Effects of LBNP + specific physical exercises on leg venous hemodynamics during a 28-days -6 degrees head-down bedrest

The mechanisms underlying increased venous distensibility during exposure to microgravity are not well known yet. However, there seems to be evidence indicating that skeletal muscle changes resulting from exposure to microgravity play a very important role. The purpose of this experiment was to test the hypothesis that leg muscles could play an important role in the changes of leg venous distensibility observed in simulated microgravity. Twelve subjects were submitted for 28 days to a -6 degrees head-down bedrest. Changes in leg vein hemodynamics (filling and emptying) have been measured by mercury strain gauge plethysmography with venous occlusion. Six of these subjects trained their lower limbs with isometric and isokinetic exercises during bedrest (group CM), while the other 6 subjects (control group, C) had no training.     

Neuromuscular adaptations during 30 days of cast-immobilization and head-down bedrest

Prolonged skeletal muscle disuse, during space flights and on Earth, produces distinct adaptive changes in the neuromuscular system of human subjects. There is a significant decline in muscle mass and strength, exercise capacity, fatigue resistance, integrated EMG (IEMG) output and time-dependent alterations in the behavior of Hoffman (H) and deep tendon reflexes. The objective of this study was to examine the changes in excitability of segmental motoneuronal network and its influence upon gastrocnemius-soleus (G-S) function in healthy male and female subjects, who underwent either 6 degrees head-down bedrest (HDB) or unilateral cast-immobilization (CIM) for a period of 30 days.     

Venous distensibility and venous emptying in the legs during a 42-day -6 degrees head-down bedrest

Exposure to actual or simulated microgravity is known to result in changes in lower limb venous compliance or distensibility which may play a role in post-bedrest or postflight orthostatic intolerance. Venous deconditioning has only been described in terms of changes in vascular compliance or distensibility. But a complete understanding of changes in venous hemodynamics and cardiovascular regulation occurring under these conditions has to take into account changes in emptying capacities of the veins which influence venous return, cardiac filling, and cardiac output regulation. Moreover, few data are available about the course of changes in venous hemodynamics for periods of simulated microgravity longer than 4 weeks. The purpose of this investigation was to measure parameters of venous compliance and venous emptying before, during, and after a 42-day period of bedrest at -6 degrees head-down tilt for a better understanding of long term venous physiological adaptation to microgravity.     

Effect of leg exercise training on vascular volumes during 30 days of 6 degrees head-down bed rest.

Plasma and red cell volumes, body density, and water balance were measured in 19 men (32-42 yr) confined to bed rest (BR). One group (n = 5) had no exercise training (NOE), another near-maximal variable-intensity isotonic exercise for 60 min/day (ITE; n = 7), and the third near-maximal intermittent isokinetic exercise for 60 min/day (IKE; n = 7). Caloric intake was 2,678-2,840 kcal/day; mean body weight (n = 19) decreased by 0.58 +/- 0.35 (SE) kg during BR due to a negative fluid balance (diuresis) on day 1. Mean energy costs for the NOE, and IKE, and ITE regimens were 83 (3.6 +/- 0.2 ml O2.min-1.kg-1), 214 (8.9 +/- 0.5 ml.min-1.kg-1), and 446 kcal/h (18.8 +/- 1.6 ml.min-1.kg-1), respectively. Body densities within groups and mean urine volumes (1,752-1,846 ml/day) between groups were unchanged during BR. Resting changes in plasma volume (ml/kg) after BR were -1.5 +/- 2.3% (NS) in ITE, -14.7 +/- 2.8% (P less than 0.05) in NOE, and -16.8 +/- 2.9% (P less than 0.05) in IKE, and mean water balances during BR were +295, -106, and +169 ml/24 h, respectively. Changes in red cell volume followed changes in plasma volume. The significant chronic decreases in plasma volume in the IKE and NOE groups and its maintenance in the ITE group could not be accounted for by water balance or by responses of the plasma osmotic, protein, vasopressin, or aldosterone concentrations or plasma renin activity. There was close coupling between resting plasma volume and plasma protein and osmotic content.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparing cardiovascular responses during exercise between head-down tilt pedaling with lower body negative pressure and upright cycling in man

If lower body negative pressure (LBNP) loaded on exercise in weightlessness environment is able to derive a comparable cardiovascular responses to these in the ground, it should be identified as an optimal LBNP for exercise in space. To investigate the LBNP, 7 young subjects were exercised 4 work rates stepping up every 50 watts from 50 watts to 200 watts every 5 minutes in the upright position or 6 degree head down tilt position with each LBNP of 20, 40, 60, 80, and 100 mmHg. Oxygen uptake during tilt exercise with over 60 mmHg LBNP was not different from it in upright exercise. Heart rate and systolic arterial pressure responses to exercise were very similar between tilt exercise with 60 mmHg LBNP and upright exercise. In conclusion, the optimal LBNP loaded on exercise in space should be around 60 mmHg.     

