Joint Variance Research Articles

Two Kinematic Synergies in Voluntary Whole-Body Movements During Standing

We used a particular computational approach, the uncontrolled manifold hypothesis, to investigate joint angle covariation patterns during whole-body actions performed by standing persons. We hypothesized that two kinematic synergies accounted for the leg/trunk joint covariation across cycles during a rhythmic whole-body motion to stabilize two performance variables, the trunk orientation in the external space and the horizontal position of the center of mass (COM). Subjects stood on a force plate and performed whole-body rhythmic movements for 45 s under visual feedback on one of the four variables, the position of the center of pressure or the angle in one of the three joints (ankle, knee, or hip). The Fitts-like paradigm was used with two target amplitudes and six indices of difficulty (ID) for each of the four variables. This was done to explore the robustness of kinematic postural synergies. A speed-accuracy trade-off was observed in all feedback conditions such that the movement time scaled with ID and the scaling differed between the two movement amplitudes. Principal-component (PC) analysis showed the existence of a single PC in the joint space that accounted for over 95% of the joint angle variance. Analysis within the uncontrolled manifold hypothesis has shown that data distributions in the joint angle space were compatible with stabilization of both trunk orientation and COM location. We conclude that trunk orientation and the COM location are stabilized by co-varied changes of the major joint angles during whole-body movements. Despite the strong effects of movement amplitude and ID on performance, the structure of the joint variance showed only minor dependence on these task parameters. The two kinematic synergies (co-varied changes in the joint angles that stabilized the COM location and trunk orientation) have proven to be robust over a variety of tasks.

Joint coordination during quiet stance: effects of vision

Stabilization of the center of mass (CM) is an important goal of the postural control system. Coordination of several joints along the human "pendulum" is required to achieve this goal. We studied the coordination among body segments with respect to horizontal CM stabilization during a quiet stance task and the effects of vision on CM stability. Subjects were asked to stand quietly on a narrow wooden block supporting only the mid-foot, with either open (EO) or closed (EC) eyes on separate trials. Instant equilibrium points (IEPs) in the center of pressure (CP) trajectory were determined when the horizontal component of the ground reaction force was zero and the CP data were decomposed into their rambling and trembling components. The joint angle, CM and CP data were divided into short cycles (time-normalized to 100 data points) or longer segments (time-normalized to 1000 data points) of equal length beginning and ending in an IEP. Motor abundance with respect to patterns of joint coordination was evaluated using the uncontrolled manifold (UCM) approach. Here, a UCM is a subspace spanning all joint combinations resulting in a given CM position. All combinations of joint angles that lie within this subspace are equivalent with respect to that CM position while joint angle combinations lying in a subspace orthogonal to the UCM lead to deviation from that CM position. UCM analysis was performed on data organized either across time within longer segments or at each point in time across multiple segments or across multiple cycles. Regardless of method of analysis, most of the variance in joint space was constrained to be within the UCM, preserving the mean CM position in both the EO and EC conditions. Joint configuration variance was significantly higher in the EC than in the EO condition although this increase occurred primarily within the UCM rather than in the orthogonal subspace that would have led to variation of the CM position. These results demonstrate the ability of the control system to selectively "channel" motor variability into directions in joint space that stabilize the CM position. This effect was enhanced when the task was made more challenging in the absence of vision. There was also a significant relationship between joint variance that led to a change in the CM position and, in particular, the rambling component of the CP path, lending some support to the idea that the CNS prescribes a certain stable trajectory of the CP during quiet stance that leads to a small controlled movement of the CM.

Learning a throwing task is associated with differential changes in the use of motor abundance

