclimation in both the moderate (4%) and hot (2%) environments. The reduction in VO2max in the hot compared to moderate environment was not related to the difference in core temperature at VO2max between moderate and hot trials, nor was it strongly related with aerobic fitness level. These findings indicate that heat stress, per se, reduced the VO2max. Further, the reduction in VO2max due to heat was not affect be state of heat acclimation, the degree of elevation in core temperature, or level of aerobic fitness.     

Effects of marked hyperthermia with and without dehydration on VO(2) kinetics during intense exercise.

This study determined whether marked hyperthermia alone or in combination with dehydration reduces the initial rate of rise in O(2) consumption (VO(2) on-kinetics) and the maximal rate of O(2) uptake (VO(2 max)) during intense cycling exercise. Six endurance-trained male cyclists completed four maximal cycle ergometer exercise tests (402 +/- 4 W) when euhydrated or dehydrated (4% body wt) with normal (starting esophageal temperature, 37.5 +/- 0.2 degrees C; mean skin temperature, approximately 31 degrees C) or elevated (+1 and +6 degrees C, respectively) thermal strain. In the euhydrated and normal condition, subjects reached VO(2 max) (4.7 +/- 0.2 l/min) in 228 +/- 34 s, with a mean response time of 42 +/- 2 s, and fatigued after 353 +/- 39 s. Hyperthermia alone or in combination with dehydration reduced mean response time (17-23%), VO(2 max) (16%), and performance time (51-53%) (all P < 0.01) but did not alter the absolute response time (i.e., the time to reach 63% response in the control trial, 3.2 +/- 0.1 l/min, 42 s). Reduction in VO(2 max) was accompanied by proportional decline in O(2) pulse and significantly elevated maximal heart rate (195 vs. 190 beats/min for hyperthermia vs. normal). Preventing hyperthermia in dehydrated subjects restored VO(2 max) and performance time by 65 and 50%, respectively. These results demonstrate that impaired high-intensity exercise performance with marked skin and internal body hyperthermia alone or in combination with dehydration is not associated with a diminished rate of rise in VO(2) but decreased VO(2 max).     

Thermoregulatory activity in the rat: effects of hypohydration, hypovolemia and hypertonicity and their interaction with short-term heat acclimation

Hypothalamic temperature thresholds to heat-induced (40 degrees C ambient temperature) tail vasodilation (Vth) and salivation (Sth) as well as salivary flow rate and volume were studied in conscious rats, hypohydrated (24 hr water deprivation), hypovolemic (20% dextran sc), hypertonic (1M NaCL po), hypertonic and hypovolemic and heat-acclimated (5 days at 34 degrees C) before and after hypohydration. Sth was elevated in hypohydrated, hypovolemic, hypertonic and heat-acclimated hypohydrated rats concomitantly with a remarkable decrease in saliva volume, flow rate and heat tolerance. Heat acclimation alone resulted in a reduction in Vth, Sth, salivary flow and volume. Vth was not affected by hypohydration, but was elevated following hypovolemia and combined hypovolemia and hypertonicity. It is concluded that alterations in both plasma volume and osmolarity, which may occur during hypohydration, play a major role in the alteration in thermoregulatory responses during hypohydration. Heat acclimation does not improve tolerance during hypohydration. Thus, during hypohydration, the control of body fluids overrides thermoregulation.     

Hypohydration and exercise: effects of heat acclimation, gender, and environment

This study examined the effects of heat acclimation and subject gender on treadmill exercise in comfortable (20 degrees C, 40% rh), hot-dry (49 degrees C, 20% rh), and hot-wet (35 degrees C, 79% rh) environments while subjects were hypo- or euhydrated. Six male and six female subjects, matched for maximal aerobic power and percent body fat, completed two exercise tests in each environment both before and after a 10-day heat acclimation program. One exercise test was completed during euhydration and one during hypohydration (-5.0% from baseline body weight). In general, no significant (P greater than 0.05) differences were noted between men and women at the completion of exercise for rectal temperature (Tre), mean skin temperature (Tsk), or heat rate (HR) during any of the experimental conditions. Hypohydration generally increased Tre and HR values and decreased sweat rate values while not altering Tsk values. In the hypohydration experiments, heat acclimation significantly reduced Tre (0.19 degrees C) and HR (13 beats X min-1) values in the comfortable environment, but only HR values were reduced in hot-dry (21 beats X min-1) and hot-wet (21 beats X min-1) environments. The present findings indicated that men and women respond in a physiologically similar manner to hypohydration during exercise. They also indicated that for hypohydrated subjects heat acclimation decreased thermoregulatory and cardiovascular strain in a comfortable environment, but only cardiovascular strain decreased in hot environments.     

