Göran M Hägg

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.

Methodologies for evaluating electromyographic field data in ergonomics

Surface electromyography (SEMG) is an important tool for work load assessment in ergonomics. Several different approaches using amplitude as well as frequency parameters give fruitful information depending on question at issue in the laboratory as well as in field studies. One basic factor determining the choice of analysis method is whether the SEMG is interpreted as an indicator of forces/torques or pure muscular activation. Two methods for occupational SEMG data reduction representing two different approaches to SEMG applications in ergonomics, Exposure Variation Analysis (EVA), and Joint Analysis of EMG Spectrum and Amplitude (JASA), applied on the same SEMG recording from three muscles during urology surgeon work, have been compared. The EVA method categorised the three muscle recordings as too static with no EMG gaps while the JASA method identifies fatigue in two of the three recordings. The practical relevance of these findings is discussed.

Forearm extensor and flexor muscle exertion during simulated gripping work -- an electromyographic study.

OBJECTIVE: To study forearm muscular exertion during intermittent gripping work at three different regimes resembling vocational work. DESIGN: An electromyographic laboratory study of forearm fatigue during intermittent gripping work at 25% MVC with 10/10, 20/10 and 30/10 s of work/rest was performed. BACKGROUND: Data from the literature indicate that forearm flexors as well as extensors are activated during gripping. However, no systematic quantitative assessment of the muscular exertion has been made hitherto. METHODS: Nine female subjects participated. EMG was detected from five forearm muscles, three extensors and two flexors. Zero crossing rate alterations were used as an index of fatigue. RESULTS: The results show significant (P < 0.05) fatigue signs in at least two of the extensor muscles at all regimes while significant signs of fatigue was seen only at the 30/10 regime for the flexor muscles. CONCLUSIONS: Fatigue effects are generally larger on the extensor side. None of the studied regimes is acceptable from EMG fatigue point of view. RELEVANCE: The muscular fatigue of the forearm extensors in relation to the flexors during gripping work has not been systematically assessed before. Forearm fatigue distribution is probably of great importance for the understanding of forearm disorders related to muscular exertion.

Corporate initiatives in ergonomics--an introduction.

Examples in the literature of corporate initiatives in ergonomics are reviewed. Different types of programmes are identified with ambitions ranging from time-limited interventions to continuous processes. Common elements are health surveillance, workstation design and choice of tools, product design, quality aspects, participative aspects and education, training and information. The implementation of ergonomics programmes varies substantially depending on the type of company, and company policies and organisation. Some of the most developed ergonomics programmes originate from the automobile industry. Other businesses with many established programmes are the electronics industry, the food industry and the office environment. A participative approach, as well as ergonomics expertise, are crucial ingredients for a successful programme. The scientific evaluation of ergonomics programmes, especially in economical terms, is in too many cases insufficient or missing. Furthermore, links to company core values such as quality improvement are often lacking. Programmes in ergonomics are still often seen as solely a matter of health and safety. Only a few companies have reached the state where ergonomics constitutes an integrated part of the overall strategy of the enterprise.

The impact on musculoskeletal disorders of changing physical and psychosocial work environment conditions in the automobile industry

Abstract In the car-body-sealing department at an automobile assembly plant in Sweden, a reorganisation of work from lineout to line production was performed. The aim of the present investigation was to study the influence of changes in physical and psychosocial conditions on musculoskeletal disorders. Working conditions were studied on two occasions, before and after the change. Physical workload regarding strenuous postures and movements was assessed by questionnaires, direct measurements recordings and by computer based observations registrations, and the psychosocial conditions by questionnaires and group discussions. The musculoskeletal disorders of the workers were studied by reports from the occupational health care centre and by self-reported musculoskeletal symptoms. A decrease in the time spent in strenuous positions was found, but the work cycle time decreased considerably and the worker felt ‘robotised’. There was a marked decrease in the perception of opportunities to influence the work, as well as the stimulation at work. The amount of musculoskeletal disorders increased for the study group, but not for a reference group from the same plant. A possible explanation for the increase in musculoskeletal disorders was the increase in perceived physical exertion and the decrease in occupational pride. Relevance to industry This paper illustrates the importance of paying attention to psychosocial issues in order to avoid musculoskeletal disorders after an intervention mainly aimed at physical workstation design.

Occupational lifting by nursing aides and warehouse workers.

