Postural specificity of cardiovascular adaptations to exercise training

The purposes of this study were to determine 1) whether posture affects the magnitude of cardiovascular adaptations to training and 2) whether cardiovascular adaptations resulting from exercise training in the supine posture transfer (generalize) to exercise in the upright posture and vice versa. Sixteen sedentary men, aged 18-33 yr, were trained using high-intensity interval and prolonged continuous cycling in the supine (STG; supine training group) or upright (UTG; upright training group) posture 4 days/wk, 40 min/day, for 8 wk, while seven male subjects served as nontraining controls. After training, maximal O2 uptake measured during supine and upright cycling, respectively, increased significantly (P less than 0.05) by 22.9 and 16.1% in the STG and by 6.0 and 14.6% in the UTG. No significant cardiovascular adaptations were observed at rest. During submaximal supine cycling at 100 W, significant increases in end-diastolic volume (21%) and stroke volume (22%) (radionuclide ventriculography and CO2 rebreathing) and decreases in heart rate, blood pressure, and systemic vascular resistance occurred in the STG, whereas only a significant decrease in blood pressure occurred in the UTG. During upright cycling at 100 W, a significant decrease in blood pressure occurred in the STG, whereas significant increases in end-diastolic volume (17%) and stroke volume (18%) and decreases in blood pressure and systemic vascular resistance occurred in the UTG. Volume of myocardial contractility, ejection fraction, and systolic blood pressure-to-end-systolic volume ratio did not change significantly after training when measured during supine and upright cycling in either training group. Blood volume increased significantly in the UTG but remained unchanged in the STG.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma atrial natriuretic peptide during brief upright and supine exercise in humans

The factors associated with the exercise-induced increase in plasma atrial natriuretic peptide (ANP) have not been clearly established. Thus the purpose of the study was to further document the stimulus for the exercise-induced release of ANP and to examine the role of ANP in the control of hydromineral balance during exercise. Eight healthy male volunteers (25.1 +/- 4.5 yr) were submitted to a graded cycling exercise in both the upright and supine positions. Venous blood was sampled at rest and at the end of each 5-min work load at 40, 60, and 80% maximal oxygen uptake (Vo2max), at maximal exercise, and during recovery through an indwelling catheter for the determination of plasma vasopressin, aldosterone, catecholamines, plasma renin activity, and ANP concentrations. Results indicate a significant increase in ANP (pg/ml) from rest to maximal exercise in the upright position [rest, 21.9 +/- 10.2; 40%, 24.7 +/- 12.6; 60%, 32.4 +/- 17*; 80%, 47.8 +/- 27.7*; 100% Vo2max, 65.9 +/- 34.5* (*P less than or equal to 0.05)]. Supine concentrations were significantly higher than upright at 40 (37.9 +/- 15.2), 60 (54.0 +/- 18.8), and 80% Vo2max (68.9 +/- 16.6). Plasma ANP during maximal exercise was similar in both positions. Plasma vasopressin, aldosterone, renin activity, and catecholamines increased with increasing exercise intensity in both positions, although lower values were systematically observed in the supine position. The association of higher plasma ANP and blunted plasma vasopressin, plasma renin activity, and norepinephrine concentrations during supine exercise suggests that ANP may exert modulatory effects on the control of the hydromineral hormonal system during exercise.     

Effect of prostaglandin inhibition by indomethacin on plasma active and inactive renin concentration in men.

The effect of inhibition of prostaglandin synthesis by indomethacin on active renin and on acid-activable inactive renin was studied in nine healthy, sodium-replete men, both at rest and exercise. These volunteers were investigated after pretreatment with placebo or indomethacin, 150 mg daily for 3 days. Indomethacin induced a decrease in active (p = 0.004), total (p less than 0.001), and inactive (p = 0.02) renin at rest recumbent on average by 42, 19, and 8%, respectively, and at rest sitting on average by 45, 15, and 3%, respectively. Inhibition of prostaglandins with indomethacin reduced (p less than 0.001) active and total renin at each level of work load but not (p = 0.32) inactive renin. However, the exercise-induced stimulation (p less than 0.05) of active and total renin still occur during indomethacin. Indomethacin reduced (p less than 0.001) at rest sitting and at maximal exercise the plasma concentrations of immunoreactive prostaglandins E2 by 50 and 54%, respectively, prostaglandin F2 alpha by 36 and 39%, respectively, and 13,14-dihydro-15-keto-prostaglandin F alpha by 38 and 60%, respectively. The urinary excretion of immunoreactive prostaglandin E2 and F2 alpha was also reduced.     

