muscular activity in simulated light work among subjects with frequent neck-shoulder pain

International Journal o/Industrial Ergonomics, 8 (1991) 157-164 157 Elsevier

Muscular activity in simulated light work among subjects with frequent neck-shoulder pain

Esa-Pekka Takala and Eira Viikari-Juntura Institute of Occupational Health, Department of Physiology, Topeliuksenkatu 41 a A, SF-00250 Helsmki, Finland

(Received November 12, 1990; accepted in revised form May 13, 1991)

Abstract

An experimental study was performed to investigate differences in shoulder muscle activity between subjects with and those without neck-shoulder pain. Ten female bank cashiers with frequent neck-shoulder pain (cases) and ten nonsymptomatic referents (matched by age, body size and work experience) with similar work performed a simulation of one of their ordinary main tasks. Surface electromyography (EMG) was recorded on the upper trapezius and rhomboideus/erector spinae muscles. The mean EMG activities were similar in both groups. The cases had a greater number of short rest periods (less than 2% of the maximal activity) for their trapezius muscles during the work cycles. Lowering the height of the service counter by 25 cm reduced the mean electromyographic activity and the duration of the work periods (over 2% of the maximal activity) in the right trapezius in both cases and referents.

Relevance to industry

Although we could not experimentally show an association between increased shoulder muscle activity and symptoms, reduction of muscular load has been recommended for the prevention and treatment of neck-shoulder symptoms at work. Our results show that a simple ergonomic alteration (lowering the service counter) can change the working pattern in favour of the muscular loading of the shoulders.

Keywords

Case-referent study, electromyography, occupational cervicobrachial disorder, tension neck syndrome, work simulation.

Introduction

Muscular pain in the neck-shoulder region is common in many sedentary occupations (Hagberg and Wegman, 1987). It is generally agreed that high muscle load resulting in injury or fatigue of the working muscles is a major factor in the development of the symptoms (Maeda, 1977; Hagberg, 1984; Edwards, 1988; Sejersted and Westgaard, 1988). Especially static load of the muscles has been regarded to be harmful (Jonsson, 1982; Westgaard, 1988).

Static load may be imposed on shoulder muscles in two ways. First, the work situation may demand holding of the arms so that the weight of the arms and of the objects held induces load (Hag-

berg, 1981). Secondly, in addition to such an external biomechanical load, the person may produce extra muscle activity due to psychophysiological mechanisms, especially in tasks requiring discrimination (Van Boxtel and Van der Ven, 1978; Weber et al., 1980; Fussler-Pfohl et al., 1984; Waeersted et al., 1987; Westgaard and Bjorklund, 1987). Static muscle load may be controlled by reducing the amount of work done with arms elevated, by shortening the static phases by pauses (Hagberg and Sundelin, 1986; Henning et al., 1989; Sundehn and Hagberg, 1989), or by work rotation.

Why some subjects acquire neck-shoulder symptoms while others in similar work remain healthy, is not fully understood. Among 96 women

0169-1936/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

158 E.-P. Takala, E. Viikari-Juntura / Muscular activity

in electronics manufacturing industry, those with neck-shoulder symptoms differed from those without symptoms in their working technique, e.g. the angle of abduction and flexion of the upper arm (Kilbom et al., 1986; Kilbom and Persson, 1987; Jonsson et al., 1988). In a study of ten women working at a chocolate packing machine, five subjects with previous episodes of neck- shoulder symptoms had higher levels of static muscle activity and fewer short relaxing periods in their muscles than the five workers without such episodes (Veiersted et al., 1990). On the other hand, among 25 assembly-plant employees, Christensen (1986) found no evidence of muscular fatigue in EMG despite the subjects' experience of fatigue in the shoulder muscles at the end of the work day.

The aim of this study was to test the following hypotheses in a simulated work task requiring attention and elevation of the arms: (1) Subjects with frequent neck-shoulder symp-

toms have higher electromyographic activity in their shoulder muscles than subjects without symptoms.

(2) Nonsymptomatic subjects have a greater number of short periods of inactivity in their shoulder muscles than subjects with symptoms.

In addition, we wanted to study the effect of lowering the height of the service counter on the shoulder muscle load in bank cashiers' work.

Table 1

Age and anthropometry of the subjects.

