Trapezius muscle load as a risk indicator for occupational shoulder-neck complaints

Shoulder and neck complaints in customer relations: individual risk factors and perceived exposures at work

Perceived psychosocial and biomechanical exposures, individual factors and pain in the shoulder and neck were recorded in two groups of female service workers (healthcare and shopping centre workers). The jobs investigated were characterized by 'much' direct human relations, 'little' sitting and 'much' standing, and were light work by physiological or biomechanical criteria but potentially psychosocially demanding. A screening survey (n = 400 females) was the basis for the selected sample (n = 66 females), which was the object of the main investigation of this study. Reliability of the questionnaires was tested in a separate group of female healthcare workers (n = 29). Heart rate recordings through the work day estimated workload. There was a high prevalence of shoulder and neck pain (> 50%) for both work groups. In the two populations it proved difficult to explain shoulder and neck pain by reported physical and psychosocial exposures or individual factors, except by the variable 'perceived general tension', which clearly differentiated workers with and without pain. The findings in this study indicated, first, that perceived general tension might be an independent risk factor for muscle pain and, second, that this might be related to personality factors. However, this putative relationship must be verified in a longitudinal study. As no variable describing exposures in the working environment was associated with shoulder and neck pain, the question is posed whether such complaints can be considered work-related. Alternatively, the variables used to describe mechanical and psychosocial exposures in this study may have low specificity in characterizing work-related risk factors for service workers with customer relations.

Trapezius muscle activity as a risk indicator for shoulder and neck pain in female service workers with low biomechanical exposure

Electromyographic activity of the upper trapezius muscles was recorded over the workday for two groups of service workers, shopping centre (n = 22) and healthcare workers (n = 44), both with low observed biomechanical exposure. Static and median EMG activity level, number of EMG gaps and gap time were determined. The variability of these variables over the workday was examined by calculating the coefficient of variation (CV) and the intraclass correlation coefficient (ICC) of 1-h consecutive recording periods. All variables except gap time showed acceptable reliability (ICC = 0.69-0.78), i.e. the largest fraction of variance in the data set was due to intersubject variance, despite relatively large hour-to-hour variation (CV = 0.21 0.62). The EMG activity level in the trapezius muscles was low (static activity level < 1% EMGmax), despite the high prevalence of shoulder and neck pain for both groups of workers. In addition to the work recordings, tests were performed to determine intersubject variation in muscle activity when adopting a standardized resting posture, and in a dynamic muscle activity pattern during paced arm movement. Neither the EMG variables from the work recordings nor the tests with EMG recording indicated higher trapezius EMG activity levels for workers with pain in the shoulders and neck in this study. The low EMG levels are interpreted to indicate a low risk of developing shoulder and neck complaints due to biomechanical exposure for both groups of workers. The possibility of pain-initiating mechanisms, associated with stress and not mediated through muscle activity, is considered in the discussion.

Work-related musculoskeletal complaints: some ergonomics challenges upon the start of a new century

Three themes likely to be important within health-related ergonomics in the coming years are discussed. The first two themes concern methods for risk analysis of low-level biomechanical and psychosocial exposures. The third theme is approaches to successful implementation of ergonomics interventions. Evidence on the assessment of low-level biomechanical and psychosocial exposures by instrumented measurements is discussed. It is concluded that, despite recent advances in our understanding of exposure-effect associations under these exposure conditions, we must at present rely on more subjective methods, employed in a collaboration between expert and worker. This approach to risk analysis identifies in most cases critical exposures in a work situation. The focus should then be on the successful implementation of measures against those exposures, as identification alone does not solve problems. The aim of improved health for the workers further requires that the full complement of risk factors be considered, including work, leisure time and person-based risk factors. Finally, the need to put ergonomics intervention initiatives in an organisational context is emphasised, and examples of approaches used by Norwegian companies are presented.