This study sought to characterize changes in the synergy of joint motions related to learning a Frisbee throwing task and, in particular, how the use of abundant solutions to joint coordination changed during the course of learning for successful performance. The latter information was helpful in determining the relative importance of different performance-related variables (PVs) to performance improvement. Following a pre-test, the main experiment consisted of six subjects practicing a Frisbee throw to a laterally-placed target for five days, 150 throws per day, followed by a post-test. A subgroup of three subjects continued to practice for an extended period of extensive practice amounting to 1800-2700 additional throws each, followed by a second post-test. Motor abundance was addressed through the uncontrolled manifold approach (UCM), which was used to partition the variance of joint configurations into two components with respect to relevant PVs, one component leading to a consistent value of the PV across repetitions, and a reflection of motor abundance, and a second component resulting in unstable values of the relevant PV. The method was used to test hypotheses about the relative importance of controlling the PVs that have an impact on successful task performance: movement extent, movement direction, hand path velocity, and the hand's orientation to the target. In addition, the amount of self-motion, or apparently extraneous joint motion having no effect on the hand's motion, compared to joint motion that does affect the hand's motion, was determined. After a week of practice, all subjects showed improvement in terms of targeting accuracy. Hand movement variability also decreased with practice and this was associated with a decrease in overall joint configuration variance. This trend continued to a greater extent in the three subjects who participated in extended practice. Although the component of joint configuration variance that was consistent with a stable value of all PVs was typically substantially higher than variance leading to unstable values of those PVs, both components decreased with practice. However, the decrease in joint configuration variance reflecting motor abundance was less than the other variance component only in relation to control of movement direction and the hand's orientation to the target. These results indicate that improvement of throwing performance in this experiment was more related to improved stabilization of movement direction and to the hand's orientation to the target than to movement extent and hand velocity. Nonetheless, the relative values of the two joint variance components were such that the instantaneous value of both hand path velocity and movement extent were stabilized throughout the experiment and showed a consistent compensatory relationship at the time of Frisbee release, despite not changing with practice. Finally, the amount of self-motion increased significantly with practice, possibly reflecting better compensation for perturbations due to the limb's dynamics. The results are consistent with other studies, suggesting the need to reevaluate Bernstein's hypothesis of freeing and freezing DOFs with learning.

Effect of mass selection on the mean and genetic variance of maize cultivar AL 25

th cycle. Randomized complete blocks were used for evaluating grain yield and ear height. Individual and joint variance analyses considering the two years of experimental duration compared the mean and variance components of both cycles. A slight reduction in additive genetic variance was observed as the selection proceeded for both analyzed traits. The increase in the means of grain yield and ear height with the selection cycles was also little expressive. The mass selection cycles did not promote significant alterations in this cultivar’s mean and variance.

Aspects of joint coordination are preserved during pointing in persons with post-stroke hemiparesis.

Understanding the fundamental deficits that underlie abnormal reaching movements in persons with hemiparesis is important to the development of rehabilitation approaches for these persons. The purpose of the present study was to investigate whether, and to what extent, persons with hemiparesis retain the ability to exploit motor abundance to coordinate their joint motions to control the hand's path during reaching--a general feature of joint coordination in age-matched control persons. Eight subjects with mild to moderate right hemiparesis following a stroke and seven age and gender matched control subjects performed pointing movements with each arm individually to targets in the contralateral and ipsilateral workspaces. Ten joint motions and characteristics of hand movement were measured over multiple repetitions. The variance (across trials) of joint combinations was partitioned into two components at every point in the hand's trajectory; joint variance that led to a consistent hand position and joint variance that led to an inconsistent hand position from trial to trial. All participants were able to limit joint configurations that would have led to trial-to-trial variance of the hand's path, while using a range of joint configurations consistent with a stable hand path. These results demonstrate that persons with mild to moderate hemiparesis and no measured sensory or perceptual deficits utilized available motor abundance to stabilize performance variables that were important to successful completion of a reaching task, just as was found for age-matched control persons. A principal components analysis of joint angle variance across several points in the movement cycle demonstrated that the persons with hemiparesis showed different patterns of joint couplings during reaching compared with the age-matched control persons; this was particularly evident for the more impaired individuals. These findings were related to the two features of synergy as proposed by other workers. It is suggested that these results support the idea that, while subjects with mild-to-moderate hemiparesis demonstrate differences in the feature of a synergy related to the specific patterns of joint coupling, they retain the feature of a synergy that is related to error compensation.

Age, aging, and the sense of control among older adults: a longitudinal reconsideration.

The purpose of this study was to first estimate the crude cross-sectional and longitudinal associations between age and the sense of control, and then to partition any joint variance attributable to a theoretically specified set of potential confounders. Sense of control was measured at baseline and at each of six bimonthly follow-up interviews among 1,662 patients at two medical centers. Potential confounders were measured at baseline. Analyses include descriptive assessments of level and normative stability, repeated measures analysis of covariance, and hierarchical multiple linear and change score regressions. Although the sense of control is relatively stable between any two successive waves of data collection, significant gradual changes are observed over a 1-year period. Compelling evidence is found for statistically and substantively significant associations between age and the sense of control at baseline, and between age and changes in the sense of control over time. The only other major predictor of the sense of control is mental well-being. Longitudinal studies with repeated assessments over prolonged observation periods are now needed to elucidate age-related trajectories in the sense of control.