Cases ( N = 10) Referents ( N = 10) Mean SD Range Mean SD Range

Age, years 36.5 3.4 29- 40 36.6 3.1 31- 40 Stature, cm 162.8 4.3 156-170 163.2 4.5 155-172 Weight, kg 58.7 12.2 48- 90 59.9 11.4 48- 85

to be located in the muscles. Ten referents (no symptoms in the neck and shoulder region during the previous 12 months) were matched for age, stature, weight and working experience (more than 10 years in similar work). Subjects with radicular pain to the arm, general diseases and acute infec- tions were excluded. In addition, subjects with possible primary fibromyalgla were excluded (per- sistent symptoms in at least three anatomical re- gions for at least three months' time) (Yunus et al., 1981). All of the subjects were right-handed.

The subjects came to the laboratory studies three to four weeks after the postal survey. None of the selected persons refused. One of the referents had got symptoms after the survey, and she was replaced by the nearest available matched pair in the referent group reservoir. Table 1 shows the characteristics of the subjects.

Laboratory procedures

Subjects and methods

Selection of subjects

A health survey questionnaire was mailed to 380 female bank cashiers, aged 20-50 years. They all worked in the same company and the work stations had been standardized a few years ago. The questions about the musculoskeletal symptoms were modified after the standardized Nordic questionnaires (Kuorinka et al., 1987). Based on the answers, 10 women with frequent symptoms in the neck and shoulder region were selected as cases (symptoms lasting altogether more than 30 days during the past 12 months and symptoms also during the past seven days). They all de- scribed their symptoms during the last seven days

The subjects arrived in the laboratory on working days either in the morning or in the afternoon, cases and referents in a random order. No one had taken pain medication on the day of the laboratory study. They were informed about the procedures, about the possibility to stop the experiment at any time, and they gave their consent for the study.

To control the selection of the cases and referents, the subjects were first asked about their present neck and shoulder symptoms. To exclude the possibility of cervical nerve root compression, the neck compression test (Viikari-Juntura, 1987) was performed in the sitting position, and none of the subjects had a positive result.

For determination of the reference level of the electromyography, the subjects performed four to six isometric maximal elevation tests of the

E.-P. Takala, E. Viikari-Juntura / Muscular activity 159

/

Fig. 1. Layout of the work station. C = counter, elevated by 25 cm for the first five work cycles. V = video display terminal, K = keyboard, Pr = printer, M s cashdrawer, P = pigeonhole

for the receipts either on the left or right.

shoulders, and a maximal isometric abduction of both arms before the work simulation. After the work simulation the subjects again performed two maximal shoulder elevation tests. The time between the single maximal tests was at least two minutes (Takala and Viikari-Juntura, 1991).

Work simulation

The work station The bank cashier's work stations of the com-

pany had been standardized a few years ago. A similar work station was constructed in the laboratory (figure 1). The standard desk height was 72 cm. Part of the simulation was performed using a 25 cm higher counter similar to those that have been installed in some work stations of the company for standing customers. The cashier worked on a video display terminal (VDT) located obliquely on the right. In this experiment a soft- ware program designed for the training of new cashiers was used, and all subjects were ac- customed to it. A cashdrawer with banknotes was located on the left of the cashier. A pigeonhole for the receipts was located either to the left or right side of the counter, and the subject was allowed to place it as in her own work station.

Work cycles The work cycle simulated was the situation in

which a customer draws money from his account with a passbook. The cycle can be divided into the following sequences: (1) Reaching out to the counter and taking the

passbook, and a slip of paper indicating the amount of money that the customer wanted to draw.

(2) Typing the account number and the sum of money on the terminal.

(3) Putting the passbook into the printer located between the keyboard and VDT-screen.

(4) Taking the passbook and putting the receipt into the printer.

(5) Taking the receipt and handing it back to the customer for the signature.

(6) Taking the bank-notes from the cashdrawer. (7) Handing the money and the passbook to the

customer. (8) Placing the receipt in the pigeonhole.

The simulation was performed for five customers on the high counter, and then repeated for five customers on the low counter. Before the first trial, one to three training cycles were performed till the cashier was sure she was able to manage. The mean duration of one work cycle was 56 s (range 40-91 s), and did not differ statistically significantly between the cases and the referents.