Interactive effects of body posture and exercise training on maximal oxygen uptake

To determine the effect of posture on maximal O2 uptake (VO2 max) and other cardiorespiratory adaptations to exercise training, 16 male subjects were trained using high-intensity interval and prolonged continuous cycling in either the supine or upright posture 40 min/day 4 days/wk for 8 wk and 7 male subjects served as non-training controls. VO2 max measured during upright cycling and supine cycling, respectively, increased significantly (P less than 0.05) by 16.1 +/- 3.4 and 22.9 +/- 3.4% in the supine training group (STG) and by 14.6 +/- 2.0 and 6.0 +/- 2.0% in the upright training group (UTG). The increase in VO2 max measured during supine cycling was significantly greater (P less than 0.05) in the STG than in the UTG. The increase in VO2 max in the UTG was significantly greater (P less than 0.05) when measured during upright exercise than during supine exercise. However, there was no significant difference in posture-specific VO2 max adaptations in the STG. A postural specificity was also evident in other maximal cardiorespiratory variables (ventilation, CO2 production, and respiratory exchange ratio). In the UTG, maximal heart rate decreased significantly (P less than 0.05) only during supine cycling; there was no significant difference in maximal heart rate after training in the STG. We conclude that posture affects maximal cardiorespiratory adaptations to cycle training. Additionally, supine training is more effective than upright training in increasing maximal cardiorespiratory responses measured during supine exercise, and the effects of supine training generalize to the upright posture to a greater extent than the effects of upright training generalize to the supine posture.(ABSTRACT TRUNCATED AT 250 WORDS)     

Maximal vascular leg conductance in trained and untrained men

Lower leg blood flow and vascular conductance were studied and related to maximal oxygen uptake in 15 sedentary men (28.5 +/- 1.2 yr, mean +/- SE) and 11 endurance-trained men (30.5 +/- 2.0 yr). Blood flows were obtained at rest and during reactive hyperemia produced by ischemic exercise to fatigue. Vascular conductance was computed from blood flow measured by venous occlusion plethysmography, and mean arterial blood pressure was determined by auscultation of the brachial artery. Resting blood flow and mean arterial pressure were similar in both groups (combined mean, 3.0 ml X min-1 X 100 ml-1 and 88.2 mmHg). After ischemic exercise, blood flows were 29- and 19-fold higher (P less than 0.001) than rest in trained (83.3 +/- 3.8 ml X min-1 X 100 ml-1) and sedentary subjects (61.5 +/- 2.3 ml X min-1 X 100 ml-1), respectively. Blood pressure and heart rate were only slightly elevated in both groups. Maximal vascular conductance was significantly higher (P less than 0.001) in the trained compared with the sedentary subjects. The correlation coefficients for maximal oxygen uptake vs. vascular conductance were 0.81 (trained) and 0.45 (sedentary). These data suggest that physical training increases the capacity for vasodilation in active limbs and also enables the trained individual to utilize a larger fraction of maximal vascular conductance than the sedentary subject.     

Reduced stroke volume during exercise in postural tachycardia syndrome.

