Exercise thermoregulation in men after 1 and 24-hours of 6 degrees head-down tilt.

BACKGROUND: Exercise thermoregulation is dependent on heat loss by increased skin blood flow (convective and conductive heat loss) and through enhanced sweating (evaporative heat loss). Reduction of plasma volume (PV), increased plasma osmolality, physical deconditioning, and duration of exposure to simulated and actual microgravity reduces the ability to thermoregulate during exercise. HYPOTHESIS: We hypothesized that 24 h of head down tilt (HDT24) would alter thermoregulatory responses to a submaximal exercise test and result in a higher exercise rectal temperature (Tre) when compared with exercise Tre after 1 h of head down tilt (HDT1). METHODS: Seven men (31+/-SD 6 yr, peak oxygen uptake (VpO2peak) of 44+/-6 ml x kg(-1) x min(-1)) were studied during 70 min of supine cycling at 58+/-SE 1.5% VO2peak at 22.0 degrees C Tdb and 47% rh. RESULTS: Relative to pre-tilt sitting chair rest data, HDT1 resulted in a 6.1+/-0.9% increase and HDT24 in a 4.3+/-2.3% decrease in PV (delta = 10.4% between experiments, p<0.05) while plasma osmolality remained unchanged (NS). Pre-exercise Tre was elevated after HDT24 (36.71 degrees C +/-0.06 HDT1 vs. 36.93 degrees C+/-0.11 HDT24, p<0.05). The 70 min of exercise did not alter this relationship (p<0.05) with respective end exercise increases in Tre to 38.01 degrees C and 38.26 degrees C (degrees = 1.30 degrees C (HDT1) and 1.33 degrees C (HDT24)). While there were no pre-exercise differences in mean skin temperature (Tsk), a significant (p<0.05) time x treatment interaction occurred during exercise: after min 30 in HDT24 the Tsk leveled off at 31.1 degrees C, while it continued to increase reaching 31.5 degrees C at min 70 in HDT1. A similar response (NS) occurred in skin blood velocity. Neither local sweating rates nor changes in body weight during exercise of -1.63+/-0.24 kg (HDT1) or - 1.33+/-0.09 kg (HDT24) were different (NS) between experiments. CONCLUSION: While HDT24 resulted in elevated pre-exercise Tre, reduced PV, attenuation of Tsk and skin blood velocity during exercise, the absolute increase in exercise Tre was not altered. But if sweat rate and cutaneous vascular responses were similar at different core temperatures (unchanged thermoregulation), the Tre offset could have been caused by the HDT-induced hypovolemia.

Body composition by air-displacement plethysmography by using predicted and measured thoracic gas volumes.

The BOD POD, a new air-displacement plethysmograph for measuring human body composition, utilizes the inverse relationship between pressure and volume (Boyle's law) to measure body volume directly. The quantity of air in the lungs during tidal breathing, the average thoracic gas volume (Vtg), is also measured by the BOD POD by using a standard plethysmographic technique. Alternatively, the BOD POD provides the use of a predicted Vtg (Vtgpred). The validity of using Vtgpred in place of measured Vtg (Vtgmeas) to determine the percentage of body fat (%BF) was evaluated in 50 subjects (36 women, 14 men; ages 18-56 yr). There was no significant difference between Vtgmeas and Vtgpred (mean difference +/- SE, 53.5 +/- 63.3 ml) nor in %BF by using Vtgmeas vs. Vtgpred (0.2 +/- 0.2 %BF). On an individual basis, %BF measured by using Vtgmeas vs. Vtgpred differed within +/-2.0% BF for 82% of the subjects; maximum differences were -2.9 to +3.0% BF. For comparison, data from 24 subjects who had undergone hydrostatic weighing were evaluated for the validity of using predicted vs. measured residual lung volume (VRpred vs. VRmeas, respectively). Differences between VRmeas and VRpred and in %BF calculated by using VRmeas vs. VRpred were significant (187 +/- 46 ml and 1.4 +/- 0.3% BF, respectively; P < 0.001). On an individual basis, %BF determined by using VRmeas vs. VRpred differed within +/-2.0% BF for 46% of the subjects; maximum differences were -2.9 to +3.8% BF. With respect to %BF measured by air displacement, our findings support the use of Vtgpred for group mean comparisons and for purposes such as screening in young to middle-aged individuals. This contrasts with the use of VRpred in hydrostatic weighing, which leads to significant errors in the estimation of %BF. Furthermore, although the use of Vtgpred has some application, determining Vtgmeas is relatively simple in most cases. Therefore, we recommend that the use of Vtgmeas remain as standard experimental and clinical practice.