The work simulation was based on the previous knowledge that the work cycle chosen was the most frequent set of tasks in bank cashier's work. After the laboratory simulation an observational analysis of one work day was performed for each subject. Observations were made by the same ob- server every 5 s for a 35 seconds time and these samples were taken with 5 min intervals. The sampling rate was chosen to serve an EMG recording during the work (Nieminen et al., 1986). Due to technical problems we could not compare these vocational EMG recordings with those of the work simulation. The tasks simulated in this study covered 36% of the activities of an average workday (table 2).

Electromyography (EMG)

EMG was recorded bilaterally over the upper part of the trapezius and the rhomboideus /

160 E.-P. Takala, E. Vitkari-Juntura / Muscular activity

thoracic erector spinae muscles. Pairs of flexible self-adhesive disposable surface electrodes (Ag- AgC1, Medicotest, 01stykke, Denmark) were used; the inter-electrode distance being about 25-30 mm. The electrodes were placed on the upper margin of the trapezius muscle at the midpoint between the acromion and the spinous process of the 7th cervical vertebra. The electrodes on the rhomboideus/ thoracic erector spinae muscles were located 2-3 cm laterally from the spinous processes of the first and second thoracic vertebra (SchiJldt, 1988). The reference electrode was placed on the spinous process of the 7th cervical vertebra. The electrodes were attached to the skin in a standardized manner (Takala and Viikari-Juntura, 1991).

The E M G signals were preamplified with light preamplifiers attached near the electrodes on the skin, amplified (bandwidth 10-450 Hz), and recorded on a cassette tape recorder (Teac R-30-E, bandwidth 0-1250 Hz) (Nieminen et al., 1986). The tape-recorded signals were low-pass filtered up to 500 Hz to remove the tape noise, digitized with a sampling frequency of 1000 Hz, and stored on a disk. Root mean squares (RMS) were calculated with a time constant of 0.2 s. During the experiment the amplified raw E M G signal was visually controlled on the graphs printed on paper.

The relative E M G activity (TAMP%, percentage of the Time-Averaged Myoelectrical Potential) (Schiildt, 1988) of each muscle was calculated by dividing the RMS values by the

maximal reference value, and was multiplied by 100. The maximal reference value was the highest mean RMS value for a 0.5 s period in the muscle in question, recorded in any of the maximal isometric tests (Takala and Viikari-Juntura, 1991).

The periods of rest and work during the experiment were selected using 2% of the isometric maximum as a discrimination level.

Outcome variables and statistical methods

The following outcome variables were com- puted for each muscle when working with the high and the low counter: (1) TAMP%, mean and percentage of low

(TAMP% below 1, 2 and 5%) and high (TAMP% over 20% of maximum) level activities

(2) Number of rest per iods/100 s (3) Mean duration of rest periods, s (4) Mean duration of work periods, s (5) Res t /work ratio: Mean duration of rest peri-

o d s / M e a n duration of work periods. For the TAMP%, differences between the cases and referents were tested with t-test. Differences between the left and right side, between the trapezius and rhomboideus muscles of the same subject, and between the cycles at the high and the low counter were tested with the paired t-test. In some instances the distributions of variables 2 - 4 were skewed, and then Mann-Whitney and Wilcoxon rank sum tests were used. Two-sided

Table 2

Frequency of different tasks in bank cashier's work (analysis of one work day of 20 subjects).

Task Frequency %

Reaching out to the counter Typing on the terminal Placing the receipt in the pigeonhole Handling the bank-notes in the cashdrawer Putting the passbook or the receipt into the printer

Speaking in the telephone Waiting Writing with a pen Typing on the typewriter Other tasks (mostly counseling customers)

Tasks outside the work station

Total

413 1517 293 506 572

447 812 528 203

2182

1758

9231

4.5 16.4 3.2 5.5 6.2

4.8 8.8 5.7 2.2

23.6

19.0

100


tests were used and p values of 0.05 or less were considered to be statistically significant.

Results

Table 3

Mean EMG activity (TAMP%) during work simulation.

