Podokinetic after-rotation in a simulated reduced gravity environment

Stepping in place on a rotating platform for a period of 15 minutes induces an adaptive response, podokinetic after-rotation (PKAR), which causes a blindfolded individual to inadvertently rotate when attempting to step in place on the floor. The purpose of this investigation was to determine whether lower extremity load receptors were involved in this adaptation. As load receptor input is critical for locomotion, we hypothesized that manipulating load via body weight support (BWS) would influence PKAR. Eleven healthy female volunteers performed 15 minutes of stepping in place on a rotating treadmill (stimulation), followed by 10 minutes of stepping in place (response) without vision on a stationary surface. Response and stimulation periods were with 50% body weight support (BWS) and without body weight support (NoBWS) in all four possible combinations (BWS-BWS, NoBWS-NoBWS, BWS-NoBWS, and NoBWS-BWS). Conditions were randomly assigned to all subjects and performed on four separate occasions at least 48 hr apart. During the 10-min PKAR response period, trunk angular velocity was calculated and plotted versus time, and exponential models were applied to the data to obtain curve-fit parameters for each condition. Despite the manipulations of BWS, no significant differences were found for any parameter value (p = 0.13-0.98). BWS applied during stimulation only, response only, or during both stimulation and response does not appear to influence PKAR. This suggests that load receptors may not play a critical role in mediating adaptive changes in locomotor trajectory control in response to walking on a rotating surface.     

Podokinetic after-rotation in a simulated reduced gravity environment

Stepping in place on a rotating platform for a period of 15 minutes induces an adaptive response, podokinetic after-rotation (PKAR), which causes a blindfolded individual to inadvertently rotate when attempting to step in place on the floor. The purpose of this investigation was to determine whether lower extremity load receptors were involved in this adaptation. As load receptor input is critical for locomotion, we hypothesized that manipulating load via body weight support (BWS) would influence PKAR. Eleven healthy female volunteers performed 15 minutes of stepping in place on a rotating treadmill (stimulation), followed by 10 minutes of stepping in place (response) without vision on a stationary surface. Response and stimulation periods were with 50% body weight support (BWS) and without body weight support (NoBWS) in all four possible combinations (BWS-BWS, NoBWS-NoBWS, BWS-NoBWS, and NoBWS-BWS). Conditions were randomly assigned to all subjects and performed on four separate occasions at least 48 hr apart. During the 10-min PKAR response period, trunk angular velocity was calculated and plotted versus time, and exponential models were applied to the data to obtain curve-fit parameters for each condition. Despite the manipulations of BWS, no significant differences were found for any parameter value (p = 0.13–0.98). BWS applied during stimulation only, response only, or during both stimulation and response does not appear to influence PKAR. This suggests that load receptors may not play a critical role in mediating adaptive changes in locomotor trajectory control in response to walking on a rotating surface.     

Humans running in place on water at simulated reduced gravity

Background

On Earth only a few legged species, such as water strider insects, some aquatic birds and lizards, can run on water. For most other species, including humans, this is precluded by body size and proportions, lack of appropriate appendages, and limited muscle power. However, if gravity is reduced to less than Earth’s gravity, running on water should require less muscle power. Here we use a hydrodynamic model to predict the gravity levels at which humans should be able to run on water. We test these predictions in the laboratory using a reduced gravity simulator.

Methodology/Principal Findings

We adapted a model equation, previously used by Glasheen and McMahon to explain the dynamics of Basilisk lizard, to predict the body mass, stride frequency and gravity necessary for a person to run on water. Progressive body-weight unloading of a person running in place on a wading pool confirmed the theoretical predictions that a person could run on water, at lunar (or lower) gravity levels using relatively small rigid fins. Three-dimensional motion capture of reflective markers on major joint centers showed that humans, similarly to the Basilisk Lizard and to the Western Grebe, keep the head-trunk segment at a nearly constant height, despite the high stride frequency and the intensive locomotor effort. Trunk stabilization at a nearly constant height differentiates running on water from other, more usual human gaits.

Conclusions/Significance

The results showed that a hydrodynamic model of lizards running on water can also be applied to humans, despite the enormous difference in body size and morphology.     