Hypohydration adversely affects lactate threshold in endurance athletes

The purpose of this investigation was to observe the effect of hypohydration (-4% body mass) on lactate threshold (LAT) in 14 collegiate athletes (8 men and 6 women; age, 20.9 +/- 0.5 years; height, 171.1 +/- 2.4 cm; weight, 64.8 +/- 2.3 kg; V(O)2 max, 62.8 +/- 1.9 ml x kg(-1) x min(-1); percentage of fat, 11.4 +/- 1.5%). Subjects performed 2 randomized, discontinuous treadmill bouts at a dry bulb temperature (T(db)) of 22 degrees C to volitional exhaustion in 2 states of hydration, euhydrated and hypohydrated. The hypohydrated condition was achieved in a thermally neutral environment (T(db), 22 degrees C; humidity, 45%), with exercise conducted at a moderate intensity as defined by rating of perceived exertion (RPE, approximately 12) 12-16 hours before testing. On average, subjects decreased 3.9% of their body mass before the hypohydration test. Blood lactate, hematocrit, V(O)2, minute ventilation (VE), R value, heart rate (HR), and RPE were measured during each 4-minute stage of testing. In the hypohydrated condition, LAT occurred significantly earlier during exercise and at a lower absolute V(O)2, VE, respiratory exchange ratio, RPE, and blood lactate concentration. Also, the blood lactate concentration was significantly lower in the hypohydrated condition (6.7 +/- 0.8 mmol) compared with the euhydrated condition (10.2 +/- 0.9 mmol) at peak exercise. There were no differences in HR or percentage of maximum HR at LAT nor did plots of V(CO2):V(O)2 reveal differences in bicarbonate buffering during exercise between the 2 conditions. From these results, we speculate that hypohydration did not significantly alter cardiovascular function or buffering capacity but did cause LAT to occur at a lower absolute exercise intensity.     

Stroke volume during exercise: interaction of environment and hydration

Euhydrated and dehydrated subjects exercised in a hot and a cold environment with our aim to identify factors that relate to reductions in stroke volume (SV). We hypothesized that reductions in SV with heat stress are related to the interaction of several factors rather than the effect of elevated skin blood flow. Eight male endurance-trained cyclists [maximal O(2) consumption (VO(2 max)) 4.5 +/- 0.1 l/min; means +/- SE] cycled for 30 min (72% VO(2 max)) in the heat (H; 35 degrees C) or the cold (C; 8 degrees C) when euhydrated or dehydrated by 1.5, 3.0, or 4.2% of their body weight. When euhydrated, SV and esophageal temperature (T(es) 38. 2-38.3 degrees C) were similar in H and C, whereas skin blood flow was much higher in H vs. C (365 +/- 64% higher; P < 0.05). With each 1% body weight loss, SV declined 6.4 +/- 1.3 ml (4.8%) in H and 3.4 +/- 0.4 ml (2.5%) in C, whereas T(es) increased 0.21 +/- 0.02 and 0. 10 +/- 0.02 degrees C in H and C, respectively (P < 0.05). However, reductions in SV were not associated with increases in skin blood flow. The reduced SV was highly associated with increased heart rate and reduced blood volume in both H (R = 0.96; P < 0.01) and C (R = 0. 85; P < 0.01). In conclusion, these results suggest that SV is maintained in trained subjects during exercise in euhydrated conditions despite large differences in skin blood flow. Furthermore, the lowering of SV with dehydration appears largely related to increases in heart rate and reductions in blood volume.     