Submaximal exercise VO2 and Qc during 30-day 6 degrees head-down bed rest with isotonic and isokinetic exercise training.

BACKGROUND: Maintaining intermediary metabolism is necessary for the health and well-being of astronauts on long-duration spaceflights. While peak oxygen uptake (VO2) is consistently decreased during prolonged bed rest, submaximal VO2 is either unchanged or decreased. METHODS: Submaximal exercise metabolism (61 +/- 3% peak VO2) was measured during ambulation (AMB day-2) and on bed rest days 4, 11, and 25 in 19 healthy men (32-42 yr) allocated into no exercise (NOE, N = 5) control, and isotonic exercise (ITE, N = 7) and isokinetic exercise (IKE, N = 7) training groups. Exercise training was conducted supine for two 30-min periods per day for 6 d per week: ITE training was intermittent at 60-90% peak VO2; IKE training was 10 sets of 5 repetitions of peak knee flexion-extension force at a velocity of 100 degrees s-1. Cardiac output was measured with the indirect Fick CO2 method, and plasma volume with Evans blue dye dilution. RESULTS: Supine submaximal exercise VO2 decreased significantly (*p < 0.05) by 10.3%* with ITE and by 7.3%* with IKE; similar to the submaximal cardiac output decrease of 14.5%* (ITE) and 20.3%* (IKE), but different from change in peak VO2 (+1.4% with ITE and -10.2%* with IKE) and decrease in plasma volume of -3.7% (ITE) and -18.0%* (IKE). Reduction of submaximal VO2 during bed rest correlated 0.79 (p < 0.01) with submaximal Qc, but was not related to change in peak VO2 or plasma volume. CONCLUSION: Reduction in submaximal oxygen uptake during prolonged bed rest is related to decrease in exercise but not resting cardiac output; perturbations in active skeletal muscle metabolism may be involved.

Aerobic walking in slowly progressive neuromuscular disease: effect of a 12-week program.

OBJECTIVE: Poor cardiorespiratory endurance is a common finding in neuromuscular disease (NMD), and the capacity of such patients to respond to aerobic training is unclear. This study was conducted to determine if a 12-week walking program results in increased aerobic capacity in slowly progressive NMD subjects, whether such a program is safely tolerated, and whether such patients can adhere to a self-monitored, home-based training program. DESIGN: Before-after trial. SETTING: Subjects' homes. PATIENTS: A cohort of 8 slowly progressive NMD subjects (4 men, 4 women) followed in the neuromuscular disease clinic participated (age, 36.6 +/- 8.0 yrs; ht, 170 +/- 11 cm; wt, 74.3 +/- 19.0 kg) (Mean +/- SD). INTERVENTION: Subjects walked 15 to 30 min 3 to 4 days a week at 50% to 60% of their heart rate reserve. MAIN OUTCOME MEASURES: Resting, submaximal, and peak heart rates, systolic and diastolic blood pressures, oxygen uptake, and peak power output. RESULTS: Graded exercise testing to volitional fatigue using a semirecumbent cycle ergometer before and after the training program found significant decreases in submaximal heart rate by 7 +/- 3 beats/min (Mean +/- SEM) (95% CI = -23 to 9) (p = .046) and submaximal systolic blood pressure by 11 +/- 4 mmHg (95% CI = -31 to 9) (p = .019), and nonsignificant increases in peak power output and VO2. CONCLUSIONS: These results suggest that moderate-intensity aerobic exercise training is well tolerated and may provide modest improvement in aerobic capacity in slowly progressive NMD subjects.

