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A case study is presented that evaluates the impact of partial automation strategies on productivity and ergonomics. A company partly automated its assembly and transportation functions while moving from a parallel-batch to a serial line-based production system. Data obtained from company records and key informants were combined with detailed video analysis, biomechanical modelling data and field observations of the system. The new line system was observed to have 51% higher production volumes with 21% less per product labour input and lower work-in-process levels than the old batch-cart system. Partial automation of assembly operations was seen to reduce the total repetitive assembly work at the system level by 34%. Automation of transportation reduced transport labour by 63%. The strategic decision to implement line-transportation was found to increase movement repetitiveness for operators at manual assembly stations, even though workstations were constructed with consideration to ergonomics. Average shoulder elevation at these stations increased 30% and average shoulder moment increased 14%. It is concluded that strategic decisions made by designers and managers early in the production system design phase have considerable impact on ergonomic conditions in the resulting system. Automation of transport and assembly both lead to increased productivity, but only elements related to the automatic line system also increased mechanical loads on operators and hence increased the risk for work-related disorders. Suggestions for integrating the consideration of ergonomics into production system design are made. [ABSTRACT FROM AUTHOR]

Purpose--The purpose of this paper is to investigate a strategic change from parallel cell-based assembly (old) to serial-line assembly (new) in a Swedish company with special reference to how production system design elements affect both productivity and ergonomics. Design/methodology/approach--Multiple methods, including records and video analysis, questionnaires, interviews, biomechanical modelling, and flow simulation were applied. Findings--The new system, unlike the old, showed the emergence of system and balance losses as well as vulnerability to disturbances and difficulty handling all product variants. Nevertheless, the new system as realised partially overcame productivity barriers in the operation and management of the old system. The new system had impaired ergonomics due to decreased physical variation and increased repetitiveness with cycle times that were 6 per cent of previous thus increasing repetitiveness, and significantly reducing perceived influence over work Workstations' uneven exposure to physical tasks such as nut running created a potential problem for workload management. The adoption of teamwork in the new system contributed to significantly increased co-worker support--an ergonomic benefit. Practical implications--Design decisions made early in the development process affect both ergonomics and productivity in the resulting system. While the time pattern of physical loading appeared to be controlled by flow and work organization elements, the amplitude of loading was determined more by workstation layout. Psychosocial conditions appear to be affected by a combination of system elements including layout, flow, and work organization elements. Strategic use of parallelisation elements in assembly, perhaps in hybrid forms from configurations observed here, appears to be a viable design option for improved performance by reducing the fragility and ergonomic problems of assembly lines. Originality/value--The interacting design elements examined here pose potential "levers" of control by which productivity and ergonomics could be jointly optimised for improved total system performance. [ABSTRACT FROM AUTHOR]

This study investigated temporal changes in movement strategy and performance during fatiguing short-cycle work. Eighteen participants performed six 7-min work blocks with repetitive reaching movements at 0.5 Hz, each followed by a 5.5-min rest break for a total duration of 1 h. Electromyography (EMG) was collected continuously from the upper trapezius muscle, the temporal movement strategy and timing errors were obtained on a cycle-to-cycle basis, and perceived fatigue was rated before and after each work block. Clear signs of fatigue according to subjective ratings and EMG manifestations developed within each work block, as well as during the entire hour. For most participants, timing errors gradually increased, as did the waiting time at the near target. Changes in temporal movement strategy were negatively correlated with changes in the level and variability of EMG, suggesting that an adaptive temporal strategy offset the development of unstable motor solutions in this fatiguing, short-cycle work., Practitioner Summary: Sustained performance of operators is essential to maintain competitiveness. In this study of repetitive work, participants gradually changed their temporal movement strategy, for possibly alleviating the effects of fatigue. This suggests that in order to effectively counteract fatigue and sustain performance, industrial production should allow extensive spatial and temporal flexibility.