Nursing aides (18 women and six men) at one traditional and one modern geriatric ward, and warehouse workers (16 men) at two different types of warehouse departments were studied during occupational work. The vertical load during manual handling was measured using strain gauges built into wooden shoes. The warehouse workers performed four times as many lifts as the nursing aides and transferred five times as great a mass per unit time. Less than 25% of the lifts were carried out with the load evenly distributed on both feet during both the upward lift and lowering. In the warehouses the lifts were short, while the nursing aides were exposed both to lifts of longer duration and to carrying, as well as a greater frequency of unexpected, sudden and high peak load. The heart rate and oxygen uptake values recorded were relatively low, and both groups utilized approximately 25% of their maximum aerobic power. According to official occupational injury statistics, both warehouse workers and nursing aides belong to risk groups with a high frequency of reported back injuries. However, more injuries are reported by nursing aides than by warehouse workers. Greater muscle strength in male workers may partly explain this difference. Qualitative differences in the design of the lifting work, with more lifts of long duration in awkward work postures, more carrying, more exertion of horizontal forces and a greater frequency of unexpected rapid lifts in the nursing aides may also contribute to the differences in risk of injury between these occupational groups. Considerable differences were found between the two hospital wards as regards lifting frequency, force, duration and the proportion of lifts with an even distribution of load on both feet during upward lift. The lifting work was approximately 50% less in the modern ward, which has easily maneuvered electric overhead hoists, spacious premises and better work organization than in the traditional ward, even though the patient handling needs were equivalent. In the warehouse department at which the wares were stored with easy access, the frequency of lifts with an even distribution of load on both feet was approximately 25%, compared to 7% in the other warehouse department. In conclusion, evaluations of occupational lifting work should include a quantitative assessment of the duration and load distribution of lifting, as well as of postures. Workstation design, technical aids and work organization have a profound effect on the lifting work load even in jobs with equal manual handling demands.

Forearm muscular load and wrist angle among automobile assembly line workers in relation to symptoms

Electromyographic activity (EMG) from m. flexor carpi radialis (FCR) and m. extensor carpi radialis longus (ECRL) of the right forearm was recorded together with wrist angles in the flexion/extension and radial/ulnar plane in 20 healthy automobile assembly line workers during work. Eleven of these were randomly recruited from assembly stations with a low prevalence of subjective wrist/forearm symptoms (LPS), while the rest came from stations with a high prevalence of symptoms (HPS). The main EMG finding was a clear difference in activation pattern between flexors and extensors. ECRL was activated more statically, while FCR had a more dynamic pattern with more pauses but also higher peak loads. The main wrist angle finding was a difference in angle distribution as well as absolute angular velocity in the radial/ulnar plane between LPS and HPS work stations. Workers in HPS stations worked longer times in a more ulnar deviated hand position and had higher absolute deviation angular velocity compared to LPS workers, indicating ulnar deviation as a risk factor. Generally, ulnar deviation from a neutral position was more frequent than angular displacement in the flexion/extension plane.

Electromyographic fatigue analysis based on the number of zero crossings

An alternative method for analysis of localized muscle fatigue as performed by Lindström et al. (1977) is presented. The method is based on counting of zero crossing of the electromyographic signal. The method is implemented on a microprocessor computer system. The analysis can be done on line and the result is presented as a plot of the normalized action potential velocity versus time.

Zero crossing rate of electromyograms during occupational work and endurance tests as predictors for work related myalgia in the shoulder/neck region

SummaryThe relationship between electromyographic signs of fatigue (ESF) during work and occupation-related myalgia in the shoulder/neck region was investigated in a longitudinal study. Forty-three healthy female assembly workers were studied over 2 years. Measurements were performed at the start of the study with follow-up measurements after 1 and 2 years. The ESF were estimated as the zero crossing rate of electromyograms (EMG) detected during short test contractions performed during short breaks in normal work. As a complement, an endurance test using EMG records was performed and analysed with the zero crossing technique. The occurrence of shoulder/neck disorders was assessed by a clinical investigation and a questionnaire. No significant relationship between ESF during work in year 0 and deterioration of the disorder was seen. On the other hand, the absolute zero crossing rate and the time constant of the zero crossing decline from the endurance test showed a significant relationship with deterioration of the disorder. The ESF during work year 2, showed a significant relationship with disorder year 2, while the endurance test parameters year 2 did not. It was concluded that ESF during work was not a predictor of muscle injury, whereas it could be useful as a diagnostic tool.

A worksite method for shoulder muscle fatigue measurements using EMG, test contractions and zero crossing technique

Local muscular fatigue is an important aspect of ergonomic studies. Spectral analysis of electromyographic (EMG) signals may be applied to provide a quantitative measure of the extent of muscular fatigue. The present study was aimed at methodological development of practical and reliable field studies for the shoulder region, EMG signals were recorded during short test contractions performed at regular intervals during work and at rest. Signal analysis was performed using the zero-crossing method. The results show that the zero-crossing method gives results which are as relevant as the MPF-method. The test contraction concept in conjunction with the zero-crossing technique is called Frequency Analysis of Test Contractions (FTC). An evaluation experiment on ten subjects at rest yielded the intrinsic standard deviation of the method and also revealed lateral covariation of the trapezius and infraspinatus muscles.