Fatigue induced by static work

Despite its low energy cost, isometric contraction can result in the onset of local muscle fatigue. The onset of fatigue occurs more rapidly when the relative force exerted is greater than 15-20% of the maximum voluntary contraction (MVC) of the muscle considered, and when the contraction time is increased. The maximum maintenance time (limit-time) and the corresponding relative force are linked by a hyperbolic relation. Ischaemia promotes accumulation of acid metabolites produced during contraction, and hinders their elimination, thus constituting the main causal factor in the onset of local muscle fatigue. The introduction of rest periods of sufficient duration to ensure restoration of normal blood flow through the muscle is an effective way of delaying, or even preventing, the onset of muscle fatigue. Other factors may also be taken into account, such as the position in which the static work is performed, and the nature and number of muscles used simultaneously, etc. Numerous laboratory and field studies have allowed the development of various models that take into account the conditions relating to isometric contractions during static work.     

Fatigue of directly wafer-bonded silicon under static and cyclic loading

 Fatigue tests were performed to investigate the reliability properties of wafer-bonded single crystalline silicon exposed to static or cyclic mechanical loading. A distinct decrease of strength with increasing load duration or cycle number was found which limited the lifetime of mechanically stressed wafer-bonded components. The occurrence of fatigue is related to siloxane bonds in the bonded interface between the silicon wafers. It was shown that fatigue is absent either if siloxane bonds are not present in the bonded interface or if local areas of pure silicon bonds were formed between the silicon wafers. As a consequence of this fatigue phenomenon, loaded wafer-bonded silicon sensors and actuators may suddenly fail during application. Therefore, appropriate fracture mechanical techniques were developed to predict either the time-to-failure or the cycles-to-failure for a given loading situation. These concepts allow for the optimization of the device layout with respect to long-term reliability, which can reduce the development time and costs.     

Independent complexity patterns in single neuron activity induced by static magnetic field

We applied a combination of fractal analysis and Independent Component Analysis (ICA) method to detect the sources of fractal complexity in snail Br neuron activity induced by static magnetic field of 2.7 mT. The fractal complexity of Br neuron activity was analyzed before (Control), during (MF), and after (AMF) exposure to the static magnetic field in six experimental animals. We estimated the fractal dimension (FD) of electrophysiological signals using Higuchi's algorithm, and empirical FD distributions. By using the Principal Component Analysis (PCA) and FastICA algorithm we determined the number of components, and defined the statistically independent components (ICs) in the fractal complexity of signal waveforms. We have isolated two independent components of the empirical FD distributions for each of three groups of data by using FastICA algorithm. ICs represent the sources of fractal waveforms complexity of Br neuron activity in particular experimental conditions. Our main results have shown that there could be two opposite intrinsic mechanisms in single snail Br neuron response to static magnetic field stimulation. We named identified ICs that correspond to those mechanisms - the component of plasticity and the component of elasticity. We have shown that combination of fractal analysis with ICA method could be very useful for the decomposition and identification of the sources of fractal complexity of bursting neuronal activity waveforms.     

The effects of strength testing schedules on muscle strength recovery following static work

In order to measure the recovery of muscle strength after static contractions, strength tests must be performed at various times recovery. This is shown to be an invasive procedure, particularly at typically-recommended schedules of one measurement every 30 s or every 60 s, as the measurement interferes with the recovery process. A predictive equation is given to correct recovery data for this invasiveness.     

Fatigue and human reliability revisited in the light of ergonomics and work psychopathology

In the process control human-beings are actually information processors, solving many problems, especially covariance-problems which involve various interconnected and interactive factors.

The purpose of this experiment was to investigate the types of cognitive strategy and information-processing procedure used in solving covariance-problems, and to evaluate and determine the best strategy.

Fifty-five subjects participated in the experiment to solve two-factor covariance-problems. Verbal protocols and structural analysis were used to record and analyse the cognitive processes of covariance-problem solving. Verbal protocols were coded and analysed to differentiate the basic operators and thence explore the various strategies.

The results indicated that under two-factor covariant conditions, there were nine kinds of cognitive strategy. The 'biconvex-lens’ strategy was the most effective. All nine strategies can be classified into three categories, trial-and-error, analytic and intuitive.

This experiment research was the first study on cognitive strategies in solving covariance-problems. New topics for further study are put forward.     