This study investigated the effect of work pace on workload, motor variability and fatigue during light assembly work. Upper extremity kinematics and electromyography (EMG) were obtained on a cycle-to-cycle basis for eight participants during two conditions, corresponding to "normal" and "high" work pace according to a predetermined time system for engineering. Indicators of fatigue, pain sensitivity and performance were recorded before, during and after the task. The level and variability of muscle activity did not differ according to work pace, and manifestations of muscle fatigue or changed pain sensitivity were not observed. In the high work pace, however, participants moved more efficiently, they showed more variability in wrist speed and acceleration, but they also made more errors. These results suggest that an increased work pace, within the range addressed here, will not have any substantial adverse effects on acute motor performance and fatigue in light, cyclic assembly work. STATEMENT OF RELEVANCE: In the manufacturing industry, work pace is a key issue in production system design and hence of interest to ergonomists as well as engineers. In this laboratory study, increasing the work pace did not show adverse effects in terms of biomechanical exposures and muscle fatigue, but it did lead to more errors. For the industrial engineer, this observation suggests that an increase in work pace might diminish production quality, even without any noticeable fatigue being experienced by the operators.

For measuring the physical exposure/workload in studies of work-related musculoskeletal disorders, direct measurements are valuable. However, the between-days and between-subjects variability, as well as the precision of the method per se, are not well known. In a laboratory, six women performed three standardised assembly tasks, all of them repeated on three different days. Triaxial inclinometers were applied to the head, upper back and upper arms. Between-days (within subjects) and between-subjects (within tasks) variance components were derived for the 10th, 50th and 90th percentiles of the angular and the angular velocity distributions, and for the proportion of time spent in predefined angular sectors. For percentiles of the angular distributions, the average between-days variability was 3.4 degrees , and the between-subjects variability 4.0 degrees . For proportion of time spent in angular sectors, the variability depended on the percentage of time spent in the sector; the relative variability was scattered and large, on average 103% between days and 56% between subjects. For the angular velocity percentiles, the average between-days variability was 7.9%, and the average between-subjects variability was 22%. The contribution of the measurement procedure per se to the between-days variability, i.e., the imprecision of the method, was small: less than 2 degrees for angles and 3% for angular velocity.

Though surface electromyography (EMG) has been widely used in studies of occupational exposure, its precision in terms of the variance between-days and between-subjects has seldom been evaluated. This study aimed at such an evaluation. Six women performed three different work tasks: 'materials picking', 'light assembly', and 'heavy assembly', repeated on 3 different days. EMG was recorded from m. trapezius, m. infraspinatus and the forearm extensors. Normalisation was made to a maximal (MVE), and a submaximal (RVE), reference contraction. Variance components between days (within subjects) and between subjects were derived for the 10th, 50th and 90th percentiles, as well as for muscular rest parameters. For the task 'heavy assembly', the coefficient of variation between days (CV(BD)) was 8% for m. trapezius (right side, 50th percentile, MVE normalised values). Larger variabilities were found for m. infraspinatus (CV(BD) 15%), and the forearm extensors (CV(BD) 33%). Between-subjects variability (CV(BS)) was greater, 16% for m. trapezius and 57% for m. infraspinatus, 29% for the forearm extensors. RVE normalisation resulted in larger CV(BD), while reducing CV(BS). The between-days and between-subjects variability may be used to optimise sampling strategy, and to assess the bias in epidemiological studies. The bias caused by measurement procedures per se is acceptable.

Three work station design studies in forestry machines were carried out with regard to shoulder/arm muscle load. (1) In an arm rest study, a movable (in the sagittal plane) arm rest was compared with a fixed one; (2) in a lever study, a mini lever with a small deflection was compared with an ordinary hand lever; (3) in a boom control study, a computer controlled boom system was compared with an ordinary boom system. In the arm rest study eight male subjects carried out lever operations in the laboratory. In the lever and boom studies, 15 male forestry students operated forwarders. Muscle load, cycle time, skill, perceived exertion, heart rate and preferred set up were recorded. The muscle loads on the upper trapezius, infraspinatus, extensor carpi ulnaris and flexor carpi radialis were recorded by electromyography. With mini lever and movable arm rest the upper trapezius load was reduced with preserved or decreased cycle time, although the grand mean of the 'static' load level was only about 2% of maximal voluntary contraction (MVC) and the effects of the new designs were small (about 0.7% MVC). The subjects preferred the lever/arm rest also giving the lowest muscle load. No differences between levers or arm rests were found in skill, perceived exertion or heart rate. It was not possible to draw any definitive conclusions regarding effects of the boom control system, due to technical problems. According to the literature, the duration of lever operation has increased considerably in forestry during recent years as part of a rationalization process. Partly due to this, it is suggested that the investigated work station improvements may not be sufficient to eliminate the risk for shoulder-neck disorders.