Evaluation of a new air displacement plethysmograph for measuring human body composition.

A new air displacement plethysmograph, the BOD POD (BP), was evaluated in comparison to hydrostatic weighing (HW). Sixty-eight adult subjects (26 F, 42 M) varying widely in age (range 20-56 yr), ethnicity, and fatness participated in this study. Same-day test-retest reliability was assessed in a subsample of 16 subjects (9 F, 7 M) and validity was assessed in all subjects (N = 68). The test-retest coefficients of variation (CV) for %FAT measured by BP (%FATBP) and HW (%FATHW) were not significantly different (1.7% +/- 1.1% and 2.3% +/- 1.9% for BP and HW, respectively (mean +/- SD)), indicating excellent reliability for both methods. Validity of percent fat measured by the BP (%FATBP) was also excellent. The mean difference in %FAT (BP - HW) was -0.3 +/- 0.2 (SEM), with a 95% confidence interval of -0.6 to 0 %FAT. The regression equation (%FATHW = 1.86 + 0.94 %FATBP; r2 = 0.93, SEE = 1.81) was not significantly different from the line of identity (%FATHW = %FATBP), and did not differ by gender. These findings indicate that the BOD POD is a highly reliable and valid method for determining %FAT in adult humans in comparison to HW. This new method has several advantages over HW in that it is quick, relatively simple to operate and may be able to accommodate special populations such as the obese, elderly, and disabled.

Calf venous compliance measured with head-up tilt equals supine calf compliance.

Elevated calf compliance may contribute to orthostatic intolerance following space flight and bed rest. Calf venous compliance is measured conventionally with venous occulusion plethysmography in supine subjects. With this well-established technique, subjects undergo inflation of a pressure cuff around the thigh just above the knee, which increases calf venous pressure. A plethysmograph simultaneously measures calf volume elevation. Compliance equals calf volume elevation per mm Hg thigh occlusion (calf venous) pressure in relaxed legs of the supine subjects. Compliance may also be measured during stepwise head-up tilt (HUT) as calf volume elevation per mm Hg gravitational venous pressure elevation produced by HUT. However, during HUT on a tilt table with a footplate, calf muscles activate to counteract gravity: this is an obvious and natural response to gravitational force. Such muscle activation conceivably could reduce calf compliance, yet relatively little calf muscle activation occurs during HUT and orthostasis (<10% of maximal voluntary levels). Also, this activation produces minimal calf volume change (<0.3%). Therefore, we hypothesized that calf compliance measured with HUT equals that measured with supine venous occlusion.

Knee-joint proprioception during 30-day 6 degrees head-down bed rest with isotonic and isokinetic exercise training.

To determine if daily isotonic exercise or isokinetic exercise training coupled with daily leg proprioceptive training, would influence leg proprioceptive tracking responses during bed rest (BR), 19 men (36 +/- SD 4 years, 178 +/- 7 cm, 76.8 +/- 7.8 kg) were allocated into a no-exercise (NOE) training control group (n = 5), and isotonic exercise (ITE, n = 7) and isokinetic exercise (IKE, n = 7) training groups. Exercise training was conducted during BR for two 30-min periods.d-1, 5 d.week-1. Only the IKE group performed proprioceptive training using a new isokinetic procedure with each lower extremity for 2.5 min before and after the daily exercise training sessions; proprioceptive testing occurred weekly for all groups. There were no significant differences in proprioceptive tracking scores, expressed as a percentage of the perfect score of 100, in the pre-BR ambulatory control period between the three groups. Knee extension and flexion tracking responses were unchanged with NOE during BR, but were significantly greater (*p < 0.05) at the end of BR in both exercise groups when compared with NOE responses (extension: NOE 80.7 +/- 0.7%, ITE 82.9* +/- 0.6%, IKE 86.5* +/- 0.7%; flexion: NOE 77.6 +/- 1.5%, ITE 80.0 +/- 0.8% (NS), IKE 83.6* +/- 0.8%). Although proprioceptive tracking was unchanged during BR with NOE, both isotonic exercise training (without additional proprioceptive training) and especially isokinetic exercise training when combined with daily proprioceptive training, significantly improved knee proprioceptive tracking responses after 30 d of BR.