Postural tachycardia syndrome (POTS) is characterized by excessive tachycardia without hypotension during orthostasis. Most POTS patients also report exercise intolerance. To assess cardiovascular regulation during exercise in POTS, patients (n = 13) and healthy controls (n = 10) performed graded cycle exercise at 25, 50, and 75 W in both supine and upright positions while arterial pressure (arterial catheter), heart rate (HR; measured by ECG), and cardiac output (open-circuit acetylene breathing) were measured. In both positions, mean arterial pressure, cardiac output, and total peripheral resistance at rest and during exercise were similar in patients and controls (P > 0.05). However, supine stroke volume (SV) tended to be lower in the patients than controls at rest (99 +/- 5 vs. 110 +/- 9 ml) and during 75-W exercise (97 +/- 5 vs. 111 +/- 7 ml) (P = 0.07), and HR was higher in the patients than controls at rest (76 +/- 3 vs. 62 +/- 4 beats/min) and during 75-W exercise (127 +/- 3 vs. 114 +/- 5 beats/min) (both P < 0.01). Upright SV was significantly lower in the patients than controls at rest (57 +/- 3 vs. 81 +/- 6 ml) and during 75-W exercise (70 +/- 4 vs. 94 +/- 6 ml) (both P < 0.01), and HR was much higher in the patients than controls at rest (103 +/- 3 vs. 81 +/- 4 beats/min) and during 75-W exercise (164 +/- 3 vs. 131 +/- 7 beats/min) (both P < 0.001). The change (upright - supine) in SV was inversely correlated with the change in HR for all participants at rest (R(2) = 0.32), at 25 W (R(2) = 0.49), 50 W (R(2) = 0.60), and 75 W (R(2) = 0.32) (P < 0.01). These results suggest that greater elevation in HR in POTS patients during exercise, especially while upright, was secondary to reduced SV and associated with exercise intolerance.     

Kinetics of ventilation and gas exchange during supine and upright cycle exercise.

The dynamics of ventilation (VE), oxygen uptake (VO2), carbon dioxide output (VCO2), and heart rate (fc) were studied in 12 healthy young men during upright and supine exercise. Responses to maximal and to two different types of submaximal exercise tests were contrasted. During incremental exercise to exhaustion, the maximal work rate, VO2max, VEmax, fc,max, and ventilatory threshold were all significantly reduced in supine compared to upright exercise (P less than 0.01-0.001). Following step increases or decreases in work rate between 25 W and 105 W, both VO2 and VCO2 responded more slowly in supine than upright exercise. Dynamics were also studied in two different pseudorandom binary-sequence (PRBS) exercise tests, with the work rate varying between 25 W and 105 W with either 5-s or 30-s durations of each PRBS unit. In both of these tests, there were no differences caused by body position in the amplitude or phase shifts obtained from Fourier analysis for any observed variable. These data show that the body position alters the dynamic response to the more traditional step increase in work rate, but not during PRBS exercise. It is speculated that the elevation of cardiac output observed with supine exercise in combination with the continuously varying work-rate pattern of the PRBS exercise allowed adequate, perhaps near steady-state, perfusion of the working muscles in these tests, whereas at the onset of a step increase in work rate, greater demands were placed on the mechanisms of blood flow redistribution.     

Relative effects of the supine posture and of immersion on the renin aldosterone system at rest and during exercise

The relative influences of the supine posture and of immersion on the renin-aldosterone system (RAS) were studied at rest and during moderate exercise in five healthy men. When supine, resting or immersion to the neck for 20 min in a thermoneutral environment both induced a decrease in plasma renin activity (PRA) when compared with the levels measured after 15 min sitting at rest (resting: -44%, p less than 0.05. Immersion: -45%, p less than 0.05). There was no significant difference in PRA decrease between the two situations. Aldosterone (ALDO) values were lower after supine rest or immersion than those observed after sitting at rest, but the difference was not significant. Two types of exercise at a constant relative work load (40-50% maximal oxygen uptake), namely cycling on an ergocycle in the supine position and free-style swimming, induced increases in PRA and ALDO when compared with the levels measured after 15 min rest when sitting (respectively, PRA = +35%, p less than 0.05, and +45%, p less than 0.05, ALDO = +32%, p less than 0.01 and +35%, p less than 0.05). Increases in PRA and ALDO did not differ between the two exercises. Thus inhibitory effects on RAS of change in external pressure are negligible during water immersion to the neck in the supine position and during swimming at moderate intensity.     

