Soft tissue wobbling affects trunk dynamic response in sudden perturbations.

Soft tissue wobbling reduces the transferred impact of external loads on lower limb joints. The present study investigated whether soft tissue wobbling has similar effects on trunk dynamic response to sudden perturbations. Three healthy males were subjected to a series of anteriorly directed trunk position perturbations at three different velocities while trunk kinematics and kinetics were measured. A nonlinear active-passive finite element model of the human trunk was then used to study the effects of soft tissue wobbling on trunk response. Also investigated were the effects on model predictions of including elements simulating the apparatus (rod-harness assembly) transferring motor-generated perturbations to the trunk. Predicted and measured trunk kinematics and kinetics, when accounting for the dynamic effects of both wobbling mass and rod-harness assembly, were in good agreement for all velocities especially early (<120 ms) after the perturbations (ρ>0.97). Root mean square errors in model predictions increased considerably when neglecting the aforementioned modeling considerations. The trunk wobbling mass and connecting elements between the trunk and the perturbing device, particularly during faster perturbations, substantially attenuated the transferred impact of external loads on the spine (by 33-90 N across perturbation velocities). Such reductions in the impacts transferred, in turn, reduced the predicted demands on the neuromuscular system for control and maintenance of spinal loads and stability. As such, these features should be considered in future biodynamic models of the human trunk aimed at estimating trunk neuromuscular behaviors during sudden perturbations.

Muscle fatigue during intermittent isokinetic shoulder abduction: age effects and utility of electromyographic measures.

Most existing evidence regarding the effects of age on muscular fatigue has focused on prolonged isometric contractions, repeated maximum dynamic contractions and individuals beyond traditional retirement age (>65 years). In the present study, age-related differences in muscle fatigue during submaximal dynamic efforts were examined. There were 24 younger (18-25 years) and 24 older (55-65 years) participants, all of whom were healthy and active, with equal numbers of each gender within each age group. Participants performed repetitive, intermittent shoulder abductions until exhaustion, at peak moments of 30% and 40% of individual maximum voluntary isokinetic contraction (MVIC) and with cycle durations of 20 and 40 s. Fatigue development was determined based on changes in MVIC, electromyographic (EMG) signals and ratings of perceived discomfort (RPD). Following the exhaustive exercises, strength recovery was monitored using a series of MVICs over a 15-min period. Results indicated the existence of an age-related fatigue resistance, with the older group demonstrating significantly slower rates of MVIC decline and RPD increase and smaller modifications in EMG-based fatigue measures. These age effects were generally more pronounced at the higher effort level. Main effects of effort level and cycle duration were also significant, while gender effects appeared to be marginal. Rates of strength recovery were not significantly influenced by age. In addition, the utility of standard EMG-based fatigue measures was assessed. Findings indicated that time-dependent changes in static and dynamic EMG-based measures were roughly comparable in terms of sensitivity and variability, supporting the use of standard EMG analyses for fatigue monitoring during intermittent dynamic contractions.

Muscle fatigue and endurance during repetitive intermittent static efforts: development of prediction models.

Localized muscle fatigue has received growing attention as a potential design variable and exposure metric in research towards prevention of musculoskeletal disorders in the workplace. While fatigue during sustained static work has been investigated extensively, effects during tasks comprising work-rest cycles are less clear. Work-rest models for static intermittent work have been presented in several reports, but the applicability is often limited to specific conditions. A study was conducted that facilitated a description of the relationships between static intermittent efforts and muscle endurance and fatigue. Exercises consisted of 1 h (maximum) of repetitive static arm abductions, involving a range of muscle contraction levels (10-30% maximum voluntary exertion), duty cycles (0.2-0.8) and cycle times (20-180 s). A between-subject central composite experimental design was used and 15 different exercise conditions were examined with six participants (three females and three males) for each. Along with endurance times, temporal changes related to fatigue were monitored using muscle strength, ratings of discomfort and electromyography (EMG) obtained from the middle-deltoid muscle during the contraction phase of the work cycles. The results of this study showed the influence of contraction level and duty cycle on the majority of fatigue measures used, while cycle time tended to affect EMG spectral measures. Using a response surface methodology, several fatigue prediction models and contour plots were developed that can be employed as an aid for design and evaluation of light repetitive static tasks. Good correspondence was generally found between discomfort rating and other measures of fatigue, suggesting the usefulness of this measure for rapid assessments of local fatigue in the workplace.

