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Biomedical subjects

D B Chaffin

Publications and source records attributed to D B Chaffin.

At least 19 recordsLinked to original sources
PubMed1997

A neural network model for simulation of torso muscle coordination.

An artificial neural network (ANN) was created to simulate lumbar muscle response to static moment loads. The network model was based on an abstract representation of a motor control system in which muscle activity is driven primarily to maintain moment equilibrium. The network model parameters were obtained by an iterative method (trained), using a modification of the standard backpropagation algorithm and moment equilibrium constraints. In contrast to previous ANN models of muscle activity, patterns of muscle activity are not target (training) values, but rather emerge as a result of moment equilibrium constraints. Assumptions regarding the moment generating capacity muscles and competitive interactions between muscles were employed and enabled the prediction of realistic patterns of muscle activity upon comparison with experimental electromyographic (EMG) data sets (r2: 0.4-0.9). The success of the simulation model suggests that a motor recruitment plan can be mimicked with relatively simple systems and that 'competition' between responsive units (muscles) may be intrinsic to the learning process. Prediction of alternative recruitment patterns and differing magnitudes of co-contractile activity were achieved by varying competition parameters within and between units.

Abdominal Muscles↗
PubMed1997

Using principal-components regression to stabilize EMG-muscle force parameter estimates of torso muscles.

Models for estimating muscle force from surface electromyographic (EMG) recordings require parameter estimates with low intertrial variability. The inclusion of multiple muscles in multivariate statistical models can lead to multicollinearity, especially when there are significant correlations between synergist muscles. One result of multicollinearity is that parameter estimates are very sensitive to changes in the independent variables. This study compared the parameter variability of multiple regression and principal-components regression techniques when applied to a six muscle EMG analysis of the lumbar region of the torso. Nine subjects participated. Twenty-three percent of the traditional multiple-regression parameters had incorrect signs, but none of the principal-components regression parameters did. The principal-components regression technique also produced parameter estimates having an order of magnitude smaller parameter variability. It was concluded that principal-components regression is an effective method of mitigating the effect of multicollinearity in torso EMG models.

Adult↗
PubMed1997

Task effects on three-dimensional dynamic postures during seated reaching movements: an investigative scheme and illustration.

In this paper we describe a new scheme for empirically investigating the effects of task factors on three-dimensional (3D) dynamic postures during seated reaching movements. The scheme relies on an underlying model that integrates two statistical procedures: (a) a regression description of the relationship between the time-varying hand location and postural angles to characterize the movement data and (b) a series of analyses of variance to test the hypothesized task effects using representative instantaneous postures. The use of this scheme is illustrated by an experiment that examines two generic task factors: hand motion direction and motion completion time. Results suggest that hand motion direction is a significant task factor in determining instantaneous postures, whereas a distinctive difference in the time to complete a motion does not appear to have a significant effect. We discuss the concept of an instantaneous posture and its utility in dynamic studies of movements, some insights into human reaching movement control strategy, and implications for the development of a 3D dynamic posture prediction model.

Adult↗
PubMed1997

Stability limits in extreme postures: effects of load positioning, foot placement, and strength.

Although injuries related to postural stability are prevalent, ergonomic job analyses traditionally have not addressed stability issues. In this research functional stability limits are quantified for persons standing in extreme postures under various external load and foot positioning conditions. Participants were asked to lean and displace their center of gravity (COG) as far as possible in eight directions to the sides and front of the body. Stability measures based on these COG displacements were calculated. All controlled variables significantly affected the stability measures. When standing unladen, participants extended their COG to within 99% of their theoretical maximum. Movement was much more restricted when handling a load (89%), especially when holding it with one hand on the shoulder (84%). On average, increased separation of the feet in a particular direction resulted in larger COG displacements in that direction. The results are discussed relative to their effects on balance and stability modeling.

Adult↗
PubMed1997

A biomechanical analysis of methods used for transferring totally dependent patients.

Lifting and transferring patients have been identified as frequent precipitating factors or causes of low back problems among nurses. This study systematically evaluated six different transfer methods (three manual and three mechanical) completed by two female nurses working as a team to transfer two totally dependent patients (heavy, 95 kg and light, 56 kg). The patient transfers were completed on a rehabilitation unit of a large university hospital. Each transfer was videotaped and the short (150 cm) and tall (178 cm) nurse each performed the lead and assist roles using all six methods for both patients for a total of 24 transfers. A biomechanical software program referred to as the "3-Dimensional Static Strength Prediction Program (3DSSPPTM)" was used to model each patient transfer, and to compute the peak compressive force on the L5/S1 disc, as well as estimate the percent of the population with sufficient strength capability to transfer patients. The results of biomechanical analysis revealed that the low back compression forces exceeded the back compression design limit recommended by the National Institute for Occupational Safety and Health (NIOSH) (3400N). For the manual transfer methods peak compressive forces greater than 10,000 N were predicted, which far exceeded the NIOSH upper limit of 6400 N. When mechanical lift devices were used, the back compression forces were below the back compression design limits. This study reinforces the need to utilize a mechanical lift device when transferring totally dependent patients with only two nurses.

