Coordinates Of Critical Point Research Articles

The effects of visual input on postural control of the lumbar spine in unstable sitting

Postural control of the lumbar spine in unstable sitting was quantified through the analysis of the center of pressure (CoP) movement recorded by a force plate situated underneath a seat that incorporated a hemisphere. Thirteen healthy subjects were tested under conditions of increasing seat instability and elimination of visual input. The purpose of this study was to determine the relative effects of visual input and support surface instability on open and closed loop postural control mechanisms in sitting and to determine the association between traditional summary statistics and random walk analysis of CoP movement. The effects of the seat instability level and visual input on the CoP movement parameters were tested with a two-factor, repeated measures ANOVA ( p<0.01). In all summary statistics CoP movement parameters increased significantly due to the seat instability level and lack of visual input. The random walk analysis identified two regions, short- and long-term, which has been postulated to represent open and closed loop control mechanism, respectively. While short-term scaling exponents were independent from visual input, CoP displacement in the short-term region was significantly increased in the eyes closed condition. Summary statistic of CoP total path length per second correlated highly with critical point coordinates and short-term diffusion coefficients. The CoP movement in the long-term region was consistent with a closed loop control mechanisms. The findings of visual influence on what is assumed as an open-loop control mechanism does not, at face value, support the hypothesis that two separate mechanisms are working to achieve postural control.

Influences of Head Orientation on the Unperturbed Postural Sway in Human Upright Stance

We examined the effects of head orientation on postural sway during quiet upright standing. Twenty healthy male subjects with diverse balance abilities were included in this study. Postural sway was estimated using an instrumented force platform to record the center-of-pressure (COP) under a subject’s feet during standing. The subjects were tested with the ‘head straight’ (normal) and with the ‘head back’ (I.e., neck extension), both conditions with eyes-open. The COP trajectories were analyzed using a technique of statistical mechanics, better known as stabilogram-diffusion analysis, proposed by Collins and De Luca (1993). The extracted parameters were the effective diffusion coefficients (D) for the short-term (less than approximately 1.0 s) and the long-term (longer than 1.0 s) intervals, respectively, as well as the Hurst exponents (H) for each of the short- and long-term intervals, and some critical point coordinates (I.e., critical mean square displacements and critical time intervals). The short-term D, short-term H, and critical mean square displacements were significantly larger with the head back condition than with the head straight condition. No significant effects of head orientation were found in the long-term D and H or in the critical time intervals. That is, head back or neck extension caused an increase in the stochastic activity and positively correlated (persistent) behavior of the COP during shorter timescales. These results suggest that head backward orientation influences the open-loop (I.e., short-term) control mechanisms, but not the corrective feedback (I.e., long-term) mechanisms used to maintain postural stability.

Changes in center-of-pressure dynamics during upright standing related to decreased balance control in young adults: fractional Brownian motion analysis.

We investigated the relationships between the ability to maintain balance in an upright stance and center-of-pressure (COP) dynamic properties in young adults. Included in this study were 10 healthy male subjects in each of two groups with respect to balance ability. Balance ability was evaluated according to the length of time a subject stood on one leg with his eyes closed. The means and ranges of this one-leg balancing time were 17.9 s (3-43 s) and 118.3 s (103-120 s) for the off-balance and balance groups, respectively. The time-varying displacements of the COP under a subject's feet during quiet two-leg (normal) standing were measured by an instrumented force platform. Each subject was tested in both the eyes-open and eyes-closed conditions. The COP trajectories were analyzed as fractional Brownian motions according to the procedure of 'stabilogram-diffusion analysis', proposed by Collins and De Luca (1993). The extracted parameters were the effective diffusion coefficients (D) for the short-term (less than about 1.0 s) and long-term intervals, respectively, as well as the Hurst exponents (H) for the short-term and long-term intervals, and some critical-point coordinates (i.e., critical mean square displacements and critical time intervals). The off-balance group showed significantly higher values for short-term D, short-term H, and critical mean square displacements than the balance group. No significant differences between the groups were found in the long-term D and H or in the critical time intervals. That is, for the off-balance subjects, an increase in the stochastic activity and positively correlated (persistent) behavior of the postural sway during shorter timescales may cause postural instability. These results suggest that the difference in balance ability for young adults is related to the open-loop (i.e., short-term) control mechanisms but not to the corrective feedback (i.e., long-term) mechanisms used to maintain balance in an upright stance.

