muscle force, endurance and neck-shoulder symptoms of sedentary workers an experimental study on...

International Journal of Industrial Ergonomics. 7 (1991 ) 123-132 123 Elsevier

Muscle force, endurance and neck-shoulder symptoms of sedentary workers

An experimental study on bank cashiers with and without symptoms

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

(Received May 10, 1990; accepted in revised form December 1, 1990)

Abstract

The purpose of our investigation was to study the relations of neck-shoulder symptoms with muscle function and pressure pain sensitivity. Ten female bank cashiers with frequent neck-shoulder symptoms were compared with nonsymptomatic referents (matched by age, body size and work experience) in similar work. Isometric elevation and abduction forces of the shoulders, and endurance time of shoulder elevation against 30% of maximum force were measured. Signs of fatigue of the upper trapezius and rhomboideus muscles were recorded with surface eleetromyography (EMG). Pressure pain threshold was measured on eight muscles in the neck-shoulder region. No significant differences between the cases and referents were detected in muscle function. Signs of fatigue in EMG increased with static holding time in all subjects. The pressure pain threshold was lower for the cases than for the referents.

Relevance to industry

Programs for increasing muscular force and endurance have been developed for the prevention and control of neck-shoulder symptoms. The results of this study suggest that the basis of such fitness programs is vague. The results support the view that tests of muscular strength or endurance have no place in the selection of workers into light sedentary tasks.

Keywords

Case-referent study, electromyography, fitness, occupational cervicobrachial disorder, pressure pain sensitivity, tension neck syndrome.

Introduction

Neck and shoulder symptoms are c o m m o n in m a n y sedentary occupat ions (Hagberg and Weg- man, 1987). Clearly defined diseases are uncom- mon, and therefore entities like ' occupa t iona l cervicobrachial disorder ' (Maeda, 1977), ' cumula - tive t rauma disorder ' , ' r epe t i t ion strain in jury '

(Nat iona l Occupa t iona l Heal th and Safety Com- mission 1985, p. 8) or ' t e n s i o n neck syndrome ' (Wads , 1979) have been used in the classification of the symptoms.

The pa t home c ha n i sms of these unspecific symptoms are unclear. Most symptoms are be- lieved to be of muscular origin. Metabol ic and morphological changes in the muscles due to

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

124 E.-P. Takala. E Viikari-Juntura / Muscle force, endurance and neck-shoulder symptoms

mechanical trauma, fatigue or cramp have been proposed to lie behind the symptoms (Hagberg, 1984: Edwards, 1988).

Muscle activity which exceeds a certain propor- tional limit of one's muscular capacity, resulting in chronic fatigue or traumatic changes of the muscles, has been proposed to be an important pathogenetic factor in occupational muscle pain (Hagberg, 1984; Maeda, 1977; Sejersted and Westgaard, 1988; Wafts, 1979). If this assumption is true, symptoms at work could be relieved either by decreasing the external work load by ergo- nomic means, or by increasing the performance capacity of the muscles, i.e. by improving physical fitness and motor skills (Edwards, 1988).

Poor muscular fitness has been associated with neck and shoulder symptoms. Patients (n = 13) at an industrial health care centre with acute non- traumatic neck-shoulder pain and without any underlying disease were weaker in several isometric tests of the shoulder muscles than their matched referents (n = 26), but the external work load seemed to be a more dominant factor separating the cases and referents (Bjelle et al., 1981). KvarnstrSm (1983) found in his study on industrial workers that the group of 112 cases with neck and shoulder symptoms had a lower mean of isometric muscle strength of the shoulders than the matched referent group. In a study of 10 patients with a history of neck and shoulder pain lasting for at least a year, the muscles on the more painful shoulder showed a shorter endurance time than the muscles on the opposite side (Hagberg and KvarnstrSm, 1984).