Effects of dynamic,static and combined work on heart rate and oxygen consumption

The effects upon heart rate and oxygen consumption of muscular exercises including simultaneous dynamic and static contractions were studied in three male subjects. Dynamic work consisted of walking at four speeds (0·56, 0·83, 1·11, 1·39 m s^?) on a horizontal treadmill; static work consisted of pushing against, pulling and holding 6, 9, 12, 18 and 24?kg; combined work associated walking with each one of the forms of static work. Physiological load is expressed in terms of cardiac cost (ΔHR) and oxygen cost (Δ[Vdot]_o2). The physiological cost of combined work increases with both the walking speed and the static load. For each parameter (HR and [Vdot]_o2) the extra-cost of combined work has been determined by computing the difference between the cost of combined work and the sum of the costs observed during static and dynamic exercises separately performed. The paired t-test shows significant differences for all of the walking-pushing tests, but only for 8 pulling tests and 2 holding tests. Linear relationships are observed between the oxygen extra-cost and load when walking at 0·56 or 0·83 ms^?1, with correlation coefficients statistically significant for pushing and pulling (p < 0·01) but not significant for holding tests. The present results show that, when the static work is combined with walking, the physiological response varies with the type of static work considered.     

A study stressing the need for a static postural force model for work analysis

The maximum endurance time (MET) in static force exertions was used as a parameter for the assessment of five working postures. By applying the methodology of Rohmert to the construction of a general model for static muscular work and evaluating the measured MET results, the need for a new static posture model has been shown.

The aim of the present pilot study was to test MET in load situations that would indicate when the general model can be used or when a new static postural force model is needed.

Subjects exerted static postural forces at different load levels until exhaustion. In the first two postures, the strain was concentrated on the upper limbs, where active forces (muscular) play a key-role and justify the use of the model. In the remaining postures, the strain affected mainly the back/trunk, where the mechanical equilibrium of the body is brought about by active (muscles) and passive (skeleton and ligaments) structures.

During the tests electromyographic (EMG) measurements of selected muscles (objective measurements) as well as rated perceived exertion (RPE; subjective measurements on Borg's CR-10 scale) were recorded.

The results show that the maximum endurance times in upper extremity postures are predicted by the general model whilst in the back/trunk postures the measured MET was longer than predicted by the model. New models are presented for static postural force on the back.

The EMG measurements supported the conclusion that the muscles studied play a key role in the chosen upper-extremity postures but gave no clear indication in the back-oriented postures. Ratings of perceived exertion coincided with the EMG measurements in upper extremity postures and proved to be a good substitute for measurement and calculation of the load levels studied. The initial RPE can therefore be used in models for predicting maximal endurance times in complex cases.

For the range of relative postural loads tested, an exponential function for predicting MET in static posture exertions produced the best fit curve.     

Effects of changes in sitting work posture on static neck and shoulder muscle activity

Abstract

In order to analyse the effect of changing the sitting posture on the level of neck and shoulder muscular activity, an electromyographic (EMG) study of ten healthy experienced female workers from an electronics plant was undertaken. A standardized, simulated task was performed in eight different sitting work postures. Using surface electrodes, the level of muscular activity was recorded as normalized, full-wave rectified low-pass filtered EMG. The results showed that the whole spine flexed sitting posture gave higher levels of static activity in several neck and shoulder muscles than the posture with a straight and vertical spine, which in turn gave higher levels than the posture with slightly backward-inclined thoraco-lumbar spine.     

Muscle- and task-dependent responses to concurrent physical and mental workload during intermittent static work

Many workers experience combined physical and mental demands in their jobs, yet the contribution of these demands to the development of musculoskeletal disorders is unclear. The purpose of this study was to investigate muscle- and task-dependent responses to concurrent demands during intermittent static work. Twenty-four participants performed shoulder, wrist, and torso exertions at three levels of physical workload (PWL) in the absence (control) and presence (concurrent) of a mental arithmetic task. Compared to the control, concurrent demand conditions resulted in decreased muscle activity (4–9% decrease), increased cardiovascular load (2–4% increase), and impaired motor co-ordination (9–24% increase in force fluctuation). Furthermore, these outcomes were more prominent at higher PWL levels and within postural (shoulder and torso) muscles. Mental task performance exhibited greater interference with the physical task at low and high PWL levels. Thus, it may be important to consider these muscle- and task-specific interactions of concurrent demands during job design to address worker health and performance issues.