The effect of a high resistance exercise program in slowly progressive neuromuscular disease.

A 12-week high resistance home exercise program was completed by 10 subjects with slowly progressive neuromuscular diseases (NMD) and 6 normal control subjects (CTL). After evaluating baseline maximal isometric and isokinetic strength of the elbow flexors and knee extensors, subjects completed the home program using adjustable ankle and wrist cuff weights. One side of the body was randomly chosen for exercise. Subjects initially performed 1 set of 10 repetitions on 3 days a week and gradually increased to 5 sets of 10 repetitions on 4 days a week. The NMD group demonstrated significant (p < 0.05) gains in several knee extension isokinetic strength measures but loss of elbow flexion eccentric peak torque and work per degree. The CTL group demonstrated significant gains in all measures of knee extension strength, with improvement of elbow isokinetic eccentric work per degree as well. There was evidence of cross training to the nonexercised limbs for both groups. A high resistance training program, although well tolerated in NMD subjects with mild to moderate weakness, may cause some deleterious effects to diseased skeletal muscle. A high resistance training program appears to offer no advantage over a moderate resistance training program in this population.

Isokinetic strength and endurance during 30-day 6 degrees head-down bed rest with isotonic and isokinetic exercise training.

The purpose of our study was to determine if an intensive, intermittent, isokinetic, lower extremity exercise training program would attenuate or eliminate the decrease of muscular strength and endurance during prolonged bed rest (BR). The 19 male subjects (36 +/- 1 yr, 178 +/- 2 cm, 76.5 +/- 1.7 kg) were allocated into a no exercise (NOE) training group (N = 5), an isotonic (lower extremity cycle ergometer) exercise (ITE) training group (N = 7), and an isokinetic (isokinetic knee flexion-extension) exercise (IKE) training group (N = 7). Peak knee (flexion and extension) and shoulder (abduction-adduction) functions were measured weekly in all groups with one 5-repetition set. After BR, average knee extension total work decreased by 16% with NOE, increased by 27% with IKE, and was unchanged with ITE. Average knee flexion total work and peak torque (strength) responses were unchanged in all groups. Force production increased by 20% with IKE and was unchanged with NOE and ITE. Shoulder total work was unchanged in all groups, while gross average peak torque increased by 27% with ITE and by 22% with IKE, and was unchanged with NOE. Thus, while ITE training can maintain some isokinetic functions during BR, maximal intermittent IKE training can increase other functions above pre-BR control levels.

Moderate resistance exercise program: its effect in slowly progressive neuromuscular disease.

A 12-week moderate resistance exercise program was performed by 27 patients with slowly progressive neuromuscular diseases (NMD) and 14 control subjects (CTL) in order to determine safety and efficacy of a strengthening program. A 3-day per week submaximal regimen of home exercise using ankle and wrist weights and hand grip exerciser was prescribed. One side of the body was randomly chosen for exercise. Subjects were tested for maximal isokinetic and isometric strength at baseline and after weeks 4 and 12 of the training protocol, and the prescribed amount of work was gradually increased throughout the program. Both the NMD and CTL groups demonstrated significant (p < .05) increases in most strength measures. Both groups responded similarly to the exercise program, and strength gains did not significantly differ between the exercised and nonexercised limbs in either group. This study provides evidence that a 12-week submaximal strength training program is practical and safe in slowly progressive NMD and produces moderate improvement in measured strength.

