Gita Murthy

Upright exercise or supine lower body negative pressure exercise maintains exercise responses after bed rest

Adaptation to bed rest or space flight is accompanied by an impaired ability to exercise in an upright position. We hypothesized that a daily, 30-min bout of intense, interval exercise in upright posture or supine against lower body negative pressure (LBNP) would maintain upright exercise heart rate and respiratory responses after bed rest. Twenty-four men (31 +/- 3 yr) underwent 5 d of 6 degree head-down tilt: eight performed no exercise (CON), eight performed upright treadmill exercise (UPex), and eight performed supine treadmill exercise against LBNP at -51.3 +/- 0.4 mm Hg (LBNPex). Submaximal treadmill exercise responses (56, 74, and 85% of VO2peak) were measured pre- and post-bed rest. In CON, submaximal heart rate, respiratory exchange ratio, and ventilation were significantly greater (P < or = 0.05) after bed rest. In UPex and LBNPex, submaximal exercise responses were similar pre- and post-bed rest. Our results indicate that a daily 30-min bout of intense, interval upright exercise training or supine exercise training against LBNP is sufficient to maintain upright exercise responses after 5 d of bed rest. These results may have important implications for the development of exercise countermeasures during space flight.

The Effect of Backpacks on the Lumbar Spine in Children: A Standing Magnetic Resonance Imaging Study

Study Design. This study is a repeated measures design to measure the lumbar spine response to typical school backpack loads in healthy children. The lumbar spine in this setting was measured for the first time by an upright magnetic resonance imaging (MRI) scanner. Objective. The purpose of this study is to measure the lumbar spine response to typical school backpack loads in healthy children. We hypothesize that backpack loads significantly increase disc compression and lumbar curvature. Summary of Background Data. Children commonly carry school backpacks of 10% to 22% bodyweight. Despite growing concern among parents about safety, there are no imaging studies which describe the effect of backpack loads on the spine in children. Methods. Three boys and 5 girls, age 11 ± 2 years (mean ± SD) underwent T2 weighted sagittal and coronal MRI scans of the lumbar spine while standing. Scans were repeated with 4, 8, and 12 kg backpack loads, which represented approximately 10%, 20%, and 30% body weight for our sample. Main outcome measures were disc compression, defined as post- minus preloading disc height, and lumbar asymmetry, defined as the coronal Cobb angle between the superior endplates of S1 and L1. Results. Increasing backpack loads significantly compressed lumbar disc heights measured in the midline sagittal plane (P < 0.05, repeated-measures analysis of variance [ANOVA]). Lumbar asymmetry was: 2.23° ± 1.07° standing, 5.46° ± 2.50° with 4 kg, 9.18° ± 2.25° with 8 kg, and 5.68° ± 1.76° with 12 kg (mean ± SE). Backpack loads significantly increased lumbar asymmetry (P < 0.03, one-way ANOVA). Four of the 8 subjects had Cobb angles greater than 10° during 8-kg backpack loads. Using a visual-analogue scale to rate their pain (0-no pain, 10-worst pain imaginable), subjects reported significant increases in back pain associated with backpack loads of 4, 8, and 12 kg (P < 0.001, 1-way ANOVA). Conclusion. Backpack loads are responsible for a significant amount of back pain in children, which in part, may be due to changes in lumbar disc height or curvature. This is the first upright MRI study to document reduced disc height and greater lumbar asymmetry for common backpack loads in children.

Asymmetric Loads and Pain Associated With Backpack Carrying by Children

Background: Shoulder and back pain in school children is associated with wearing heavy backpacks. Such pain may be attributed to the magnitude of the backpack load and the manner by which children distribute the load over their shoulders and back. The purpose of this study is to quantify the pressures under backpack straps of children while they carried a typical range of loads during varying conditions. Methods: Ten healthy children (aged, 12-14 years) wore a backpack loaded at 10%, 20%, and 30% body weight (BW). Backpacks were carried under 2 conditions, low on back or high on back. Pressure sensors (0.1 mm thick) measured pressures beneath the shoulder straps. Results: When walking with the backpack straps over both shoulders, contact pressures were significantly greater in the low-back condition than in the high-back condition (P = 0.004). In addition, when children carried the backpack in the low-back condition, mean pressures (±SE) over the right shoulder were as follows: 98 ± 31, 153 ± 48, and 170 ± 54 mm Hg at 10%, 20%, and 30% BW, respectively, which were significantly higher (P < 0.001) than those over the left shoulder (46 ± 14, 92 ± 29, and 90 ± 29 mm Hg, respectively). Perceived pain with the backpack over 1 shoulder was significantly greater (P = 0.002) than that for donning with both shoulders in the low-back condition. Conclusions: Pressures at 10%, 20%, and 30% BW loads on the right or left shoulder, during low-back or high-back conditions, are higher than the pressure thresholds (approximately 30 mm Hg) to occlude skin blood flow. Furthermore, asymmetric and high pressures exerted for extended periods of time may help explain the shoulder and back pain attributed to backpacks. Clinical Relevance: Randomized controlled trial. Grade 1 level of evidence.

