Engineering topics

Hargens, Alan R.

Publications and source records attributed to Hargens, Alan R..

At least 73 records · Page 4

NASA NTRS1995

Simulated Microgravity Increases Cutaneous Blood Flow in the Head and Leg of Humans

The cutaneous microcirculation vasodilates during acute 6 degree head-down tilt (HDT, simulated microgravity) relative to upright conditions, more in the lower body than in the upper body. Cutaneous microvascular blood flow was measured with laser-Doppler flowmetry at the leg (over the distal tibia) and cheek (over the zygomatic arch) of eight healthy men before, during, and after 24 h of HDT. Results were calculated as a percentage of baseline value (100% measured during pre-tilt upright sitting). Cutaneous blood flow in the cheek increased significantly to 165 +/- 37% (mean +/- SE, p less than 0.05) at 9-12 h HDT, then returned to near baseline values by 24 h HDT (114 +/- 29%, NSD), despite increased local arterial pressure. Microvascular flow in the leg remained significantly elevated above baseline througout 24 h HDT (427 +/- 85% at 3 h HDT and 215 +/- 142% at 24 h HDT, p less than 0.05). During the 6-h upright sitting recovery period, cheek and leg blood flow levels returned to near pre-tilt baseline values. Because hydrostatic effects of HDT increase local arterial pressure at the carotid sinus, baroreflex-mediated withdrawal of sympathetic tone probably contributed to increased microvascular flows at the head and leg during HDT. In the leg baroreflex effects combined with minimal stimulation of local veno-arteriolar and myogenic autoregulatory vasoconstriction to elicit relatively larger and more sustained increases in cutaneous flow during HDT. In the cheek, delayed myogenic vasoconstriction and/or hurmonal effects apparently compensated for flow elevation by 24 h of HDT. Therefore, localized vascular adaptations to gravity probably explain differences in acclimation of lower and upper body blood flow to HDT and actual microgravity.

Stout, M. Shannon↗

NASA NTRS1995

Pharmacologic Atrial Natriuretic Peptide Reduces Human Leg Capillary Filtration

Atrial natriuretic peptide (ANP) is produced and secreted by atrial cells. We measured calf capillary filtration rate with prolonged venous-occlusion plethysmography of supine healthy male subjects during pharmacologic infusion of ANP (48 pmol/kg/min for 15 min; n = 6) and during placebo infusion (n = 7). Results during infusions were compared to prior control measurements. ANP infusion increased plasma (ANP) from 30 +/- 4 to 2,568 +/- 595 pmol/L. Systemic hemoconcentration occurred during ANP infusion: mean hematocrit and plasma colloid osmotic pressure increased 4.6 and 11.3%, respectively, relative to preinfusion baseline values (p less than 0.05). Mean calf filtration, however, was significantly reduced from 0.15 to 0.08 ml/100 ml/min with ANP. Heart rate increased 20% with ANP infusion, whereas blood pressure was unchanged. Calf conductance (blood flow/ arterial pressure) and venous compliance were unaffected by ANP infusion. Placebo infusion had no effect relative to prior baseline control measurements. Although ANP induced systemic capillary filtration, in the calf, filtration was reduced with ANP. Therefore, pharmacologic ANP infusion enhances capillary filtration from the systemic circulation, perhaps at upper body or splanchnic sites or both, while having the opposite effect in the leg.

Watenpaugh, Donald E.↗

NASA NTRS1995

Pharmacologic Atrial Natriuretic Peptide Reduces Human Leg Capillary Filtration

Atrial natriuretic peptide (ANP) is produced and secreted by atrial cells. We measured calf capillary filtration rate with prolonged venous-occlusion plethys-mography of supine health male subjects during pharmacologic infusion of ANP (48 pmol/kg/min for 15 min; n equals 6) and during placebo infusion (n equals 7). Results during infusions were compared to prior control measurements. ANP infusion increased plasma (ANP) from 30 plus or minus 4 to 2,568 plus or minus 595 pmol/L. Systemic hemoconcentration occurred during ANP infusion; mean hematocrit and plasma colloid osmotic pressure increased 4.6 and 11.3 percent respectively, relative to pre-infusion baseline values (p is less than 0.05). Mean calf filtration, however was significantly reduced from 0.15 to 0.08 ml/100 ml/min with ANP. Heart rate increased 20 percent with ANP infusion, wheras blood pressure was unchanged. Calf conductance (blood flow/arterial pressure) and venous compliance were unaffected by ANP infusion. Placebo infusion had no effect relative to prior baseline control measurements. Although ANP induced systemic capillary filtration, in the calf, filtration was reduced with ANP. Therefore, phamacologic ANP infusion enhances capillary filtration from the systemic circulation, perhaps at upper body or splanchic sites or both, while having the opposite effect in the leg.

