Jörgen Winkel

Normalization of surface EMG amplitude from the upper trapezius muscle in ergonomic studies — A review

Surface electromyographic (EMG) amplitude from the upper trapezius muscle is widely used as a measure of shoulder-neck load in ergonomic studies. A variety of methods for normalizing EMG amplitude from the upper trapezius (EMGamp(ut)) have been presented in the literature. This impedes meta-analyses of, for instance, upper trapezius load in relation to development of shoulder-neck disorders. The review offers a thorough discussion of different normalization procedures for EMGamp(ut). The following main issues are focused: output variable, location of electrodes, posture and attempted movement during normalization, load and duration of reference contractions, signal processing and test-retest repeatability. It is concluded that translations of EMGamp(ut) into biomechanical variables, for example relative force development in the shoulder or in the upper trapezius itself, suffer from low validity, especially if used in work tasks involving large and/ or fast arm movements. The review proposes a standard terminology relating to normalization of EMGamp(ut) and concludes in a concrete suggestion for a normalization procedure generating bioelectrical variables which reflect upper trapezius activation.

ASSESSMENT OF PHYSICAL WORK LOAD IN EPIDEMIOLOGIC STUDIES : CONCEPTS, ISSUES AND OPERATIONAL CONSIDERATIONS

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.

Ergonomic intervention research for improved musculoskeletal health: A critical review

Abstract This literature review of ergonomic intervention studies aims to identify effective ergonomic interventions for improved musculoskeletal health in the workplace and to make recommendations for quality criteria in ergonomic intervention research. To avoid ambiguity in terminology a list of definitions of the ergonomic terms used in this paper is provided in an appendix. Models were developed for use in the classification of ergonomic intervention research and to illustrate the problems in interpreting ergonomic intervention data. The relevant literature was identified by a two-step process. First the relevant literature was identified by inclusion criteria, then, quality criteria were applied to identify studies of good quality for effective intervention. These appear to be firstly “organizational culture” and secondly modifier interventions, the former using multiple interventions with high stakeholder commitment to reduce identified risk factors, and the latter especially focusing workers at risk and using measures which actively involve the individual. A list of recommendations is provided.

Incidence of shoulder and neck pain in a working population: effect modification between mechanical and psychosocial exposures at work? Results from a one year follow up of the Malmö shoulder and neck study cohort

Study objective: To assess the impact of mechanical exposure and work related psychosocial factors on shoulder and neck pain. Design: A prospective cohort study. Participants: 4919 randomly chosen, vocationally active men and women ages 45–65 residing in a Swedish city. Neck and shoulder pain were determined by the standardised Nordic questionnaire. Mechanical exposure was assessed by an index based on 11 items designed and evaluated for shoulder and neck disorders. Work related psychosocial factors were measured by the Karasek and Theorell demand-control instrument. Main results: High mechanical exposure was associated with heightened risk for shoulder and neck pain among men and women during follow up. Age adjusted odds ratios (OR) were 2.17 (95% confidence intervals (CI): 1.65, 2.85) and 1.59 (95% CI: 1.22, 2.06), respectively. In women, job strain (high psychological job demands and low job decision latitude) correlated with heightened risk (OR = 1.73, 95% CI: 1.29, 2.31). These risk estimates remained statistically significant when controlled for high mechanical exposure regarding job strain (and vice versa), and for sociodemographic factors. Testing for effect modification between high mechanical exposure and job strain showed them acting synergistically only in women. Conclusion: Job related mechanical exposure in both sexes, and psychosocial factors in women, seem independently of each other to play a part for development of shoulder and neck pain in vocationally active people. The effect of psychosocial factors was more prominent in women, which could be the result of biological factors as well as gender issues. These results suggest that interventions aiming at reducing the occurrence of shoulder and neck pain should include both mechanical and psychosocial factors.

Guidelines for occupational musculoskeletal load as a basis for intervention: a critical review.