Spatial uncertainty analysis for mapping soil erodibility based on joint sequential simulation

Soil erodibility (susceptibility of soil to be lost to erosion) is one of the components of the universal soil loss equation (USLE). In the USLE, erodibility is known as the K factor, which in turn is a function of these soil properties: particle size distribution, organic matter content, structure, and permeability. The traditional approach for estimating soil erodibility does not account for spatial variability of individual soil properties or spatial correlation among those properties. Our objectives in this study were to evaluate the use of joint sequential simulation for mapping soil erodibility, as well as to partition the individual and joint variance contribution of soil properties for predicting soil erodibility. We collected 192 usable soil samples across Fort Hood, Texas in the summer of 1999. For each of those samples, we obtained an estimate of particle size distribution, organic matter content, structure, permeability, and calculated soil erodibility. We carried out both independent and joint sequential simulation to generate spatially explicit predictions and variance of all soil properties as well as covariance between pairs of soil properties for each cell within our simulation area. We used the program GCOSIM3D to conduct those simulations. On average, joint sequential simulation resulted in a K factor variance of less than half the variance obtained from independent simulation. Using the results from joint sequential simulation, we partitioned the contribution of each soil property and pair of properties using first-order Taylor series expansion of the soil erodibility function. Individually, Very-Fine-Sand-and-Silt contributed the most (46.19%), whereas Structure contributed the least (6.53%) to the K factor variance. Jointly, Permeability/Structure contributed the most (9.32%), whereas Sand/Very-Fine-Sand-and-Silt caused the largest reduction (−19.19%) in the K factor variance. We conclude that joint sequential simulation provided approximately twice as much precision as independent simulation for the spatially explicit prediction of soil erodibility. Likewise, first-order Taylor series expansion offered an accurate approach for partitioning the individual and joint contribution of soil properties to soil erodibility variance. This partitioning allowed us to identify large sources of uncertainty and suggest efficient approaches for further improving the precision of K value predictions.

On market efficiency of Asian foreign exchange rates: evidence from a joint variance ratio test and technical trading rules

Abstract This study employs a joint variance ratio test and technical trading rules to examine the random walk behavior for nine Asian foreign exchange rates for the period 1988–1995. The joint variance ratio test results suggest that there is little evidence of serial correlations in the daily exchange rate series. The results also indicate that, in general, the moving average and channel trading rules do not generate significant, positive profits.

Multivariate image regression (MIR): implementation of image PLSR?first forays

In the effort of analysing multivariate images, image PLS has been considered interesting. In this paper, image PLS (MIR) is compared with image PCA (MIA) by studying a comparison data set. While MIA has been commercially available for some time, image PLS has not. The kernel PLS algorithm of Lindgren has been implemented in a development environment which is a combination of G (LabVIEW) and MATLAB. In this presentation the power of this environment, as well as an early example in image regression, will be demonstrated. With kernel PLS, all PLS vectors (eigenvectors and eigenvalues) can be calculated from the joint variance–covariance (X′Y and Y′X) and association (Y′Y and X′X) matrices. The dimensions of the kernel matrices X′YY′X and Y′XX′Y are K × K (K is the number of X-variables) and M × M (M is the number of Y-variables) respectively. Hence their size is dependent only on the number of X and Y-variables and not on the number of observations (pixels), which is crucial in image analysis. The choice of LabVIEW as development platform has been based on our experience of a very short implementation time combined with user-friendly interface possibilities. Integrating LabVIEW with MATLAB has speeded up the decomposition calculations, which otherwise are slow. Also, algorithms for matrix calculations are easier to formulate in MATLAB than in LabVIEW. Applying this algorithm on a representative test image which shows many of the typical features found in technical imagery, we have shown that image PLS (MIR) decomposes the data differently than image PCA (MIA), in accordance with chemometric experience from ordinary two-way matrices. In the present example the Y-reference texture-related image used turned out to be able to force a rather significant ‘tilting’ compared with an ‘ordinary MIA’ of the primary structures in the original, spectral R/G image. Copyright © 2000 John Wiley & Sons, Ltd.

