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.

Slower fatigue and faster recovery of the adductor pollicis muscle in women matched for strength with men

In previous gender comparisons of muscle performance, men and women rarely have been closely matched, absolute force has not been equalized, and rates of fatigue and early recovery have not been determined. We compared adductor pollicis muscle performance at a similar absolute force development in healthy men and women (both n=9) matched for adductor pollicis maximal voluntary contraction (MVC) force (132 +/- 5 N for women and 136 +/- 4 N for men, mean +/- SE, P > 0.05). Subjects repeated static contractions at a target force of approximately 50% of MVC force of rested muscle (68 +/- 3 N or 51.9 +/- 1.0% MVC for women and 72 +/- 2 N or 53.0 +/- 2.0% MVC for men, P > 0.05) for 5 s followed by 5 s rest until exhaustion, i. e. inability to maintain the target force for 5 s. MVC force was measured following each minute of exercise, at exhaustion, and after each minute for 3 min of passive recovery. For women compared with men: MVC force fell less after 1 min of exercise (to 93 +/- 1% vs. 80 +/- 3% of MVC force of rested muscle, respectively, P < 0.05); MVC force (N min-1) fell approximately 2-fold slower (P < 0.05); and endurance time to exhaustion was nearly two times longer (14.7 +/- 1. 6 min vs. 7.9 +/- 0.7 min, P < 0.05). After declining to a similar level of MVC force of rested muscle at exhaustion (56 +/- 1% for women and 56 +/- 3% for men), MVC force rose faster in women than in men (to 71 +/- 2% vs. 65 +/- 3% of MVC force of rested muscle, respectively; P < 0.05) during the first minute of recovery. The findings are consistent with the hypothesis that slower adductor pollicis muscle fatigue in women is linked with differences between men and women both in impairment of force generating capacity, per se, and in rates of recovery between contractions.

Comparison of diaphragm strength between healthy adult elderly and young men

To test the hypothesis that aging is associated with a reduction in the diaphragm's force-generating capacity, we compared the maximum transdiaphragmatic pressure (Pdimax) obtained during voluntary maximal inspiratory efforts in nine young (19-28 yr) and ten elderly (65-75 yr) subjects. The relationship between Pdi and lung volume was compared in the two groups by having subjects make maximal inspiratory maneuvers at specified lung volumes (i.e., 20, 40, and 80% vital capacity). Subjects underwent symptom-limited exercise tests to characterize their aerobic capacities and evaluate the relationship between aerobic capacity and Pdimax. The average Pdimax of the elderly subjects (128 +/- 9 cm H2O) was significantly lower (p < 0.003) than the average Pdimax of the younger subjects (171 +/- 8 cm H2O). In the elderly, Pdi was lower across the range of lung volumes tested (p < 0.001), and Pdimax occurred at similar relative lung volumes (elderly, at 47% total lung capacity [TLC]; young, at 50% TLC) in both groups. The elderly subjects were quite fit based on their VO2max, and there was no significant relationship between Pdimax and VO2max. This study suggests that diaphragm strength is reduced in elderly individuals. This age-related decrease in diaphragm strength may predispose elderly patients to diaphragm fatigue in the presence of conditions that impair inspiratory muscle function or increase ventilatory load.

Comparison of scales used to quantitate the sense of effort to breathe in patients with chronic obstructive pulmonary disease

Several different scaling techniques, i.e., Borg category (BC) and visual analogue (VA) scales have been used to quantitate the intensity of the respiratory sensations elicited during exercise, but their relationship is unclear. Six subjects with stable chronic obstructive lung disease (FEV1 = 1.2 +/- 0.1 SE L) simultaneously rated the sense of effort to breathe with both BC and VA scales during progressive, maximal exercise tests performed three to five times on a cycle ergometer. The VA scores correlated linearly with minute ventilation in all subjects in all trials (r = 0.98 +/- 0.01), and when converted to common units (i.e., Z scores) correlated closely with simultaneous scores obtained using the Borg scale (r = 0.99 +/- 0.01). Furthermore, VA scores varied minimally over several trials. Coefficient of variation for the maximal VA scores was 6 +/- 1%, which was similar to the variation in maximal Borg score (i.e., 3 +/- 1%). We conclude that the visual analogue scale is reproducible and correlates closely with the Borg score when scaling the sense of effort to breathe during exercise in subjects with stable chronic obstructive pulmonary disease.