Effects of lumbar extensor fatigue and fatigue rate on postural sway.

Falls from heights resulting from a loss of balance are a major concern in the occupational setting. Previous studies have documented a deleterious effect of lower extremity fatigue on balance. The purpose of this study was to investigate the effect of lumbar extensor fatigue on balance during quiet standing. Additionally, the effects of fatigue rate on balance and balance recovery rate were assessed. Eight center-of-pressure-based measures of postural sway were collected from 13 participants, both before and after a protocol that fatigued the lumbar extensors to 60% of their unfatigued maximum voluntary exertion force. In addition, postural sway was measured for 30 min after the fatiguing protocol, at 5-min intervals, to quantify balance recovery rate during recovery from fatigue. Two different fatigue rates were achieved by fatiguing participants over either 10 min or 90 min. Results show an increase up to 58% in time-domain postural sway measures with lumbar extensor fatigue, but no change in frequency-domain measures. Fatigue rate did not affect the magnitude of these postural sway increases, nor did it affect the rate of balance recovery following fatigue. Statistical power for the latter result, however, was low. These results show that lumbar extensor fatigue increases postural sway and may contribute to fall-from-height accidents.

Determination of relative UV response factors for HPLC by use of a chemiluminescent nitrogen-specific detector.

Ultraviolet (UV) absorbance is the most widely used detection method for high-performance liquid chromatography (HPLC) separations. In pharmaceutical analysis, purity determinations often include quantitation of related impurities based on relative HPLC peak areas obtained at a specific wavelength. In order for this quantitation to accurately reflect weight percentages of impurities, the relative UV response factors (absorptivities) at the given wavelength must be known. In this work, we present a convenient method for determining relative UV response factors on-line, without isolation or purification of impurities, without standards, and without requiring known analyte concentrations. The procedure described makes use of a chemiluminescent nitrogen-specific HPLC detector (CLND) in conjunction with a UV detector. The CLND response is directly proportional to the number of moles of nitrogen in each eluting peak, and can, therefore, be used to determine relative amounts of each nitrogen-containing impurity present in the sample, provided the molecular formulas are known (e.g. from exact mass LC-MS). It is a simple matter, then, to determine the relative UV response factors from the UV area ratios obtained for the same sample. The feasibility and accuracy of this method is demonstrated for gradient HPLC separations of commercially available compounds of widely varying structures. Finally, the method's utility in obtaining accurate mass balance is demonstrated by application to photodegradation of nifedipine.

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.

Static and dynamic myoelectric measures of shoulder muscle fatigue during intermittent dynamic exertions of low to moderate intensity.

Despite extensive research on muscular fatigue during prolonged static efforts, there have been relatively few studies of more complex tasks (dynamic and intermittent). A laboratory study of overhead work tasks was conducted to investigate whether electromyographic (EMG) measures can potentially serve as indicators of fatigue, particularly for ergonomic tasks analysis. Sixteen participants performed the tasks until they either developed substantial discomfort or reached a 3-h limit. EMG signals were obtained at intervals throughout the experiment from four shoulder muscles, both statically (during fixed-level test contractions) and dynamically (during task performance). Both EMG root mean square (RMS) amplitude and spectral content (mean and median power frequencies) were examined and compared in terms of their variability and sensitivity. In addition, a new fatigue index was developed to allow for the estimation of substantial fatigue onset. Variability was found to differ significantly between muscles and EMG measures, and was generally lowest for mean power frequencies obtained during static test contractions. Sensitivity was typically greatest for RMS versus spectral measures, and slightly higher for median than mean power frequencies. The results suggest that fatigue during dynamic tasks, while a complex phenomenon, can be monitored and quantified using EMG.

Motion times, hand forces, and trunk kinematics when using material handling manipulators in short-distance transfers of moderate mass objects.

The risk of musculoskeletal injury associated with manual materials handling tasks has led in part to the use of material handling manipulators, yet there is limited empirical data to facilitate selection, design, and evaluation of these devices. A laboratory study of two types of mechanical manipulators (articulated arm and overhead hoist) was conducted of short-distance transfers of moderate loads, and the influence of various task parameters and transfer method on motion times, peak hand forces, and torso kinematics was obtained. Use of manipulators increased elemental motion times for symmetric sagittal plane transfers by 36-63%, and asymmetric transfers (in the frontal plane) by 62-115%, compared to similar transfers performed manually. Peak hand forces were significantly lower with both manipulators (40-50%), and approximately 10% higher for asymmetric versus symmetric transfers. Overall torso kinematics were grossly similar with and without a manipulator. These results suggest that for self-paced job tasks, moderate mass objects will be transferred slower over short distances and with lower levels of external (hand) forces when using mechanical aids. These simple effects, however, were influenced by object mass and transfer height.