Effect of calcium channel blockade and beta-adrenoceptor blockade on short graded and single-level endurance exercises in normal men

The effect of verapamil (240 mg) on exercise capacity was studied during a short graded and a single-level endurance exercise test in 12 normal volunteers; it was compared to the effects of atenolol (100 mg x day-1). Intake of verapamil, atenolol and placebo, administered according to a randomized, double-blind cross-over design, was started 3 days before the exercise tests. Compared to placebo, verapamil did not affect peak oxygen uptake in the graded test or exercise duration in the endurance test. Heart rate, systolic blood pressure, rating of perceived exertion and respiratory data at submaximal and peak exercise were unaffected in either test. On the other hand atenolol reduced maximal oxygen uptake by 5% (p less than 0.001) and endurance exercise duration by 17% (p less than 0.05). Besides marked decreases in heart rate and systolic blood pressure during the two types of exercise, atenolol also reduced oxygen uptake at submaximal exercise levels and it increased the rating of perceived exertion (p less than 0.05), the latter only during the endurance exercise test.     

Influence of body position and pre-exercise activity on cardiac output and oxygen uptake following step changes in exercise intensity.

Parallel measurements of breath-by-breath oxygen uptake, cardiac output (Doppler technique), blood pressure (Finapres technique) and heart rate were performed in nine subjects during cycle ergometer exercise in the upright and supine positions. Transients were monitored during power steps starting from and leading to either rest or lower levels of exercise intensity. Oxygen uptake (VO2) and cardiac output kinetics were markedly faster than in all other conditions when exercise was started from rest. In contrast to exercise-exercise on steps, the computed arteriovenous difference in O2 content increased almost immediately in this situation, indicating that not only the additional energy expenditure due to the acceleration of the flywheel but also an increased venous admixture from non-exercising parts of the body contributed to the early kinetics. The off kinetics generally showed a more uniform pattern and did not simply mirror the on transients. The present findings indicate that transitions from rest should be avoided when muscle VO2 kinetics are to be assessed on the basis of VO2 measurements at the mouth.     

Cardiopulmonary baroreflex is reset during dynamic exercise.

The purpose of this study was to examine the hypothesis that the operating point of the cardiopulmonary baroreflex resets to the higher cardiac filling pressure of exercise associated with the increased cardiac filling volumes. Eight men (age 26 +/- 1 yr; height 180 +/- 3 cm; weight 86 +/- 6 kg; means +/- SE) participated in the present study. Lower body negative pressure (LBNP) was applied at 8 and 16 Torr to decrease central venous pressure (CVP) at rest and during steady-state leg cycling at 50% peak oxygen uptake (104 +/- 20 W). Subsequently, two discrete infusions of 25% human serum albumin solution were administered until CVP was increased by 1.8 +/- 0.6 and 2.4 +/- 0.4 mmHg at rest and 2.9 +/- 0.9 and 4.6 +/- 0.9 mmHg during exercise. During all protocols, heart rate, arterial blood pressure, and CVP were recorded continuously. At each stage of LBNP or albumin infusion, forearm blood flow and cardiac output were measured. During exercise, forearm vascular conductance increased from 7.5 +/- 0.5 to 8.7 +/- 0.6 U (P = 0.024) and total systemic vascular conductance from 7.2 +/- 0.2 to 13.5 +/- 0.9 l.min(-1).mmHg(-1) (P < 0.001). However, there was no significant difference in the responses of both forearm vascular conductance and total systemic vascular conductance to LBNP and the infusion of albumin between rest and exercise. These data indicate that the cardiopulmonary baroreflex had been reset during exercise to the new operating point associated with the exercise-induced change in cardiac filling volume.     