Women at altitude: carbohydrate utilization during exercise at 4,300 m

To evaluate the hypothesis that exposure to high altitude would reduce blood glucose and total carbohydrate utilization relative to sea level (SL), 16 young women were studied over four 12-day periods: at 50% of peak O(2) consumption in different menstrual cycle phases (SL-50), at 65% of peak O(2) consumption at SL (SL-65), and at 4,300 m (HA). After 10 days in each condition, blood glucose rate of disappearance (R(d)) and respiratory exchange ratio were measured at rest and during 45 min of exercise. Glucose R(d) during exercise at HA (4.71 +/- 0.30 mg. kg(-1). min(-1)) was not different from SL exercise at the same absolute intensity (SL-50 = 5.03 mg. kg(-1). min(-1)) but was lower at the same relative intensity (SL-65 = 6.22 mg. kg(-1). min(-1), P < 0.01). There were no differences, however, when glucose R(d) was corrected for energy expended (kcal/min) during exercise. Respiratory exchange ratios followed the same pattern, except carbohydrate oxidation remained lower (-23.2%, P < 0.01) at HA than at SL when corrected for energy expended. In women, unlike in men, carbohydrate utilization decreased at HA. Relative abundance of estrogen and progesterone in women may partially explain the sex differences in fuel utilization at HA, but subtle differences between menstrual cycle phases at SL had no physiologically relevant effects.

Exercise VE and physical performance at altitude are not affected by menstrual cycle phase

We hypothesized that progesterone-mediated ventilatory stimulation during the midluteal phase of the menstrual cycle would increase exercise minute ventilation (VE; l/min) at sea level (SL) and with acute altitude (AA) exposure but would only increase arterial O2 saturation (SaO2, %) with AA exposure. We further hypothesized that an increased exercise SaO2 with AA exposure would enhance O2 transport and improve both peak O2 uptake (VO2 peak; ml x kg-1 x min-1) and submaximal exercise time to exhaustion (Exh; min) in the midluteal phase. Eight female lowlanders [33 +/- 3 (mean +/- SD) yr, 58 +/- 6 kg] completed a VO2 peak and Exh test at 70% of their altitude-specific VO2 peak at SL and with AA exposure to 4,300 m in a hypobaric chamber (446 mmHg) in their early follicular and midluteal phases. Progesterone levels increased (P < 0.05) approximately 20-fold from the early follicular to midluteal phase at SL and AA. Peak VE (101 +/- 17) and submaximal VE (55 +/- 9) were not affected by cycle phase or altitude. Submaximal SaO2 did not differ between cycle phases at SL, but it was 3% higher during the midluteal phase with AA exposure. Neither VO2 peak nor Exh time was affected by cycle phase at SL or AA. We conclude that, despite significantly increased progesterone levels in the midluteal phase, exercise VE is not increased at SL or AA. Moreover, neither maximal nor submaximal exercise performance is affected by menstrual cycle phase at SL or AA.

Women at altitude: short-term exposure to hypoxia and/or alpha(1)-adrenergic blockade reduces insulin sensitivity

After short-term exposure to high altitude (HA), men appear to be less sensitive to insulin than at sea level (SL). We hypothesized that the same would be true in women, that reduced insulin sensitivity would be directly related to the rise in plasma epinephrine concentrations at altitude, and that the addition of alpha-adrenergic blockade would potentiate the reduction. To test the hypotheses, 12 women consumed a high-carbohydrate meal at SL and after 16 h at simulated 4,300-m elevation (HA). Subjects were studied twice at each elevation: once with prazosin (Prz), an alpha(1)-adrenergic antagonist, and once with placebo (Pla). Mathematical models were used to assess insulin resistance based on fasting [homeostasis model assessment of insulin resistance (HOMA-IR)] and postprandial [composite model insulin sensitivity index (C-ISI)] glucose and insulin concentrations. Relative to SL-Pla (HOMA-IR: 1.86 +/- 0.35), insulin resistance was greater in HA-Pla (3.00 +/- 0.45; P < 0.05), SL-Prz (3.46 +/- 0.51; P < 0.01), and HA-Prz (2.82 +/- 0.43; P < 0.05). Insulin sensitivity was reduced in HA-Pla (C-ISI: 4.41 +/- 1.03; P < 0.01), SL-Prz (5.73 +/- 1.01; P < 0.05), and HA-Prz (4.18 +/- 0.99; P < 0.01) relative to SL-Pla (8.02 +/- 0.92). Plasma epinephrine was significantly elevated in HA-Pla (0.57 +/- 0.08 ng/ml; P < 0.01), SL-Prz (0.42 +/- 0.07; P < 0.05), and HA-Prz (0.82 +/- 0.07; P < 0.01) relative to SL-Pla (0.28 +/- 0.04), but correlations with HOMA-IR, HOMA-beta-cell function, and C-ISI were weak. In women, short-term exposure to simulated HA reduced insulin sensitivity compared with SL. The change does not appear to be directly mediated by a concurrent rise in plasma epinephrine concentrations.

