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ABSTRACT: The currently approved objective clinical measure of standing balance in astronauts after space flight is the Sensory Organization Test battery of computerized dynamic posturography. No tests of walking balance are currently approved for standard clinical testing of astronauts. This study determined the sensitivity and specificity of standing and walking balance tests for astronauts before and after long-duration space flight. Astronauts were tested on an obstacle avoidance test known as the Functional Mobility Test (FMT) and on the Sensory Organization Test using sway-referenced support surface motion with eyes closed (SOT 5) before and six months after (n=15) space flight on the International Space Station. They were tested two to seven days after landing. Scores on SOT tests decreased and scores on FMT increased significantly from pre- to post-flight. In other words, post-flight scores were worse than pre-flight scores. SOT and FMT scores were not significantly related. ROC analyses indicated supra-clinical cut-points for SOT 5 and for FMT. The standard clinical cut-point for SOT 5 had low sensitivity to post-flight astronauts. Higher cut-points increased sensitivity to post-flight astronauts but decreased specificity to pre-flight astronauts. Using an FMT cut-point that was moderately highly sensitive and highly specific plus SOT 5 at the standard clinical cut-point was no more sensitive than SOT 5, alone. FMT plus SOT 5 at higher cut-points was more specific and more sensitive. The total correctly classified was highest for FMT, alone, and for FMT plus SOT 5 at the highest cut-point. These findings indicate that standard clinical comparisons are not useful for identifying problems. Testing both standing and walking balance will be more likely to identify balance deficits.
Journal of Vestibular Research 01/2012; 22(4):191-6. · 1.35 Impact Factor
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ABSTRACT: We studied 15 men (8 treatment, 7 control) before and after 21 days of 6º head-down tilt to determine whether daily, 1-h exposures to 1.0 G(z) (at the heart) artificial gravity (AG) would prevent bed rest-induced cardiovascular deconditioning. Testing included echocardiographic analysis of cardiac function, plasma volume (PV), aerobic power (VO(2)pk) and cardiovascular and neuroendocrine responses to 80º head-up tilt (HUT). Data collected during HUT were ECG, stroke volume (SV), blood pressure (BP) and blood for catecholamines and vasoactive hormones. Heart rate (HR), cardiac output (CO), total peripheral resistance, and spectral power of BP and HR were calculated. Bed rest decreased PV, supine and HUT SV, and indices of cardiac function in both groups. Although PV was decreased in control and AG after bed rest, AG attenuated the decrease in orthostatic tolerance [pre- to post-bed rest change; control: -11.8 ± 2.0, AG: -6.0 ± 2.8 min (p = 0.012)] and VO(2)pk [pre- to post-bed rest change; control: -0.39 ± 0.11, AG: -0.17 ± 0.06 L/min (p = 0.041)]. AG prevented increases in pre-tilt levels of plasma renin activity [pre- to post-bed rest change; control: 1.53 ± 0.23, AG: -0.07 ± 0.34 ng/mL/h (p = 0.001)] and angiotensin II [pre- to post-bed rest change; control: 3.00 ± 1.04, AG: -0.63 ± 0.81 pg/mL (p = 0.009)] and increased HUT aldosterone [post-bed rest; control: 107 ± 30 pg/mL, AG: 229 ± 68 pg/mL (p = 0.045)] and norepinephrine [post-bed rest; control: 453 ± 107, AG: 732 ± 131 pg/mL (p = 0.003)]. We conclude that AG can mitigate some aspects of bed rest-induced cardiovascular deconditioning, including orthostatic intolerance and aerobic power. Mechanisms of improvement were not cardiac-mediated, but likely through improved sympathetic responsiveness to orthostatic stress.
Arbeitsphysiologie 05/2011; 112(2):605-16. · 2.15 Impact Factor
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ABSTRACT: We propose a technique to estimate functional limits of stability (LOS) during bipedal stance using a controlled, low speed, voluntary leaning protocol requiring feet to remain in contact with the ground. LOS are estimated from ellipses fit to center-of-mass position data obtained during the leaning protocol. The LOS of nine healthy subjects were found to be 20-59% closer to the center of stance than the more frequently used anatomical boundaries and were reduced by closing the eyes. We conclude that functional stability boundaries should be used when the outcome measure is related to fall risk.