Due to unsatisfactory productivity, a large company in the Swedish manufacturing industry decided to rationalize their assembly system. The intended rationalization comprised several changes with deliberate ergonomic implications. The main aim of the present study was to evaluate the impact of the rationalization on the physical work load of the operators. The work load was assessed before and after the changes using expert observations, company records and direct technical measurements. The results indicate that the intervention led to only minor changes in muscle load, body postures and movement patterns. Several of the planned initiatives were never implemented, e.g. teaching the workers multiple skills and designing work stations at which a major part of the assembly sequence could be performed. This was mainly due to a policy revision caused by changes in the market situation. In spite of the company's original intentions, the revised production system contained only minor ergonomic improvements. Thus, the realization of the ergonomic potential in a rationalization seems to depend on management culture, as well as factors outside the company.

Ergonomic epidemiology is a rapidly increasing field of research providing data on the occurrence of musculoskeletal disorders and possible risk factors. The present paper states, on the basis of a literature overview, that physical work load (mechanical exposure) is poorly defined and measured in most studies on ergonomic epidemiology. On this background the paper: (1) suggests adjustments of mechanical exposure concepts and terminology; (2) concludes that invalid exposure assessment may, to a large extent, explain the lack of quantitative data on relationships between mechanical exposures and musculoskeletal disorders; and (3) suggests some guidelines for future quantitative assessments of mechanical exposure in large populations.

Prolonged, repetitive handling of light material is known to increase the risk of shoulder-neck disorders. Biological risk indicators related to musculoskeletal exposure, applicable by the general practitioner in the workplace, could provide an instrument for early intervention and rehabilitation. Eight women were studied, all full-time workers performing assembly tasks associated with a high prevalence of shoulder-neck complaints. All subjects were more tender in the shoulder region than young women in low-risk occupations. Heart rate recordings indicated a low general metabolic load during work. Concentrations in antecubital venous blood of several markers for metabolic stress and cellular damage (lactate, ammonia, hypoxanthine, urate, malondialdehyde, potassium, creatine kinase) were normal for all subjects, and showed no increase during 3 consecutive working days. Thus, the blood markers did not reflect hazardous shoulder-neck exposure.

Six female subjects, aged 24-34 years, performed shoulder-neck exercise for 1 h or until they were exhausted by holding out their arms horizontally at 60 degrees to the sagittal plane. One continuous and six intermittent protocols were applied, all with a mean load corresponding to the torque of the arms, i.e. about 15% maximal voluntary contraction (MVC). The intermittent protocols varied according to cycle time (10 s, 60 s, 360 s) and duty cycle (0.33, 0.50, 0.67, 0.83). Electromyogram (EMG), mean arterial blood pressure (BPa), heart rate (fc) and perceived fatigue were monitored at regular intervals during exercise. Blood concentrations of potassium, lactate and ammonia were determined in pre- and postexercise samples of venous blood. Before and up to 4 h after exercise, measurements were made of MVC, pressure pain threshold, proprioceptive performance, and of EMG, BPa and fc during 1-min arm-holding at 25% MVC. Endurance times ranged from about 10 min to more than 1 h, significantly relating to both cycle time and duty cycle. The BPa, fc, EMG amplitude and perceived fatigue increased early during all protocols and continued to increase throughout the exercise period. Duty cycle influenced all of these variables, while only BPa and fatigue perception were related to cycle time. Cardiovascular and neuromuscular recovery was incomplete for hours after several of the protocols, as indicated for example by a sensitized response to the 1-min arm-holding. The protocols differed substantially as regards the relationship between different responses. Thus, ranking of the protocols in terms of physiological strain was different, depending on the criterion variable. The result stresses the relevance of applying a comprehensive selection of variables when evaluating the responses to intermittent shoulder-neck exercise.

Variation has in recent years become an important concept in the analysis of loads on the locomotor system in occupational life. Studies of controlled variation in isometric exercise have concentrated on the effects of introducing pauses at regular intervals into an otherwise constant load. A complete description of the cyclic character of such regimens requires a set of three parameters. We suggest the use of the set mean load/cycle time/duty cycle. The design of laboratory studies should more often be based upon the activity patterns found in occupational life. In studies observing exposure, the variation concept introduces problems concerning how to define and how to quantify 'variation'. It is shown that present methods (APDF analysis and contraction frequency analysis) individually fail to pick up important aspects of variation. A new method for exposure variation analysis (EVA) is proposed. Examples are presented of EVA performed on vocational EMG registrations.