Human thermoregulatory responses to cold air are altered by repeated cold water immersion

The effects of repeated cold water immersion on thermoregulatory responses to cold air were studied in seven males. A cold air stress test (CAST) was performed before and after completion of an acclimation program consisting of daily 90-min cold (18 degrees C) water immersion, repeated 5 times/wk for 5 consecutive wk. The CAST consisted of resting 30 min in a comfortable [24 degrees C, 30% relative humidity (rh)] environment followed by 90 min in cold (5 degrees C, 30% rh) air. Pre- and postacclimation, metabolism (M) increased (P less than 0.01) by 85% during the first 10 min of CAST and thereafter rose slowly. After acclimation, M was lower (P less than 0.02) at 10 min of CAST compared with before, but by 30 min M was the same. Therefore, shivering onset may have been delayed following acclimation. After acclimation, rectal temperature (Tre) was lower (P less than 0.01) before and during CAST, and the drop in Tre during CAST was greater (P less than 0.01) than before. Mean weighted skin temperature (Tsk) was lower (P less than 0.01) following acclimation than before, and acclimation resulted in a larger (P less than 0.02) Tre-to-Tsk gradient. Plasma norepinephrine increased during both CAST (P less than 0.002), but the increase was larger (P less than 0.004) following acclimation. These findings suggest that repeated cold water immersion stimulates development of true cold acclimation in humans as opposed to habituation. The cold acclimation produced appears to be of the insulative type.

Volumetric quantification of brain swelling after hypobaric hypoxia exposure

We applied a novel MR imaging technique to investigate the effect of acute mountain sickness on cerebral tissue water. Nine volunteers were exposed to hypobaric hypoxia corresponding to 4572 m altitude for 32 h. Such an exposure may cause acute mountain sickness. We imaged the brains of the volunteers before and at 32 h of hypobaric exposure with two different MRI techniques with subsequent data processing. (1) Brain volumes were calculated from 3D MRI data sets by applying a computerized brain segmentation algorithm. For this specific purpose a novel adaptive 3D segmentation program was used with an automatic correction algorithm for RF field inhomogeneity. (2) T(2) decay rates were analyzed in the white matter. The results demonstrated that a significant brain swelling of 36.2 +/- 19.6 ml (2.77 +/- 1.47%, n = 9, P < 0.001) developed after the 32-h hypobaric hypoxia exposure with a maximal observed volume increase of 5.8% (71.3 ml). These volume changes were significant only for the gray matter structures in contrast to the unremarkable changes seen in the white matter. The same study repeated 3 weeks later in 6 of 9 original subjects demonstrated that the brains recovered and returned approximately to the initially determined sea-level brain volume while hypobaric hypoxia exposure once again led to a significant new brain swelling (24.1 +/- 12.1 ml, 1.92 +/- 0.96%, n = 6, P < 0.005). On the contrary, the T(2) mapping technique did not reveal any significant effect of hypobaria on white matter. We present here a technique which is able to detect reversible brain volume changes as they may occur in patients with diffuse brain edema or increased cerebral blood volume, and which may represent a useful noninvasive tool for future evaluations of antiedematous drugs.

Altitude acclimatization and blood volume: effects of exogenous erythrocyte volume expansion

We studied sea-level residents during 13 days of altitude acclimatization to determine 1) altitude acclimatization effects on erythrocyte volume and plasma volume, 2) if exogenous erythrocyte volume expansion alters subsequent erythrocyte volume and plasma volume adaptations, 3) if an increased blood oxygen content alters erythropoietin responses during altitude acclimatization, and 4) mechanisms responsible for plasma loss at altitude. Sixteen healthy men had a series of hematologic measurements made at sea level, on the first and ninth days of altitude (4,300 m) residence, and after returning to sea level. Twenty-four hours before the ascent to altitude, one group received a 700-ml infusion of autologous erythrocytes (42% hematocrit), whereas the other group received only a saline infusion. Erythrocyte infusion increased erythrocyte volume by approximately 10%, whereas saline infusion had no effect; in addition, initially at altitude, blood oxygen content was 8% higher in erythrocyte-infused than in saline-infused subjects. The new findings regarding altitude acclimatization are summarized as follows: 1) erythrocyte volume does not change during the first 13 days and is not affected by prior exogenous expansion, 2) a modest increase in blood oxygen content does not modify erythropoietin responses, 3) plasma losses are related to vascular protein losses, and 4) exogenous erythrocyte volume expansion coincides with transient increases in plasma loss, vascular protein loss, and mean arterial pressure elevation. These findings better define human blood volume responses during altitude acclimatization.

Women at altitude: ventilatory acclimatization at 4,300 m

Women living at low altitudes or acclimatized to high altitudes have greater effective ventilation in the luteal (L) compared with follicular (F) menstrual cycle phase and compared with men. We hypothesized that ventilatory acclimatization to high altitude would occur more quickly and to a greater degree in 1) women in their L compared with women in their F menstrual cycle phase, and 2) in women compared with men. Studies were conducted on 22 eumenorrheic, unacclimatized, sea-level (SL) residents. Indexes of ventilatory acclimatization [resting ventilatory parameters, hypoxic ventilatory response, hypercapnic ventilatory response (HCVR)] were measured in 14 women in the F phase and in 8 other women in the L phase of their menstrual cycle, both at SL and again during a 12-day residence at 4,300 m. At SL only, ventilatory studies were also completed in both menstrual cycle phases in 12 subjects (i.e., within-subject comparison). In these subjects, SL alveolar ventilation (expressed as end-tidal PCO(2)) was greater in the L vs. F phase. Yet the comparison between L- and F-phase groups found similar levels of resting end-tidal PCO(2), hypoxic ventilatory response parameter A, HCVR slope, and HCVR parameter B, both at SL and 4,300 m. Moreover, these indexes of ventilatory acclimatization were not significantly different from those previously measured in men. Thus female lowlanders rapidly ascending to 4,300 m in either the L or F menstrual cycle phase have similar levels of alveolar ventilation and a time course for ventilatory acclimatization that is nearly identical to that reported in male lowlanders.

