Time-temperature studies of gellan polysaccharide-high sugar mixtures: effect of sodium ions on structure formation.

Changes to the viscoelastic storage and loss moduli were measured as a function of temperature and oscillatory frequency for 0.5% (w/w) gellan:80% (w/w) cosolute dispersions with added Na(+) (40 to 160 mM). Isothermal frequency (0.15 to 15 Hz) and thermal scans (at 0.15 Hz) were performed over a decreasing then increasing temperature range of 85 to 5 degrees C and 5 to 85 degrees C, respectively. Moduli were found to increase in magnitude with decreasing temperature and increasing levels of Na(+) during cooling, then remained relatively thermally irreversible upon heating. Isothermal frequency (ITF) data were described using the time-temperature superposition (TTS) principle and the modified Cole-Cole (MCC) analysis. Both TTS and the MCC analyses successfully described the behavior of samples containing 40 mM added Na(+) during cooling and heating, and at the 100-mM Na(+) level during cooling. TTS superposed ITF data over the entire temperature range, whereas successful superposition was restricted to lower temperatures in the MCC analysis, where the viscoelastic response was dominated by the long-range relaxation of gellan chains between junction zones. Failure of both analyses was attributed to the formation of junction zones composed of polymer-polymer associations. It is proposed that the addition of Na(+) promotes the formation of a weakly cross-linked gellan network.

Contributions of altered sensation and feedback responses to changes in coordination of postural control due to aging.

We used multivariate kinematics and joint torque measurements during dynamic posturography to determine the relative contributions of changes in overall control gain, relative weighting of sensors, and noise-like effects on posture control in the elderly. Our results show that sway coordination and amplitude both change with age, but that changes in overall feedback gains do not explain these differences. We propose that increased sway of elderly subjects in platform sway-referenced conditions is due to sensory noise or decreased ability to detect small motions of the platform, while increased sway during visual sway-referencing is due to re-weighting of the various sensors.

Modeling of orthokinetic flocculation of Saccharomyces cerevisiae.

This study examined the flocculation behavior of two Saccharomyces cerevisiae strains expressing either Flo1 (LCC1209) genotype or NewFlo (LCC125) phenotype in a laminar flow field by measurement of the fundamental flocculation parameter, the orthokinetic capture coefficient. This orthokinetic capture coefficient was measured as a function of shear rate (5.95-223 s(-1)) and temperature (5-45 degrees C). The capture coefficients of these suspensions were directly proportional to the inverse of shear rate, and exhibited an increase as the temperature was increased to 45 degrees C. The capture coefficient of pronase-treated cells was also measured over similar shear rate and temperature range. A theory, which predicts capture coefficient values due to zymolectin interactions, was simplified from that developed by Long et al. [Biophys. J. 76: (1999) 1112]. This new modified theory uses estimates of: (1) cell wall densities of zymolectins and carbohydrate ligands; (2) cell wall collision contact area; and (3) the forward rate coefficient of binding to predict theoretical capture coefficients. A second model that involves both zymolectin interactions and DLVO forces was used to describe the phenomenon of yeast flocculation at intermediate shear ranges, to explain yeast flocculation in laminar flow.

EquiTest modification with shank and hip angle measurements: differences with age among normal subjects.

The Sensory Organization Test protocol of the EquiTest system (NeuroCom International, Clackamas Oregon) tests utilization of visual, vestibular, and proprioceptive sensors by manipulating the accuracy of visual and/or somatosensory inputs during quiet stance. In the standard Sensory Organization Test, both manipulation of sensory input (sway-referencing) and assessment of postural sway are based on ground reaction forces measured from a forceplate. The purpose of our investigation was to examine the use of kinematic measurements to provide a more direct feedback signal for sway-referencing and for assessment of sway. We compared three methods of sway-referencing: the standard EquiTest method based on ground reaction torque, kinematic feedback based on servo-controlling to shank motion, and a more complex kinematic feedback based on servo-controlling to follow position of the center of mass (COM) as calculated from a two-link biomechanical model. Fifty-one normal subjects (ages 20-79) performed the randomized protocol. When using either shank or COM angle for sway-referencing feedback as compared to the standard EquiTest protocol, the Equilibrium Quotient and Strategy Score assessments were decreased for all age groups in the platform sway-referenced conditions (SOT 4, 5, 6). For all groups of subjects, there were significant differences in one or more of the kinematic sway measures of shank, hip, or COM angle when using either of the alternative sway-referencing parameters as compared to the standard EquiTest protocol. The increased sensitivities arising from use of kinematics had the effect of amplifying differences with age. For sway-referencing, the direct kinematic feedback may enhance ability to reduce proprioceptive information by servo-controlling more closely to actual ankle motion. For assessment, kinematics measurements can potentially increase sensitivity for detection of balance disorders, because it may be possible to discriminate between body sway and acceleration and to determine the phase relationship between ankle and hip motion.