Other studies have failed to show a difference in muscle function between those with and without neck-shoulder symptoms. Kilbom et al. (1986) found no relationship between neck and shoulder symptoms and maximal static elevation strength or static endurance in abduction of the arm in a study of 96 female employees in the electronics industry. Only a weak correlation was found between the maximal abduction force of the upper arm and symptoms of the shoulder. In the pro- spective part of the study (Kilbom and Persson, 1987; Jonsson et al., 1988) neither low isometric forces nor short endurance of the muscles was a risk factor for neck and shoulder symptoms. In contrast, subjects with high values in the maximal shoulder elevation test seemed to be at an in-

creased risk. In another longitudinal study of 32 car motor assemblers whose work demanded high external forces, low isometric strength of shoulder muscles was a risk factor for neck and shoulder symptoms (Kilbom, 1988).

The aim of this study was to test the following hypotheses: (1) the shoulder musculature of nonsymptomatic women in sedentary work is stronger, and (2) the endurance of these muscles is longer than those of women with frequent neck and shoulder symptoms. In addition we wanted to study the relations of pressure pain sensitivity with neck-shoulder symptoms and muscle function.

Subjects

Selection and characteristics 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 described their symptoms during the last seven days 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-

Table 1

Age and anthropometry of the subjects.

Cases (n = I0) Referents (n = 10) Mean SD Range Mean SD Range

Age, years 36.5 3.4 29- 40 36.6 3.1 31- 40 Stature, crn 162.8 4.3 156-170 163.2 4.5 155-172 Weight, kg 58.7 12.2 48- 90 5 9 . 9 11 .4 48- 85

tions were excluded. In addition, subjects with possible primary fibromyalgia 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.

Follow-up of symptoms

Fig. 1. Left: Points for pressure pain threshold measurements (upper trapezius, levator scapulae, infraspinatus and

rhomboideus muscles). Right: Location of the electrodes.

The questionnaire was mailed to the subjects three, six and eight months after the first survey. The questions were the same as in the first survey, except that the time period of the symptoms was not 12 months but the time after the previous survey. Nineteen subjects replied to all three questionnaires. One of the cases responded only to the first follow-up survey. During the follow-up seven of the cases had a similar frequency of symptoms in all cross-sections, two had less frequent symptoms after six months, and one had a similar frequency of symptoms after three months, but she did not reply to the questionnaires after six and eight months. Nine of the referents had no symptoms and one had mild symptoms in the follow-up.

Methods

Laboratory procedures

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,

E.-P. Takala, E Viikari-Juntura /Musc le force, endurance and neck-shoulder symptoms 125

the neck compression test (Viikari-Juntura, 1987) was performed in the sitting position, and none of the subjects had a positive result.

Pressure pain threshold was measured with a mechanical algometer (Ametek LNS0) (Takala, 1990), bilaterally on the upper trapezius, levator scapulae, rhomboideus and infraspinatus muscles, and on the sternal manubrium, in a random order. Figure 1 shows the measuring points.

After the application of the surface EMG electrodes, the subjects performed several isometric maximal elevation tests of the shoulders, and a maximal isometric abduction of both arms. They then performed a standardized work simulation ('solitaire'-test) (Takala and Viikari-Juntura, submitted to Clinical Biomechanics), modified after the studies of Schiildt (1988), and a set of tasks simulating their ordinary cash work for about l0 rain (Takala and Viikari-Juntura, submitted to Int. J. Ind. Ergon.). After these simulations the subjects again performed two maximal elevation tests for the determination of the force level in the isometric endurance test. After the endurance test the study was finished by pressure pain measurements as in the beginning.

126 E.-P. Takala, E Viikari-Juntura / Muscle force, endurance and neck-shoulder symptoms

Electromyography (EMG)

EMG was recorded bilaterally over the upper part of the trapezius and the rhomboideus/ thoracic erector spinae muscles (figure 1). Pairs of flexible self-adhesive disposable surface electrodes (Ag-AgC1, Medicotest, Olstykke, Denmark) were used with an inter-electrode distance of about 25-30 mm. The electrodes were placed on the upper margin of the trapezius muscle at the mid- point 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 (Schiildt, 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: the skin was rubbed with a piece of sandpaper and cleaned with alcohol. Elec- trode paste was used, and the skin resistance was measured to be less than 10 kohm before the beginning of the recordings.