Muscle activity as a releasing factor for pain in the shoulder and neck

In this review, the evidence for trapezius muscle activity as a releasing factor for shoulder and neck pain is considered, mainly on the basis of studies in our laboratory. Two lines of evidence are produced, (i) vocational studies in an occupational setting, where muscle activity pattern is recorded by surface EMG and a clinical examination of the shoulder region of the subjects performed; and (ii) laboratory studies where muscle activity patterns and pain development are recorded in an experimental situation with mental stress and minimal physical activity. The vocational studies demonstrate pain development in the shoulder and neck despite very low muscle activity recorded, making it very difficult to assume muscular involvement for all cases with such complaints. However, the hypothesis of pain development through overexertion of a subpopulation of low-threshold motor units also makes it difficult to draw a firm negative conclusion. The laboratory experiments, on the other hand, show that trapezius activity patterns in response to stress have many features that would be expected if muscle activation induces pain symptoms. It is further noted that the trapezius is the only muscle with activity patterns that show these features. Possibly, we observe the effects of parallel physiological phenomena, e.g., a systemic autonomic activation that induces pain symptoms and also facilitates the motor response of some muscles. Evidence of autonomic activation of trapezius is presented by the observation of low-level, rhythmic EMG activity during sleep. However, this is not firm evidence for the above hypothesis, which at present best serves as a basis for further experimentation.

Upper trapezius muscle activity patterns during repetitive manual material handling and work with with a computer mouse

Firstly, upper trapezius EMG activity patterns were recorded on the dominant side of 6 industrial production workers and on the side operating a computer mouse of 14 computer-aided design (CAD) operators to study differences in acute muscular response related to the repetitiveness of the exposure. The work tasks were performed with median arm movement frequencies ranging from 5 min(-1) to 13 min(-1) and were characterized by work cycle times ranging from less than 30 sec to several days. However, the static and median EMG levels and EMG gap frequencies were similar for all work tasks indicating that shoulder muscle loads may be unaffected by large variations in arm movement frequencies and work cycle times. An exposure variation analyses (EVA) showed that the EMG activity patterns recorded during production work were more repetitive than during CAD work, whereas CAD work was associated with more static muscle activity patterns, both may be associated with a risk of developing musculoskeletal symptoms. Secondly, upper trapezius EMG activity patterns recorded on the mouse side of the CAD operators were compared with those recorded on the non-mouse side to study differences in muscular responses potentially related to the risk of developing shoulder symptoms which were more prevalent on the mouse side. The number of EMG gaps on the mouse side were significantly lower than the values for the upper trapezius on the non-mouse side indicating that more continuous activity was present in the upper trapezius muscle on the mouse side and EVA analyses showed a more repetitive muscle activity pattern on the mouse side. These findings may be of importance to explain differences in the prevalence of shoulder symptoms.

Postural load during VDU work: a comparison between various work postures

The aim of this study was to compare the postural load during VDU work in the following work postures: (1) Supporting and not supporting the forearms on the table top, (2) Sitting and standing positions, and (3) Sightline to the centre of the screen at an angle of 15 degrees and 30 degrees below the horizontal. The muscle load from the upper part of musculus trapezius and from the lumbar part of musculus erector spinae (L3 level) was measured by electromyography (EMG). Postural angles of head, upper arm and back were measured by inclinometers. The load on m. trapezius when using the keyboard was significantly less in sitting with supported forearms compared to sitting and standing without forearm support. Further, the time and number of periods when the trapezius load was below 1% MVC was significantly greater with support versus no support. The load on the right erector spinae lumbalis was also significantly less and the time when the load was below 1% MVC was significantly longer in a sitting work position with support versus standing without support. In addition, when using a mouse supporting the forearms reduced the static trapezius load in sitting. The results from this study document clearly the importance of giving the operator the possibility of supporting the forearms on the table top.

Arm and trunk posture during work in relation to shoulder and neck pain and trapezius activity