Adductor pollicis muscle fatigue during acute and chronic altitude exposure and return to sea level

Large muscle exercise performance is impaired during acute exposure to normobaric or hypobaric hypoxia, but the effects of hypoxic conditions on fatigue of isolated smaller muscle groups per se are poorly defined. We studied how acute and chronic altitude (ALT) exposure and post-ALT return to sea level (SL) affects voluntary strength and fatigue of the adductor pollicis muscle. Eight healthy men (mean age 28 yr) were studied on five separate occasions: at SL, on days 1 (acute) and 13 (chronic) at ALT (4,300 m), and on days 1 (post 1) and 3 or 4 (post 2) at SL after 20 days of residence at ALT. On each day, maximal voluntary contractions (MVCs) of the adductor pollicis were obtained before and at the end of each minute of submaximal intermittent contractions of the adductor pollicis (50% of MVC of rested muscle, 5 s of contraction/5 s of rest) until exhaustion, defined as the inability to exert or maintain 50% of rested MVC. MVC of rested muscle did not differ among days. Time to exhaustion was shorter at acute ALT [5.1 +/- 0.5 (SE) min] than at SL (7.4 +/- 0.8 min, P < 0.05) and tended to be shorter than at chronic ALT (6.6 +/- 0.7 min, P > 0.05). Compared with acute and chronic ALT, time to exhaustion was prolonged during post 1 (9.0 +/- 1.2 min, P < 0.05) but not post 2 (6.1 +/- 0.5 min, P > 0.05). We conclude that 1) MVC of rested adductor pollicis muscle is not impaired during or after ALT exposure, 2) compared with SL conditions, acute but not chronic ALT exposure leads to a more rapid decline in adductor pollicis MVC associated with submaximal contractions, and 3) time to exhaustion is prolonged for > or = 1 day after return from ALT.

Exercise responses after altitude acclimatization are retained during reintroduction to altitude

Following 2 to 3 wk of altitude acclimatization, ventilation is increased and heart rate (HR), plasma volume (PV), and lactate accumulation ([La]) are decreased during submaximal exercise. The objective of this study was to determine whether some degree of these exercise responses associated with acclimatization would be retained upon reintroduction to altitude (RA) after 8 d at sea level (SL). Six male lowlanders (X +/- SE; 31 +/- 2 yr, 82.4 +/- 4.6 kg) exercised to exhaustion at the same relative percentages of peak oxygen uptake (VO2peak) at SL, on acute altitude (AA) exposure, after a 16-d chronic altitude (CA) exposure on Pikes Peak (4,300 m), and during a 3- to 4-h RA in a hypobaric chamber (4,300 m; 446 mm Hg) after 8 d at SL. The submaximal exercise to exhaustion time (min) was the same at SL (66.0 +/- 1.6), AA (67.7 +/- 7.3), CA (79.9 +/- 6.2), and RA (67.9 +/- 1.9). At 75% VO2peak: (1) arterial oxygen saturation (SaO2) increased from AA to CA (67.0 +/- 1.5 vs 78.5 +/- 1.8%; P < 0.05) and remained increased at RA (77.0 +/- 2.0%); (2) HR decreased from SL to CA (171 +/- 6 vs 152 +/- 9 beats x min-1; P < 0.05) and remained decreased at RA (157 +/- 5 beats x min-1); (3) calculated PV decreased 6.9 +/- 10.0% at AA, 21.3 +/- 11.1% at CA, and 16.7 +/- 5.4% at RA from SL baseline values, and (4) [La] decreased from AA to CA (5.1 +/- 0.9 vs 1.9 +/- 0.4 mmol x L-1; P < 0.05) and remained decreased at RA (2.6 +/- 0.6 mmol x L-1). Upon RA after 8 d at SL, the acclimatization responses were retained 92 +/- 9% for SaO2, 74 +/- 8% for PV, and 58 +/- 3% for [La] at 75% VO2peak. In conclusion, although submaximal exercise to exhaustion time is not improved upon reintroduction to altitude after 8 d at sea level, retention of beneficial exercise responses associated with altitude acclimatization is likely in individuals whose work, athletic competition, or recreation schedules involve intermittent sojourns to high elevations.

