Shoulder joint kinetics during the push phase of wheelchair propulsion

The purpose of this investigation was to quantify the forces and moments at the shoulder joint during free, level wheelchair propulsion and to document changes imposed by increased speed, inclined terrain, and 15 minutes of continuous propulsion. Data were collected using a six-camera VICON motion analysis system, a strain gauge instrumented wheel, and a wheelchair ergometer. Seventeen men with low level paraplegia participated in this study. Shoulder joint forces and moments were calculated using a three-dimensional model applying the inverse dynamics approach. During free propulsion, peak shoulder joint forces were in the posterior (46 N) and superior directions (14 N), producing a peak resultant force of 51 N at an angle of 185 degrees (180 degrees = posterior). Peak shoulder joint moments were greatest in extension (14 Newton-meters [Nm]), followed by abduction (10 Nm), and internal rotation (6 Nm). With fast and inclined propulsion, peak vertical force increased by greater than 360%, and the increase in posterior force and shoulder moments ranged from 107% to 167%. At the end of 15 minutes of continuous free propulsion, there were no significant changes compared with short duration free propulsion. The increased joint loads documented during fast and inclined propulsion could lead to compression of subacromial structures against the overlying acromion.     

Heterogeneous catalysis: looking forward with molecular simulation

Some of the areas in which we anticipate, over the next five years, notable advances in the application of molecular simulation to problems in heterogeneous catalysis are considered, in the context of recent progress to date. The areas specifically addressed are:

• expanding access to methods,

• quantitative structure-property relationships,

• building structural models to focus or pre-screen experiments,

• confidence in predicting local and extended structure

• reaction mechanisms, barriers and kinetics, and

• data for chemical process simulations.

In each of these areas, we indicate why we consider the topic significant, provide reference to topical work and suggest opportunities for future developments.     

Stochastic model-based processing for detection of small targets innon-Gaussian natural imagery

Stochastic background models incorporating spatial correlations can be used to enhance the detection of targets in natural terrain imagery, but are generally difficult to apply when the statistics are non-Gaussian. Chapple and Bertilone (see Opt. Commun., vol.150, p.71-76, 1998) proposed a simple stochastic model for images of natural backgrounds based on the pointwise nonlinear transformation of Gaussian random fields, and demonstrated its effectiveness and computational efficiency in modeling the textures found in natural terrain imagery acquired from airborne IR sensors. In this paper, we show how this model can be used to design algorithms that detect small targets (up to several pixels in size) in natural imagery by identifying anomalous regions of the image where the statistics differ significantly from the rest of the background. All of the model-based algorithms described here involve nonlinear spatial processing prior to the final decision threshold. Monte Carlo testing reveals that the model-based algorithms generally perform better than both the adaptive threshold filter and the generalized matched filter for detecting low-contrast targets, despite the fact that they do not require the target statistics needed for constructing the matched filter.     

Fast evaluation of radial basis functions: I

This paper describes some new techniques for the rapid evaluation and fitting of radial basic functions. The techniques are based on the hierarchical and multipole expansions recently introduced by several authors for the calculation of many-body potentials. Consider in particular the N term thin-plate spline, s(x) = Σ_j=1^N d_jφ(x−x_j), where φ(u) = |u|²log|u|, in 2-dimensions. The direct evaluation of s at a single extra point requires an extra O(N) operations. This paper shows that, with judicious use of series expansions, the incremental cost of evaluating s(x) to within precision ε, can be cut to O(1+|log ε|) operations. In particular, if A is the interpolation matrix, a_i,j = φ(x_i−x_j, the technique allows computation of the matrix-vector product Ad in O(N), rather than the previously required O(N²) operations, and using only O(N) storage. Fast, storage-efficient, computation of this matrix-vector product makes pre-conditioned conjugate-gradient methods very attractive as solvers of the interpolation equations, Ad = y, when N is large.     

Density functional calculations on agostic ethyl-Ti (IV) complexes

First principles, density functional theory embodied in the DMol program has been applied to agostic ethyl-Ti-complexes, including the dmpe complex, [Ti(-CH₂CH₃)C1₃(dmpe)], where DMPE=(Me₂PCH₂)₂ and its model complex, [Ti(−CH₂CH₃)Cl₃(PH₃)₂]. The ethyl moiety of the complexes can adopt two limiting conformations, staggered and eclipsed. In the model complex, [Ti(−CH₂CH₃)C1₃(PH₃)₂], both conformers are found to form agostic structures upon geometry optimization subject to Cs symmetry constraint, with the agostic eclipsed structure being the lower in energy. Full geometry optimization of the dmpe complex, [Ti(−CH₂CH₃)C1₃(dmpe)], yields an agostic structure with geometrical features similar to those measured by single crystal X-ray analysis. It is shown that the HOMO orbital contributes substantially to the agostic bonding.     

Efficient calculation of finite Gabor transforms

The Gabor transform may be viewed as a collection of localized Fourier transforms and as such is useful for analysis of nonstationary signals and images. We present a new approach to analyzing the Gabor transform and use it to study the various critically sampled discretizations that form the infinite-discrete, periodic finite-discrete, and nonperiodic finite-discrete versions of the transform. In particular, we distinguish between the analysis and synthesis forms of the transform, and introduce an intermediate operation that decomposes both forms into collections of independent Toeplitz operators. In the continuous, the infinite-discrete, and the periodic finite-discrete cases, this decomposition allows us to show that, for appropriate windows, the analysis and synthesis transforms are inverses of each other. In the nonperiodic finite-discrete case this relation no longer holds, but we are still able to use the decomposition and results on Toeplitz matrices to show that both the transform and the inverse transform of P discrete samples are computable in O(P log P) operations (after a setup cost of O(Plog²P)). Furthermore, we use the decomposition to study in detail the differences between the periodic and nonperiodic versions of the transform and to compare their conditioning     

Crystal structure solution and prediction via global and local optimization

Simulations machinery broadly available today allows direct-space solutions of crystal structures based on high quality powder diffraction data for systems with up to some 15–20 degrees of freedom. Analogous machinery for predicting new structures is rarely unsuccessful, but has had limited systematic application. The prime challenges for structure solution are for degrees of freedom beyond some 15–20, and for powder diffraction data which is of insufficient quality to allow an unambiguous indexing. In structure prediction, the corresponding issues are (i) enumeration strategies, (ii) collated access to the results of prior predictions, (iii) relating results in some fashion to likelihood of occurrence, encompassing finite temperature issues, and (iv) rational approaches to the synthesis of designed structures. Recent progress in the field has been steady and, while solutions to these key problems are not immediately obvious, the pace of progress over the past 5 years suggests major headway is imminent in a number of these areas.     

Information on catalyst surface structure from crystallite morphologies observed by scanning electron microscopy (SEM)

The shapes of small crystals grown under equilibrium conditions are governed by the stabilities of their exposed faces. Computer simulation methods can be used to calculate surface energies and hence crystallite morphologies, illustrated by results for Cr₂O₃. Such calculations can include the effects of surface impurity segregation. Comparison of the resulting calculated crystal morphologies with those directly observed in the SEM thus provides a direct link between atomistic simulation and experiment.