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At least 1,135 records · Page 63Linked to original sources

PubMed2000

The gravity sag view: a simple radiographic technique to show posterior laxity of the knee.

We introduce a simple radiographic technique of taking a "gravity sag view" to show sagittal laxity secondary to posterior cruciate ligament injury. Side-to-side differences of the tibia-femur step-off measured with these radiographs have proved to be useful parameters for evaluating posterior laxity. This radiographic technique is recommended instead of routine lateral radiographs taken with the knee kept recumbent.

Humans↗

PubMed1989

Eccentric and concentric torque-velocity characteristics, torque output comparisons, and gravity effect torque corrections for the quadriceps and hamstring muscles in females.

The purpose of this study was to assess and compare eccentric (ECC) and concentric (CONC) torque output of the quadriceps and hamstring muscles and to analyze the effect of gravity effect torque (GET) correction on the calculation of the hamstring/quadriceps peak torque quotient (H/Q quotient). Twenty female subjects performed maximal voluntary CONC and ECC contractions of the quadriceps and hamstring muscles at five isokinetic lever arm velocities from 60 degrees/s to 360 degrees/s. Peak torque was measured and corrected for GET. Mean ECC torque did not significantly change with increasing ECC velocity for either the quadriceps or hamstring muscles (P greater than 0.05). Mean CONC torques were significantly lower than the corresponding ECC torques (P less than 0.05) and decreased with increasing CONC velocity. At each test velocity, the CONC H/Q quotient was significantly lower than the corresponding ECC H/Q quotient (P less than 0.05). Mean H/Q quotients did not significantly change with increasing velocity for either the CONC or ECC tests (means: 0.46 and 0.57; P greater than 0.05). Mean H/Q quotients not corrected for GET significantly increased with increasing velocity for the CONC (0.61 to 0.78; P less than 0.05), but not ECC tests (0.66 to 0.71; P greater than 0.05). The results indicate that the ECC torque-velocity curve is essentially level for both quadriceps and hamstring muscles. The present findings point strongly toward the necessity of correcting for GET when calculating both CONC and ECC H/Q quotients.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1997

Quantitative evaluation of the effects of gravity and dependency on microvascular tissue transfer to the lower limb, with clinical applications.

The authors have attempted quantitatively to evaluate the effects of gravity and dependency on free-tissue transfers to the lower limb, particularly in the immediate postoperative period, using the diagnostic modalities of Doppler ultrasonography and interstitial compartment pressure measurements. Results of these quantitative evaluations promoted a change in postoperative management among patients undergoing lower-limb tissue transfer. The experience in two patient cohorts is presented.

Adult↗

PubMed2004

Chaotic attitude oscillations of a satellite filled with a rotating ellipsoidal mass of liquid subject to gravity-gradient torques.

The Hamiltonian equations of a liquid-filled satellite subject to gravity-gradient torques in terms of the generalized Deprit variables are established for applying the Melnikov theory formally. The heteroclinic orbits of the torque-free symmetric liquid-filled satellite are found. A criterion for the heteroclinic transversal intersections at the onset of chaotic attitude is formulated using Melnikov's integral. The results from the Melnikov theory are crosschecked with simulation.

Acceleration↗

PubMed1994

The coherent structures of shallow-water turbulence: Deformation-radius effects, cyclone/anticyclone asymmetry and gravity-wave generation.

Over a large range of Rossby and Froude numbers, we investigate the dynamics of initially balanced decaying turbulence in a shallow rotating fluid layer. As in the case of incompressible two-dimensional decaying turbulence, coherent vortex structures spontaneously emerge from the initially random flow. However, owing to the presence of a free surface, a wealth of new phenomena appear in the shallow-water system. The upscale energy cascade, common to strongly rotating flows, is arrested by the presence of a finite Rossby deformation radius. Moreover, in contrast to near-geostrophic dynamics, a strong asymmetry is observed to develop as the Froude number is increased, leading to a clear dominance of anticyclonic vortices over cyclonic ones, even though no beta effect is present in the system. Finally, we observe gravity waves to be generated around the vortex structures, and, in the strongest cases, they appear in the form of shocks. We briefly discuss the relevance of this study to the vortices observed in Jupiter's atmosphere.

Journal Article↗

PubMed2004

Gravity-induced liquid crystal phase transitions of colloidal platelets.

The influence of gravity on a suspension of sterically stabilized colloidal gibbsite platelets is studied. An initially isotropic-nematic biphasic sample of such a suspension develops a columnar phase on the bottom on prolonged standing. This phenomenon is described using a simple osmotic compression model. We performed Monte Carlo simulations of cut spheres with aspect ratio L/D=1/15 and took data from the literature to supply the equations of state required for the model. We find that the model describes the observed three-phase equilibrium quite well.

Journal Article↗

PubMed2006

Monte Carlo simulation of crystal-fluid coexistence states in the hard-sphere system under gravity with stepwise control.