Gait & posture 01/2011; 33(4):715-7. · 2.58 Impact Factor
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ABSTRACT: Astronauts face transient disruptions of sensorimotor functions after spaceflight. Computerized dynamic posturography (CDP) testing has been used to document functional recovery; however, its objective value in return-to-duty decision-making has not been established. Therefore, we studied the diagnostic accuracy of CDP to determine the most effective test components for probing post-spaceflight sensorimotor deficits.
There were 11 first-time astronauts and 11 matched controls who were evaluated by CDP before and after spaceflight (controls did not fly). All CDP testing was conducted with eyes closed while standing on a computer-controlled force plate. Somatosensory influences were either unperturbed (stationary force plate) or altered (unstable force plate), and vestibular influences were either unperturbed (head erect) or altered by static (head pitched forward or back by 200) or dynamic (head pitched voluntarily in cadence with an auditory signal: +/- 20 degrees at 0.33 Hz) challenges. Using equilibrium (EQ) scores derived from peak A-P sway as the dependent measure, we determined the sensitivity and specificity of each test condition and then constructed receiver operator characteristic (ROC) curves to determine their diagnostic accuracies.
The greatest diagnostic accuracy was obtained from the test requiring the subject to make dynamic head movements while standing on an unstable force plate (94.9% sensitivity 96.6% specificity, area under ROC curve = 0.991). By contrast, the estimated ROC area for the standard clinical Romberg test (fixed support, head erect), which is often used to make postflight return-to-duty decisions, was 0.718.
We recommend that results from this test paradigm be considered during postflight return-to-duty decision-making.
Aviation Space and Environmental Medicine 07/2010; 81(7):625-31. · 0.88 Impact Factor
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ABSTRACT: Spaceflight has functionally significant effects on sensorimotor behavior, but it is difficult to separate the effects of ascending somatosensory changes caused by postural muscle and plantar surface unloading from descending visual-vestibular neural changes. To differentiate somatosensory changes from graviceptor changes in post-spaceflight sensorimotor behavior, bed rest may serve as an exclusionary analog to spaceflight.
Four separate tests were used to measure changes in sensorimotor performance: 1) the monosynaptic stretch reflex (MSR); 2) the functional stretch reflex (FSR); 3) balance control parameters associated with computerized dynamic posturography (CDP); and 4) a functional mobility test (FMT).
A mixed model regression analysis showed significant increases in median MSR start and peak latencies, while the median FSR latency showed no significant increase. Median MSR peak magnitude showed a significant increase during the middle bed rest period (19-60 d). There were no significant effects of bed rest on balance control, but some indication that dynamic head movements may affect posture after bed rest. Time to complete the course for the FMT increased significantly with bed rest.
The four primary tests indicate that long-duration head-down bed rest, through unloading and modification of the body's support surface, serves as an exclusionary analog for sensorimotor responses to spaceflight. Furthermore, the data suggest that procedures designed to alleviate modifications to the sensory substrate serving the soles of the feet may provide a countermeasure to help maintain support afferentation of the postural muscles.
Aviation Space and Environmental Medicine 06/2009; 80(5 Suppl):A45-54. · 0.88 Impact Factor
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ABSTRACT: Excessive, chronic whole-body vibration (WBV) has a number of negative side effects on the human body, including disorders of the skeletal, digestive, reproductive, visual, and vestibular systems. Whole-body vibration training (WBVT) is intentional exposure to WBV to increase leg muscle strength, bone mineral density, health-related quality of life, and decrease back pain. The purpose of this study was to quantitatively evaluate vibration exposure and biodynamic responses during typical WBVT regimens.
Healthy men and women (N = 16) were recruited to perform slow, unloaded squats during WBVT (30 Hz; 4 mm(p-p)), during which knee flexion angle (KA), mechanical impedance, head acceleration (Ha(rms)), and estimated vibration dose value (eVDV) were measured. WBVT was repeated using two forms of vibration: 1) vertical forces to both feet simultaneously (VV), and 2) upward forces to only one foot at a time (RV).
Mechanical impedance varied inversely with KA during RV (effect size, eta(p)(2): 0.668, P < 0.01) and VV (eta(p)(2): 0.533, P < 0.05). Ha(rms) varied with KA (eta(p)(2): 0.686, P < 0.01) and is greater during VV than during RV at all KA (P < 0.01). The effect of KA on Ha(rms) is different for RV and VV (eta(p)(2): 0.567, P < 0.05). The eVDV associated with typical RV and VV training regimens (30 Hz, 4 mm(p-p), 10 min.d(-1)) exceeds the recommended daily vibration exposure as defined by ISO 2631-1 (P < 0.01).