OBJECTIVE: To investigate work technique in relation to work-related shoulder and neck pain (SNP) and upper trapezius muscle activity. DESIGN: A matched pair, case-control field study of female employees with and without SNP. BACKGROUND: It has proved difficult to distinguish subjects with SNP from those without by vocational electromyographic recordings from the upper trapezius muscle. Other potential risk indicators include psychosocial factors and work technique. This study focuses on the latter. METHODS: Manual (14 pairs) and office workers (24 pairs) were recorded during a 30-min work period. Simultaneous recordings of upper trapezius activity by surface electromyography and arm and upper back postures by inclinometers were analysed. RESULTS: Cases and controls were not differentiated on the basis of arm elevation or of trunk posture in the sagittal plane. No significant correlations were found between variables averaging the muscle activity and the arm elevation over the recording period. Statistically significant correlations were, however, found between these variables when analysing recordings at high time resolution (0.2 s) and adjusting for the delay in arm elevation relative to the upper trapezius muscle activity (r = 0.43, manual group; r = 0.32, office group). CONCLUSIONS: Factors other than arm elevation probably contribute more significantly to the load in the upper trapezius muscle, and to the development of work-related SNP in work situations with moderate arm elevation. RELEVANCE: The study suggests that arm and trunk posture recordings are not a sensitive indicator of risk of shoulder and neck complaints in work with low to moderate biomechanical demands.

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.

Attention-related muscle activity in different body regions during VDU work with minimal physical activity

The purpose of this study was to characterize regional variation in task-irrelevant muscle activity. Surface EMG was recorded at 20 different muscle sites during a complex choice-reaction task. Up to ten muscle sites were recorded in parallel and always including the upper trapezius muscles. The experimental setting was designed to minimize the physical activity while performing a VDU-based task demanding continuing attention. Consequently, the label 'attention-related muscle activity' has been adopted. The presence of this activity varied between subjects and between the different muscle sites. It tended to decrease in a caudal direction and to be low in the muscles of the extremities, with the exception of the extensor muscles of the hand and foot. Attention-related activity was prevalent in the frontalis and upper trapezius muscles. This result is discussed with respect to a potentially harmful health effect of task-irrelevant muscle activity.

Activity of single motor units in attention-demanding tasks: firing pattern in the human trapezius muscle

Activity of single motor units in relation to surface electromyography (EMG) was studied in 11 subjects in attention-demanding work tasks with minimal requirement of movement. In 53 verified firing periods, single motor units fired continuously from 30 s to 10 min (duration of the experiment work task) with a stable median firing rate in the range of 8-13 Hz. When the integrated surface EMG were stable, the motor units identified as a rule were continuously active with only small modulations of firing rate corresponding to low-amplitude fluctuations in surface EMG. Marked changes in the surface EMG, either sudden or gradual, were caused by recruitment or derecruitment of motor units, and not by modulations of the motor unit firing rate. Motor unit firing periods (duration 10 s-35 s) in low-level voluntary contractions (approximately 1%-5% EMGmax) performed by the same subjects showed median firing rates (7-12 Hz) similar to the observations in attention-related activation.

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.

Is there a reduction in postural load when wearing progressive lenses during VDT work over a three-month period?

In an initial study, 18 VDT operators were found to have a higher muscle load in the neck/ shoulder region when using progressive lenses compared with single-vision lenses, while working on a VDT unit. For single-vision lenses, a higher flexion angle in the neck was also discovered. The present study investigates whether this higher muscle load is reduced when a progressive correction is worn for a three-month period. Adaptation in terms of reduced muscle load measured by electromyography (EMG) was not discovered. Head flexion angle was smaller when using progressive correction as compared with single-vision lenses. Some practical implications for occupational optometry are discussed.

A case-control study of trapezius muscle activity in office and manual workers with shoulder and neck pain and symptom-free controls

A case-control study with matched pairs was initiated to investigate the relationship between shoulder-neck complaints and activity in the upper trapezius muscle. The matching was done so that the physical demands from work (external exposure) were equal for both the case and the control. Each pair was also matched for gender, age, working hours, and employment time. Male (n = 18) and female workers (n = 78) employed in both manual and office work were included. Muscle activation levels and pause patterns during work and muscle activity during tests of attention, coordination, and rest were recorded by surface electromyography. The results showed consistent associations between pain and signs of increased activation of the trapezius for the cases in the manual group. No such associations were observed in the office group. The results are consistent with the hypothesis that muscle activation patterns may in some instances, but not in all, explain why some workers develop pain while others do not in work situations where the physical demands are similar.