Impact of body position on central and peripheral hemodynamic contributions to movement-induced hyperemia: implications for rehabilitative medicine

This study used alterations in body position to identify differences in hemodynamic responses to passive exercise. Central and peripheral hemodynamics were noninvasively measured during 2 min of passive knee extension in 14 subjects, whereas perfusion pressure (PP) was directly measured in a subset of 6 subjects. Movement-induced increases in leg blood flow (LBF) and leg vascular conductance (LVC) were more than twofold greater in the upright compared with supine positions (LBF, supine: 462 ± 6, and upright: 1,084 ± 159 ml/min, P < 0.001; and LVC, supine: 5.3 ± 1.2, and upright: 11.8 ± 2.8 ml·min?1 ·mmHg?1, P < 0.002). The change in heart rate (HR) from baseline to peak was not different between positions (supine: 8 ± 1, and upright: 10 ± 1 beats/min, P = 0.22); however, the elevated HR was maintained for a longer duration when upright. Stroke volume contributed to the increase in cardiac output (CO) during the upright movement only. CO increased in both positions; however, the magnitude and duration of the CO response were greater in the upright position. Mean arterial pressure and PP were higher at baseline and throughout passive movement when upright. Thus exaggerated central hemodynamic responses characterized by an increase in stroke volume and a sustained HR response combined to yield a greater increase in CO during upright movement. This greater central response coupled with the increased PP and LVC explains the twofold greater and more sustained increase in movement-induced hyperemia in the upright compared with supine position and has clinical implications for rehabilitative medicine.     

Effects of autonomic blockade on cardiac function at rest and during upright exercise in humans

The cardiac function was studied by radionuclide cardiography in eight healthy subjects at rest and during submaximal upright exercise before and after autonomic blockade with metoprolol and atropine. At rest the median stroke volume was reduced by 21% during autonomic blockade (P less than 0.01), but cardiac output was maintained by a concomitant increase in heart rate. The systolic blood pressure was reduced from 120 to 105 mmHg (P less than 0.01), and left ventricular ejection fraction was reduced from 61 to 56% (P less than 0.05). After autonomic blockade the heart rate reached during exercise was the same. Stroke volume and cardiac output were maintained through cardiac dilation. The increase in left ventricular end-diastolic volume was 31 vs. 10% during control conditions (P less than 0.01). The systolic blood pressure was reduced from 174 to 135 mmHg (P less than 0.01). Left ventricular ejection fraction was reduced from 75 to 67% (P less than 0.05), but the increase from rest to exercise was preserved. Total peripheral resistance was reduced by 17% (P less than 0.05). These findings suggest that the heart possesses intrinsic mechanisms to maintain cardiac output during submaximal upright exercise. End-diastolic dilation results in a preserved stroke volume despite a reduced contractility.     

Beta-adrenoceptors and the regulation of blood pressure and plasma renin during exercise

The relative role of beta 1- and beta 2-adrenoceptors in the regulation of blood pressure and plasma renin at rest and during exercise was studied in 17 normal male volunteers. They performed, in a randomized order and according to a double-blind crossover study design, three graded and uninterrupted exercise tests until exhaustion after being pretreated during 3 consecutive days with a placebo, with a predominantly beta 1-blocker (atenolol, 50 mg once/day), or with a predominantly beta 2-blocker (ICI 118551, 20 mg 3 times/day). Both drugs caused a decrease of heart rate, but the reduction by ICI 118551 was less pronounced at rest and no additional decline occurred at exercise. ICI 118551 did not affect blood pressure at rest, but during exercise diastolic blood pressure was higher than after a placebo. Atenolol lowered systolic blood pressure at rest and suppressed the exercise-induced increase in systolic blood pressure. At rest and during exercise plasma renin activity was lowered by predominantly beta 1-blockade and unchanged during beta 2-antagonism. The exercise-induced increase in plasma renin was, however, not affected by the beta 1-blocker. After atenolol the urinary excretion of aldosterone was decreased but the plasma aldosterone concentration was not changed. ICI 118551 did not alter plasma or urinary aldosterone. Our results therefore provide further evidence that the adrenoceptors mediating the release of renin at rest and during exercise in humans are partially of the beta 1-subtype, whereas beta 2-adrenergic receptors probably play only a minor role in the control of renin secretion, especially at exercise.(ABSTRACT TRUNCATED AT 250 WORDS)     

Supine lower body negative pressure exercise during bed rest maintains upright exercise capacity.