Effects of erythrocyte infusion on VO2max at high altitude

This study investigated whether autologous erythrocyte infusion would ameliorate the decrement in maximal O2 uptake (VO2max) experienced by lowlanders when they ascend to high altitude. VO2max was measured in 16 men (treadmill running) at sea level (SL) and on the 1st (HA1) and 9th (HA9) days of high-altitude (4,300 m) residence. After VO2max was measured at SL, subjects were divided into two matched groups (n = 8). Twenty-four hours before ascent to high altitude, the experimental group received a 700-ml infusion of autologous erythrocytes and saline (42% hematocrit), whereas the control group received only saline. The VO2max of erythrocyte-infused [54 +/- 1 (SE) ml.kg-1.min-1] and control subjects (52 +/- 2 ml.kg-1.min-1) did not differ at SL before infusion. The decrement in VO2max on HA1 did not differ between groups, averaging 26% overall, despite higher (P < 0.01) arterial hematocrit, hemoglobin concentration, and arterial O2 content in the erythrocyte-infused subjects. By HA9, there were no longer any differences in hematocrit, hemoglobin concentration, or arterial O2 content between groups. No change in VO2max occurred between HA1 and HA9 for either group. Thus, despite increasing arterial O2-carrying capacity, autologous erythrocyte infusion did not ameliorate the decrement in VO2max at 4,300-m altitude.

Carbohydrate supplementation improves time-trial cycle performance during energy deficit at 4,300-m altitude

Carbohydrate supplementation (CHOS) typically improves prolonged time-trial (TT) performance at sea level (SL). This study determined whether CHOS also improves TT performance at high altitude (ALT; 4,300 M) despite increased hypoxemia and while in negative energy balance (∼1,250 kcal/day). Two groups of fasting, fitness-matched men performed a 720-kJ cycle TT at SL and while living at ALT on days 3 (ALT3) and 10 (ALT10). Eight men drank a 10% carbohydrate solution (0.175 g/kg body wt) and eight drank a placebo (PLA; double blind) at the start of and every 15 min of the TT. Blood glucose during each TT was higher ( P < 0.05) for CHOS than for PLA. At SL, TT duration (∼59 min) and watts (∼218 or ∼61% of peak watts; %SL Wpeak) were similar for both groups. At ALT, the TT was longer for both groups ( P < 0.01) but was shorter for CHOS than for PLA on ALT3 (means ± SE: 80 ± 7 vs. 105 ± 9 min; P < 0.01) and ALT10 (77 ± 7 vs. 90 ± 5 min; P < 0.01). At ALT, %SL Wpeak was reduced ( P < 0.01) with the reduction on ALT3 being larger for PLA (to 33 ± 3%) than for CHOS (to 43 ± 2%; P < 0.05). On ALT3, O2 saturation fell similarly from 84 ± 2% at rest to 73 ± 1% during the TT for both groups ( P < 0.05), and on ALT10 O2 saturation fell more ( P < 0.02) for CHOS (91 ± 1 to 76 ± 2%) than for PLA (90 ± 1 to 81 ± 1%). %SL Wpeak and O2 saturation were inversely related during the TT for both groups at ALT ( r ≥ −0.76; P ≤ 0.03). It was concluded that, despite hypoxemia exacerbated by exercise, CHOS greatly improved TT performance at ALT in which there was a negative energy balance.

Gender alters impact of hypobaric hypoxia on adductor pollicis muscle performance

Recently, we reported that, at similar voluntary force development during static submaximal intermittent contractions of the adductor pollicis muscle, fatigue developed more slowly in women than in men under conditions of normobaric normoxia (NN) (Acta Physiol Scand 167: 233-239, 1999). We postulated that the slower fatigue of women was due, in part, to a greater capacity for muscle oxidative phosphorylation. The present study examined whether a gender difference in adductor pollicis muscle performance also exists during acute exposure to hypobaric hypoxia (HH; 4,300-m altitude). Healthy young men (n = 12) and women (n = 21) performed repeated static contractions at 50% of maximal voluntary contraction (MVC) force of rested muscle for 5 s followed by 5 s of rest until exhaustion. MVC force was measured before and at the end of each minute of exercise and at exhaustion. Exhaustion was defined as an MVC force decline to 50% of that of rested muscle. For each gender, MVC force of rested muscle in HH was not significantly different from that in NN. MVC force tended to decline at a faster rate in HH than in NN for men but not for women. In both environments, MVC force declined faster (P < 0.01) for men than for women. For men, endurance time to exhaustion was shorter (P < 0.01) in HH than in NN [6.08 +/- 0.7 vs. 8.00 +/- 0.7 (SE) min]. However, for women, endurance time to exhaustion was similar (not significant) in HH (12.86 +/- 1.2 min) and NN (13.95 +/- 1.0 min). In both environments, endurance time to exhaustion was longer for women than for men (P < 0.01). Gender differences in the impact of HH on adductor pollicis muscle endurance persisted in a smaller number of men and women matched (n = 4 pairs) for MVC force of rested muscle and thus on submaximal absolute force and, by inference, ATP demand in both environments. In contrast to gender differences in the impact of HH on small-muscle (adductor pollicis) exercise performance, peak O(2) uptake during large-muscle exercise was lower in HH than in NN by a similar (P > 0.05) percentage for men and women (-27.6 +/- 2 and -25.1 +/- 2%, respectively). Our findings are consistent with the postulate of a higher adductor pollicis muscle oxidative capacity in women than in men and imply that isolated performance of muscle with a higher oxidative capacity may be less impaired when the muscle is exposed to HH.