Cardiac responses to lower body negative pressure and dynamic leg exercise

Cardiac responses to dynamic leg exercise at 0, 50, and 100 W in the supine position were investigated with and without the lower portion of the body exposed to a pressure of -6.6 kPa (Lower Body Negative Pressure, LBNP). Resting values for heart rate (HR) and stroke volume (SV) were considerably higher and lower, respectively, during LBNP than in the control condition. At the transition from rest to the mildest exercise during LBNP SV showed a prompt increase by about 40%, but no significant change in the control condition. HR, which increased by 17 beats X min-1 in the control condition, showed during LBNP no change initially and subsequently a small but significant drop below its resting value. Steady-state values for HR at the various levels of exercise were not significantly affected by LBNP, whereas corresponding values for SV were considerably lowered, so that exercise values for cardiac output were about 3 l X min-1 less during LBNP than in the control condition. The reductions in SV and cardiac output indicate residual pooling of blood in intra- and extramuscular capacitance vessels of the legs. With a change from rest to exercise at 100 W during LBNP mean systolic ejection rate (MSER) increased by 67%, the relations between SV and MSER suggesting that ventricular performance was maintained by a combination of the Frank-Starling mechanism and enhanced contractile strength.     

Sympathetic nerve activity in arm and leg muscles during lower body negative pressure in humans

Nonhypotensive lower body negative pressure (LBNP) is reported to decrease forearm but not calf blood flow as measured by strain-gauge plethysmography. This suggests that unloading of cardiopulmonary receptors increases sympathetic outflow to arm but not to leg. To test this hypothesis we measured muscle sympathetic nerve activity (MSA) in the arm (radial nerve) and leg (peroneal nerve) simultaneously during LBNP. In eight healthy subjects, we measured heart rate, blood pressure, and radial and peroneal MSA during LBNP at 10 and 20 mmHg. There was no difference between radial and peroneal MSA at rest, and there were successive parallel increases of MSA in both nerves during LBNP at 10 and 20 mmHg. These data indicate that there are nearly identical increases of sympathetic outflow to the arm and leg during mild to moderate degrees of orthostatic stress.     

Effect of lower body negative pressure on orthostatic tolerance and cardiac function during 21 days head-down tilt bed rest

The purpose of the present study was to investigate the changes of orthostatic tolerance and cardiac function during 21 d head-down tilt (HDT) bed rest and effect of lower body negative pressure in the first and the last week in humans. Twelve healthy male volunteers were exposed to -6 degrees HDT bed rest for 21 d. Six subjects received -30 mmHg LBNP sessions for 1 h per day from the 1st to the 7th day and from the 15th to the 21st day of the HDT, and six others served as control. Orthostatic tolerance was assessed by means of standard tilt test. Stroke volume (SV), cardiac output (CO), preejection period (PEP) and left ventricular ejection time (LVET) were measured before and during HDT. Before HDT, all the subjects in the two groups completed the tilt tests. After 10 d and 21 d of HDT, all the subjects of the control group and one subject of the LBNP group could not complete the tilt test due to presyncopal or syncopal symptoms. The mean upright time in the control group (15.0 +/- 3.2 min) was significantly shorter than those in the LBNP group (19.7 +/- 0.9 min). SV and CO decreased significantly in the control group on days 3 and 10 of HDT, but remained unchanged throughout HDT in the LBNP group. A significant increase in PEP/LVET was observed on days 3 and 14 of HDT in both groups. The PEP/LVET in the LBNP group was significantly lower on day 3 of HDT, while LVET in the LBNP group was significantly higher on days 3, 7 and 14 of HDT than those in the control group. The results of this study suggest that brief daily LBNP sessions used in the first and the last weeks of 21 d HDT bed rest were effective in diminished the effect of head-down tilt on orthostatic tolerance, and LBNP might partially improve cardiac pumping function and cardiac systole function.     

间断下体负压暴露方式对下体负压耐力的影响

目的：探讨不同方式反复下体负压锻炼对下体负压耐力的影响,以期筛选最佳的负压锻炼方式。方法：27名男性健康受试者随机分成3组,分别进行－5.33kPa8min(A组）、6.67kPa4min（B组）、6.67kPa8min（C组）的下体负压锻炼后累积应激指数（CSI）、总耐受时间（DNP）较锻炼前显著提高,A、B组上述指标无显著变化,下体负压暴露时的心率较平静状态显著升高,收缩压显著降低,舒张压无显著变化。结论：经过－6.67kPa/d8min连续8d的间断下体负压可以显著提高下体负压耐力。     

Forearm skin and muscle vasoconstriction during lower body negative pressure

In view of conflicting reports of skeletal muscle and skin blood flow participation in baroreceptor-mediated reflexes, we studied the effects of graded lower body negative pressure (LBNP) on cutaneous and muscular components of forearm blood flow (FBF) in seven male subjects at 28 degrees C. FBF was measured by venous occlusion plethysmography and cutaneous flow by laser-Doppler velocimetry, the difference being the muscular flow. Mean FBF decreased by 39 and 56% from control at LBNP of 20 and 50 Torr, respectively. Skin flow decreased linearly with graded LBNP contributing 32% of the decrease of total blood flow at 20 Torr and then 50% of total decrease of blood flow at 50 Torr. Conversely, the decrease in muscle flow represented 68% of the total decrease at LBNP of 20 Torr and then 50% of the total decrease at LBNP of 50 Torr. We concluded that both skin and muscle circulations participate in sustained peripheral vasoconstriction during LBNP, with muscle flow achieving near maximum vasoconstriction by 20 Torr and skin showing a graded vasoconstriction to decreases in LBNP.     