ISO standards indicate that 10 min.d(-1) WBVT is potentially harmful to the human body; the risk of adverse health effects may be lower during RV than VV and at half-squats rather than full-squats or upright stance. More research is needed to explore the long-term health hazards of WBVT.
Medicine & Science in Sports & Exercise 11/2007; 39(10):1794-800. · 4.43 Impact Factor
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ABSTRACT: Leg muscle strength and power are increased after whole-body vibration (WBV) exercise. These effects may result from increased neuromuscular activation during WBV; however, previous studies of neuromuscular responses during WBV have not accounted for motion artifact.
Sixteen healthy adults performed a series of static and dynamic unloaded squats with and without two different directions of WBV (rotational vibration, RV; and vertical vibration, VV; 30 Hz; 4 mmp-p). Activation of unilateral vastus lateralis, biceps femoris, gastrocnemius, and tibialis anterior was recorded using EMG. During RV and VV, increases in EMG relative to baseline were compared over a range of knee angles, contraction types (concentric, eccentric, isometric), and squatting types (static, dynamic).
After removing large, vibration-induced artifacts from EMG data using digital band-stop filters, neuromuscular activation of all four muscles increased significantly (P<or=0.05) during RV and VV. Average responses of the extensors were significantly greater during RV than VV, whereas responses of the tibialis anterior were significantly greater during VV than RV. For all four muscles, responses during static squatting were greater than or equal to responses during dynamic squatting, whereas responses during eccentric contractions were equal to or smaller than responses during concentric and isometric contractions. Neuromuscular responses of vastus lateralis, gastrocnemius, and tibialis anterior were affected by knee angle, with greatest responses at small knee angles.
Motion artifacts should be removed from EMG data collected during WBV. We propose that neuromuscular responses during WBV may be modulated by leg muscle cocontraction as a postural control strategy and/or muscle tuning by the CNS intended to minimize soft-tissue vibration.
Medicine & Science in Sports & Exercise 09/2007; 39(9):1642-50. · 4.43 Impact Factor
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ABSTRACT: The NASA Artificial Gravity Bed Rest Pilot Study was the first cross-institutional study to investigate the effectiveness of intermittent artificial gravity (AG) as a multi-system countermeasure to bed rest deconditioning in human subjects. Daily treatments by 60 min exposures to short radius centrifugation (SRC) were used to load the longitudinal body axis of eight male human subjects during 21 days of head down tilt bed rest as a means of protecting the bone, muscle, and cardiovascular systems from deconditioning. Data from these treatment subjects were compared with those from seven male human control subjects who were not exposed to SRC loading. This paper reports on implementation issues and lessons learned during the conduct of this complex study.
Journal of gravitational physiology: a journal of the International Society for Gravitational Physiology 08/2007; 14(1):P1-4.
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ABSTRACT: We have implemented a 41-day ground-based study to investigate the effects of daily artificial gravity loading on bed rest deconditioned human subjects. Each subject underwent 21 days of 6 degree head-down bed rest. Treatment subjects received 60 min daily doses of inertial mechanical loading (2.5 G at the feet decreasing to 1 G at the heart) produced by a short radius centrifuge. During rotation, the subject's cardiovascular responses were monitored via ECG, blood pressure and pulse oximetry, and subjective assessment of motion sickness and overall health were periodically requested. The subject's weight distribution at the feet was measured using a force plate, and lower leg muscle activity was monitored via surface electromyography. Control subjects were instrumented but did not receive any centrifugation. This paper provides details on the centrifuge protocol development and efficacy.
Journal of gravitational physiology: a journal of the International Society for Gravitational Physiology 08/2007; 14(1):P5-8.
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ABSTRACT: Head down bed-rest for 21 days was used as a ground analog of space flight in a pilot study to assess the pros and cons of daily 1 hr short radius centrifugation as a flight countermeasure. Positive results were found for cardiovascular and muscle protection, effectiveness was indicated for bone protection, and no negative issues interfering with high speed centrifugation were uncovered. The directions for extension of artificial gravity (AG) research to make it practical for extended space voyages are presented.
Journal of gravitational physiology: a journal of the International Society for Gravitational Physiology 08/2007; 14(1):P31-3.