Bed rest and spaceflight reduce exercise fitness. Supine lower body negative pressure (LBNP) treadmill exercise provides integrated cardiovascular and musculoskeletal stimulation similar to that imposed by upright exercise in Earth gravity. We hypothesized that 40 min of supine exercise per day in a LBNP chamber at 1.0-1.2 body wt (58 +/- 2 mmHg LBNP) maintains aerobic fitness and sprint speed during 15 days of 6 degrees head-down bed rest (simulated microgravity). Seven male subjects underwent two such bed-rest studies in random order: one as a control study (no exercise) and one with daily supine LBNP treadmill exercise. After controlled bed-rest, time to exhaustion during an upright treadmill exercise test decreased 10%, peak oxygen consumption during the test decreased 14%, and sprint speed decreased 16% (all P < 0.05). Supine LBNP exercise during bed rest maintained all the above variables at pre-bed-rest levels. Our findings support further evaluation of LBNP exercise as a countermeasure against long-term microgravity-induced deconditioning.     

Perceived exertion and gas exchange after calcium and beta-blockade in atrial fibrillation

Nine male patients (mean age 65 yr) with chronic atrial fibrillation underwent maximal exercise testing during placebo, beta-adrenergic (celiprolol, 600 mg), or calcium (diltiazem, 30 or 60 mg four times daily) channel blockade. The results were analyzed to determine which factors most closely related to ratings of perceived exertion (RPE) during exercise. Heart rate (HR), blood pressure (BP), oxygen uptake (VO2), minute ventilation (VE), and carbon dioxide production (VCO2) were evaluated at rest, 3.0 mph/0% grade, the gas exchange anaerobic threshold (ATge), 80% of placebo maximal O2 uptake, and maximal exercise. Both beta-adrenergic and calcium channel blockade significantly reduced heart rate and systolic blood pressure relative to placebo; these effects were more profound during beta-adrenergic blockade and as exercise progressed. Correlation coefficients and estimates of slope were derived for changes in RPE during exercise vs. changes in HR, VO2, VE, and VCO2 during the three treatments (r = 0.76 to 0.92, P less than 0.001). Although RPE was significantly correlated with HR during placebo and diltiazem therapy (r = 0.45, P less than 0.01), this was not the case during beta-adrenergic blockade (r = 0.31, NS). Slope of the regression lines between RPE and VO2, VE, and VCO2 did not differ between the three treatments. Slope of the regression lines between RPE and HR differed only during calcium channel blockade. Because the presence of atrial fibrillation and beta-adrenergic blockade altered the associations between RPE, VO2, and HR, these results suggest that VE is more closely related to RPE than the other parameters.     

Effects of 20 days horizontal bed rest on mechanical efficiency during steady state exercise at mild-moderate work intensities in young subjects

Delta efficiency(DE) at mild-moderate (31%-56% of maximum oxygen uptake) bicycle exercise in the upright sitting and in the supine position was measured in 10 young males and 5 females before and after 20 days bed rest (BR). Total muscle mass in the both legs(TMML) were measured by dual-energy x-ray absorptiometry. After BR, oxygen uptake decreased at all intensities during upright exercise and at 100 W during supine exercise (p<0.05) in the males but cardiac output (acetylene rebreathing) was almost unchanged. As defined as increase in work over the corresponding increase in energy liberation, DE significantly increased by 20.1% during upright exercise and 18.4% during supine exercise in males, but unchanged in females. TMML was decreased in both males and females. The differences in DE between males and females could not be explained in the present study. One possibility might be that the decreased DE was due to a simultaneous decrease in slow twitch muscle fiber content which might be responsible for the decreased TMML induced by BR in the males.     