Isometric and isokinetic measurement of hamstring and quadriceps strength.

This study evaluates quadriceps strength at varying movement velocities in a group of healthy subjects. Specific objectives included (1) investigating the effect of angular velocity on peak torque (PT) (2) investigating the effect of angular velocity on joint angle at peak torque (JAPT) and (3) evaluating whether the use of a relatively new isokinetic testing device yields new insights to previously accepted relationships between angular velocity and PT or JAPT. Twenty healthy subjects were tested for isometric and isokinetic knee extension and flexion strength on a LIDO Active Isokinetic Rehabilitation System at velocities of 0 degrees, 60 degrees, 120 degrees, and 180 degrees/sec. Isometric PT (0 degrees/sec) was found to be highly correlated with isokinetic PT for both extensors and flexors at all velocities (r = .88-.93, p less than .01). PT declined significantly as angular velocity increased for both extension (p less than .01) and flexion (p less than .05). A relationship between isometric and isokinetic JAPT was significant for extension only at 60 degrees and 180 degrees/sec (r = .48-.52, p less than .05). These results indicate that knee isometric and isokinetic PT as measured on the LIDO Active system are highly related for both extension and flexion. PT declined as isokinetic angular velocity increased. Isometric and isokinetic JAPT are significantly related only for extension.

Plasma volume shifts with immersion at rest and two exercise intensities.

Eight men were studied to determine the effect of cycling exercise on plasma volume (PV) during water immersion to the xiphoid process (WIX). In all protocols the subjects were seated upright. After 30 min of rest, subjects were immersed in 34.5 degrees C water and seated on a cycling ergometer. During three 1 h WIX protocols, subjects either remained at rest (No Ex) or pedaled from minutes 20 to 30 at 38% (Ex1) or 62% (Ex2) of peak oxygen consumption (VO2peak). Hematocrit (Hct) and hemoglobin concentration [( Hb]) from venous blood samples were compared pre-WIX and at minutes 20, 30, 40, and 60. Percent change in PV (delta PV) was calculated from pre-WIX Hct and [Hb] within each protocol. Hct and [Hb] decreased after 20 min of resting WIX (P less than 0.017). In the No Ex protocol, there were no further significant changes in these variables, with delta PV values of +10.4% at minute 20 and at a peak of +13.5% at minute 40. In Ex1 and Ex2, cycling increased Hct and [Hb] (P less than 0.01, minute 30 vs No Ex), with delta PV values at minute 30 of +3.7% and -0.9%, respectively, vs +12.8% in No Ex. Minute 60 values between protocols were not significantly different (mean delta PV of +10.8 +/- 0.6% SD). The hemodilution associated with WIX was either partially or completely attenuated by cycling exercise; the degree of hemoconcentration was related to exercise intensity. The exercise-induced hemoconcentration was reversed by 30 min of resting WIX. Exercise during WIX appears to cause similar decreases in PV, as does exercise in air provided that postural hemoconcentration prior to exercise is not already maximal.

Exercise-training protocols for astronauts in microgravity.

The question of the composition of exercise protocols for use by astronauts in microgravity is unresolved. Based on our knowledge of physical working requirements for astronauts during intra- and extravehicular activity and on the findings from bed-rest studies that utilized exercise training as a countermeasure for the reduction of aerobic power, deterioration of muscular strength and endurance, decrements in mood and cognitive performance, and possibly for bone loss, two exercise protocols are proposed. One assumes that, during microgravity, astronaut exercise physiological functions should be maintained at 100% of ground-based levels; the other assumes that maximal aerobic power in flight can be reduced by 10% of the ground-based level. A recommended prescription for in-flight prevention or partial suppression of calcium (bone) loss cannot be written until further research findings are obtained that elucidate the site, the magnitude, and the mechanism of the changes. Hopefully these proposed exercise prescriptions will stimulate further research and discussion resulting in even more efficient protocols that will help ensure the optimal health and well-being of our astronauts.