Thermoregulation during cold water immersion is unimpaired by low muscle glycogen levels

This investigation studied the importance of muscle glycogen levels for body temperature regulation during cold stress. Physiological responses of eight euglycemic males were measured while they rested in cold (18 degrees C, stirred) water on two separate occasions. The trials followed a 3-day program of diet and exercise manipulation designed to produce either high (HMG) or low (LMG) preimmersion glycogen levels in the muscles of the legs, arms, and upper torso. Preimmersion vastus lateralis muscle glycogen concentrations were lower during the LMG trial (144 +/- 14 mmol glucose/kg dry tissue) than the HMG trial (543 +/- 53 mmol glucose/kg dry tissue). There were no significant differences between the two trials in shivering as reflected by aerobic metabolic rate or in the amount of body cooling as reflected by changes in rectal temperature during the immersions. Postimmersion muscle glycogen levels remained unchanged from preimmersion levels in both trials. Small but significant increases in plasma glucose and lactate concentration occurred during both immersions. Plasma glycerol increased during immersion in the LMG trial but not in the HMG trial. Plasma free fatty acid concentration increased during both immersion trials, but the change was apparent sooner in the LMG immersion. It was concluded that thermoregulatory responses of moderately lean and fatter individuals exposed to cold stress were not impaired by a substantial reduction in the muscle glycogen levels of several major skeletal muscle groups. Furthermore, the data suggest that, depending on the intensity of shivering, other metabolic substrates are available to enable muscle glycogen to be spared.

Plasma hormonal responses at graded hypohydration levels during exercise-heat stress

The effects of graded levels of hypohydration (3, 5, and 7% of body weight) on hormonal responses to exercise in the heat were examined in six heat-acclimated male volunteers. On the day following dehydration, subjects performed light (approximately 25% maximal O2 consumption, 1.03 1 X min-1) exercise in a hot (49 degrees C, 20% relative humidity) environment for four consecutive 25-min intervals interspaced by 10-min rests; blood was obtained before exercise and at approximately 10 min before completion of each exercise period. During euhydration, plasma cortisol (PC) levels manifested significant decrements over time (e.g., time 0, 14.2 micrograms X 100 ml-1 vs. time 2, 8.9 micrograms X 100 ml-1), probably related to its diurnal periodicity. However, during hypohydration, levels of PC were increased and correlated with hypohydration intensity (e.g., time 0, 0, 3, 5, and 7% hypohydration, 14.2, 16.5, 19.8, and 36.2 micrograms X 100 ml-1, respectively). Plasma renin activity (PRA) was increased significantly by hypohydration (e.g., time 0, euhydrated vs. 3%, 3.7 vs. 6.2 units) but was unaffected by exercise in the heat. Plasma aldosterone (ALD) levels were generally increased by exercise in the heat (e.g., time 0 vs. time 4, 3% hypohydration, 12.1 vs. 18.7 ng X 100 ml-1). Regression analysis illustrated that graded intensities of hypohydration were correlated with incremented PRA and ALD through 5% hypohydration. Conversely, PC was incrementally elevated through 7% hypohydration.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of polycythemia on blood volume and thermoregulation during exercise-heat stress

We studied the effects of autologous erythrocyte infusion on blood volume and thermoregulation during exercise in the heat. By use of a double-blind design, nine unacclimated male subjects were infused with either 600 ml of a NaCl-glucose-phosphate solution containing a approximately 50% hematocrit (n = 6, reinfusion) or 600 ml of this solution only (n = 3, saline). A heat stress test (HST) was attempted approximately 2-wk pre- and 48-h postinfusion during the late spring months. After 30 min of rest in a 20 degrees C antechamber, the HST consisted of a 120-min exposure (2 repeats of 15 min rest and 45 min treadmill walking) in a hot (35 degrees C, 45% rh) environment while euhydrated. Erythrocyte volume (RCV, 51Cr) and plasma volume (PV, 125I) were measured 24 h before each HST, and maximal O2 uptake (VO2max) was measured 24 h after each HST. Generally, no significant effects were found for the saline group. For the reinfusion group, RCV (11%, P less than 0.01) and VO2max (11%, P less than 0.05) increased after infusion, and the following observations were made: 1) the increased RCV was associated with a reduction in PV to maintain the same blood volume as during the preinfusion measurements; 2) polycythemia reduced total circulating protein but did not alter F-cell ratio, plasma osmolality, plasma protein content, or plasma lactate at rest or during exercise-heat stress; 3) polycythemia did not change the volume of fluid entering the intravascular space from rest to exercise-heat stress; and 4) polycythemia tended to reduce the rate of heat storage during exercise-heat stress.