The physiological response to continuous and intermittent handgrip exercise was evaluated. Three experiments were performed until exhaustion at 25% of maximal voluntary contraction (MVC): experiment 1, continuous handgrip (CH) (n = 8); experiment 2, intermittent handgrip with 10-s rest pause every 3 min (IH) (n = 8); and experiment 3, as IH but with electrical stimulation (ES) of the forearm extensors in the pauses (IHES) (n = 4). Before, during, and after exercise, recordings were made of heart rate (HR), arterial blood pressure (BP), exercising forearm blood flow, and concentrations of potassium [K+] and lactate [La-] in venous blood from both arms. The electromyogram (EMG) of the exercising forearm extensors and perceived exertion were monitored during exercise. Before and up to 24 h after exercise, observations were made of MVC, of force response to electrical stimulation and of the EMG response to a 10-s test contraction (handgrip) at 25% of the initial MVC. Maximal endurance time (tlim) was significantly longer in IH (23.1 min) than in CH (16.2 min). The ES had no significant effect on tlim. During exercise, no significant differences were seen between CH and IH in blood flow, venous [K+] and [La-], or EMG response. The HR and BP increased at the same rate in CH and IH but, because of the longer duration of IH, the levels at exhaustion were higher in this protocol. The subjects reported less subjective fatigue in IH. During recovery, return to normal MVC was slower after CH (24 h) than after IH (4 h).(ABSTRACT TRUNCATED AT 250 WORDS)

The electromyographical (EMG) response to isometric ramp contractions of the right arm, the left arm, and both arms was studied using four pairs of surface electrodes above the right upper trapezius muscle (UT) of six men and six women. Contractions were made against gravity with the active arm(s) in eight positions, ranging from flexion to abduction. To describe arm positions, new, simple terminology was developed. Root mean square (rms)-converted EMG-signals were normalized (EMGnorm) with respect to a reference contraction. The EMGnorm corresponding to a 15 N.m torque in the right glenohumeral (GH) joint was strongly related to the position of the right arm (P less than 0.001). The shape of this relationship depended on the electrode position (P less than 0.001). The ratio between EMGnorm at 30 N.m and 15 N.m GH torques was related to arm position (P less than 0.001) and differed between electrodes (P less than 0.001). A left-side GH torque resulted in right-side (contralateral) EMG activity, typically corresponding to 20%-30% of that obtained during similar right-side GH torque. Bilateral GH torque implied 0%-50% increase in EMG activity as compared to that obtained with the right arm alone. The results have shown that signals from one pair of surface electrodes above UT cannot be taken as representative of the EMG activity from electrodes located elsewhere above UT. The EMG recordings reflected a complex pattern of muscular activation, significantly related to both outwardly visible factors (arm position, GH torque), and within-body servosystems (motor control reflexes).

Six sedentary students, six orienteers, and six soccer players were each subjected to 15 tests, comprising 120 s of repeated, maximal isokinetic knee extensions. The tests differed with respect to movement velocity (30 degrees.s-1, 120 degrees.s-1, and 300 degrees.s-1), and movement frequency (5 at each velocity). At a certain velocity, a rectilinear relationship was found between muscular performance intensity (expressed either as average power output or as exercise time ratio) and development of fatigue (expressed either as an absolute or as a fractional decline in work output). Significant inter-velocity differences existed between the slopes of these lines at some combinations of performance and fatigue expressions. Only tendencies towards a difference in x-intercept values were found. This x-intercept value can be taken as a measure of the greatest attainable intensity level of performance without the development of fatigue. This suggestion is valuable both in basic physiological research, and as a possible criterion for optimization of muscular performance. At a given exercise time ratio, increasing movement velocity produced increasing fatigue. However, at a given muscular power output--above 15 W approximately--fatigue developed to a greater extent at the low velocity than at the two higher ones, which did not differ significantly. Substantial individual variation was seen in the positions of the low-, medium-, and high-velocity lines. These variations did not depend on the training background. This implies that the validity of using single-velocity, single-frequency tests in determining isokinetic endurance is doubtful.