Walking in simulated reduced gravity: mechanical energy fluctuations and exchange

Walking humansconserve mechanical and, presumably, metabolic energy with an invertedpendulum-like exchange of gravitational potential energy and horizontalkinetic energy. Walking in simulated reduced gravity involves arelatively high metabolic cost, suggesting that the inverted-pendulummechanism is disrupted because of a mismatch of potential and kineticenergy. We tested this hypothesis by measuring the fluctuations andexchange of mechanical energy of the center of mass at differentcombinations of velocity and simulated reduced gravity. Subjects walkedwith smaller fluctuations in horizontal velocity in lower gravity, suchthat the ratio of horizontal kinetic to gravitational potential energyfluctuations remained constant over a fourfold change in gravity. Theamount of exchange, or percent recovery, at 1.00 m/s was notsignificantly different at 1.00, 0.75, and 0.50 G (average 64.4%),although it decreased to 48% at 0.25 G. As a result, the amount ofwork performed on the center of mass does not explain the relatively high metabolic cost of walking in simulated reduced gravity.

     

Intraspecific differences in bacterial responses to modelled reduced gravity

AIMS: Bacteria are important residents of water systems, including those of space stations which feature specific environmental conditions, such as lowered effects of gravity. The purpose of this study was to compare responses with modelled reduced gravity of space station, water system bacterial isolates with other isolates of the same species. METHODS AND RESULTS: Bacterial isolates, Stenotrophomonas paucimobilis and Acinetobacter radioresistens, originally recovered from the water supply aboard the International Space Station (ISS) were grown in nutrient broth under modelled reduced gravity. Their growth was compared with type strains S. paucimobilis ATCC 10829 and A. radioresistens ATCC 49000. Acinetobacter radioresistens ATCC 49000 and the two ISS isolates showed similar growth profiles under modelled reduced gravity compared with normal gravity, whereas S. paucimobilis ATCC 10829 was negatively affected by modelled reduced gravity. CONCLUSIONS: These results suggest that microgravity might have selected for bacteria that were able to thrive under this unusual condition. These responses, coupled with impacts of other features (such as radiation resistance and ability to persist under very oligotrophic conditions), may contribute to the success of these water system bacteria. SIGNIFICANCE AND IMPACT OF THE STUDY: Water quality is a significant factor in many environments including the ISS. Efforts to remove microbial contaminants are likely to be complicated by the features of these bacteria which allow them to persist under the extreme conditions of the systems.     

Mechanics of running under simulated low gravity.

Using a linear mass-spring model of the body and leg (T. A. McMahon and G. C. Cheng. J. Biomech. 23: 65-78, 1990), we present experimental observations of human running under simulated low gravity and an analysis of these experiments. The purpose of the study was to investigate how the spring properties of the leg are adjusted to different levels of gravity. We hypothesized that leg spring stiffness would not change under simulated low-gravity conditions. To simulate low gravity, a nearly constant vertical force was applied to human subjects via a bicycle seat. The force was obtained by stretching long steel springs via a hand-operated winch. Subjects ran on a motorized treadmill that had been modified to include a force platform under the tread. Four subjects ran at one speed (3.0 m/s) under conditions of normal gravity and six simulated fractions of normal gravity from 0.2 to 0.7 G. For comparison, subjects also ran under normal gravity at five speeds from 2.0 to 6.0 m/s. Two basic principles emerged from all comparisons: both the stiffness of the leg, considered as a linear spring, and the vertical excursion of the center of mass during the flight phase did not change with forward speed or gravity. With these results as inputs, the mathematical model is able to account correctly for many of the changes in dynamic parameters that do take place, including the increasing vertical stiffness with speed at normal gravity and the decreasing peak force observed under conditions simulating low gravity.     

间断性人工重力作用对模拟失重大鼠股骨的防护效应

目的：比较三种间断性人工重力对抗措施对模拟失重大鼠的影响。方法：SD雄性大鼠30只,按体重配对后随机等分为5组：对照组（C）,悬吊组（S）,站立组（G1）,1.5G（G2）和2.6G人工重力组（G3）。对抗组大鼠每日分别给予1h的站立、1.5和2.6G人工重力作用。利用物理测量和三点弯曲等实验,观察了3周间断性人工重力对大鼠股骨生长、生物力学特性的影响。结果：与S组大鼠比较：G1组弹性载荷、最大和刚性系数显著恢复（P＜0.05）;G2组直径（P＜0.01）和干重、密度（P＜0.05）,弹性载荷和最大载荷显著提高（P＜0.05）;G3组弹性载荷和最大载荷显著恢复（P＜0.01）。结论：三种对抗措施均显著改善了尾吊大鼠承重骨的生长力学特性,而1.5G的1h/d人工重力作用是较理想的对抗方法。     