Adaptabilidade e estabilidade fenotípica em genótipos de feijão de cor (Phaseolus vulgaris L.) em três ambientes distintos

No biênio 1996/97 foram testados, em três ambientes, vinte e um genótipos de feijão de cor, com o objetivo de identificar e avaliar os parâmetros de adaptabilidade e estabilidade fenotípica para a produtividade de grãos. Foi utilizado o delineamento experimental, blocos casualizados, com três repetições por tratamento. Os genótipos revelaram diferenças altamente significativas entre si, em todos os ambientes avaliados. Na análise de variância conjunta, os efeitos das causas de variação foram altamente significativos, evidenciando diferenças entre genótipos, ambientes e a existência de um comportamento diferenciado entre os genótipos frente às variações de ambientes. O rendimento médio de grãos variou de 1321kg/ha a 801kg/ha, com média geral de 1081kg/ha. A maioria dos genótipos demonstraram alta estabilidade fenotípica, merecendo destaque os genótipos LP 93-15, LP 93,2, LP 93-38 e Rudá, os quais evidenciaram rendimento médio superior à média geral dos ensaios, adaptabilidade geral e comportamento previsível em todos os ambientes estudados. Entre os genótipos com rendimento de grãos abaixo da média geral podem ser destacados os genótipos AN 9022180, LM 93204319, LR 91155315, PF 9029975, PF 9029984 e TB 94-05, com adaptabilidade geral e alta estabilidade fenotípica nos ambientes avaliados.

ESTRATIFICAÇÃO DE AMBIENTES EM EXPERIMENTOS DE FEIJOEIRO NO ESTADO DE SÃO PAULO

Efetuaram-se análises da variância conjunta em três experimentos de feijoeiro instalados em várias localidades paulistas, em 1990-93, na época da seca (plantio em janeiro--fevereiro) e na de inverno (maio-junho) e, em 1990-92, na das águas (setembro-outubro), objetivando-se quantificar os valores da interação genótipo por ambiente (GA) e identificar os locais onde essa interação se mostrou menos pronunciada ou, ao contrário, mais contrastante. Os experimentos E1, E2 e E3, delineados em blocos ao acaso com cinco repetições, continham 17, 13 e 13 genótipos respectivamente. Os resultados identificaram Pariquera-Açu, Capão Bonito e Itaberá como ambientes contrastantes, ou seja, com interação GA altamente significativa. Em Ribeirão Preto, Votuporanga, Aguaí, Mococa e Riversul, verificou-se a formação de vários grupos de ambientes com interações GA não significativas; sendo assim, esses locais foram considerados os mais homogêneos para análise de produtividade e avaliação de genótipos.

Heritability estimates for callus growth and regeneration in desmodium

The F2 and F3 generations of two crosses (6123×13083 and 6123×144, with 6123 the regenerating parent) were evaluated for callus growth and regeneration capacity. Based on joint scaling tests and variance partitioning, neither callus growth nor regeneration fitted a simple additive-dominant genetic model. Heritability estimates obtained from parent-offspring regression analyses ranged from 0.65 to 0.77 for callus growth and from 0.19 to 0.46 for regeneration, with the range in both influenced by the cross and numerical scale employed. Members of two F3 families exhibited much more vigorous and prolific regeneration than the regenerating parental genotype. Because many individuals in the segregating generations showed no evidence of regeneration, population distributions for this trait were severely truncated, or censored. Regression-order analysis was used to estimate means and variances of these censored populations. The association between poor callus growth and high regeneration capacity observed in the parental lines was absent from the F2 and F3 generations, indicating that no association between callus growth and regeneration was present.

Note on the interpretation of standard cost variances

The interpretation of variances in a standard cost system is troublesome for many students. For those students who are not quantitatively oriented a graphical analysis seems to be a logical way of clearly demonstrating the source of variances and their interpretation. The most commonly used graph for standard cost variance analysis is a one-quadrant area graph. If applied to the unfavorable variance case the one-quadrant area graph is an adequate vehicle for demonstrating relationships between costs and variances. However, when variances are favorable the one-quadrant graph becomes difficult to interpret. In this case the joint variance appears to be included in the quantity variance rather than the joint variance. The difficulty is caused by the sign change of the joint variance which cannot be shown in the one-quadrant area graph. A four-quadrant area graph is presented as an alternative pedagogical tool. Variances in quadrants I and III are unfavorable and variances in quadrants II and IV are favorable. The four-quandrant area graph is consistent with the underlying algebra and aids the student in developing an intuitive understanding and ability to interpret standard cost variances.