Watenpaugh, Donald E.↗

NASA NTRS1995

Space Station Biological Research Project

NASA Ames Research Center is responsible for the development of the Space Station Biological Research Project (SSBRP) which will support non-human life sciences research on the International Space Station Alpha (ISSA). The SSBRP is designed to support both basic research to understand the effect of altered gravity fields on biological systems and applied research to investigate the effects of space flight on biological systems. The SSBRP will provide the necessary habitats to support avian and reptile eggs, cells and tissues, plants and rodents. In addition a habitat to support aquatic specimens will be provided by our international partners. Habitats will be mounted in ISSA compatible racks at u-g and will also be mounted on a 2.5 m diameter centrifuge except for the egg incubator which has an internal centrifuge. The 2.5 m centrifuge will provide artificial gravity levels over the range of 0.01 G to 2 G. The current schedule is to launch the first rack in 1999, the Life Sciences glovebox and a second rack early in 2001, a 4 habitat 2.5 in centrifuge later the same year in its own module, and to upgrade the centrifuge to 8 habitats in 2004. The rodent habitats will be derived from the Advanced Animal Habitat currently under development for the Shuttle program and will be capable of housing either rats or mice individually or in groups (6 rats/group and at least 12 mice/group). The egg incubator will be an upgraded Avian Development Facility also developed for the Shuttle program through a Small Business and Innovative Research grant. The Space Tissue Loss cell culture apparatus, developed by Walter Reed Army Institute of Research, is being considered for the cell and tissue culture habitat. The Life Sciences Glovebox is crucial to all life sciences experiments for specimen manipulation and performance of science procedures. It will provide two levels of containment between the work volume and the crew through the use of seals and negative pressure. The glovebox will accommodate use by two crew persons simultaneously and the capability for real time video down-link and data acquisition. In house testbeds and Phase B studies of the centrifuge validated the concepts of vibration isolation and autobalancing systems to meet the ISSA microgravity requirements. The vibration isolation system is effective above the centrifuge rotation frequency while the autobalancing system on the rotor removes vibration at and below the rotation rate. Torque of the Station, induced by spin-up/spindown of the centrifuge, can be minimized by controlling spin-up/spin-down rates. The SSBRP and ISSA will provide the opportunity to perform long-term, repeatable and high quality science. The long duration increments available on the Station will permit multigeneration studies of both plants and animals which have not previously been possible. The u-g habitat racks and the eight habitat centrifuge will accommodate sufficient number of specimens to permit statistically significant sampling of specimens to investigate the time course of adaptation to altered gravity environments. The centrifuge will, for the first time, permit investigators to use gravity itself as a tool to investigate fundamental processes, to investigate the intensity and duration of gravity to maintain normal structure and function, to separate the effects of u-g from other environmental factors and to examine artificial gravity as a potential countermeasure for the physical deconditioning observed during space flight.

Johnson, Catherine C.↗

NASA NTRS1995

Exercise Training During Bed Rest Attenuates Deconditioning

A 30-day 6 deg. head-down bed rest study was conducted to evaluate high-intensity, short-duration, alternating isotonic cycle ergometer exercise (ITE) training and high-intensity intermittent isokinetic exercise (IKE) training regiments designed to maintain peak VO2 and muscle mass, strength, and endurance at ambulatory control levels throughout prolonged bed rest. Other elements of the deconditioning (acclimation) syndrome, such as proprioception, psychological performance, hypovolemia, water balance, body composition, and orthostatic tolerance, were also measured. Compared with response during bed rest of the no exercise (NOE) control group: the ITE training regimen (a) maintained work capacity (peak VO2), (b) maintained plasma and red cell volume, (c) induced positive body water balance, (d) decreased quality of sleep and mental concentration, and (e) had no effect on the decrease in orthostatic tolerance; the IKE training regimen (a) attenuated the decrease in peak VO2 by 50%, (b) attenuated loss of red cell volume by 40%, but had no effect on loss of plasma volume, (c) induced positive body water balance, (d) had no adverse effect on quality of sleep or concentration, and (e) had no effect on the decrease in orthostatic tolerance. These findings suggest that various elements of the deconditioning syndrome can be manipulated by duration and intensity of ITE or IKE training regiments, and that several different training protocols will be required to maintain or restore physiological and psychological performance of individuals confined to prolonged bed rest.