The EMG 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). During the experiment the amplified raw EMG signal was visually controlled on the graphs printed on paper.

For the analysis, 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. Two kinds of fatigue signs were analyzed: decrease in the zero Crossing frequencies (H~igg, 1981; H~igg et al., 1987), and increase in the amplitudes (eg. Kadefors et al., 1978; Hagberg, 1981a, b; Petrofsky et al., 1982).

For the calculation of the zero crossings, a moving window technique was used (window length 1 s, overlap between two adjacent window positions 0.5 s). The zero crossing rate (ZCR) is the number of zero crossings inside a moving window. The endurance test was analysed in 5 s sequences (mean ZCR within each sequence). The first 5 s period in the beginning of the endurance test was used as the reference level. The zero crossings were expressed as normalized values by dividing each value with this reference value.

The fatigue time estimated by zero crossings (Tzcr) of each muscle was determined to be the start of the first 5 s time sequence where the level of the normalized zero crossing value was constantly below 0.9 (10% below the reference level).

For the analysis of the amplitude changes, root mean squares (RMS) were calculated with a time constant of 0.2 s. Like ZCR the RMS was calculated for 5 s periods and normalized against the first 5 s time sequence at the beginning of the endurance test. The fatigue time (Trms) of each muscle was determined to be the start of the first 5 s time sequence where the level of the normalized RMS value was constantly over 1.1 (10% above the reference level).

If there was no 10% decrease of 5CR or no 10% increase of RMS in the fatigue test, the fatigue time (Tzcr, Trms) of the muscle in question was recorded as the end of the endurance test.

The relative EMG activity (TAMP%, per- centage of the Time-Averaged Myoelectrical Potential, Schiildt, 1988) of each muscle for the first 5 s period in the beginning of the endurance test (3096 of the maximal shoulder elevation force) was calculated by dividing these RMS values by the maximal reference value, and was multiplied by 100. The maximal reference value was the highest mean RMS value for the muscle in question for a 0.5 s period, recorded in any of the maximal isometric tests.

Maximal test contractions

The subjects performed several isometric maximal tests. During the shoulder elevation test (figure 2) the subject sat on a stool with straightened back, arms hanging by the side. A sling was placed over both acromions and connected to the strain gauge force transducer. The subject was asked to elevate both shoulders with a maximal force for 2 -5 s. To increase motivation the force was monitored for the subject on a computer screen in front of her face. No lateral bending was allowed. First the force transducer was connected to the right or left sling, and the force of that shoulder was recorded. After two or three bouts the force transducer was changed to the opposite sling to measure the force of this shoulder, and again two or three bouts were performed. The time between the single bouts was at least two minutes.

E.-P. Takala. E Viikari-Juntura / Muscle force, endurance and neck-shoulder symptoms 127

Fig. 2. Shoulder elevation test.

After the work simulation the subjects performed again two maximal elevations of the shoulders. Now both slings were connected to the force transducer to record the maximal elevation force of both shoulders. The highest of the two or three forces was recorded as the maximum of each trial. The recordings in different trials were stable, and only three of the referents and one of the cases increased their force more than 10% in the second trial.

Because individuals activate their trapezius muscles maximally either in shoulder elevation or abduction (Schiildt and Harms-Ringdahl, 1988; Westgaard, 1988), also abduction tests were performed: after the elevation tests for each shoulder the subject remained sitting on the stool. She abducted one arm 90 ° in the frontal plane, and the sling was set on the lateral epicondyle of the elbow. An assistant supported the arm in this position and the subject was asked to relax her shoulders before the maximal test. The opposite arm was set on her lap. The maximal abduction against the sling was performed for 2-5 s. The procedure was then repeated with the opposite arm.