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ABSTRACT: The NASA artificial gravity-bed rest pilot study (AGPS) was designed to investigate the efficacy of daily exposure to a +Gz acceleration gradient for counteracting the physiologic decrements induced by prolonged bed rest. Test subjects were continuously monitored by a physician for signs and symptoms of pre-syncope, motion sickness, and arrhythmias while on the centrifuge. In this article, we have summarized the medical monitoring observations that were made during the AGPS and included an assessment of the relative usefulness of the information provided by the various monitoring tools in making a decision to terminate a centrifuge spin.
Journal of gravitational physiology: a journal of the International Society for Gravitational Physiology 08/2007; 14(1):P9-13.
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ABSTRACT: In preparing for the NASA Artificial Gravity Pilot Study the planned centrifuge loading protocol was tested in 11 ambulatory subjects (6 men, 5 women). Each was subjected to a single 60 min trial with 2.5G of equivalent gravitational load feet and 1G at the level of the mediastinum. Amongst the men, 5/6 completed the trial successfully with no adverse sequelae. However, amongst the women, 4/5 tests were stopped early because of presyncope. Women are known to have a greater predisposition to syncope following orthostatic stress under normal tilt table conditions, during lower body negative pressure and following space flight. Amongst the factors which may have contributed to their lower tolerance to centrifugation are anthropometric factors, the vasoactive effects of sex hormones, catecholamine levels, ability to augment total peripheral resistance in response to orthostatic stress, and structural differences in cardiac anatomy and physiology. However, determining the true cause of this difference will require further investigation.
Journal of gravitational physiology: a journal of the International Society for Gravitational Physiology 08/2007; 14(1):P15-9.
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ABSTRACT: Increased postural instability and the subsequent elevation in fall incidence with increasing age are important contributors for hip fractures and developing frailty. When testing for such instability, most studies characterize balance in terms of center-of-mass (COM) deviation from a finite point, the "equilibrium point", located at the center of a subject's stance. For example, the clinically accepted equilibrium score (EQscore) represents instability as the maximum peak-to-peak sway about the "equilibrium point". An alternative theory views balance as being controlled within a "stability margin" in which all corrective actions are based on the time to contact (TTC) of the body's COM with that margin. This study examines the differences offered by evaluating balance control using the EQscore and TTC approach across several age groups and sessions. Consenting subjects from the Baltimore Longitudinal Study of Aging were recruited (N=155) from each age decade (20s-80s) who were generally healthy and free from neurological diagnoses. Results showed TTC tests detected significant variations in eyes open versus eyes closed testing that were unpredictable by EQscore. Further, TTC produced differences in age-related stability threats not seen using EQscore. The TTC data also provided a discriminating difference between subjects who fell in the difficult tests and those who maintained posture. Overall, these data suggest EQscore might not sufficiently account for dynamic control components the body may be using to maintain balance. TTC may offer a more accurate estimate of postural stability (functional ability) than EQscore based on its inclusion of a velocity component to detect dynamic changes.
Gait & Posture 02/2007; 25(1):56-62. · 2.12 Impact Factor
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Annals of the New York Academy of Sciences 12/2006; 656(1):747 - 754. · 3.15 Impact Factor
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ABSTRACT: To better understand the effects of varying head movement frequencies on human balance control, 12 healthy adult humans were studied during static and dynamic (0.14, 0.33, 0.6 Hz) head tilts of +/- 30 degrees in the pitch and roll planes. Postural sway was measured during upright stance with eyes closed and altered somatosensory inputs provided by a computerized dynamic posturography (CDP) system. Subjects were able to maintain upright stance with static head tilts, although postural sway was increased during neck extension. Postural stability was decreased during dynamic head tilts, and the degree of destabilization varied directly with increasing frequency of head tilt. In the absence of vision and accurate foot support surface inputs, postural stability may be compromised during dynamic head tilts due to a decreased ability of the vestibular system to discern the orientation of gravity. This instability may compound the risk of falling following recovery from balance disorders or adaptation to altered gravity conditions such as space flight. Thus, dynamic head tilts may improve the diagnostic sensitivity of computerized dynamic posturography, particularly for healthy subjects recovering from temporary balance control deficits.
Gait & Posture 05/2006; 23(3):315-23. · 2.12 Impact Factor
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ABSTRACT: Square-wave jerks (SWJs) are small, involuntary saccades that disrupt steady fixation. We report the case of an astronaut (approximately 140 d on orbit) who showed frequent SWJs, especially postflight, but who showed no impairment of vision or decrement of postflight performance. These data support the view that SWJs do not impair vision because they are paired movements, consisting of a small saccade away from the fixation position followed, within 200 ms, by a corrective saccade that brings the eye back on target. Since many returning astronauts show a decrement of dynamic visual function during postflight locomotion, it seems possible that frequent SWJs improved this astronaut's visual function by providing postsaccadic enhancement of visual fixation, which aided postflight performance. Certainly, frequent SWJs did not impair performance in this astronaut, who had no other neurological disorder.