Greenleaf, J. E.↗

NASA NTRS1995

Direction of Perceived Motion and Eye Movements Show Similar Biases for Asymmetrically Windowed Moving Plaids

We have shown that moving a plaid in an asymmetric window biases the perceived direction of motion (Beutter, Mulligan & Stone, ARVO 1994). We now explore whether these biased motion signals might also drive the smooth eye-movement response by comparing the perceived and tracked directions. The human smooth oculomotor response to moving plaids appears to be driven by the perceived rather than the veridical direction of motion. This suggests that human motion perception and smooth eye movements share underlying neural motion-processing substrates as has already been shown to be true for monkeys.

Beutter, B. R.↗

NASA NTRS1995

Effects of Chronic 2.0% and 0.7% CO2 Exposures on the Well-Being, Growth and Renal Function of Rats

On the Space Shuttle and MIR, mean CO2 levels have been 0.3% which is ten times that of normal air. There have also been extended periods with levels of 0.7% CO2 with peak concentrations at 2.0%. The Space Station program had proposed that CO2 concentration levels be maintained, on average, at 0.7%, and not to exceed 1.0%. To ensure that these levels of CO2 would not compromise the integrity of the science performed on the Space Station, the effects of chronic exposure of rats to 2.0% and 0.7% CO2 were investigated. Ten male rats per group were placed in individual metabolic cages for monitoring of food and water consumption, as well as fecal and urine production. Cages were placed in a large (4W x 10L x 4H ft.) plexiglass chamber with a controlled atmospheric environment. Following 7 days of cage adaptation, animals were exposed to experimental (2.0% or 0.7% CO2) or control (ambient air) conditions for 30 days. Daily body weight, food and water intake, and fecal and urine excretions were measured for the last three days of adaptation and the first ten days of exposure and then every three to four days for the remaining three weeks. Urine was measured for pH and total CO2. During 2.0% and 0.7% CO2 exposures, animal growth, fecal production and food and water consumption were within normal ranges. Urine excretion was significantly (p less than 0.05) higher in both experimental groups compared to controls. Urine pH of animals exposed to 2.0% CO2 was decreased by 0.32 over the first 6 days of exposure, followed by a 0.63 increase by day 30. In animals exposed to 0.7% CO2, urine pH did not decrease early in the exposure period, but did increase by 0.37 by day 30. Urine CO2 excretion did not change the first 6 days of exposure, but significantly increased in both 2.0% and 0.7% CO2 by day 30 (897 and 402 mmol/day, respectively). These results of chronic exposure to 2.0% and 0.7% CO2 are consistent with renal compensation in response to an altered acid-base homeostasis. These findings may impact science conducted on the Space Shuttle or the Space Station if CO2 concentrations reach 0.7%.

Lang, C. K.↗

NASA NTRS1995

Calf Venous Compliance in Supine Posture Equals that Measured with Head-up Tilt

Calf venous compliance is measured conventionally with venous occlusion, where compliance equals calf volume elevation per mmHg thigh occlusion pressure in relaxed legs of supine subjects. Compliance may also be measured during stepwise head-up tilt (HUT) as calf volume elevation per mmHg gravitational venous pressure elevation produced by HUT. Although HUT on a tilt table with a footplate activates calf muscles which could reduce compliance, this muscle activation is relatively small (less than or = 10% of maximal voluntary levels). Therefore, we hypothesized that calf compliance measured with HUT equals that measured with conventional supine venous occlusion. The venous occlusion protocol consisted of 20, 40, 60 and 80 mmHg thigh cuff inflations held for 1, 2, 3 and 4 min, respectively, in 14 supine subjects. One min of cuff deflation to 0 mmHg separated occlusions. Nine other subjects underwent HUT through the following angles held for 30s each: -12 deg., -6 deg., 0 deg., 12 deg., 30 deg., 54 deg., and 90 deg. HUT calf venous pressure data were interpolated from Katkov and Chestukhin; these data correspond to 70-80% of calculated calf venous pressure changes (pgh). Liquid metal-in-silastic strain gauges measured calf volume for both venous occlusion and HUT compliance. Relaxed calf compliance in supine subjects equaled 4.6 +/- 0.4 ml/mmHg (X +/- SE), as measured with venous occlusion. Calf compliance during HUT equaled 4.6 +/- 1.0 ml/mmHg. Therefore, calf compliance in supine, relaxed subjects equals that measured during head-up tilt. We conclude that the minimal calf muscle activation which occurs during HUT does not importantly affect calf compliance.