Multivariate changes in coordination of postural control following spaceflight.

Postural and gait instabilities in astronauts returning from spaceflight are thought to result from in-flight adaptation of central nervous system processing of sensory inputs from the vestibular, proprioceptive, and visual systems. We hypothesized that reorganization of posture control relying on these multiple inputs would result in not only greater amounts of sway, but also changes in interjoint coordination. We tested this hypothesis by examining the multivariate characteristics of postural sway and comparing the postural control gain used for maintenance of upright stance during the altered sensory conditions of the Sensory Organization Test (EquiTest, Neurocom Intl.). We used the covariance of hip and ankle kinematics as a measure of joint motion and interjoint coordination, and then utilized discriminant analysis to further examine these characteristics in a group of 10 first-time astronauts. In five of the six conditions, the most important difference was an increased relative utilization of the hip strategy, which would not be evident using conventional balance measures such as peak or root-mean-square sway. This finding was supported by indications of increased hip torque gains relative to lower extremity and neck motion in at least four conditions (p < 0.05). In contrast, ankle torque gains to these motions did not appear to change. These results suggest that after spaceflight, astronauts exhibit significant multivariate changes in multijoint coordination, of which increased sway is only one component. These changes are consistent with reweighting of vestibular inputs and changes in control strategy in a multivariable control system.

Effect of altered sensory conditions on multivariate descriptors of human postural sway.

Multivariate descriptors of sway were used to test whether altered sensory conditions result not only in changes in amount of sway but also in postural coordination. Eigenvalues and directions of eigenvectors of the covariance of shnk and hip angles were used as a set of multivariate descriptors. These quantities were measured in 14 healthy adult subjects performing the Sensory Organization test, which disrupts visual and somatosensory information used for spatial orientation. Multivariate analysis of variance and discriminant analysis showed that resulting sway changes were at least bivariate in character, with visual and somatosensory conditions producing distinct changes in postural coordination. The most significant changes were found when somatosensory information was disrupted by sway-referencing of the support surface (P = 3.2 x 10(-10)). The resulting covariance measurements showed that subjects not only swayed more but also used increased hip motion analogous to the hip strategy. Disruption of vision, by either closing the eyes or sway-referencing the visual surround, also resulted in altered sway (P = 1.7 x 10(-10)), with proportionately more motion of the center of mass than with platform sway-referencing. As shown by discriminant analysis, an optimal univariate measure could explain at most 90% of the behavior due to altered sensory conditions. The remaining 10%, while smaller, are highly significant changes in posture control that depend on sensory conditions. The results imply that normal postural coordination of the trunk and legs requires both somatosensory and visual information and that each sensory modality makes a unique contribution to posture control. Descending postural commands are multivariate in nature, and the motion at each joint is affected uniquely by input from multiple sensors.

Density of lectin-like receptors in the FLO1 phenotype of Saccharomyces cerevisiae.

An improved lectin assay was recently developed to determine the density of lectin sites present on the surface of a haploid strain 1209 of Saccharomyces cerevisiae exhibiting the Flo1 phenotype. The assay is based on the use of FITC-avidin which contains a mannose side chain which functions as a binding site to the cell wall lectins termed zymolectins. An examination of the effect of culture time on the zymolectin density indicated no significant effect (P > 0.05) confirming other findings that Flo1 acts in a constitutive manner. Zymolectin densities for this strain averaged 5.4 x 10(6) sites cell-1.

Hamstrings and psoas lengths during normal and crouch gait: implications for muscle-tendon surgery.

Crouch gait, one of the most common movement abnormalities among children with cerebral palsy, is characterized by persistent flexion of the knee during the stance phase. Short hamstrings are thought to be the cause of crouch gait; thus, crouch gait is often treated by surgical lengthening of the hamstrings. In this study, a graphics-based model of the lower extremity was used in conjunction with three-dimensional kinematic data obtained from gait analysis to estimate the lengths of the hamstrings and psoas muscles during normal and crouch gaits. Only three of 14 subjects with crouch gait (four of 20 limbs with knee flexion of 20 degrees or more throughout stance) had hamstrings that were shorter than normal by more than 1 SD during walking. Most (80%) of the subjects with crouch gait had hamstrings of normal length or longer, despite persistent knee flexion during stance. This occurred because the excessive knee flexion was typically accompanied by excessive hip flexion throughout the gait cycle. All of the subjects with crouch gait had a psoas that was shorter than normal by more than 1 SD during walking. These results emphasize the need to consider the geometry and kinematics of multiple joints before performing surgical procedures aimed at correcting crouch gait.