A case-control study of psychological and psychosocial risk factors for shoulder and neck pain at the workplace

Risk factors associated with work-related shoulder and neck myalgia were investigated in a case-control study with pairs matched for age, gender, and physical exposure. Guided interviews with standardized and self-constructed questionnaires were performed among manual (n = 15 pairs) and office (n = 24 pairs) workers. Perceived general tension was the variable with the strongest association with shoulder and neck pain in both work groups. Otherwise, the results in the two groups were very different, indicating that different risk factors and mechanisms were associated with shoulder and neck pain in the two work groups. The study provides background information for future attempts to establish causal relationships between physical and psychosocial exposure and shoulder and neck pain, which can be more accurately investigated in a longitudinal rather than a cross-sectional experimental design.

Muscle coordination and choice-reaction time tests as indicators of occupational muscle load and shoulder-neck complaints

The use was explored of psychomotor tests as indicators of the risk of shoulder-neck disorders in workers with low-level static loads on the shoulder muscles. Two groups of workers performing office work and light production work were studied. A muscle coordination test with continuous movement of the arm and hand between three target areas and a psychogenic tension test, posing mental demands and with minimal requirement for body movements, aimed to quantify muscle activity in excess of that needed for biomechanical purposes. The electromyogram (EMG) recording of the active trapezius muscle in the muscle coordination test correlated with the median and static EMG values of the vocational (i.e. during the normal work task) trapezius recording both for the office and production workers, but showed no correlation with shoulder-neck complaints. The EMG responses in the psychogenic tension test and of the passive (contralateral) trapezius in the muscle coordination test correlated best with the parameters showing short, spontaneous pauses in the EMG recording of occupational load. For the office workers, but not for the production workers these parameters also correlated with shoulder-neck complaints and the presence of psychosocial problems. Psychomotor tests may thus be useful as indicators of the risk of shoulder-neck complaints in certain occupations, but further experimentation is needed to validate this conclusion.

Individual and work-related risk factors associated with symptoms of musculoskeletal complaints

Individual and work-related risk factors in the development of occupational musculoskeletal complaints were investigated in a cross-sectional study of 52 female production workers and 34 female office workers. The work tasks of the production workers were considered to generate shoulder muscle loads of low amplitude and high repetitiveness, and the work tasks of the office workers, muscle loads of low amplitude and low repetitiveness. The symptom scores were similar in the two groups, with the highest score for both groups in the shoulder-neck region. Previous pain symptoms were an important risk factor for musculoskeletal pain in all body regions, whereas psychosocial problems at work were a risk factor for complaints in the shoulder-neck region. For the office workers, 27% of the variance in shoulder-neck symptoms was explained by the variance in the parameters "previous pain symptoms" and "psychosocial problems" in a multilinear regression model. In three groups of workers with different physical loads on the shoulder muscles the symptom scores for workers without previous pain symptoms and psychosocial problems were related to the physical load. For workers with previous pain symptoms and psychosocial problems, the symptom scores were high and similar for all three groups.

The influence of electrode position on bipolar surface electromyogram recordings of the upper trapezius muscle

The effect of electrode position on the upper trapezius muscle on the myo-electric signal amplitude was investigated with special reference to arm position and estimate of force output. Previously, a depression of the electromyogram (EMG) signal has been reported midway between the seventh cervical vertebrae (C7) and acromion (Veiersted 1991, Eur J Appl Physiol 62:91-98) although this electrode position has been recommended (Zipp 1982, Eur J Appl Physiol 50: 41-54). Ten healthy subjects performed maximal shoulder elevations with the arm in vertical, abducted and flexed positions and they performed a dynamic movement test. The myo-electric signal was recorded along the length of the right upper trapezius muscle by a 16-channel bipolar array electrode and was integrated with a 0.2-s time resolution. A region just lateral to the midpoint between C7 and the lateral edge of acromion was found with high and stable amplitudes (% coefficient of variation equalled 5.6). At the midpoint a dip in the amplitude profile appeared which was slightly displaced by arm abduction or flexion probably due to sliding of the skin relative to the muscle. A linear EMG-force relationship was found in the region with high signal amplitudes, whereas the more lateral and the dip region showed highly variable EMG-force relationships. Thus, it was found that when using bipolar surface electrodes with an interelectrode distance of 2 cm a centre position 2 cm lateral to the midpoint between C7 and acromion provided good repeatability and high signal yield.