Monte Carlo (MC) simulations were performed for hard spheres (with diameter sigma and mass m) placed between well-separated upper and lower hard walls. A periodic boundary condition was imposed in the horizontal direction. The system was exposed to the gravitational field with the acceleration due to gravity g. After preparing a melt as the initial state, g was increased stepwise up to mgsigma/k(B)T(identical with g(*))=1.5 or 2.0 with an increment Deltag(*) = 0.1; k(B)T is the temperature multiplied by Boltzmann's constant. We maintained g(*) at each value for 2.0 x 10(5) MC cycles. The transition of the system into a metastable state such as a polycrystalline state due to trapping phenomena was successfully avoided. A monotonic increase and subsequent saturation were observed for the development of the crystalline region formed at the bottom of the system. The development of this region accompanied a shrinkage of the defective (or less ordered) crystalline region that was formed between the bottom region and the fluid phase. As the development of the bottom region almost saturated, the defective region grew upward again.

Journal Article↗

PubMed2006

The density profile of hard sphere liquid system under gravity.

The density profile of hard sphere liquid under gravity is calculated by using density functional theory and Monte Carlo simulation method. The two methods give consistent results for a wide range of parameters. Meanwhile, the validity range of the density functional theory is also established. The results are quite different from the barometric height distribution rho(z)=rho(0) exp(-zL(G)) in almost all cases studied, which indicates that the interaction between particles plays an important role in the density distribution under external fields. Moreover, the crystallizing phenomenon is also predicted at the bottom part of the system under strong gravitation.

Journal Article↗

PubMed2000

Portal vein blood flow-effects of pregnancy, gravity, and exercise.

OBJECTIVE: We sought to test the hypotheses that pregnancy increases portal vein blood flow and that regular exercise training during pregnancy limits the flow redistribution away from the splanchnic and uterine circulations in response to either gravitational or exercise-induced hemodynamic stress. STUDY DESIGN: Portal vein blood flow, which probably reflects changes in uterine blood flow, was estimated with ultrasonography in 6 regularly exercising and 6 physically active control subjects before and during pregnancy after 15 minutes of rest in the left lateral recumbent position, after 5 minutes of standing rest, and immediately and 5 minutes after 20 minutes of treadmill exercise at 55% +/- 3% of maximal aerobic capacity. RESULTS: Portal vein blood flow rose significantly during early and mid pregnancy at recumbent rest (from 660 +/- 110 to 1090 +/- 120 mL/min), standing rest (580 +/- 70 to 790 +/- 120 mL/min), immediately after exercise (160 +/- 30 to 360 +/- 60 mL/min), and at 5 minutes of recovery (520 +/- 60 to 760 +/- 110 mL/min). Before pregnancy, exercise training did not blunt the decremental effects of either gravity or exercise on portal vein blood flow. During mid and late pregnancy, exercise training had no effect on the fall in portal vein blood flow with gravitational stress, but it markedly reduced the decremental effects of exercise (average for the 2 time points, -510 +/- 80 vs -840 +/- 100 mL/min) and improved recovery at 5 minutes after exercise (940 +/- 140 vs 600 +/- 130 mL/min). CONCLUSIONS: Portal vein blood flow rises significantly during pregnancy, and flow redistribution away from the splanchnic and uterine circulations in response to severe hemodynamic stress is reduced by exercise training in mid and late pregnancy.

Adult↗

PubMed1982

Spatial aggregation in gravity models: 2. One-dimensional population density models.

"This paper, the second of four, is concerned with applying a methodology for analysing the spatial aggregation problem in gravity models outlined in the first paper. The methodology is based on a consistent framework for linking measures of pattern in interaction data to the derivation and estimation of related interaction models using spatial information. In this quest, a link is forged between information in data and the parameters of an associated model.... This relationship can be formalised for the case of the continuous one-dimensional interaction model such as the population density model, and this paper is concerned with demonstrating such an application to aggregations of zones in the Reading [England] region."

Demography↗

PubMed1982

Spatial aggregation in gravity models: 4. Generalisations and large-scale applications.

"This paper is concerned with applying and extending a methodology for analysing spatial aggregation in gravity models developed in three earlier papers to a larger scale and hence more realistic example of spatial interaction than has been treated so far." Seven models are generated and applied to four levels of aggregation of the spatial interaction pattern in Edmonton, Alberta.

Americas↗

PubMed2005

Two reference frames for visual perception in two gravity conditions.

The processing and storage of visual information concerning the orientation of objects in space is carried out in anisotropic reference frames in which all orientations are not treated equally. The perceptual anisotropies, and the implicit reference frames that they define, are evidenced by the observation of 'oblique effects' in which performance on a given perceptual task is better for horizontally and vertically oriented stimuli. The question remains how the preferred horizontal and vertical reference frames are defined. In these experiments cosmonaut subjects reproduced the remembered orientation of a visual stimulus in 1g (on the ground) and in 0g, both attached to a chair and while free-floating within the International Space Station. Results show that while the remembered orientation of a visual stimulus may be stored in a multimodal reference frame that includes gravity, an egocentric reference is sufficient to elicit the oblique effect when all gravitational and haptic cues are absent.