Aviation Space and Environmental Medicine 09/2004; 75(8):700-4. · 0.88 Impact Factor
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ABSTRACT: To examine the sensorimotor adaptive response of a 77-year-old man exposed to the gravito-inertial challenges of orbital space flight.
Prospective case study with retrospective comparisons.
NASA Neurosciences Laboratory (Johnson Space Center) and Baseline Data Collection Facility (Kennedy Space Center). PRIMARY PARTICIPANT: One 77-year-old male shuttle astronaut.
Insertion into low Earth orbit was used to remove gravitational stimuli and thereby trigger sensorimotor adaptation to the microgravity environment. Graviceptor stimulation was reintroduced at landing, and sensorimotor readaptation to the terrestrial environment was tracked to completion.
Computerized dynamic posturography tests were administered before and after orbital flight to determine the magnitude and time course of recovery.
The elderly astronaut exhibited balance control performance decrements on landing day; however, there were no significant differences between his performance and that of younger astronauts tested on the same shuttle mission or on previous shuttle missions of similar duration.
These results demonstrate that the physiological changes attributed to aging do not necessarily impair adaptive sensorimotor control processes.
Ontology & Neurotology 02/2004; 25(1):53-6. · 1.90 Impact Factor
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ABSTRACT: Computerized dynamic posturography (CDP) is widely used for assessment of altered balance control. CDP trials are quantified using the equilibrium score (ES), which ranges from zero to 100, as a decreasing function of peak sway angle. The problem of how best to model and analyze ESs from a controlled study is considered. The ES often exhibits a skewed distribution in repeated trials, which can lead to incorrect inference when applying standard regression or analysis of variance models. Furthermore, CDP trials are terminated when a patient loses balance. In these situations, the ES is not observable, but is assigned the lowest possible score--zero. As a result, the response variable has a mixed discrete-continuous distribution, further compromising inference obtained by standard statistical methods. Here, we develop alternative methodology for analyzing ESs under a stochastic model extending the ES to a continuous latent random variable that always exists, but is unobserved in the event of a fall. Loss of balance occurs conditionally, with probability depending on the realized latent ES. After fitting the model by a form of quasi-maximum-likelihood, one may perform statistical inference to assess the effects of explanatory variables. An example is provided, using data from the NIH/NIA Baltimore Longitudinal Study on Aging.
IEEE Transactions on Biomedical Engineering 05/2002; 49(4):300-9. · 2.28 Impact Factor
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ABSTRACT: In a previous study from our laboratory, we observed carbon dioxide (CO2) accumulation in the helmet of the NASA Launch and Entry Suit (LES) during a simulated emergency egress from the Space Shuttle. Eight of 12 subjects were unable to complete the egress simulation with a G-suit inflation pressure of 1.5 psi. The purpose of this report was to compare CO2 accumulation and egress walking time in the new Advanced Crew Escape Suit (ACES) with that in the LES.
02/2000;
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ABSTRACT: NASA's vision for space exploration includes missions of unprecedented distance and duration. However, during 30 years of human space flight experience, including numerous long-duration missions, research has not produced any single countermeasure or combination of countermeasures that is completely effective. Current countermeasures do not fully protect crews in low-Earth orbit, and certainly will not be appropriate for crews journeying to Mars and back over a three-year period. The urgency for exploration-class countermeasures is compounded by continued technical and scientific successes that make exploration class missions increasingly attractive. The critical and possibly fatal problems of bone loss, cardiovascular deconditioning, muscle weakening, neurovestibular disturbance, space anemia, and immune compromise may be alleviated by the appropriate application of artificial gravity (AG). However, despite a manifest need for new countermeasure approaches, concepts for applying AG as a countermeasure have not developed apace. To explore the utility of AG as a multi-system countermeasure during long-duration, exploration-class space flight, eighty-three members of the international space life science and space flight community met earlier this year. They concluded unanimously that the potential of AG as a multi-system countermeasure is indeed worth pursuing, and that the requisite AG research needs to be supported more systematically by NASA. This presentation will review the issues discussed and recommendations made.
02/2000;