Construction of a lower body negative pressure chamber

Lower body negative pressure (LBNP) is an established and important technique used to physiologically stress the human body, particularly the cardiovascular system. LBNP is most often used to simulate gravitational stress, but it has also been used to simulate hemorrhage, alter preload, and manipulate baroreceptors. During experimentation, the consequences of LBNP and the reflex increases in heart rate and blood pressure can be manipulated and observed in a well-controlled manner, thus making LBNP an important research tool. Numerous laboratories have developed LBNP devices for use in research settings, and a few devices are commercially available. However, it is often difficult for new users to find adequately described design plans. Furthermore, many available plans require sophisticated and expensive materials and/or technical support. Therefore, we have created an affordable design plan for a LBNP chamber. The purpose of this article was to share our design template with others. In particular, we hope that this information will be of use in academic and research settings. Our pressure chamber has been stress tested to 100 mmHg below atmospheric pressure and has been used successfully to test orthostatic tolerance and physiological responses to -50 mmHg.     

Leg blood flow during slow head-down tilt with and without leg venous congestion

The effects of slow changes in body position on leg blood flow (LBF) were studied in nine healthy male subjects. Using a tilt table, sitting volunteers were tilted about 60° backwards to a supine position within 40 s. To modify the venous filling in the legs, the tilt manoeuvre was repeated with congestion of the leg veins induced by two thigh cuffs inflated to a subdiastolic pressure of 60 mmHg. Doppler measurements in the femoral artery were used to estimate LBF. Additional Doppler measurements at the aortic root in five of the subjects were taken for the determination of cardiac output. The LBF was influenced by body position. In the control experiment it increased from 500 ml · min⁻¹ in the upright to 780 ml · min^–1 after 15 min in the supine position. A mean maximal value of 950 ml · min⁻¹ was observed 20 s after the tilt. Heart rate remained almost constant during the tilt phase, whereas stroke volume increased from 90 ml to 120 ml and it remained at that level after the cessation of the tilt. Congestion of the leg veins had no significant effect on heart rate, stroke volume and mean blood pressure. However, it increased vascular resistance of the leg during and after the tilt. After 15 min in the tilted position LBF amounted to 600 ml · min⁻¹. The results suggest that the filling of the leg veins is inversely related to leg blood flow. The most likely mechanism underlying this observation is a local effect of venous filling on vasomotor tone. Accepted: 20 May 1998     

Effects of lower body negative pressure on sympathetic discharge to leg muscles in humans

Recent studies indicate that nonhypotensive orthostatic stress in humans causes reflex vasoconstriction in the forearm but not in the calf. We used microelectrode recordings of muscle sympathetic nerve activity (MSNA) from the peroneal nerve in conscious humans to determine if unloading of cardiac baroreceptors during nonhypotensive lower body negative pressure (LBNP) increases sympathetic discharge to the leg muscles. LBNP from -5 to -15 mmHg had no effect on arterial pressure or heart rate but caused graded decreases in central venous pressure and corresponding large increases in peroneal MSNA. Total MSNA (burst frequency X mean burst amplitude) increased by 61 +/- 22% (P less than 0.05 vs. control) during LBNP at only -5 mmHg and rose progressively to a value that was 149 +/- 29% greater than control during LBNP at -15 mmHg (P less than 0.05). The major new conclusion is that nonhypotensive LBNP is a potent stimulus to muscle sympathetic outflow in the leg as well as the arm. During orthostatic stress in humans, the cardiac baroreflex appears to trigger a mass sympathetic discharge to the skeletal muscles in all of the extremities.     

Effects of leg venous hemodynamics on cardiovascular responses to lower body negative pressure and aging

In aged people, decreases in stroke volume and cardiac output during orthostatic challenge are less. It is suggested that the stiffness of blood vessels is greater in the elderly, blunting leg venous pooling and drop in central blood volume in an upright position. Leg venous hemodynamics plays an important role in human cardiovascular homeostasis against gravitational stress. This study aimed to clarify how aging influences the leg venous hemodynamics and its contribution to cardiovascular homeostasis during lower body negative pressure (LBNP) in humans.     

Effect of long-term bedrest on lower leg muscle activation patterns during quiet standing

It has been well known that balance instabilities after long-term exposure to microgravity (e.g., Anderson et al. 1986) or bedrest (BR) can be related to alterations and/or adaptations to postural control strategies. Little is known, however, how the reduced muscular activity affects the activation pattern of the lower limb muscles during quiet standing (QS). The purpose of this study was to investigate whether or not any changes in the lower limb muscle activation patterns during QS would occur after BR.