Activities of Daily Living↗
PubMed1996

Kinematics, kinetics, and psychophysical perceptions in symmetric and twisting pushing and pulling tasks.

The use of material handling devices (MHDs) to eliminate the repetitive lifting components of industrial jobs has introduced a new set of issues for industrial ergonomics. Worker performance and safety when using MHDs depends on the mass and distance moved, the asymmetry of the postures, and the positioning accuracy required at each endpoint of the movement. Ten participants moved loads of up to 68 kg and placed them in targets of different sizes. In one experiment they maintained sagittally symmetric postures; in the another they were required to rotate their entire body 180 degrees to complete the task. Measured peak push and pull hand forces ranged from 40 to 120 N and peak hand velocities ranged from 1.0 to 1.6 m/s in both studies. Psychophysical ratings were significantly correlated with peak push and pull hand forces but not peak velocity. Averages of peak hand force and velocity are used to develop some general recommendations for improved implementation of MHDs in industry.

Adult↗
PubMed1996

Distributed moment histogram: a neurophysiology based method of agonist and antagonist trunk muscle activity prediction.

A neurocortical-based technique of muscle recruitment is presented to solve the muscle indeterminacy problem for lumbar torso modeling. Cortical recordings from behaving primates have established motor cortex cells that respond to a wide range of task directions, but are tuned to a preferred direction. A characteristic activity pattern of these neurons seems to be associated with effort direction. It was hypothesized that a model which recruits muscles based on a similar distribution would predict antagonistic muscle activity with greater realism than a widely referenced optimization formulation. The predictions of the Distributed Moment Histogram (DMH) method were evaluated under common speed (< 30 degrees s-1) sagittal plane lifting conditions using five subjects. The predicted forces showed high correspondence with agonist and antagonist myoelectric patterns. The mean coefficient of determination for the erector spinae was r2 = 0.91, and 0.41 for the latissimus. For the antagonistic muscles, the rectus abdominus was found to be electrically silent (< 3% MVC) and no activity was predicted by the method. The external oblique muscle was observed to be minimally active (< 16% MVC), and the DMH method predicted its mostly constant activity with a mean standard error of 1.6% MVC. The realistic antagonistic predictions supported the hypothesis and justify this cortical based technique as an alternative for muscle tension estimation in biomechanical torso modeling. A primary advantage of this method is its computational simplicity and direct physiologic analog.

Abdominal Muscles↗
PubMed1996

Support for a linear length-tension relation of the torso extensor muscles: an investigation of the length and velocity EMG-force relationships.

This study investigated the hypothesis that the length-tension relation of the torso erectors would be linear, mirroring the observed linear increase in extension strength capability toward full flexion. The effect of torso extension velocity on the tension capability of these muscles was also investigated for common motion speeds. A myoelectric-based approach was used wherein a dynamic biomechanical model incorporating active and passive tissue characteristics provided muscle kinematic estimates during controlled sagittal plane extension motions. A double linear optimization formulation from the literature provided muscle tension estimates. The data of five male subjects supported the hypothesis of a linear length-tension relation toward full flexion for both the erector spinae and latissimus muscles. Velocity trends agreed with the predicted by Hill's exponential relation, although linear trends were found to fit the data almost as well. The results have implications for muscle tension estimation in biomechanical torso modeling, and suggest a possible low back pain injury mechanism through tissue strain while lifting in fully flexed postures.

Abdominal Muscles↗
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

The effect of strict muscle stress limits on abdominal muscle force predictions for combined torsion and extension loadings.

The objective of this study was to determine to what extent the central nervous system activates torso muscles so as to equalize the largest muscle stresses. Two optimization models that treat large muscle stresses differently were formulated. One model minimized spinal compression force subject to the lowest possible muscle stress limit, and the other model minimized the sum of cubed muscle stresses. Experimental conditions were determined for which the two models made different muscle force predictions. Specifically, the models predicted different rectus abdominis activity levels for tasks involving torsion and extension moment loadings. Surface electromyography was used to evaluate the model predictions. Applied loads were chosen to assure that the rectus abdominis EMG exceeded 30% MVC. Analysis of variance indicated that rectus abdominis activity was not affected by torsion loading at the p < 0.05 level of significance in a statistical design having 90% power, which was consistent with the predictions of the model that minimized the sum of cubed muscle stresses. Thus, it was concluded that equalization of the largest muscles stress was not the paramount objective of the central nervous system in the tasks studied.

Abdominal Muscles↗
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

Evaluating the effect of co-contraction in optimization models.