Intramuscular Pressures Beneath Elastic and Inelastic Leggings

Leg compression devices have been used extensively by patients to combat chronic venous insufficiency and by astronauts to counteract orthostatic intolerance following spaceflight. However, the effects of elastic and inelastic leggings on the calf muscle pump have not been compared. The purpose of this study was to compare in normal subjects the effects of elastic and inelastic compression on leg intramuscular pressure (IMP), an objective index of calf muscle pump function. IMP in soleus and tibialis anterior muscles was measured with transducer-tipped catheters. Surface compression between each legging and the skin was recorded with an air bladder. Subjects were studied under three conditions: (1) control (no legging), (2) elastic legging, and (3) inelastic legging. Pressure data were recorded for each condition during recumbency, sitting, standing, walking, and running. Elastic leggings applied significantly greater surface compression during recumbency (20±1 mm Hg, mean±SE) than inelastic leggings (13±2 mm Hg). During recumbency, elastic leggings produced significantly higher soleus IMP of 25±1 mm Hg and tibialis anterior IMP of 28±1 mm Hg compared to 17±1 mm Hg and 20±2 mm Hg, respectively, generated by inelastic leggings and 8±1 mm Hg and 11±1 mm Hg, respectively, without leggings. During sitting, walking, and running, however, peak IMPs generated in the muscular compartments by elastic and inelastic leggings were similar. Our results suggest that elastic leg compression applied over a long period in the recumbent posture may impede microcirculation and jeopardize tissue viability. On the other hand, inelastic leggings do not compress leg tissues at levels above 20 mm Hg during recumbency. Therefore inelastic leggings may be more effective in improving venous circulation in the legs of patients with chronic venous insufficiency.

Exercise against lower body negative pressure as a countermeasure for cardiovascular and musculoskeletal deconditioning

Exposure to lower body negative pressure (LBNP) with oral salt and water ingestion has been tested by astronauts as a countermeasure to prevent postflight orthostatic intolerance. Exercise is another countermeasure that astronauts commonly use during spaceflight to maintain musculoskeletal strength. We hypothesize that a novel combination of exercise and simultaneous exposure to lower body negative pressure during spaceflight will produce Earth-like musculoskeletal loads as well as cardiovascular stimuli to maintain adaptation to Earths gravity. Results from recent studies indicate that leg exercise within a LBNP chamber against the suction force of 100 mmHg LBNP in horizontal-supine posture produces an equivalent, if not greater exercise stress compared to similar leg exercise in upright posture (without LBNP) against Earths gravity. Therefore, the concept of LBNP combined with exercise may prove to be a low cost and low mass technique to stress the cardiovascular and the musculoskeletal systems simultaneously.

Ultrasound as a Noninvasive Method to Assess Changes of Intracranial Volume and Pressure During Simulated Microgravity

Headaches are commonly experienced by astronauts in microgravity and by subjects undergoing head-down tilt (simulated microgravity on Earth). Exposure to microgravity probably elevates blood pressure and flow in the head which may increase intracranial volume (ICV) and pressure (ICP) and in turn cause headache. Due to the slightly compliant nature of the cranial vault and the encasement of brain and its vasculature within this vault, any increase of ICV will increase ICP and slightly distend the cranium. Previous studies document perivascular edema and increased ICP in rhesus monkeys during head-down tilt. Elevated ICP has also been reported in humans during head-down tilt. ICP measurements in healthy humans are rare because of the invasiveness of currently-available measurement techniques. Therefore, we proposed a noninvasive ultrasound technique to assess changes of ICV and JCP. The ultrasound principle is based on compliance of the cranial vault. A 450 kHz ultrasound stimulus is transmitted through the cranium by a transducer every 7.5-10 msec. The ultrasound wave enters the brain tissue, reflects off the opposite side of the cranium and is received by the same transducer. The detected wave is compared for phase quadrature (90 deg.to transmitted wave). Because the electronic circuitry of the device maintains a 90 deg. phase (phi), any alterations in the detected wave caused by an increase of ICV and ICP will be reflected as a change in the wave frequency. Phase shift is directly proportional to path length of the wave, DELTA x, which is expressed as DELTA x = phi lambda/2 pi where lambda is wavelength. Elevated ICV and ICP expand the cranial vault and increase path length of the wave (a measure of intracranial distance). Increased path length equals reduced frequency of the detected wave. Reduced frequency is then related to elevated ICP. This technique has potential uses for ICP studies of astronauts in space and head trauma patients on Earth.

Mechanism of headward fluid shift during exposure to microgravity

A prominent feature of early cardiovascular adaptation to the microgravity of space flight is a shift of blood and tissue fluid from the lower body to the upper body. Symptoms of this fluid shift include facial edema, nasal congestion, and headache. Normally on Earth, the human body is exposed to hydrostatic (gravitational) blood pressure gradients during upright posture. In this posture, mean arterial pressures at head, heart, and foot levels are approximately 70, 100, and 200 mm Hg, respectively. Theoretically, all hydrostatic pressures within arteries and veins are lost during exposure to microgravity so that mean arterial pressure in all regions of the body is uniform and approximately equal to that at heart level (100 mm Hg). Acute studies of 60 head-down tilt (simulated microgravity on Earth) indicate that facial edema is caused by: 1) elevation of capillary blood pressure from 28 to 34 mm Hg, 2) reduction of blood colloid osmotic pressure 22 to 18 mm Hg, and 3) 50% increase of blood perfusion in tissues of the head. Furthermore, as compared to microvasculature in the feet, microvessels of the head have a low capacity to constrict and diminish local perfusion. Elevation of blood and tissue fluid pressures/flow in the head may also explain the higher headward bone density associated with long-term head-down tilt. These mechanistic studies of head-down tilt, along with a better understanding of the relative stresses involved with upright posture and lower body negative pressure, have facilitated development of physiologic countermeasures to maintain astronaut health during microgravity. Presently no exercise hardware is available to provide a blood pressure gradient from head to feet in space. However, recent studies in our laboratory suggest that treadmill exercise within lower body negative pressure provides equivalent or greater physiologic stress as compared to similar upright exercise on Earth.