Practitioner Summary: Occupational tasks place both physical and mental demands on workers. These demands can adversely affect physiological responses and performance, and are muscle- and task-dependent. Findings from this research may facilitate the development of ergonomics interventions, such as task redesign and tool/workstation design, that may help reduce risk of workplace injuries.     

Quantification of the static load component in muscle work using nonlinear filtering of surface EMG

Prolonged static strain on the muscles of the neck-shoulder region is believed to be linked to the development of musculoskeletal problems. To quantify the static strain on the basis of EMG, the level as well as the duration of the muscle load should be analysed on temporal basis. In this paper, some methods for the temporal analysis of EMG recordings are proposed with an aim of quantifying the long-term static strain on the muscle. The use of nonlinear median prefilters for decomposing the EMG activity according both to amplitude level and duration of the activity at different levels is proposed. The prefiltering methods were also evaluated using laboratory studies. The main aim of the studies was to compare the estimation errors between EMG and force for different types of prefilters especially when the static load component was analysed. The average estimation error for sequences having a duration longer than 1 s was found to be 8% of MVC in the case of trapezius muscle and 14% of MVC in the case of biceps brachii muscle. Linear relation was found on the basis of linear least squares curve fitting to give the largest correlation coefficients between EMG and force, when the static load component was analysed.     

Fatigue during prolonged intermittent overhead work: reliability of measures and effects of working height

Shoulder pain is prevalent among industrial workers and existing evidence supports that overhead work is an important specific risk factor. Existing guidelines are limited, with overhead work typically recommended to be avoided, and research on overhead work has been mixed in terms of the effects of increasing arm reach. A laboratory-based simulation of overhead work was conducted, at three working heights, in order to facilitate improved guidelines and to identify potential non-linear effects of overhead work height. Several indicators of shoulder fatigue served as outcome measures and a preliminary study was performed to assess the reliability of several of these measures. Fatigue measures based on electromyography (EMG) generally had low reliability, whereas excellent reliability was exhibited for ratings of perceived discomfort (RPD). Consistent with this, no effects of overhead work height were found on EMG-based measures, yet clear non-linear effects were found on RPD and task performance. The source of the effects of work height appeared to be related to a combination of muscle activation levels and demands on precision/control at the highest location. These results support the utility of subjective measures for relatively low-level intermittent exertions and demonstrate increasingly detrimental fatigue and performance effects at extremes in reach during overhead work.     

Fatigue during prolonged intermittent overhead work: reliability of measures and effects of working height

Shoulder pain is prevalent among industrial workers and existing evidence supports that overhead work is an important specific risk factor. Existing guidelines are limited, with overhead work typically recommended to be avoided, and research on overhead work has been mixed in terms of the effects of increasing arm reach. A laboratory-based simulation of overhead work was conducted, at three working heights, in order to facilitate improved guidelines and to identify potential non-linear effects of overhead work height. Several indicators of shoulder fatigue served as outcome measures and a preliminary study was performed to assess the reliability of several of these measures. Fatigue measures based on electromyography (EMG) generally had low reliability, whereas excellent reliability was exhibited for ratings of perceived discomfort (RPD). Consistent with this, no effects of overhead work height were found on EMG-based measures, yet clear non-linear effects were found on RPD and task performance. The source of the effects of work height appeared to be related to a combination of muscle activation levels and demands on precision/control at the highest location. These results support the utility of subjective measures for relatively low-level intermittent exertions and demonstrate increasingly detrimental fatigue and performance effects at extremes in reach during overhead work.     

Squaring down by static and dynamic compensators

Given a system, which is not necessarily invertible and which has an unequal number of inputs, a method of `squaring down', that is a method of designing pre-compensators and postcompensators such that the resulting system has an equal number of inputs and outputs and is invertible, is presented. The compensators are, in general, dynamic and have the property that the additional finite zeros induced by them are assignable to the open left-half complex plan. Furthermore, the compensators are asymptotically stable, and hence do not deteriorate the robustness and performance of an eventual feedback design. Also, the compensator design preserves the stabilizability, detectability, and minimum-phase properties and the infinite zero structure of the original system. Thus, a method of designing nonsquare systems by converting them to square invertible systems is introduced. Two applications of such a design philosophy, (1) diagonal decoupling with state feedback; and (2) almost disturbance decoupling with output feedback, are pointed out     