Deflection in center of gravity at the simulated operations of cabinet in the aged

The study projected a scenario where the aged handled home furniture in their daily lives. With the furniture designated at 5 different heights for task performance, different aspects of the center of gravity(CG) deflection in the young and the aged were investigated. The following points which should be considered were suggested; (1) The findings indicated that remarkable increases in CG deflection were detected in performing tasks on cabinet as age advanced. The age factor appeared to yield the major effect in all indices measured, whether the drawer was being drawn open or closed. (2) The major effects with age accompanied by significant mutual interactions between age and operation height were established when the drawer was drawn open. (3) In cases where the aged drew open the drawer, the LR deflections with reference to the CG were pronounced. When performing the task especially at greater heights, the deflections in the aged were enlarged, a tendency that was in contrast to the extended LR deflections (cf. at lower heights) in the young, contributing therefore to the increases in CG-DD and CG-DA, accordingly.     

Denatured proteins and early folding intermediates simulated in a reduced conformational space

Conformations of globular proteins in the denatured state were studied using a high-resolution lattice model of proteins and Monte Carlo dynamics. The model assumes a united-atom and high-coordination lattice representation of the polypeptide conformational space. The force field of the model mimics the short-range protein-like conformational stiffness, hydrophobic interactions of the side chains and the main-chain hydrogen bonds. Two types of approximations for the short-range interactions were compared: simple statistical potentials and knowledge-based protein-specific potentials derived from the sequence-structure compatibility of short fragments of protein chains. Model proteins in the denatured state are relatively compact, although the majority of the sampled conformations are globally different from the native fold. At the same time short protein fragments are mostly native-like. Thus, the denatured state of the model proteins has several features of the molten globule state observed experimentally. Statistical potentials induce native-like conformational propensities in the denatured state, especially for the fragments located in the core of folded proteins. Knowledge-based protein-specific potentials increase only slightly the level of similarity to the native conformations, in spite of their qualitatively higher specificity in the native structures. For a few cases, where fairly accurate experimental data exist, the simulation results are in semiquantitative agreement with the physical picture revealed by the experiments. This shows that the model studied in this work could be used efficiently in computational studies of protein dynamics in the denatured state, and consequently for studies of protein folding pathways, i.e. not only for the modeling of folded structures, as it was shown in previous studies. The results of the present studies also provide a new insight into the explanation of the Levinthal's paradox.     

Effect of modeled reduced gravity conditions on bacterial morphology and physiology

Background  

Bacterial phenotypes result from responses to environmental conditions under which these organisms grow; reduced gravity has been demonstrated in many studies as an environmental condition that profoundly influences microorganisms. In this study, we focused on low-shear stress, modeled reduced gravity (MRG) conditions and examined, for Escherichia coli and Staphlyococcus aureus, a suite of bacterial responses (including total protein concentrations, biovolume, membrane potential and membrane integrity) in rich and dilute media and at exponential and stationary phases for growth. The parameters selected have not been studied in E. coli and S. aureus under MRG conditions and provide critical information about bacterial viability and potential for population growth.     

"Demasking" the temperature rhythm after simulated time zone transitions.

Simulated time zone transitions were performed in an isolation unit upon groups of one to four human subjects. In the first series of experiments, the adjustment of the circadian rhythm of body temperature, measured in the presence of sleep and other masking factors, was assessed by cosinor analysis and by cross-correlation methods. These methods modeled the circadian timing system either as a single component or as the sum of two components, those due to exogenous and endogenous influences. The one-component models described a more rapid adjustment of the temperature rhythm to the time zone transition than did the two-component models; we attribute this difference to the masking effects of the exogenous component. In a second series of experiments, we showed that the shift of the endogenous component, as assessed by the two-component models, was not significantly different from that measured during constant routines. The results also showed that, if the zeitgebers were phased in advance of the endogenous component, then advances of the endogenous component were produced only if this mismatch was less than about 10 hr. Mismatches greater than this, and cases where the zeitgebers were delayed with respect to the endogenous component, both produced delays of the endogenous component. We conclude that the two-component cross-correlation methods can be used to estimate shifts of the endogenous component of a circadian rhythm in the presence of masking factors. They are therefore an alternative to constant routines when these latter are impracticable to carry out.     