Cases ( N = 10) Referents ( N = 10) Mean SD Mean SD

High desk Trapezius, right 6.2 a 3.5 6.5 c

There were no statistically significant dif- Trapezius, left 4.0 2.3 4.5 e

ferences in the mean EMG activity (TAMP%) Rhomboideus, right 12.6 4.4 13.4 Rhomboideus, left 10.2 3.7 9.0

(table 3) nor in the distribution of low or high TAMP% between the cases and referents. EMG Low desk

Trapezius, right 5.5 b 3.6 5.5 a activity in the rhomboideus/ thoracic erector Trapezius, left 4.1 2.5 3.9 t spinae muscles was higher than that in the Rhomboideus, right 12.6 4.5 13.4 trapezius muscles ( p < 0.01, paired t-test) (table Rhomboideus, left 10.1 3.8 9.0 3).

The cases had a greater number of rest periods during the work cycles, and this difference was statistically significant in the left trapezius (table 4). There was a tendency of the rest /work ratio also to be higher among the cases. One referent and one case had no rest periods in their trapezius muscles during the experiments. Only one referent

2.6 2.0 4,8 4.2

2.3 1.7 4.8 4.2

a vs. b , c vs . a p <0.02 • vs. rp < 0.01, paired t-test

and one case had rest periods in their rhomboideus muscles during the work cycles.

Lowering the height of the counter by 25 cm reduced the mean TAMP% statistically signifi-

Table 4

Number and duration of rest and work periods (below and over 2% of maximal TAMP%) in the upper trapezius muscles. Medians and lower (Q1) and upper (Q3) quartiles.

Cases (N = 10) Referents (N = 10) Median Q1-Q3 Median Q1 -Q3

High desk Right Trapezius

Number of rest periods/100 s 4.6 1.8 - 9.0 1.2 0 - 3.5 Duration of rest periods, s 2.3 2.0 - 4.2 1.4 0 - 2.4 Duration of work periods, s 21.6 9.1 -53.4 10.2 0 -81.8 Rest /work ratio 0.12 0.04- 0.23 0.02 0 - 0.22

Left Trapezius Number of rest periods/100 s 7.3 5.2 - 8.9 3.7 0.8 - 4.2 a Duration of rest periods, s 5.6 3.1 - 8.0 4.6 2.8 - 6.3 Duration of work periods, s 3.9 2.6 -12.8 18.8 2.2 -23.1 Rest /work ratio 1.02 0.17- 2.16 0.16 0.08- 0.41

Low desk Right Trapezius

Number of rest periods/100 s 5.3 2.8 - 8.9 2.3 0.3 - 4.1 Duration of rest periods, s 2.2 1.5 - 3.3 2.3 1.2 - 2.9 Duration of work periods, s 9.8 4.2 -28.5 13.3 0 -22.3 Rest /work ratio 0.12 0.05- 0.79 0.08 0.05- 0.24

Left Trapezius Number of rest periods/100 s 6.5 2.8 - 8.9 2.6 0.4 - 4.3 a Duration of rest periods, s 4.7 2.0 - 7.4 3.7 1.4 -11.4 Duration of work periods, s 4.5 2.7 -15.9 14.2 1.4 -33.6 Rest /work ratio 0.80 0.13- 2.48 0.21 0.04- 1.01

a P < 0.05, Mann-Whitney

162 E.-P. Takala, E. Viikari-Juntura / Muscular acti¢,ity

cantly in the right trapezius in both groups, and among the referents also in the left trapezius (table 3). With the low counter also the duration of the work periods tended to be shorter (p--0.05, Wilcoxon) in the fight trapezius in the whole study group (table 4).

Five of the cases had neck-shoulder pain when the experiment was started. This group did not differ from the rest of the cases.

Discussion

The results did not confirm the hypothesis that subjects with frequent neck-shoulder symptoms have higher activity in their shoulder muscles than nonsymptomatic subjects in a simulated work task. In regard to rest periods, the results even con- tradicted the hypothesis, as the referents had fewer rest periods than the cases.

In the study of Veiersted et al. (1990), five female packers with neck-shoulder complaints during the last 12 months had higher levels of static muscle activity and fewer EMG gaps (below 0.5% of maximum activity) than five women without such episodes. For all subjects the gaps appeared more frequently and lasted longer when heavier (7 vs. 2 kg) loads were handled. In our study, weights of the external loads were less than 50 g, and the short rest periods were fewer than in their study. It is possible that working with heavier loads is a dynamic task with high muscular activity followed by short rest periods.