Rationalization efforts in industry, both in the Scientific Management tradition and also based on recent rationalization theories, have as one of their main aims to increase the utilization of workers. Clearly, there is a limit to the amount of physical work each employee can perform without developing musculoskeletal disorders. Such limits are generally set by guidelines for acceptable work load. This paper reviews the physical work load concept, the historical development of guidelines, and current guidelines as found in ergonomics textbooks. The focus is on the change in the aim of the guidelines over time: increased productivity, reduced fatigue and finally improved musculoskeletal health. Current guidelines for physical work load mostly emphazise a reduction in the level of work load, while there are few guidelines that consider the repetitiveness and duration of work load. As the guidelines in general only consider one exposure variable, this is a particular problem in rationalization where all three exposure dimensions may be changed simultaneously. Present guidelines are mainly based on laboratory studies aiming to eliminate short-term physiological or psychological responses. These guidelines are clearly inadequate and may be misleading in view of recent research regarding the relationship between physical work load exposure and the development of musculoskeletal complaints at the work place.

A portable ergonomic observation method (PEO) for computerized on-line recording of postures and manual handling

A new portable ergonomic observation method (PEO) is presented. It is applicable to most professions and work tasks and requires only moderate human resources for data collection and analysis. Observations are made in real time directly at the workplace using a portable personal or hand-held computer, and data are accessible for immediate analysis and presentation. Duration and number of events are calculated for postures at four body regions (arms, neck, trunk and knee) as well as for manual handling. An evaluation of the PEO method, assessing some important aspects of internal validity as well as intra- and inter-observer reliability, was carried out using video recordings. It showed acceptable validity for some types of physical exposure, and high intra- and inter-observer reliability. Practical experiences from using the PEO method in a field study and further improvements of the method are discussed.

Electromyographic activity in the shoulder-neck region according to arm position and glenohumeral torque

SummaryThe 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, a 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<0.001). The shape of this relationship depended on the electrode position (P<0.001). The ratio between EMGnorm at 30 N · m and 15 N · m GH torques was related to arm position (P < 0.001) and differed between electrodes (P< 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).

Physiological comparison of three interventions in light assembly work: reduced work pace, increased break allowance and shortened working days

An industrial assembly task known to imply a high risk for shoulder-neck disorders was simulated in the laboratory. Eight females (aged 22–32 years) were trained to manage industrial work pace (120 according to the methods-time measurement system, MTM). They carried out seven work protocols at different days with different combinations of work pace (120 or 100 MTM), break allowance (20 min of active or passive breaks added every 2 h), and duration of the working day (2, 4 or 6 h). During 6 h of work at 120 MTM the electromyographic (EMG) amplitude from the upper trapezius muscle increased by about 11%, the EMG zero crossing rate decreased by about 2.5%, and perceived fatigue increased by about 4 CR10 scale units. When work pace was reduced to 100 MTM, the upper trapezius EMG amplitude decreased by 20% and became less variable. Heart rate decreased by about 10 bpm, perceived fatigue decreased by about 1 CR10 scale units, and shoulder tenderness was reduced by about 5%. However, the work task could still not be performed in a physiological steady state. Added breaks, whether active or passive, had no apparent effects on upper trapezius load during work or on physiological responses. Recovery of EMG, maximal strength, heart rate and blood pressure sensitivity, and tenderness was complete 4 h after work, independent of the preceding work conditions. These findings suggest that a limitation of the daily duration of assembly work may be more effective in limiting acute fatigue than reduced work pace or increased break allowance.

Evaluation of foot swelling and lower-limb temperatures in relation to leg activity during long-term seated office work

Seven healthy females were studied during three experimental days, each comprising 8 hours of seated office work at different well-defined levels of leg activity. The mean foot swelling was 4·8% in the case of no leg activity (‘inactive sitting’), 2·3% when the legs were moved freely (‘ semi-active sitting’) and 0·8% at the highest activity level (‘active sitting’). The activity of the soleus muscle was evaluated by EMG and discussed in relation to the function of the musculovenous pump. The decrease in skin temperature (Tsk ) of the foot and lower leg was completed after half the working day, and on average it was 4·6°C/2·8°C (foot) and 3·3°C/l·6°C (lower leg) during ‘inactive sitting’/‘active sitting’. The mean temperature in the flexor hallucis longus muscle (Tm ) decreased by 3·7°C during the 8 hours of ‘inactive sitting’, but was almost constant during ‘active sitting’. The discomfort of the lower leg and foot was significantly correlated to foot swelling and Tm , but not to Tsk . The mean heart rate...

A case study evaluating the ergonomic and productivity impacts of partial automation strategies in the electronics industry

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.