Back lift versus leg lift: an index and visualization of dynamic lifting strategies.

The description of a lifting strategy is typically provided in qualitative terms. A quantitative static descriptor or index differentiates the starting postures but not the primary moving segments. This technical note proposes an index that quantitatively characterizes different dynamic postural strategies employed during sagittal plane lifting. Dynamic lifting strategies are modeled in the velocity domain as different schemes of partitioning postural changes between the torso and leg segments. The index consists of two parameters, assigned to two leg segments, quantifying their contributions relative to the torso. Given a measured lifting movement, its index parameters values, ranging from 0.1 to 10, are estimated through an enumeration search process with the objective of minimizing the fitting error. The use of this index is illustrated by applying it to 24 lifting movements performed by six subjects assuming either a back-lift or a leg-lift strategy. Results indicate that a lifting strategy, in terms of whether the leg or the back is generally the prime mover, can be differentiated and visualized using this simple two-parameter index. In addition, indistinct intermediate strategies are also discerned, as the involvement of each segment in a lifting movement is quantified. The index is however limited in that it does not accommodate arm motion contributions to a lift nor possible time-dependent strategic changes during a lift. Potential future applications include time-efficient movement prediction and simulation for computerized biomechanical or ergonomic analysis.

The determination of oxalic acid, oxamic acid, and oxamide in a drug substance by ion-exclusion chromatography.

Oxalic acid, oxamic acid and oxamide are potential impurities in some active pharmaceutical ingredients (API). The retention and separation of oxalic and oxamic acids are particularly challenging using conventional reversed-phase HPLC due to their high polarity. An ion-exclusion chromatography (IEC) method has been shown to provide good separation and sensitivity for the three oxalate-related impurities in a hydrophobic API matrix. The method uses a Dionex IonPac ICE-ASI column with 95/5 (v/v) 0.1% sulfuric acid/acetonitrile as the mobile phase and UV detection at 205 nm. Development and validation of this method are described.

When a truck becomes a motorcycle: the impact of sample load on a chiral capillary electrophoresis separation using mixtures of neutral and sulfated cyclodextrins.

Chiral capillary electrophoresis (CE) separations are useful for monitoring the presence of a minor isomer at low levels (e.g., <0.5%) in the presence of the major form. In order to quantitate these low levels, it is necessary to inject large amounts of sample. Separations which appear to have more than enough resolution ("big enough to drive a truck through) for dilute, equal-concentration mixtures of isomers can become inadequately resolved when the necessary amount of sample is injected. This paper addresses some important considerations in maintaining adequate resolution at high sample loads for chiral separations involving a dual-cyclodextrin (CD) system. For hydrophobic compounds, the use of both a neutral and a sulfated CD can be helpful in achieving a chiral separation. In such a system, the migration time and resolution can be controlled by varying the ratio of neutral to charged CD concentrations. It is demonstrated here that not only the ratio, but also the total CD concentration can significantly affect the separation. In this paper, the impact of the total CD concentration in a dual-CD system (with the concentration ratio constant) is examined with respect to peak shape and resolution. The influences of temperature, capillary diameter, and current are also considered. The corresponding impact on the amount of sample which can be loaded and successfully separated determines the limit of quantitation of the minor isomer. Thus, this information is important in making such chiral separations applicable to determinations of low levels of minor isomer in the presence of large amounts of the major form.

Effects of pacing when using material handling manipulators.

Common manipulator-assisted materials handling tasks were performed in a laboratory simulation at self-selected and faster (paced) speeds. The effects of pacing on peak hand forces, torso kinematics, spine moments and forces, and muscle antagonism were determined, along with any influences of several task variables on these effects. The faster trials were performed 20% more rapidly than the self-paced trials. It was found that (a) achieving this level of performance required approximately 10% higher hand forces and 5%-10% higher torso moments, (b) consistent torso postures and motions were used for both speed conditions, and (c) the faster trials resulted in approximately 10% higher spine forces and approximately 15% higher levels of lumbar muscle antagonism. On whole, these results suggest a higher risk of musculoskeletal injury associated with performance of object transfers at faster than self-selected speeds with and without a manipulator. Further analysis provided evidence that the use of manipulators involves higher levels of motor coordination than do manual tasks. Several implications regarding the use of material handling manipulators in paced operations are discussed. Results from this investigation can be used in the design, evaluation, and selection of material handling manipulators.