Blood flow to contracting human muscles: influence of increased sympathetic activity

The purpose of this study was to examine the effects of the increased sympathetic activity elicited by the upright posture on blood flow to exercising human forearm muscles. Six subjects performed light and heavy rhythmic forearm exercise. Trials were conducted with the subjects supine and standing. Forearm blood flow (FBF, plethysmography) and skin blood flow (laser Doppler) were measured during brief pauses in the contractions. Arterial blood pressure and heart rate were also measured. During the first 6 min of light exercise, blood flow was similar in the supine and standing positions (approximately 15 ml.min-1.100 ml-1); from minutes 7 to 20 FBF was approximately 3-7 ml.min-1.100 ml-1 less in the standing position (P less than 0.05). When 5 min of heavy exercise immediately followed the light exercise, FBF was approximately 30-35 ml.min-1.100 ml-1 in the supine position. These values were approximately 8-12 ml.min-1.100 ml-1 greater than those observed in the upright position (P less than 0.05). When light exercise did not precede 8 min of heavy exercise, the blood flow at the end of minute 1 was similar in the supine and standing positions but was approximately 6-9 ml.min-1.100 ml-1 lower in the standing position during minutes 2-8. Heart rate was always approximately 10-20 beats higher in the upright position (P less than 0.05). Forearm skin blood flow and mean arterial pressure were similar in the two positions, indicating that the changes in FBF resulted from differences in the caliber of the resistance vessels in the forearm muscles.(ABSTRACT TRUNCATED AT 250 WORDS)     

Exercise stroke volume relative to plasma-volume expansion

The effects of plasma-volume (PV) expansion on stroke volume (SV) (CO2 rebreathing) during submaximal exercise were determined. Intravenous infusion of 403 +/- 21 ml of a 6% dextran solution before exercise in the upright position increased SV 11% (i.e., 130 +/- 6 to 144 +/- 5 ml; P less than 0.05) in untrained males (n = 7). Further PV expansion (i.e., 706 +/- 43 ml) did not result in a further increase in SV (i.e., 145 +/- 4 ml). SV was somewhat higher during supine compared with upright exercise when blood volume (BV) was normal (i.e., 138 +/- 8 vs. 130 +/- 6 ml; P = 0.08). PV expansion also increased SV during exercise in the supine position (i.e., 138 +/- 8 to 150 +/- 8 ml; P less than 0.05). In contrast to these observations in untrained men, PV expansion of endurance-trained men (n = 10), who were naturally PV expanded, did not increase SV during exercise in the upright or supine positions. When BV in the untrained men was increased to match that of the endurance-trained subjects, SV was observed to be 15% higher (165 +/- 7 vs. 144 +/- 5 ml; P less than 0.05), whereas mean blood pressure and total peripheral resistance were significantly lower (P less than 0.05) in the trained compared with untrained subjects during upright exercise at a similar heart rate. The present findings indicate that exercise SV in untrained men is preload dependent and that increases in exercise SV occur in response to the first 400 ml of PV expansion. It appears that approximately one-half of the difference in SV normally observed between untrained and highly endurance-trained men during upright exercise is due to a suboptimal BV in the untrained men.     

Cardiorespiratory deconditioning with static and dynamic leg exercise during bed rest.

Bed rest deconditioning was assessed in seven healthy men (19-22 yr) following three 14-day periods of controlled activity during recumbency by measuring submaximal and maximal oxygen uptake (VO2), ventilation (VE), heart rate, and plasma volume. Exercise regimens were performed in the supine position and included a) two 30-min periods daily of intermittent static exercise at 21% of maximal leg extension force, and b) two 30-min periods of dynamic bicycle ergometer exercise daily at 68% of VO2max. No prescribed exercise was performed during the third bed rest period. Compared with their respective pre-bed rest control values, VO2max decreased (P less than 0.05) under all exercise conditions; -12.3% with no exercise, -9.2% with dynamic exercise, but only -4.8% with static exercise. Maximal heart rate was increased by 3.3% to 4.9% (P less than 0.05) under the three exercise conditions, while plasma volume decreased (P less than 0.05) -15.1% with no exercise and -10.1% with static, but only -7.8% (NS) with dynamic exercise. Since neither the static nor dynamic exercise training regimes minimized the changes in all the variables studied, some combination of these two types of exercise may be necessary for maximum protection from the effects of the bed deconditioning.