Postural instability and acute mountain sickness during exposure to 24 hours of simulated altitude (4300 m)

Short exposures to severe or moderate hypoxia can have detrimental effects on postural stability. We hypothesized that continuous 24-h exposure to simulated 4300-m altitude (446 mmHg) would adversely affect postural stability and that this change in postural stability would be related to the severity of acute mountain sickness (AMS). On two different studies with similar experimental designs, postural instability was measured after approximately 3 and approximately 24 h of exposure using a computer-controlled unstable platform system in a total of 19 volunteers on three consecutive, 30-sec tests: eyes open (EO), eyes closed (EC), and a dynamic test involving tracking a circular moving object. Compared to baseline sea-level results, increases in postural instability were obtained with the EO test after 2 to 3 h (30%, p = 0.002) and 23 to 24 h (21%, p = 0.036) of altitude exposure. Similar increases were obtained on the EC test: 2 to 3 h (25%, p < 0.001) and 23 to 24 h (31%, p < 0.001). Although absolute instability values were higher on the EC test, the ratio EC/EO and the relative temporal changes with altitude exposure were similar. There were no significant altitude-stability effects on the target-tracking task. Sixty-three percent of the subjects (12 of 19) exhibited significant AMS (> 0.7 ESQ-C score) at some point during the 24-h exposure. No statistically significant correlations were obtained between the ESQ-C and any of the postural instability tests. These results indicate that postural stability is adversely affected during a 24-h exposure to 4300 m; however, there does not appear to be a correlation with the incidence or severity of AMS.

Ventilatory responses to static handgrip exercise

Previous research indicates that fatiguing static exercise causes hyperventilation and a decrease of end-tidal CO2 partial pressure PETCO2. The objectives of this study were 1) to examine the changes in pattern of breathing during static exercise, and 2) to define the isocapnic ventilatory response. Six healthy males were studied once a week at one of three levels of static handgrip exercise: 15, 25, or 30% maximum voluntary contraction (MVC) was sustained for 5 min while holding PETCO2 constant or allowing it to run free. During 25 and 30% MVC, we observed 1) progressive increases in mean tidal volume (VT), inspiratory ventilation (VI), VT/TI, heart rate (HR), and arterial BP, 2) increased breath-to-breath variability of VT, 3) no significant changes in respiratory frequency (f), and 4) progressive decreases in PETCO2. Keeping PETCO2 constant at preexercise levels did not change the pattern or magnitude of the ventilatory response to exercise. The time course and magnitude of the subjects' perceived effort resembled the time course and magnitude of the ventilatory response. The variability of VT during the response to static exercise suggests an element of control instability. The identical ventilatory responses during hypocapnic and isocapnic conditions may result from the slow response of the central chemoreceptors; an overriding influence of muscle afferents; and/or increased central command arising with fatigue.

Erythropoiesis in women during 11 days at 4,300 m is not affected by menstrual cycle phase

Because the ovarian steroid hormones, progesterone and estrogen, have higher blood levels in the luteal (L) than in the follicular (F) phase of the menstrual cycle, and because of their known effects on ventilation and hematopoiesis, we hypothesized that less hypoxemia and less erythropoiesis would occur in the L than the F phase of the cycle after arrival at altitude. We examined erythropoiesis with menstrual cycle phase in 16 women (age 22.6 +/- 0.6 yr). At sea level, 11 of 16 women were studied during both menstrual cycle phases, and, where comparison within women was available, cycle phase did not alter erythropoietin (n = 5), reticulocyte count (n = 10), and red cell volume (n = 9). When all 16 women were taken for 11 days to 4,300-m altitude (barometric pressure = 462 mmHg), paired comparisons within women showed no differences in ovarian hormone concentrations at sea level vs. altitude on menstrual cycle day 3 or 10 for either the F (n = 11) or the L (n = 5) phase groups. Arterial oxygen saturation did not differ between the F and L groups at altitude. There were no differences by cycle phase on day 11 at 4,300 m for erythropoietin [22.9 +/- 4.7 (L) vs. 18.8 +/- 3.4 mU/ml (F)], percent reticulocytes [1.9 +/- 0.1 (L) vs. 2.1 +/- 0.3% (F)], hemoglobin [13.5 +/- 0.3 (L) vs. 13.7 +/- 0.3 g/100 ml (F)], percent hematocrit [40.6 +/- 1.4 (L) vs. 40.7 +/- 1.0% (F)], red cell volume [31.1 +/- 3.6 (L) vs. 33.0 +/- 1.6 ml/kg (F)], and blood ferritin [8.9 +/- 1.7 (L) vs. 10.2 +/- 0.9 microg/l (F)]. Blood level of erythropoietin was related (r = 0.77) to arterial oxygen saturation but not to the levels of progesterone or estradiol. We conclude that erythropoiesis was not altered by menstrual cycle phase during the first days at 4,300-m altitude.