Measuring aerobic cycling power as an assessment of childhood fitness

The emergence of obesity, insulin resistance, and type 2 diabetes in children requires a rational, effective public health response. Physical activity remains an important component of prevention and treatment for obesity, type 2 diabetes, and insulin resistance. Studies in adults show cardiovascular fitness to be more important than obesity in predicting insulin resistance. We recently demonstrated that a school-based fitness intervention in children who are overweight could improve cardiovascular fitness, body composition, and insulin sensitivity, but it remains unclear whether accurate assessment of fitness could be performed at the school or outside of an exercise laboratory. To determine whether new methodology using measurement of cycling power could estimate cardiovascular aerobic fitness (as defined by VO2max) in middle school children who were overweight. Thirty-five middle school children (mean age 12 +/- 0.4 years) who were overweight underwent testing on a power sensor-equipped Cycle Ops indoor cycle (Saris Cycling Group, Fitchburg, WI) as well as body composition by dual x-ray absorptiometry and VO2max by treadmill determination. Insulin sensitivity was also estimated by fasting glucose and insulin. Maximal heart rate (MHR) was determined during VO2max testing, and power produced at 80%MHR was recorded. Spearman's rank correlation was performed to evaluate associations. Mean power determined on the indoor cycle at 80% of MHR was 129 +/- 77 watts, and average power at 80% MHR divided by total body weight was 1.5 +/- 0.5. A significant correlation between watts and total body weight was seen for VO2max (P = 0.03), and significant negative correlation was seen between watts/total body weight and fasting insulin (P < 0.05). Among middle school children who were overweight, there was a significant relationship between the power component of fitness and cardiovascular aerobic fitness (measured by VO2max). This more accessible and less intimidating field-based measure of power may prove useful in predicting changes in cardiovascular fitness. Thus, accurate assessment of childhood aerobic fitness may be achievable by measurement of power, possibly within the school environment, at substantially less cost and effort than laboratory-based measurements.     

Thermoregulation at rest and during exercise in prepubertal boys

Thermal balance was studied in 11 boys, aged 10-12 years, with various values for maximal oxygen uptake (VO2max), during two standardized sweating tests performed in a climatic chamber in randomized order. One of the tests consisted in a 90-min passive heat exposure [dry bulb temperature (Tdb) 45 degrees C] at rest. The second test was represented by a 60-min ergocycle exercise at 60% of individual VO2max (Tdb 20 degrees C). At rest, rectal temperature increased during heat exposure similar to observations made in adults, but the combined heat transfer coefficient reached higher values, reflecting greater radiative and convective heat gains in the children. Children also exhibited a greater increase in mean skin temperature, and a greater heat dissipation through sweating. Conversely, during the exercise sweating-test, although the increase in rectal temperature did not differ from that of adults for similar levels of exercise, evaporative heat loss was much lower in children, suggesting a greater radiative and convective heat loss due to the relatively greater body surface area. Thermophysiological reactions were not related to VO2max in children, in contrast to adults.     

Validity of VO(2 max) in predicting blood volume: implications for the effect of fitness on aging

A multiple regression model was constructed to investigate the premise that blood volume (BV) could be predicted using several anthropometric variables, age, and maximal oxygen uptake (VO(2 max)). To test this hypothesis, age, calculated body surface area (height/weight composite), percent body fat (hydrostatic weight), and VO(2 max) were regressed on to BV using data obtained from 66 normal healthy men. Results from the evaluation of the full model indicated that the most parsimonious result was obtained when age and VO(2 max) were regressed on BV expressed per kilogram body weight. The full model accounted for 52% of the total variance in BV per kilogram body weight. Both age and VO(2 max) were related to BV in the positive direction. Percent body fat contributed <1% to the explained variance in BV when expressed in absolute BV (ml) or as BV per kilogram body weight. When the model was cross validated on 41 new subjects and BV per kilogram body weight was reexpressed as raw BV, the results indicated that the statistical model would be stable under cross validation (e.g., predictive applications) with an accuracy of +/- 1,200 ml at 95% confidence. Our results support the hypothesis that BV is an increasing function of aerobic fitness and to a lesser extent the age of the subject. The results may have implication as to a mechanism by which aerobic fitness and activity may be protective against reduced BV associated with aging.     

Branched-chain amino acid supplementation and human performance when hypohydrated in the heat.

The serotonin system may contribute to reduced human performance when hypohydrated in the heat. This study determined whether branched-chain amino acid (BCAA) supplementation could sustain exercise and cognitive performance in the heat (40 degrees C dry bulb, 20% relative humidity) when hypohydrated by 4% of body mass. Seven heat-acclimated men completed two experimental trials, each consisting of one preparation and one test day. On day 1, a low-carbohydrate diet was eaten and subjects performed exhaustive cycling (morning) and treadmill exercise in the heat (afternoon) to lower muscle glycogen and achieve the desired hypohydration level. On day 2, subjects consumed an isocaloric BCAA and carbohydrate (BC) or carbohydrate-only drink during exercise. Experimental trials included 60 min of cycle ergometry (50% peak oxygen uptake) followed by a 30-min time trial in the heat. A cognitive test battery was completed before and after exercise, and blood samples were taken. BC produced a 2.5-fold increase (P < 0.05) in plasma BCAA and lowered (P < 0.05) the ratios of total tryptophan to BCAA and large neutral amino acid. Blood prolactin, glucose, lactate, and osmolality were not different between trials but increased over time. Cardiovascular and thermoregulatory data were also similar between trials. BC did not alter time-trial performance, cognitive performance, mood, perceived exertion, or perceived thermal comfort. We conclude that BCAA does not alter exercise or cognitive performance in the heat when subjects are hypohydrated.     