Watenpaugh, Donald E.↗

NASA NTRS1995

Body Weight Gain during Altered Gravity: Spaceflight, Centrifugation and Transitions

Gravity is a force that influences all living systems, and is often disregarded in the study of environment on growth and development. To assess the effect of gravity exposure on growth, immature rats (130-200 g) were evaluated during chronic altered gravity exposure and during transition between gravity fields. The effects of 14 days of spaceflight on body weight gain were evaluated (n=12) and compared to controls. Spaceflight did not affect weight gain. In 6 rats, the transition from spaceflight to 1 G showed a significant (p less than 0.05) post flight weight loss over 48 hr of 13 g compared to controls. Over subsequent days this loss was compensated for with no difference noted after 5 days. Exposure to hypergravity, 2 G for 16 days, was evaluated in groups of n=6 (Control; On Center Control (OCC); Centrifuged). With centrifugation or OCC there was a reduction in body weight within 24 hr. The OCC regained control weights within 13 days. The weight difference, 26 +/- 1 g, persisted with 2 G with no subsequent difference in weight gain over days 3-16 compared to controls; 3.7 +/- 0.1 versus 3.9 +/- 0.1 g/day respectively. Transition from centrifugation to 1 G resulted in a weight increase within 48 hours. Over 16 days the rate of gain was increased 3.1 +/- 0.1 g/day for centrifuge compared to 2.1 +/- 0.1 g/day for controls between Day 3 to 16. However, differences from control were still noted on Day 16. Transition from one gravity field to another causes acute changes in body weight. Transition to microgravity or 1 G, following the acute changes, results in adjustments to attain a normal weight. In hypergravity the acute reduction in body weight persist, but weight gain is normal. Transitioning from hypergravity to 1G results in an increased weight gain to compensate for the persistent reduction during exposure.

Wade, Charles E.↗

NASA NTRS1995

Spatial Factors in the Integration of Speed Information

We reported that, for a 21FC task with multiple Gabor patches in each interval, thresholds for speed discrimination decreased with the number of patches, while simply increasing the area of a single patch produced no such effect. This result could be explained by multiple patches reducing spatial uncertainty. However, the fact that thresholds decrease with number even when the patches are in fixed positions argues against this explanation. We therefore performed additional experiments to explore the lack of an area effect. Three observers did a 21FC speed discrimination task with 6 Gabor patches in each interval, and were asked to pick the interval in which the gratings moved faster. The 50% contrast patches were placed on a circle at 4 deg. eccentricity, either equally spaced and maximally separated (hexagonal array), or closely-spaced, in consecutive positions (string of pearls). For the string-of-pearls condition, the grating phases were either random, or consistent with a full-field grating viewed through multiple Gaussian windows. When grating phases were random, the thresholds for the hexagonal and string-of-pearls layouts were indistinguishable. For the string-of-pearls layout, thresholds in the consistent-phase condition were higher by 15 +/- 6% than in the random-phase condition. (Thresholds increased by 57 +/- 7% in going from 6 patches to a single patch of equivalent area.). For random-phase patches, the lower thresholds for 6 patches does not depend on a specific spacing or spatial layout. Multiple, closely-spaced, consistent-phase patches that can be interpreted as a single grating, result in thresholds closer to that produced by a single patch. Together, our results suggest that object segmentation may play a role in the integration of speed information.

