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M A Nussbaum

Publications and source records attributed to M A Nussbaum.

26 records · Page 2Linked to original sources
PubMed1995

Muscle lines-of-action affect predicted forces in optimization-based spine muscle modeling.

This study describes the effects of varied torso muscle geometries commonly assumed in optimization-based muscle force prediction models. Specifically, the sensitivity of predicted muscle and spinal forces to assumed muscle lines-of-action (LOA) is systematically examined. The practical significance of varied muscle LOAs is addressed by determining the relative precision needed for individual muscle LOAs and assessing which muscles are more critical to accurate prediction of spinal forces. To perform this analysis a nonlinear optimization model was used to generate muscle force predictions during combined frontal and sagittal plane moment loadings with an assumed erect posture. The LOAs of the erector spinae, rectus abdominus, internal and external oblique, and latissimus dorsi were systematically varied in the frontal and sagittal planes over an anatomically feasible range. The results indicated that moderate changes in the assumed LOA could substantially alter the magnitudes of predicted muscle and spinal forces. The estimated activity level of a muscle, as well as the predicted active/silent state could be affected by the LOA of that muscle and others. The patterns of predicted muscle activity, with respect to load orientation, underwent only minor alterations with changing LOA. The relative activation of several muscles, however, was dependent on LOA, and frequently led to variations in predicted spinal compression (> 100 N change) and shear forces (> 50 N change). This dependence of estimated spinal forces on assumed muscle geometry was most pronounced for the obliques and minimal for the more vertically oriented muscles and when loads were sagittally symmetric. This study suggests that muscle LOAs are critical inputs when interpreting absolute muscle and spinal force values predicted by models of physical exertions.

Algorithms↗
PubMed1995

A back-propagation neural network model of lumbar muscle recruitment during moderate static exertions.

A model employing artificial neural networks (ANNs) is developed for the prediction of lumbar muscle activity in response to steady-state static external moment loads. The model is constructed using standard feedforward networks and trained with available data using the standard back-propagation algorithm. Training with a limited set of exemplars allowed accurate prediction of muscle activity for novel moment loads (generalization). Sensitivity analyses during training and testing phases showed that the choice of specific network parameters was not critical except at extreme values of those parameters. Model predictions were better correlated with experimental data than predictions made using two optimization-based methods (average r2 = 0.83 using ANNs and 0.65 using optimization). The results suggest that lumbar muscle response varies smoothly and consistently with respect to the magnitude and orientation of external moments, and they also imply an upper limit on the accuracy of muscle activity prediction using only moment loads as input. ANNs present a useful alternative to EMG- and optimization-based approaches by being both 'reality-based' and predictive.

Biomechanical Phenomena↗
PubMed1995

Control of enantiomer migration order in capillary electrophoresis separations using sulfobutyl ether beta-cyclodextrin.

The control of enantiomer migration order in capillary electrophoresis (CE) by use of sulfobutyl ether beta-cyclodextrin (SBE-beta-CD) was investigated. At high pH, electroosmotic flow (EOF) dominated and the enantiomer most strongly associated with the anionic cyclodextrin was detected last. At low pH (and reversed polarity), EOF was minimal and SBE-beta-CD functioned as a carrier. Under such conditions, the enantiomer migration order for a neutral chiral compound was reversed. Factors involved in optimization (cyclodextrin and organic modifier concentration) were studied. The impact of migration order on quantitation of low levels (below 1%) of one enantiomer in the presence of the other was also investigated. The precision of peak area ratios (minor/major enantiomer) was evaluated for samples of both enantiomers run by each method. The migration of minor before major enantiomer yielded better quantitation precision in each case.

Borates↗
PubMed1993

Torso muscle moment arms at intervertebral levels T10 through L5 from CT scans on eleven male and eight female subjects.

Moment arms for eight pairs of torso muscles were estimated based on data obtained from 19 sets of computed tomography (CT) scans. Muscle centroid locations of the rectus abdominis, external oblique, internal oblique, transversus abdominis, latissimus dorsi, psoas, quadratus lumborum, and the erector spinae mass were identified and digitized relative to vertebral body centers, which were also determined from the scans of the eight females and eleven males. Muscle moment arms were then calculated as the distance between the muscle and vertebral body centroids. The centroids for each torso muscle were plotted at each intervertebral level from T10-11 to L4-L5. When these sections were ordered in a cephalocaudad manner, the centroid-line paths, essential to the determination of muscle force lines-of-action, could be traced. Finally, anthropometric variables such as height, weight, torso depth, breadth, and age were regressed against the moment arm group means to develop several prediction equations that would determine moment arm lengths based on these anthropometric variables.

Biomechanical Phenomena↗
PubMed1992

Electrochemical characteristics of amino acids and peptides derivatized with naphthalene-2,3-dicarboxaldehyde: pH effects and differences in oxidation potentials.

The electrochemical oxidations of the cyanobenz[f]isoindole (CBI) derivatives of 18 amino acids and 15 peptides, including enkephalins and several enkephalin fragments, were studied. Cyclic voltammetry indicated that the oxidation potentials of derivatized amino acids were virtually independent of pH. The utility of this pH-independence was demonstrated by controlling, through varying the pH, the selectivity with which CBI derivatives could be detected in the presence of phenolic compounds. In addition, hydrodynamic voltammograms of derivatized amino acids and peptides were constructed from chromatographic data and compared. The E1/2 values among the derivatized amino acids covered a range of 215 mV, with the derivatives of the basic amino acids being the easiest to oxidize and those of the acidic amino acids being the most difficult to oxidize. The E1/2 values of the derivatized peptides examined varied by 270 mV, with minor variations in structure capable of producing marked changes in oxidation potential. These results indicate that voltammetry can aid in identification or selective detection of CBI derivatives of amino acids or peptides.

Amino Acid Sequence↗
PubMedDate not supplied

Fatigue and endurance limits during intermittent overhead work.

Shoulder problems are prevalent in industrial work, particularly when tasks require the hands to be used at or above shoulder level. Although extensive research has been conducted on prolonged static exertions, and several guidelines for such efforts exist, there is insufficient information for ergonomic evaluation of tasks that are intermittent and/or dynamic. A laboratory simulation was conducted of overhead assembly work that was both intermittent and dynamic, and which varied the duty cycle (work/rest ratio), arm reach, and hand orientation of a tapping task. Results consisted of endurance times and also the times of fatigue onset as indicated by perceived discomfort and declines in muscle strength. Females exhibited longer (22%) endurance times, delayed reports of discomfort, and slower declines in strength. Significant influences of duty cycle were found on both endurance and fatigue times, yet arm reach and hand orientation did not have consistent effects. Distributions of endurance and fatigue times are presented as criteria for preliminary evaluation of overhead work. Endurance times could be predicted with only moderate accuracy from earlier indicators of fatigue onset. Existing guidelines, albeit developed for static tasks, appeared unsuitable for the simulated overhead assembly efforts examined. Furthermore, such guidelines may fail to capture the substantial interindividual variability observed in this experiment.

Adolescent↗