Analysis of Variance↗

PubMed1981

On the importance of changes in the gravity field on seismic recording at ultralong periods.

The effect of changes in gravity induced by the Earth's deformation on seismic recording at ultralong periods is studied quantitatively for the low-order spheroidal modes of the Earth. Because this effect can either enhance or reduce the recorded amplitude of a mode, depending on its geometry, it may become nontrivial at the longest free-oscillation periods, and contribute to relative deviations of up to 17%.

Journal Article↗

PubMed2000

Influence of gravity on the spatial orientation of eye nystagmus induced by unilateral lesion of horizontal semicircular canal.

The influence of gravity in the orientation and slow phase eye velocity of the ocular nystagmus following unilateral damage of the cupula in the ampulla of the horizontal semicircular canal (UHCD) was investigated. The nystagmus was analysed at different sagittal head positions using the x-y infrared eye monitor technique. The nystagmus was almost horizontal at 0 degrees head pitch angle and remained partially fixed in space when the head was pitched upward or downward. The reorientation gain of the slow and quick phases was high (about 0.75) within +/- 45 degrees of head pitch angle, but beyond this range, it decreased greatly. The gain value depended on the lesion extension to otolithic receptors. The absolute value of the slow phase eye velocity of UHCD nystagmus was also modified systematically by the head pitch, showing a reduction in the upward and an increase in the downward.

Animals↗

PubMed2000

Center of gravity motions and ankle joint stiffness control in upright undisturbed stance modeled through a fractional Brownian motion framework.

The authors modeled the center of gravity vertical projection (CG(v)) and the difference, CP - CG(v), which, combined, constitute the center of pressure (CP) trajectory, as fractional Brownian motion in order to investigate their relative contributions and their spatiotemporal articulation. The results demonstrated that CG(v) and CP - CG(v) motions are both endowed in complementary fashion with strong stochastic and part-deterministic behaviors. In addition, if the temporal coordinates remain similar for all 3 trajectories by definition, the switch between the successive control mechanisms appears for shorter displacements for CP - CG(v) and CG(v) than for CP trajectories. Results deduced from both input (CG(v)) and muscular stiffness (CP - CG(v)) thus provide insight into the way the central nervous system regulates stance control and in particular how CG and CP - CG are controlled.

Adult↗

PubMed1977

Rapid separation of cells at unit gravity.

An apparatus is described for the rapid separation of cells at unit gravity. The apparatus is filled with a discontinuous density gradient, followed by the sample and then an overlay. The sedimentation chamber is turned from a vertical to a horizontal position before sedimentation takes place in order, (a) to form a continuous gradient out of the discontinuous gradient and, (b) to reduce the time required for the separation. After the sedimentation is completed the chamber is returned to a vertical position and the gradient fractionated by hydrostatic pressure using a newly developed floating device. The usefulness of the method is illustrated by the separation of leukemia cells in different phases of the life cycle.

Animals↗

PubMed2003

Head and body center of gravity control strategies: adaptations following vestibular rehabilitation.

OBJECTIVE: We present for the first time evidence that vestibulopathy impairs coordination of the head with the body center of gravity (CG) during free speed gait over ground. Vestibulopathic individuals demonstrate uncoordinated movement and gait due, at least in part, to impaired head stability and visual fixation. Vestibular rehabilitation increases speed and stability during gait and stair climbing, although the underlying mechanisms are poorly understood. MATERIAL AND METHODS: To determine whether these locomotor improvements are due to reorganized coordination of the head with whole body CG, three-dimensional kinematics were obtained from 10 vestibulopathic individuals before and after vestibular rehabilitation and from 10 matched healthy control subjects during unconstrained, paced and in-place gait. Head control patterns were characterized using both qualitative pattern analysis and quantification of coherence between head and body CG displacements. RESULTS: Patterns of head-CG coordination differ between normal and vestibulopathic individuals in all three directions of head rotation--pitch, roll and yaw--before rehabilitation. Following vestibular rehabilitation, subjects with vestibulopathy demonstrate more normal patterns in pitch and improvements toward normal in roll and yaw. CONCLUSION: These data strongly suggest that compensatory mechanisms, obtained during vestibular rehabilitation, mediate head-CG coordination.

Adaptation, Physiological↗

PubMed2004

Measles metapopulation dynamics: a gravity model for epidemiological coupling and dynamics.

Infectious diseases provide a particularly clear illustration of the spatiotemporal underpinnings of consumer-resource dynamics. The paradigm is provided by extremely contagious, acute, immunizing childhood infections. Partially synchronized, unstable oscillations are punctuated by local extinctions. This, in turn, can result in spatial differentiation in the timing of epidemics and, depending on the nature of spatial contagion, may result in traveling waves. Measles epidemics are one of a few systems documented well enough to reveal all of these properties and how they are affected by spatiotemporal variations in population structure and demography. On the basis of a gravity coupling model and a time series susceptible-infected-recovered (TSIR) model for local dynamics, we propose a metapopulation model for regional measles dynamics. The model can capture all the major spatiotemporal properties in prevaccination epidemics of measles in England and Wales.

Demography↗