Verghese, P.↗

NASA NTRS1995

Measurement of Transcranial Distance During Head-Down Tilt Using Ultrasound

Exposure to microgravity probably elevates blood pressure and flow in the head which may increase intracranial volume (ICV) and pressure (ICP). Due to the slightly compliant nature of the cranium, any increase of ICP will increase ICV and transcranial distance. We used a noninvasive ultrasound technique to measure transcranial distance (frontal to occipital) during head-down tilt. Seven subjects (ages 26-53) underwent the following tilt angles: 90 deg. upright, 30 deg., 0 deg., -6 deg., -10 deg., -6 deg., 0 deg., 30 deg., and 90 deg. Each angle was maintained for 1 min. Ultrasound wave frequency was collected continuously and transcranial distance was calculated (Delta(x) = x(Delta)f/f, where x is path length and f is frequency of the wave) for each tilt angle. Frequency decreased from 503.687 kHz (90 deg. upright) to 502.619 kHz (-10 deg.). These frequencies translated to an increased transcranial distance of 0.403 mm. Although our data suggest a significant increase in transcranial distance during head-down tilt, this apparent increase may result, in part, from head-down tilt-induced subcutaneous edema or cutaneous blood volume elevation. In three subjects, when the above protocol was repeated with an ace bandage wrapped around the head to minimize such edema, the increased transcranial distance from 90 deg. to -10 deg. was reduced by 0.174 mm. Further development of the technique to quantify bone-to-bone expansion unconfounded by cutaneous fluid is necessary. Therefore, this ultrasound technique may provide measurements of changes in cranial dimensions during microgravity.

Torikoshi, Shigeyo↗

NASA NTRS1995

The Biology and Space Exploration Video Series

The Biology and Space Exploration video series illustrates NASA's commitment to increasing the public awareness and understanding of life sciences in space. The video series collection, which was initiated by Dr. Joan Vernikos at NASA headquarters and Dr. Alan Hargens at NASA Ames Research Center, will be distributed to universities and other institutions around the United States. The video series parallels the "Biology and Space Exploration" course taught by NASA Ames scientists at Stanford University, Palo Alto, California. In the past, students have shown considerable enthusiasm for this course and have gained a much better appreciation and understanding of space life sciences and exploration. However, due to the unique nature of the topics and the scarcity of available educational materials, most students in other universities around the country are unable to benefit from this educational experience. Therefore, with the assistance of Ames experts, we are producing a video series on selected aspects of life sciences in space to expose undergraduate students to the effects of gravity on living systems. Additionally, the video series collection contains space flight footage, graphics, charts, pictures, and interviews to make the materials interesting and intelligible to viewers.

William, Jacqueline M.↗

NASA NTRS1995

Sex Differences in Tibial Bone Strength

We have used an instrument (MRTA or Mechanical Response Tissue Analyzer) that measures bending stiffness (EI) non-Invasively to evaluate the strength of the tibia, a long bone in the weightbearing skeleton highly vulnerable to mineral loss during space flight. In healthy men, we found asymmetry in EI consistent with the bone's support function (L greater than R). In this study, we analyzed EI in women and compared the results to those in men.

Arnaud, Sara B.↗

NASA NTRS1995

Validation of Long Bone Mechanical Properties from Densitometry

The objective of this study was to assess whether cross-sectional areal properties, calculated from densitometry, correlate to the true flexural properties. Right and left male embalmed tibiae were used in the study. Prior to scanning, the proximal end of each tibia was potted in a fixture with registration pins, flushed thoroughly with water under pressure to remove trapped air, and then placed in a constant thickness water bath attached to a precision indexer. Two sets of three scans of the entire tibia were taken with an Hologic QDR 1000/W densitometer at rotations of 0, 45, and 90 degrees about the tibia long axis. An aluminum step phantom and a bone step phantom, machined from bovine cortical bone, were also in the bath and scanned separately. Pixel attenuation data from the two sets of scans were averaged to reduce noise. Pixel data from the high energy beam were then converted to equivalent thicknesses using calibration equations. Cross-sectional areal properties (centroid, principal area moments and principal angle) along the length were computed from the three registered scans using methods developed in our laboratory. Flexural rigidities. Four strain gages were bonded around the circumference of each of 5 cross-sections encompassing the entire diaphysis. A known transverse load was then applied to the distal end and the bone was rotated 360 degrees in eight increments of 45 degrees each. Strains from the eight orientations were analyzed along with the known applied bending moments at each section to compute section centroids, curvatures, principal flexural rigidities and principal angle. Reference axes between the two methods were maintained within +/- 0.5 degrees using an electronic inclinometer. Principal angles (flexural - areal) differed by -2.0 +/- 4.0 degrees, and 1.0 +/- 2.5 degrees for the right and left tibia, respectively. Section principal flexural rigidities were highly correlated to principal areal moments (right: r(sup 2)= 0.997; left: r(sup 2)= 0.978) indicating a nearly constant effective flexural modulus. Right and left tibia exhibited a very high degree of symmetry when comparing either flexural or areal properties. To our knowledge this is the first study to validate the use of densitometry (DXA) to predict three dimensional structural properties of long bones. Our initial results support the conclusion that bone mineral and its distribution are the primary determinants of flexural modulus and rigidity.