The effect of co-contraction of antagonist muscles on spinal compression force is estimated using Karush-Kuhn-Tucker (K-K-T) multipliers. Co-contraction is modelled as an incremental increase in the lower bounds on the allowable muscle forces in an optimization model formation. The K-K-T multipliers associated with each lower bound provide an estimate of the partial derivate of the optimal objective function value with respect to a change in the lower bound. A model whose objective function is spinal compression force is analyzed to estimate the effect of co-contraction on spinal compression force. While the effect depends on the specific muscle and task under consideration, the marginal effect of co-contraction on spinal compression force can be as high as 5.52 N additional spinal compression force for every additional N of muscle force. Paradoxically, the co-contraction may slightly decrease predicted spinal compression in special circumstances.

Algorithms↗
PubMed1994

Evaluation of muscle force prediction models of the lumbar trunk using surface electromyography.

Optimization-based models for prediction of muscle forces in the lumbar region of the torso are used to estimate the forces acting on spinal motion segments, especially for asymmetric tasks. The objectives of this study were to determine (a) which of four torso model formulations best predicted the electromyographic data, (b) the difference in muscular contribution to spinal compression force for the four models, and (c) the effect of using the lowest possible muscle stress bound in the model formulation. An approach for the investigation of competing optimization model formulations was developed and was illustrated with electromyographic data from static asymmetric loading conditions. This method is based on (a) the choice of experimental conditions in which models predict decidedly different muscle forces, and (b) the ability to ensure that the experimental conditions are such that the minimum number of assumptions about the force-electromyogram relationship must be made in order to choose between competing model predictions. Of the four models analyzed, only the formulation with an objective function that was the sum of cubed muscle stresses predicted the electromyographic data acceptably. The muscular contribution to spinal compression force predicted by these models differed by as much as 160% for some experimental conditions. The use of the lowest possible muscle stress bound does not appear to predict muscle forces that are in agreement with electromyographic data.

Adult↗
PubMed1994

Postural effects on biomechanical and psychophysical weight-lifting limits.

Many believe that biomechanical models and data are not sufficiently refined and validated to be used as the basis for setting 'safe' population load lifting limits. Rather, they advocate that such limits should be based on what a sample of the population demonstrate to be 'maximum acceptable weight limits' (MAWL) resulting from psychophysical tests performed with different populations. Yet, biomechanical models are becoming more robust and valid. Computerized versions of these models are readily available, and these models now can provide a more complete understanding of the pathophysiology of a person's spinal column and supporting structures, thereby providing a means to predict the risk of tissue trauma in given lifting situations for large size populations. NIOSH has recognized the validity of both biomechanics and psychophysics for establishing a Recommended Weight Limit (RWL) in the United States. The implications of using both types of methods and data are explored in this paper. Three case studies are presented that involve infrequent lifting of varied size boxes from near floor level to explore how MAWL and biomechanically determined limits differ. The cases include symmetric, sagittal plane lifting using a freestyle posture, a similar lift but with freestyle and squat lift postures, and an asymmetric load lifting task. In all three case comparisons it is shown that a recommended load to be lifted based solely on a spinal disc compression force tolerance of 3400 N would be much lower than published MAWL values representing 75% or even 90% of young males under 40 years old (i.e. MAWL values may not be protective of some young males). It also is shown that MAWL values do not have as much population variance as the spinal motion segment failure values. Hence, many younger males would be expected to have spinal columns that can tolerate the highest published MAWL values. Several mitigating factors, including load lifting trajectories, lifting dynamics, and biomechanical modeling limitations are discussed, as well as differences in compressive force tolerances and MAWLs associated with age and gender.

Adult↗
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

Floor/shoe slip resistance measurement.

A variety of slip measurement devices exist that provide estimates of both static and dynamic coefficient-of-friction (COF) values between one's shoes and the floor. Unfortunately, different shoe sole/heel materials, floor conditions, and contaminants will affect the tests in ways that result in widely varying COF estimates. This paper reviews the basic physics of such tests and describes a set of experiments to determine the static and dynamic COF values under operating conditions known to exist in different jobs. The results define a set of conditions wherein low (hazardous) COF values would exist (e.g., hard Neolite shoe material in contact with a wet, smooth walking surface). The results also question the use of light-load testing devices and static and slow speed reference COF values in the literature.

Accidental Falls↗
PubMed1992

Coactivation of the trunk muscles during asymmetric loading of the torso.

Materials handling tasks in industry are rarely performed in the midsagittal plane. Often these tasks, labeled nonsagittally symmetric or asymmetric lifting tasks, can be expected to lead to an unequal distribution of forces between the left and right sides of the body. Because of the large number of muscles capable of resisting loads in the torso, researchers are forced to make simplifications when using biomechanical models to estimate mechanical loading of the spine during such tasks. Simplifications and assumptions regarding the coactivation of antagonistic muscles are frequently used because sufficient experimental data do not exist. The present study was designed to quantify coactivation of the trunk musculature in response to applied asymmetric loads. This load was varied in direction from an anterior midsagittal plane orientation to a posterior midsagittal plane orientation in 15-deg increments. The results showed little coactivation when the applied load directions were anterior and within 45 deg of the midsagittal orientation. When load directions were greater than 45 deg, coactivation was quantifiable in ipsilateral and posterior muscle groups.

Electromyography↗