Low-back stresses when learning to use a materials handling device.

This study examines the potential effect of short-term practice on low-back stresses during manual lifting and lowering of a 15 kg load, and while using two different types of materials handling devices (MHDs) to lift and lower a 40 kg load. The two MHDs used were an articulated balance arm and a pneumatic hoist. The expectation was that low-back dynamic moments, EMG measured torso muscle antagonism, and EMG predicted L4/L5 disc compression forces would rapidly decrease with practice, and that the manual lift-lower activities would be learned faster than the MHD-assisted exertions. Four naïve male college age subjects performed 40 lift and lower exertions, both manually and with the two MHDs for a total of 24 experiments. Non-linear regressions of the peak and average low-back moments, EMGs and disc compression values revealed only small decreases in the values (from 2 to 14%) over the 40 trials, and it was only statistically significant for five of the 48 regressions. This would seem to indicate that if learning is present in these tasks it is going to be very slow learning, and thus future studies will need to include a much larger number of trials. The effects of MHDs on the learning rates when compared to manual lifting learning rates was not statistically significant. It was shown, however, that MHDs had a particularly beneficial effect on reducing L4/L5 compression forces during load lowering activities despite the MHD load being much heavier than the manual load. It also was found that the level of torso muscle co-contraction increased significantly (2-4 times) when MHD handling was involved compared to manual lifting and lowering.

Biomechanical analysis of materials handling manipulators in short distance transfers of moderate mass objects: joint strength, spine forces and muscular antagonism.

Although often suggested as a control measure to alleviate musculoskeletal stresses, the use of mechanical assistance devices (i.e. manipulators) in load transfers has not been extensively studied. Without data describing the biomechanical effects of such devices, justification for decisions regarding implementation of such tools is difficult. An experimental study of two types of mechanical manipulators (articulated arm and overhead hoist) was conducted to determine whether biomechanical stresses, and hence injury risk, would be alleviated. Short distance transfers of loads with moderate mass were performed both manually and with manipulator assistance under a variety of task conditions. Using analysis and output from new dynamic torso models, strength demands at the shoulders and low back, lumbar spine forces, and lumbar muscle antagonism were determined. Strength requirements decreased significantly at both the shoulders and low back when using either manipulator in comparison with similar transfers performed manually. Peak spine compression and anterior-posterior (a-p) shear forces were reduced by about 40% on average, and these reductions were shown to be primarily caused by decreases in hand forces and resultant spinal moments. Two metrics of muscular antagonism were defined, and analysis showed that torso muscle antagonism was largest overall when using the hoist. The results overall suggest that hoist-assisted transfers, although better in reducing spine compression forces, may impose relatively higher demands on coordination and/or stability at extreme heights or with torso twisting motions. The relatively higher strength requirements and spine compression associated with the articulated arm may be a result of the high inertia of the system. Potential benefits of practice and training are discussed, and conclusions regarding implementation of mechanical manipulators are given.

Systematic screening approach for chiral separations of basic compounds by capillary electrophoresis with modified cyclodextrins.

A simple, systematic method was developed for rapidly screening potential capillary electrophoresis (CE) separation conditions for small, amine-containing enantiomers. During method development, 39 pairs of enantiomers were investigated and partial or complete separation was achieved in every case. Baseline resolution was achieved by these initial screening conditions in over half of the cases. The screening strategy uses a bare fused silica capillary and a pH 2.5 amine-modified phosphate buffer containing one of the selected cyclodextrins (CD): dimethyl-beta-CD, hydroxypropyl-beta-CD, hydroxypropyl-alpha-CD, hydroxypropyl-gamma-CD and sulfated-beta-CD. An additional set of compounds have been screened by this approach to demonstrate the validity of the method. The paper outlines the experimental work carried out to develop the screen and describes how one might implement it for a new compound.

Comparison of free solution capillary electrophoresis and size exclusion chromatography for quantitating non-covalent aggregation of an acylated peptide.