The discrimination level of rest and work periods was 2% of the maximal EMG activity in our study. This level was based on the proposals of Jonsson (1982) for the maximal static activity level allowed at work. Veiersted et al. (1990) used a lower discrimination level (0.5% of maximum). We analyzed our subjects also with 0.5% and 1% levels, but most of our subjects had no EMG gaps below these levels in their right trapezius muscles during the work simulations. These levels were therefore not suitable for comparisons between the cases and the referents.

The amount of relative EMG activity (TAMP%) depends on the activity level in the maximal tests. Differences in the number of work and rest periods based on a TAMP% level of 2% could reflect the fact that our subjects (mainly referents) did

not produce their real maximal activity in force tests. This is not very likely, as the maximal reference level was the highest of at least eight tests, and all subjects were well motivated. In addition, the recording of maximal activity was based on a window length of 0.5 s which generates a lower maximum than with a window length of 0.2 s used in the study of Veiersted et al., (1990). The longer window length makes the recordings of repeated trials more consistent and reduces the effect of occasional very short and high artificial signals in the recording.

The work cycle of bank cashiers is similar to the payment procedure in shop cashiers' work studied by Lannersten and Harms-Ringdahl (1990). The results of our study are similar to the results of their study, despite some differences in the presentation of EMG. In both studies, activities of rhomboideus/thoracic erector spinae muscles were higher than those of the upper trapezius. The recommended limits for the duration of the static work (Jonsson, 1982) are based mainly on endurance studies of limb muscles. Later, endurance of neck and trunk extensor muscles has been shown to be longer than the endurance of handgrip (Petrofsky and Phillips, 1982) or arm flexor (Grunow et al., 1985) muscles. It is therefore possible that the relatively high activities of rhomboideus/thoracic erector spinae muscles are of minor importance in the development of fatigue or neck-shoulder symptoms.

It may be argued that some differences in muscular activity between the cases and referents could have been recorded if the work time had been longer. The simulated work cycle was the most frequent set of tasks in bank cashier's work. It also demanded frequent reaching of the upper arm and probably imposed the highest biomechanical static load to the shoulder muscles. In addition, high attention was demanded in this task. Our assumption was that differences in muscular activity patterns would precede symptoms. As all our subjects performed their ordinary work tasks, ten work cycles should be enough to detect differences in muscular activity patterns.

The upper trapezius and rhomboideus muscles are active rotators and elevators of the scapula in the movements of the upper arm. In addition to these muscles, the middle and lower part of the trapezius, levator scapulae and serratus anterior


muscles are active in the e levat ion of the upper l imb (Basmaj ian, 1978, pp. 189-192; Bateman, 1978, pp. 99-113 ; DeFre i t a s et al., 1980; Bagg and Forres t , 1986). It is poss ib le that some of the subjects with s y m p t o m s in the uppe r t rapezius reduced the load of these muscles b y recrui t ing o ther muscles ins tead of the pa infu l ones. The shor t rest per iods de tec ted more of ten a m o n g the cases may also reflect an a l tered pa t t e rn of muscle act ivi ty a d a p t e d due to the symptoms .

Al l of the cases repor ted their neck-shou lde r symptoms to be re la ted to their work. A n un- answered quest ion remains whether the s imula ted work cycle m a y be regarded to descr ibe bank cashiers ' work load in general , a l though it was the most f requent and p r o b a b l y the most load ing par t of their work and the muscu la r act ivi ty pa t t e rns were very s imi lar f rom cycle to cycle. O u r inten- t ion was to compare muscular act ivi ty of real work with that of s imula ted work, bu t due to technical p rob lems in voca t iona l E M G recordings we were not able to do it.

A l though the course of events f rom high external load to increased muscle activity, fat igue, and symptoms has not been exper imenta l ly well es tab- l ished, and was ne i ther suppo r t ed by our results, reduc t ion of ex terna l load has been r e c o m m e n d e d for the prevent ion and t r ea tment of neck-shou lder s y m p t o m s at work. We found, however, that a s imple e rgonomic a l te ra t ion ( lowering the service counter) can change the work ing pa t t e rn in favour of the muscular load ing of the shoulders .

Acknowledgements

W e thank Ms M a r j a Miet t inen , MSc, and M r M a r k k u R a u t a n e n for their ass is tance in the l abo ra to ry tests. This s tudy was f inancia l ly suppo r t ed by the F inn i sh W o r k Env i ronmen t Fund .

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muscular activity in simulated light work among subjects with frequent neck-shoulder pain

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