Richards Plant Growth Model

AbstractIn this paper, the Richards function in the form y(t) = A(I +be−kt)1/(1‐m) was used to precisely analyze plant growth. The first, second and third derivative formulae of the above function were given. A new parameter, G12= f(m) was derived, enabling the calculation of the coordinates of critical points which mark the principal growth phases. The coefficients of Richards model describing the dry matter accumulation in buckwheat plants were numerically calculated. The growth curve, growth rate curve and the growth acceleration curve were also drawn, A high usefulness of approximation of the growth process of buckwheat plants by means of Richards function was confirmed statistically.

Estimation of pressure-temperature critical loci of CO 2 binary mixtures with methyl- tert-butyl ether, ethyl acetate, methyl-ethyl ketone, dioxane and decane

Pressure-temperature coordinates of critical points for binary mixtures of CO 2 and five co-solvents are estimated using a peak-shape-sensitive flow-injection procedure. CO 2 is pumped continuously under pressure control into a thermostated capillary tube which is restricted at its outlet and interfaced to a flame ionization detector. The co-solvent is injected into the capillary tube inlet at ambient temperature, where the CO 2 is a liquid at the test pressures and transported to the thermostated section of the capillary tube. The shape of the co-solvent peak as it exits the capillary tube and enters the detector is dependent on mass transport rates and phase behavior occurring earlier in the system.

An approach to determine the second virial coefficient for calculation of phase equilibria in water-protein-neutral polymer systems

A method to determine the apparent coordinates of critical points and effective values of the second virial coefficients in water-protein-neutral polymer systems enabling one to obtain the above values from data on the phase analysis within a restricted region of two-phase compositions has been proposed. The data obtained by this method for the coordinates of the critical points and binodals in water-bovine serum albumin-dextran (W-BSA-D) and waterbovine serum albumin-polyethylene glycol (W-BSA-PEG) systems are compared with experimentally determinated. Several peculiarities of the mechanism of the phase separation in above systems have been outlined in the course of this study.

The unusual coexistence curve of 2-butanol + water. An experimental and theoretical study

The coexistence curve of the 2-butanol + water system has been measured at P ≈ 1 bar, with a higher precision and a more detailed description of the pinch-off region than in previous experimental studies. The data points in the vicinity of the UCST have been fitted to a simple scaling expression to obtain the critical point coordinates and an estimation of the range of validity of simple scaling. The experimental binodal has been used to test two Ising-like models including directional interactions: an asymmetric decorated lattice one and a lattice-gas model with fluctuating orientational degrees of freedom. Both models offered comparable results when a symmetrized composition variable was used.

Spinodal decomposition and the liquid–vapour equilibrium in charged colloidal dispersions

The possibility of observing a phase separation between a high-density (‘liquid’) phase and a low-density (‘vapour’) phase in charge-stabilized aqueous dispersions of monodisperse spherical colloidal particles which are assumed to interact via the standard DLVO potential has been examined. The free energy of the system is calculated by thermodynamic perturbation theory. The contribution of the repulsive part of the potential is reduced to that of an effective hard-sphere fluid, while the van der Waals attraction is treated in the ‘high-temperature approximation’. The critical-point coordinates and spinodal and coexistence curves have been determined as functions of the potential parameters and show that a reversible ‘liquid–gas’ transition should be observable for physically reasonable values of these parameters. In a well defined range of salt concentrations it is shown that this reversible transition should be clearly distinct from irreversible coagulation, which will always occur beyond a critical salt concentration.