There are few methods available for the rapid and precise quantitation of non-covalent aggregation. The very methods used to measure the aggregation can easily disrupt the weak forces holding an aggregate together. This paper describes the novel application of free solution capillary electrophoresis (CE) for the quantitation of a biologically inactive non-covalent aggregate of C8GLIP (Des-amino-histidine-7-arginine-26 N(epsilon)-octanoyl-lysine-34-human glucagon-like insulinotropic peptide), an acylated peptide. The CE results are compared to a more traditional approach using size exclusion chromatography (SEC). Under the conditions explored in this paper, SEC showed a significantly slower apparent rate of aggregation than CE. This is due to the disruption of the aggregate during the SEC process. The cause of the disruption is complex and is potentially related to the separation process itself, on-column dilution effects, and/or interactions of the aggregate with the column packing or SEC components. Analysis times and dilution are greatly reduced by CE, and, because there is no potentially interactive stationary phase and because both the protein and the walls of the capillary are negatively charged, potential disaggregation due to surface interactions is reduced. Thus, CE is shown to be superior to SEC for this peptide in that disruption of the aggregate is minimized.

Lumbar muscle force estimation using a subject-invariant 5-parameter EMG-based model.

The use of electromyographic measures, in concert with modeled or empirical representations of muscle physiology, is a common approach for estimation of muscle force. Existing models of the lumbar musculature have allowed model parameters to vary for an individual subject. While this approach improves apparent predictive ability, it loses some degree of construct validity since parameter variability may not be physiologically justifiable. An EMG-based five-parameter model, adapted and generalized from earlier reports, is presented here. Inherent in the model is the requirement of subject-invariant modeling parameters. As a practical analysis tool was desired, the model relies on relatively few calibration constants whose determination is described. Empirical evaluation was undertaken using a database of 398 experimental trials involving lifting and transferring objects of moderate mass. Model performance, evaluated by comparison of measured and predicted lumbar moments, was comparable to earlier models, with r2 mean (S.D.) values of 0.76(0.15) for sagittal plane moments, and rms mean (S.D.) errors of 14.1(7.4), 9.7(5.3), and 8.6(3.6) Nm in the sagittal, frontal, and horizontal planes, respectively. These empirical results and the argument of physiological veracity support the use of a subject-invariant model.

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.

Pattern classification reveals intersubject group differences in lumbar muscle recruitment during static loading.

OBJECTIVE: This paper examines interindividual differences in the patterns of torso muscle recruitment during 3-dimensional static moment loading of the lumbar spine. DESIGN: A mathematical model (artificial neural network) was used to differentiate individual patterns of muscle response. BACKGROUND: Traditionally, experimental myoelectric data is averaged over subjects, assuming an ideal mean response to a given loading. However, averaging may overlook important information and implications associated with interindividual variability. METHODS: In this study a simple classification tool in the form of a competitive neural network model is developed and used to evaluate lumbar muscle recruitment patterns. RESULTS: Subjects formed consistent and denumerable clusters, and could be categorized as either 'majority' or 'minority' type responders, based on their individual muscle response patterns as discerned from the output of the competitive network model. The practical significance of these differences is shown by comparison of muscle activity with more established optimization-based force predictions. Those subjects categorized as majority-type responders had muscle activity in better correspondence with optimization-based predicted forces. Subjects in minority categories displayed more variance in their response patterns and larger degrees of antagonistic cocontraction. CONCLUSIONS: The implications for deterministic (e.g. optimization-based) biomechanical modelling are discussed. It is speculated that interindividual muscle recruitment differences may be important for assessing individual musculoskeletal risk.

Evaluation of artificial neural network modelling to predict torso muscle activity.

Owing to the complexities involved in obtaining direct measures of in vivo muscle forces, validation of predictive models of muscle activity has been difficult. An artificial neural network (ANN) model had been previously developed for the estimation of lumbar muscle activity during moderate levels of static exertion. The predictive ability of this model is evaluated in this study using several techniques, including comparison of response surfaces and composite statistical tests of values derived from model output, with multiple EMG experimental datasets. ANN-predicted activation levels were accurately modelled to within 3% across a range of experiments and levels of combined flexion/extension and lateroflexion loadings. The results indicate both a high degree of consistency in the averaged muscle activity measured in several different experiments, and substantiate the ability of the ANN model to predict generalized recruitment patterns. It also is suggested that the use of multiple comparison methods provides a better indication of model behaviour and prediction accuracy than a single evaluation criterion.