Measuring aerobic cycling power as an assessment of childhood fitness

The emergence of obesity, insulin resistance (IR), and type-2 diabetes (T2DM) in children requires a rational, effective public health response. Physical activity remains an important component of prevention and treatment for obesity, T2DM, and IR. Studies in adults show cardiovascular fitness (CVF) to be more important than obesity in predicting IR. We recently demonstrated that a school-based fitness intervention in children who were overweight can improve cardiovascular fitness, body composition, and insulin sensitivity, but it remains unclear whether accurate assessment of fitness could be performed at the school or outside of an exercise laboratory. The purpose of the study was to determine if a new methodology using measurement of cycling power could estimate cardiovascular aerobic fitness (as defined by maximum oxygen consumption; VO(2)max) in middle school children who were overweight. Thirty-five middle school children who were overweight (mean age 12 +/- 0.4 years) underwent testing on a power sensor- equipped Cycle Ops Indoor Cycle (IC), as well as body composition by dual x-ray absorptiometry (DXA), and VO(2)max by treadmill determination. Insulin sensitivity was also estimated by fasting glucose and insulin. Maximal heart rate (MHR) was determined during VO(2)max testing, and power produced at 80% of MHR was recorded. Spearman's rank correlation was performed to evaluate associations. Mean power determined on the IC at 80% of MHR was 129 +/- 77 watts, and average power at 80% MHR divided by total body weight (TBW) was 1.5 +/- 0.5. A significant correlation between watts/TBW was seen for VO(2)max (ml/kg/min) (p = 0.03), and significant negative correlation was seen between watts/TBW and fasting insulin (p < 0.05). In middle-school children who were overweight, there was a significant relationship between the power component of fitness and cardiovascular aerobic fitness (measured by VO(2)max). This more accessible and less intimidating field-based measure of power may prove useful in predicting changes in cardiovascular fitness. Thus, accurate assessment of childhood aerobic fitness may be achievable by measurement of power, possibly within the school environment at substantially less cost and effort than laboratory-based measurements.     

Influence of aerobic fitness and body fatness on tolerance to uncompensable heat stress.

This study examined the independent and combined importance of aerobic fitness and body fatness on physiological tolerance and exercise time during weight-bearing exercise while wearing a semipermeable protective ensemble. Twenty-four men and women were matched for aerobic fitness and body fatness in one of four groups (4 men and 2 women in each group). Aerobic fitness was expressed per kilogram of lean body mass (LBM) to eliminate the influence of body fatness on the expression of fitness. Subjects were defined as trained (T; regularly active with a peak aerobic power of 65 ml x kg LBM(-1) x min(-1)) or untrained (UT; sedentary with a peak aerobic power of 53 ml x kg LBM(-1) x min(-1)) with high (High; 20%) or low (Low; 11%) body fatness. Subjects exercised until exhaustion or until rectal temperature reached 39.5 degrees C or heart rate reached 95% of maximum. Exercise times were significantly greater in T(Low) (116 +/- 6.5 min) compared with their matched sedentary (UT(Low); 70 +/- 3.6 min) or fatness (T(High); 82 +/- 3.9 min) counterparts, indicating an advantage for both a high aerobic fitness and low body fatness. However, similar effects were not evident between T(High) and UT(High) (74 +/- 4.1 min) or between the UT groups (UT(Low) and UT(High)). The major advantage attributed to a higher aerobic fitness was the ability to tolerate a higher core temperature at exhaustion (the difference being as great as 0.9 degrees C), whereas both body fatness and rate of heat storage affected the exercise time as independent factors.     