Work capacity during 30 days of bed rest with isotonic and isokinetic exercise training.

The purpose was to test the hypothesis that twice daily, short-term, variable intensity isotonic and intermittent high-intensity isokinetic leg exercise would maintain peak O2 uptake (VO2) and muscular strength and endurance, respectively, at or near ambulatory control levels during 30 days of -6 degrees head-down bed rest (BR) deconditioning. Nineteen men (aged 32-42 yr) were divided into no exercise control (peak VO2 once/wk, n = 5), isokinetic (Lido ergometer, n = 7), and isotonic (Quinton ergometer, n = 7) groups. Exercise training was conducted in the supine position for two 30-min periods/day for 5 days/wk. Isotonic training was at 60-90% of peak VO2, and isokinetic training (knee flexion-extension) was at 100 degrees/s. Mean (+/- SE) changes (P less than 0.05) in peak VO2 (ml.m-1.kg-1) from ambulatory control to BR day 28 were 44 +/- 4 to 36 +/- 3, -18.2% (3.27-2.60 l/m) for no exercise, 39 +/- 4 to 40 +/- 3, +2.6% (3.13-3.14 l/min) for isotonic, and 44 +/- 3 to 40 +/- 2, -9.1% (3.24-2.90 l/min) for isokinetic. There were no significant changes in any groups in leg peak torque (right knee flexion or extension), leg mean total work, arm total peak torque, or arm mean total work. Mean energy costs for the isotonic and isokinetic exercise training were 446 kcal/h (18.8 +/- 1.6 ml.min-1.kg-1) and 214 kcal/h (8.9 +/- 0.5 ml.m-1.kg-1), respectively. Thus near-peak, variable intensity, isotonic leg exercise maintains peak VO2 during 30 days of BR, while this peak, intermittent, isokinetic leg exercise protocol does not.

Exercise reverses depressed metabolic rate produced by severe caloric restriction.

The effects of caloric restriction and exercise on resting metabolic rate (RMR) were studied in five obese humans. Subjects consumed a 500 kcal.d-1 diet for 4 wk, with the subjects remaining sedentary during the first 2 wk and then exercising 30 min daily at 60% VO2max during the last 2 wk of caloric restriction. After 2 wk of dieting, RMR decreased to approximately 87% of the pre-dieting control value. Over the last 2 wk of dieting with the addition of daily exercise, the fall in RMR was reversed as it returned to the pre-dieting level. In summary, daily exercise reversed the drop in RMR associated with severe caloric restriction.

The effectiveness of specific weight training regimens on simulated aerial combat maneuvering G tolerance.

To assess the effectiveness of muscle-strength (weight training) on simulated aerial combat maneuvering (SACM) G tolerance, seven young men were exposed to a 12-week program of whole-body weight training in which were measured, strengths of various muscle groups, body circumferences, body mass, and the percentage of body fat. The magnitudes of the weights used in training were used to measure muscle strength and were compared and correlated with each subject's SACM tolerance--defined as the total time that a subject could withstand continuous exposure to a 4.5 and 7.0 + Gz centrifuge profile using fatigue as his voluntary endpoint. Chest and biceps circumferences increased 4.2% and 3.1%, respectively; abdomen and thigh circumferences did not significantly change; body fat decreased 16.8%; and body mass increased 2.3%. Abdominal (sit ups) and biceps (arm curl) strengths increased 99% and 26.2%, respectively, and were highly correlated with SACM tolerance time (p less than 0.01); leg (leg press) and chest strengths (bench press) made less significant contributions to the SACM tolerance time. A net increase in SACM tolerance times of 53% resulted from weight-training. Multiple regression analysis of all four muscle groups between weeks 1 and 12 with the SACM tolerance had a correlation of determination of 0.61.