Methods for detecting early warnings of critical transitions in time series illustrated using simulated ecological data

Many dynamical systems, including lakes, organisms, ocean circulation patterns, or financial markets, are now thought to have tipping points where critical transitions to a contrasting state can happen. Because critical transitions can occur unexpectedly and are difficult to manage, there is a need for methods that can be used to identify when a critical transition is approaching. Recent theory shows that we can identify the proximity of a system to a critical transition using a variety of so-called 'early warning signals', and successful empirical examples suggest a potential for practical applicability. However, while the range of proposed methods for predicting critical transitions is rapidly expanding, opinions on their practical use differ widely, and there is no comparative study that tests the limitations of the different methods to identify approaching critical transitions using time-series data. Here, we summarize a range of currently available early warning methods and apply them to two simulated time series that are typical of systems undergoing a critical transition. In addition to a methodological guide, our work offers a practical toolbox that may be used in a wide range of fields to help detect early warning signals of critical transitions in time series data.     

Regional difference of blood flow in anesthetized rats during reduced gravity induced by parabolic flight.

To examine a hypothesis that change in regional blood flow due to decreased hydrostatic pressure gradient and redistribution of blood during reduced gravity (rG) is different between organs, changes in cerebrocortical blood flow (CBF) and blood flow in the temporal muscle (MBF) with exposure to rG were measured in anesthetized rats in head-up tilt and flat positions during parabolic flight. Carotid arterial pressure (CAP), jugular venous pressure (JVP), and abdominal aortic pressure were also measured simultaneously. In the head-up tilt group, CBF increased by 15 +/- 3% within 3 s of entry into rG and rapidly recovered during rG. MBF also increased, but the change was significantly greater than that of CBF. JVP increased by 1.8 +/- 0.5 mmHg, probably due to loss of hydrostatic pressure gradient, since the measuring point of JVP was 2-3 cm above the hydrostatic indifference point. CAP and abdominal aortic pressure increased by 16.7 +/- 2 and 7.7 +/- 2 mmHg, respectively, compared with the 1-G condition. Muscle vascular resistance [(CAP-JVP)/MBF] decreased on entry into rG, but no significant change was observed in cerebrocortical vascular resistance [(CAP-JVP)/CBF]. In the flat group, no significant change was observed in all the variables. The results indicate that arteriolar vasodilatation occurs in the temporal muscle but not in the cerebral cortex. Thus the blood flow control mechanism at the onset of rG is different between intra- and extracranial organs.     

Ambulation in simulated fractional gravity using lower body positive pressure: cardiovascular safety and gait analyses.

The purpose of this study is to assess cardiovascular responses to lower body positive pressure (LBPP) and to examine the effects of LBPP unloading on gait mechanics during treadmill ambulation. We hypothesized that LBPP allows comfortable unloading of the body with minimal impact on the cardiovascular system and gait parameters. Fifteen healthy male and female subjects (22-55 yr) volunteered for the study. Nine underwent noninvasive cardiovascular studies while standing and ambulating upright in LBPP, and six completed a gait analysis protocol. During stance, heart rate decreased significantly from 83 +/- 3 beats/min in ambient pressure to 73 +/- 3 beats/min at 50 mmHg LBPP (P < 0.05). During ambulation in LBPP at 3 mph (1.34 m/s), heart rate decreased significantly from 99 +/- 4 beats/min in ambient pressure to 84 +/- 2 beats/min at 50 mmHg LBPP (P < 0.009). Blood pressure, brain oxygenation, blood flow velocity through the middle cerebral artery, and head skin microvascular blood flow did not change significantly with LBPP. As allowed by LBPP, ambulating at 60 and 20% body weight decreased ground reaction force (P < 0.05), whereas knee and ankle sagittal ranges of motion remained unaffected. In conclusion, ambulating in LBPP has no adverse impact on the systemic and head cardiovascular parameters while producing significant unweighting and minimal alterations in gait kinematics. Therefore, ambulating within LBPP is potentially a new and safe rehabilitation tool for patients to reduce loads on lower body musculoskeletal structures while preserving gait mechanics.     

Gait analysis

Gait analysis, or the study of locomotion, has changed dramatically over the last few decades. Advances in computer technology and data analysis techniques have contributed greatly to the progress of this field. Gait analysis has become a valuable tool in the clinical setting. The ability to objectively quantify motion is essential to our understanding of normal and abnormal movement patterns and the evaluation of treatment effectiveness. This paper will discuss the various experimental and analytical techniques currently used for performing clinical gait analyses at the University of New Brunswick, Fredericton, New Brunswick, Canada.