On constraints in pole assignment by static feedback

A design method based on pole assignment considerations for an nth order linear time-invariant single-input p-output system by means of static (constant gain) feedback is presented for the case in which the rank p of an output matrix C is less than the number of system states n. The constraints in pole assignment are described in the form of linear relationships between the coefficients of the closed-loop characteristic polynomial and some open-loop system parameters. It has been shown that the problem of pole assignment is equivalent to the problem of solving a set of n linear equations with p unknowns in such a way that n – p equations are made linearly dependent by proper choice of coefficients of the closed-loop characteristic polynomial. As a result, all n system poles can be shifted to selected locations which satisfy the constraints. Of particular interest are the design methods for the cases when the number of arbitrarily assigned poles are equal to p and p – I.     

On estimating the useful work distribution of parallel programs under P3T: a static performance estimator

In order to improve a parallel program's performance it is critical to evaluate how even the work contained in a program is distributed over all processors dedicated to the computation. Traditional work distribution analysis is commonly performed at the machine level. The disadvantage of this method is that it cannot identify whether the processors are performing useful or redundant (replicated) work. The paper describes a novel method of statically estimating the useful work distribution of distributed-memory parallel programs at the program level, which carefully distinguishes between useful and redundant work. The amount of work contained in a parallel program, which correlates with the number of loop iterations to be executed by each processor, is estimated by accurately modeling loop iteration spaces, array access patterns and data distributions. A cost function defines the useful work distribution of loops, procedures and the entire program. Lower and upper bounds of the described parameter are presented. The computational complexity of the cost function is independent of the program's problem size, statement execution and loop iteration counts. As a consequence, estimating the work distribution based on the described method is considerably faster than simulating or actually compiling and executing the program. Automatically estimating the useful work distribution is fully implemented as part of P³T, which is a static parameter based performance prediction tool under the Vienna Fortran Compilation System (VFCS). The Lawrence Livermore Loops are used as a test case to verify the approach.     

Detection of Driver Fatigue Caused by Sleep Deprivation

This paper aims to provide reliable indications of driver drowsiness based on the characteristics of driver-vehicle interaction. A test bed was built under a simulated driving environment, and a total of 12 subjects participated in two experiment sessions requiring different levels of sleep (partial sleep-deprivation versus no sleep-deprivation) before the experiment. The performance of the subjects was analyzed in a series of stimulus-response and routine driving tasks, which revealed the performance differences of drivers under different sleep-deprivation levels. The experiments further demonstrated that sleep deprivation had greater effect on rule-based than on skill-based cognitive functions: when drivers were sleep-deprived, their performance of responding to unexpected disturbances degraded, while they were robust enough to continue the routine driving tasks such as lane tracking, vehicle following, and lane changing. In addition, we presented both qualitative and quantitative guidelines for designing drowsy-driver detection systems in a probabilistic framework based on the paradigm of Bayesian networks. Temporal aspects of drowsiness and individual differences of subjects were addressed in the framework.     

Evaluating shakedown under proportional loading by non-linear static analysis

A lower bound method for calculating shakedown loads under proportional loading by static non-linear finite element analysis is presented. Stress fields obtained by static analysis and stress superposition are substituted into Melan’s lower bound shakedown theorem. The proposed method is applied to two sample problems: a thick cylinder under internal pressure and a square plate with a central hole under proportional biaxial loading. The results indicate that the method gives accurate lower bound shakedown loads for these problems.     

Stabilization of discrete time linear systems by static outputfeedback

This note proposes a systematic approach for the static output feedback control design for discrete time linear systems. It is shown that if the open loop system satisfies some particular structural conditions, a static output feedback gain can be calculated easily, using a formula only involving the original system matrices. Among the conditions the system has to satisfy, the stronger relies on a minimum phase argument. Square and nonsquare systems are considered     

Road-traffic monitoring by knowledge-driven static and dynamic image analysis

This article presents a visual application which allows a study and analysis of traffic behavior on major roads (more specifically freeways and highways), using as the main surveillance artefact a video camera mounted on a relatively high place (such as a bridge) with a significant image analysis field. The system described presents something new which is the combination of both traditional traffic monitoring systems, that is, monitoring to get information on different traffic parameters and monitoring to detect accidents automatically. Therefore, we present a system in charge of compiling information on different traffic parameters. It also has a surveillance module for that traffic, which can detect a wide range of the most significant incidents on a freeway or highway.