Carbohydrate supplementation and endurance performance of moderate altitude residents at 4300 m

Recent work from our laboratory demonstrated that carbohydrate supplementation (CHOS) during exercise improved prolonged time-trial (TT) performance of sea-level residents (SLR) living at 4300 m while they were in daily negative energy balance (- 1250 kcal x day (-1); [ ]). The purposes of the current study were to determine during initial exposure to 4300 m:1) whether CHOS also improves TT performance of moderate altitude residents (MAR) who are in energy balance and 2) if acclimatization to moderate elevations benefits TT performance. Fifteen Air Force Academy (AFA) active duty members (age: 30 +/- 1 yrs; mean +/- SE), who had been living at approximately 2000 m for 21 +/- 3 months performed a maximal-effort 720-kJ cycle TT at the AFA and at Pikes Peak (PP), CO, (4300 m) on days 1 (PP1) and 3 (PP3). Daily energy intake and expenditure were maintained similarly at the AFA and PP. At the start of the TTs at PP, and then every 15 min thereafter, 9 subjects drank a 10 % CHO solution (0.175 g x kg (-1) body weight) and 6 subjects drank a placebo (PLA) solution. All subjects were allowed to freely adjust the power output of the cycle ergometer and drank water AD LIBITUM. Performance time did not differ between groups on PP1 (CHOS vs. PLA; 101 +/- 8 vs. 116 +/- 10 min) or PP3 (95 +/- 8 vs. 107 +/- 12 min). For both groups, cycle times on PP1 and PP3 were longer compared to the AFA (p<0.01) and were improved from PP1 to PP3 (p<0.05). Exercise intensity (i.e., % peak oxygen uptake) was maintained similarly at approximately 62 % during the TTs at the AFA and PP. Blood glucose was 1.5 to 2.0 mmol x L (-1) higher for CHOS vs. PLA (p<0.01). It was concluded that CHOS provided no TT performance benefit for MAR at 4300 m when energy balance was maintained. However, the decrements in TT performance and exercise intensity were attenuated at 4300 m in MAR compared to those of SLR as a result of acclimatization attained while living for nearly 2 years at approximately 2000 m.

Relationship of transdiaphragmatic pressure and latencies for detecting added inspiratory loads

The purpose of this investigation was to measure changes in transdiaphragmatic pressure (Pdi) developed during graded elastic (E) and resistive (R) loaded breaths and to correlate the emergence of such changes with the load-dependent alterations in latency for detection (Tdet). Five healthy adults were studied using three protocols, i.e., graded E, graded R, and graded R in the presence of elevated background R. In each protocol, loads were added for single inspirations, 10 times in random order and separated by three to five unloaded breaths. Subjects pressed a signal marker as soon as loads were detected. Inspiratory flow (VI), inspired volume (VI), mouth pressure, and Pdi of loaded breaths and the preceding unloaded breaths were recorded and computer averaged. Patterns of VI and VI were not altered prior to detection of the smallest added E and R loads but decreased with the higher loads. Group mean patterns of Pdi showed graded increases during loaded breaths. Augmentation of Pdi preceded Tdet and occurred earlier as Tdet decreased with graded E and R loads. Elevating the background R delayed both Tdet of added R and the augmentation of Pdi. Results are consistent with the hypothesis that load-induced changes in diaphragmatic tension may play a sensory role in detection of inspiratory loads.

Polycythemia and hydration: effects on thermoregulation and blood volume during exercise-heat stress

We studied the effects of autologous erythrocyte infusion on thermoregulation and blood volume during exercise in the heat. Specifically, we wanted to determine whether heat-acclimated subjects, as well as hypohydrated subjects, would have a thermoregulatory advantage from acute polycythemia during exercise in the heat. Five heat-acclimated males attempted four heat stress tests (HSTs): two pre- and two postinfusion. Autologous erythrocyte infusion was accomplished with 500 ml of a NaCl-glucose-phosphate solution containing approximately 60% hematocrit. One HST, both pre- and postinfusion, was done while subjects were euhydrated, and one HST was done while subjects were hypohydrated (-5% of body wt). After 30 min of rest in a 20 degrees C antechamber, the HST consisted of a 120-min exposure (2 repeats of 15 min rest and 45 min walking) in a hot (35 degrees C, 45% relative humidity) environment. The findings concerning acute polycythemia in heat-acclimated subjects are summarized: 1) polycythemia increased (P less than 0.05) sweating rate and reduced (P less than 0.01) core temperature during exercise-heat stress for both euhydrated and hypohydrated subjects; 2) the erythrocyte infusion caused an increased (P less than 0.05) plasma volume and increased (P less than 0.01) blood volume; 3) the increased plasma volume was associated with an increased (P less than 0.05) total circulating protein mass; 4) the increased total circulating protein mass tended to better maintain plasma volume when hypohydrated; and 5) heat acclimation may increase extravascular protein mass. Therefore, it is concluded that erythrocyte infusion provides a thermoregulatory advantage during exercise in the heat for heat acclimated subjects when both euhydrated and hypohydrated.