Whalen, R.↗

NASA NTRS1995

Adaptation to Space: An Introduction

The cardiovascular and musculoskeletal systems are normally exposed to gradients of blood pressure and weight on Earth. These gradients increase blood pressure and tissue weight in dependent tissues of the body. Exposure to actual and simulated microgravity causes blood and tissue fluid to shift from the legs to the head. Studies of humans in space have documented facial edema, space motion sickness, decreased plasma volume, muscle atrophy, and loss of bone strength. Return of astronauts to Earth is accompanied by orthostatic intolerance, decreased neuromuscular coordination, and reduced exercise capacity. These factors decrease performance during descent from orbit and increase risk during emergency egress from the spacecraft. Models of simulated microgravity include 6 deg head-down tilt, immersion, and prolonged horizontal bedrest. Head-down tilt is the most accepted model and studies using this model of up to one year have been performed in Russia. Animal models which offer clear insights into the role of gravity on vertebrates include the developing giraffe and snakes from various habitats. Finally, possible countermeasures to speed readaptation of astronauts to gravity after prolonged space flight will be discussed.

Hargens, Alan R.↗

NASA NTRS1995

Near-Infrared Monitoring of Model Chronic Compartment Syndrome In Exercising Skeletal Muscle

Chronic compartment syndrome (CCS) is characterized by muscle ischemia, usually in the anterior oompartment of the leg, caused by high intramuscular pressure during exercise. Dual-wave near-infrared (NIR) spectroscopy is an optical technique that allows noninvasive tracking of variations in muscle tissue oxygenation (Chance et al., 1988). We hypothesized that with a model CCS, muscle tissue oxygenation will show a greater decline during exercise and a slower recovery post-exercise than under normal conditions.

Hargens, Alan R.↗

NASA NTRS1994

Comparison of Gait During Treadmill Exercise While Supine in Lower Body Negative Pressure (LBNP), Supine with Bungee Resistance and Upright in Normal Gravity

Boda, Wanda↗

NASA NTRS1994

Comparison of Gait During Treadmill Exercise While Supine in Lower Body Negative Pressure (LBNP), Supine with Bungee Resistance and Upright in Normal Gravity

The purpose of this study is to compare footward forces, gait kinematics, and muscle activation patterns (EMG) generated during supine treadmill exercise against LBNP with the same parameters during supine bungee resistance exercise and upright treadmill exercise. We hypothesize that the three conditions will be similar. These results will help validate treadmill exercise during LBNP as a viable technique to simulate gravity during space flight. We are evaluating LBNP as a means to load the musculoskeletal and cardiovascular systems without gravity. Such loading should help prevent physiologic deconditioning during space flight. The best ground-based simulation of LBNP treadmill exercise in microgravity is supine LBNP treadmill exercise on Earth because the supine footward force vector is neither directed nor supplemented by Earth's gravity. Previous results from HR-95 ("Dynamics of footward force and leg intramuscular pressure during exercise against supine LBNP and upright standing in normal gravity") indicate that supine plantar-/dorsiflexion exercise in LBNP at 100 mm Hg produces similar ground reaction forces, musculoskeletal stress, and VO2 to those during upright exercise against Earth's gravity. However, elevations of leg volume and heart rate indicate that cardiovascular stress during 100 mm Hg LBNP exercise exceeds that during 1 g exercise. Therefore, the need arose to reduce the cardiovascular stress of LBNP, while maintaining LBNP-induced reaction forces. To this end, we determined that mild plantar-/dorsiflexion exercise during LBNP significantly improves tolerance to LBNP via musculovenous pumping and sympathoexcitation; more intense exercise such as walking and running may further improve LBNP tolerance. In addition, two methodological advances have permited us to simulate upright 1 g exercise better with supine LBNP exercise. First, a newly-designed waist seal allows decreased levels of LBNP (50-60 mm Hg) to produce a footward force equaling one body weight

Boda, Wanda↗