Heat loss responses in rats acclimated to heat loaded intermittently

The present study examined the heat loss response of heat-acclimated rats to direct body heating with an intraperitoneal heater or to indirect warming by elevating the ambient temperature (Ta). The heat acclimation of the rats was attained through exposure to Ta of 33 or 36 degrees C for 5 h daily during 15 consecutive days. Control rats were kept at Ta of 24 degrees C for the same acclimation period. Heat acclimation lowered the body core temperature at Ta of 24 degrees C, and the core temperature level was lowered as acclimation temperature increased. When heat was applied by direct body heating, the threshold hypothalamic temperature (Thy) for the tail skin vasodilation was also lower in heat-acclimated rats than in the control rats. However, the amount of increase in Thy from the resting level to the threshold was the same in all three groups. When heat was applied by indirect warming, threshold Thy was slightly higher in heat-acclimated than in control rats. The amount of increase in Thy from the resting level to the threshold was significantly greater in heat-acclimated rats. In addition, Ta and the skin temperature at the onset of skin vasodilation were significantly higher in heat-acclimated rats. The results indicate that heat-acclimated rats were less sensitive to the increase in skin temperature in terms of threshold Thy. The gain constant of nonevaporative heat loss response was assessed by plotting total thermal conductance against Thy.(ABSTRACT TRUNCATED AT 250 WORDS)     

The relative influence of physical fitness, acclimatization state, anthropometric measures and gender on individual reactions to heat stress

An experiment was set up to quantify the relative influence of fitness, acclimatization, gender and anthropometric measures on physiological responses to heat stress. For this purpose, 12 male and 12 female subjects were exposed to a neutral [ambient temperature (Ta) 21 degrees C, relative humidity (r.h. 50%)], a warm, humid (Ta 34 degrees C, r.h. 80%) and a hot, dry (Ta 45 degrees C, r.h. 20%) climate at rest and at two exercise intensities [25%, and 45% maximal O2 intake (VO2max)], seated seminude in a net chair behind a cycle ergometer. Their physiological responses were recorded and the data submitted to a multiple regression analysis. It was shown that for the variance in heat storage, the percentage of body fat and the surface to mass ratio had relatively the largest influence of all the individual parameters, followed by VO2max and the sweat rate versus increase in core temperature (total r2 = 92%). For the skin temperature variation, the relative influence of individual parameters (sweat gain, VO2max) was small. For body core temperatures, individual parameters had a large influence. The largest effect was due to the percentage of fat and the surface to mass ratio, followed by the sweating setpoint and, finally, VO2max (total r2 = 54%-70%). For the variance in heart rate the VO2max was the most relevant parameter, followed by the setpoint of the sweat rate:rectal temperature relationship (total r2 = 88%). Blood pressure and skin blood flow predictions were also shown to improve by the addition of individual characteristics to the model.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of Hypohydration on Peripheral and Corticospinal Excitability and Voluntary Activation

We investigated whether altered peripheral and/or corticospinal excitatory output and voluntary activation are implicated in hypohydration-induced reductions in muscle isometric and isokinetic (90°.s⁻¹) strength. Nine male athletes completed two trials (hypohydrated, euhydrated) comprising 90 min cycling at 40°C, with body weight losses replaced in euhydrated trial. Peripheral nerve and transcranial magnetic stimulations were applied during voluntary contractions pre- and 40 min post-exercise to quantify voluntary activation and peripheral (M-wave) and corticospinal (motor evoked potential) evoked responses in m. vastus medialis. Both maximum isometric (−15.3±3.1 vs −5.4±3.5%) and isokinetic eccentric (−24.8±4.6 vs −7.3±7.2%) torque decreased to a greater extent in hypohydrated than euhydrated trials (p<0.05). Half relaxation time of the twitch evoked by peripheral nerve stimulation during maximal contractions increased after exercise in the hypohydrated (21.8±9.3%) but stayed constant in the euhydrated (1.6±10.7%; p = 0.017) condition. M-wave amplitude during maximum voluntary contraction increased after exercise in the heat in hypohydrated (10.7±18.0%) but decreased in euhydrated condition (−17.4±16.9%; p = 0.067). Neither peripheral nor cortical voluntary activation were significantly different between conditions. Motor evoked potential amplitude increased similarly in both conditions (hypohydrated: 25.7±28.5%; euhydrated: 52.9±33.5%) and was accompanied by lengthening of the cortical silent period in euhydrated but not hypohydrated condition (p = 0.019). Different neural strategies seem to be adopted to regulate neural drive in the two conditions, with increases in inhibitory input of either intracortical or corticospinal origin during the euhydrated trial. Such changes were absent in the hypohydrated condition, yet voluntary activation was similar to the euhydrated condition, perhaps due to smaller increases in excitatory drive rather than increased inhibition. Despite this maximal isometric and eccentric strength were impaired in the hypohydrated condition. The increase in peripheral muscle excitability evident in the hypohydrated condition was not sufficient to preserve performance in the face of reduced muscle contractility or impaired excitation-contraction coupling.