The influence of differential physical conditioning regimens on simulated aerial combat maneuvering tolerance.

The influence of physical conditioning on tolerance to a centrifugation profile called the Simulated Aerial Combat Maneuvering (SACM)--was determined using 24 young men as subjects. These subjects were assigned to groups as controls (no physical training, C), runners (R), and weight trainers (W). They followed a 12-week protocol of specified physical training. During this study, tolerance to the SACM, maximum oxygen consumption, muscle strength, and body composition were periodically determined. SACM tolerance was defined as the total time that a subject could withstand continuous exposure to a 4.5 and 7.0 +Gx centrifugation profile as determined by his voluntary endpoint of fatigue. The +Gx tolerance of the runners and controls increased at an average rate of 4 s/week during the course of the experiment. On the other hand, the weight trainers increased their G tolerance at an average rate of 15 s/week. The difference between group W compared with groups C and R was statistically significant at the 5% level. Fatigue scores indicate that group W subjects take longer to reach a given level of fatigue than did the subjects of the other groups. It appears therefore that a physical conditioning program of weight training will improve human tolerance to aerial combat maneuvers.

Plasma volume, osmolality, vasopressin, and renin activity during graded exercise in man.

Plasma volume (PV), renin activity (PRA), and osmotic (Osm), sodium (Na+), and arginine vasopressin (AVP) concentrations were measured in venous blood samples taken before and after three levels of cycle ergometer exercise (100, 175, and 225 W) in 15 young male volunteers. Plasma volume and solute concentrations changed significantly (P less than 0.05, denoted by *) with work intensity. The % delta PV was -3.7%* at 100 W, -8.8%* at 175 W, and -12.4%* at 225 W. Plasma Na+ concentration, Osm, and AVP increase were curvilinear with graded exercise and were significant only when work intensity exceeded 40% VO2max. PRA increase was linear and significant at all work levels. The % delta PV was significantly correlated with delta Osm (r = 0.99*) and delta Na+ (r = 0.89*) but had low correlations with delta AVP (r = 0.22, NS) and delta PRA (r = 0.12, NS). However, delta AVP was significantly correlated with delta Na+ (r = 0.86*) and delta Osm (r = 0.83*), whereas delta PRA had low correlations with delta Na+ (r = 0.33, NS), delta Osm (r = 0.29, NS), and delta AVP (r = 0.43, NS). The data support the hypothesis that a) with exercise, AVP release is a primary factor for fluid and electrolyte regulation as it is highly correlated with the plasma hyperosmolality produced by a net hypotonic plasma efflux; b) an exercise intensity greater than 40% VO2max is required to change plasma osmolality and, thus, stimulate significant AVP release; and c) the stimulation of the renin-angiotensin system is a more general stress response, which responds to increasing sympathetic nervous activity.

Role of thermal and exercise factors in the mechanism of hypervolemia.

Our purpose was to determine whether the chronic increase in plasma volume (PV), resulting from heat exposure (HE) and exercise training (ET), was due only to elevated rectal temperature (Tre) or whether there were additional nonthermal factors related to the exercise. Eight men were divided into two groups. The HE group sat for 2 h/day (Tdb = 42 degrees C, 93% rh) for 8 consecutive days; Tre was raised by 1.72 +/- 0.04 degrees C to 38.5 degrees C each day. The ET group rode a bicycle ergometer for 2 h/day for 8 days (Tdb = 25 degrees C, 60% rh) at a load (60-65 Vo2max) that gave the same area under their Tre curve. PV increased by 177 ml (4.9%, P less than 0.05) in the HE group and by 427 ml (12.0%, P less than 0.05) in the ET group. This exercise-induced hypervolemia was associated with thermal factor(s) that contributed 40% and nonthermal factors that accounted for the remaining 60%. Some nonthermal, exercise-induced factors were twofold greater increases in plasma osmotic and vasopressin levels during exercise, and a fivefold increase in resting plasma protein (albumin) content.