The maximal EMG activity of the trapezius muscles was recorded most often (73%) in the shoulder elevation test. The maximal activity of the rhomboideus/ thoracic erector spinae muscles occurred most often (66%) in the abduction test.

Endurance test

After the maximal isometric elevation tests for both shoulders the subject sat in the same posture (figure 2). She elevated both shoulders to 30% of the maximal elevation force as long as she could. The 30% force level was monitored on the screen and the subject was encouraged verbally. The end point of the test was when (1) the subject volun- tarily stopped the test, or (2) she could no longer hold on the 30% line, or (3) the end of the screen was reached after 255 s from the beginning.

Outcome variables and statistical methods

The following outcome variables were used in the analysis:

(1) Maximal forces of right, left and both shoulders.

(2) Endurance time at 30% of maximal elevation force of both shoulders.

(3) Fatigue time calculated by ZCR (Tzcr): the shortest and the mean of the four muscles.

(4) Fatigue time calculated by RMS (Trms): the shortest and the mean of the four muscles.

(5) Average time of Tzcr and Trms: the shortest and the mean of the four muscles.

(6) Relative RMS (TAMP96) at the beginning of the endurance test.

(7) Relative change of ZCR during the endurance test (normalized ZCR at the end of the test).

(8) Relative change of RMS during the endurance test (normalized RMS at the end of the test).

(9) Relative change of both ZCR and RMS during the endurance test (difference between normalized ZCR and RMS at the end of the test).

(10) Pressure pain sensitivity on the trapezius and rhomboideus muscles.

Differences between the cases and referents were tested with Student's t-test. Differences between the left and right side and between the trapezius and rhomboideus muscles of the same subject were tested with the paired t-test. Pearson's correlation coefficient and regression analysis were used to measure associations between the variables. In- dividual muscles as well as the mean of the four muscles in each subject were considered in the

128 E.-P. Takala, E Viikari-Juntura / Muscle force, endurance and neck-shouMer symptoms

analysis. P values of 0.05 or less were considered to be statistically significant.

Results

The variables of muscular force and endurance showed no statistically significant differences between the cases and referents (table 2). Five of the cases had present symptoms, and they also had the lowest mean of the isometric forces of both shoulders (700 N, SD 184 N) and of the endurance times (102 s, SD 33 s), but these values were not significantly different from those of the referents.

The mean increase in amplitude (RMS) during the endurance test was 33% (SD 28%, range -17-138%). A paradoxical decrease of 5-17% was seen in five muscles of different individuals. The mean decrease in ZCR during the test was 23% (SD 14%, range 0-66%).

The fatigue times determined by ZCR and RMS correlated with each other. However, there were individual variations within the muscles. The best correlation (r = 0.48) between the different fatigue time variables (Tzcr and Trms) was obtained when the means of the four muscles were used. The

Normal ized values

2.2

2.0

1.8

1.6

1.4

1.2

1.0

.8

.6

.4

.2

13

m ~ mnt• ~ []

rl E3

5 0 1 0 0 1 5 0 2 0 0 2 5 0

Endurance time, s

Fig. 3. Relative change of RMS (121) and 5CR (E) during the endurance test (normalized RMS and 5CR at the end of the test). The regression line for RMS (y = 0.36x +91.3, r = 0.73, p = <0.001) and Z C R ( y = - 0 . 2 1 x + 1 0 2 . 6 , r = 0 . 6 8 , p = 0.003) has been calculated for 17 subjects. The three subjects whose tests were stopped at 255 s were excluded from the

regression.

fatigue times of the four muscles did not differ significantly from each other.

The relative changes in ZCR and RMS during the endurance test were significantly correlated with the endurance time (figure 3). The best correlation was obtained by combining these variables

Table 2

Maximal forces and endurance times of shoulders.

Cases (n = 10) Referents (n = 10)

Mean SD Mean SD

Maximal isometric elevation force Both shoulders (N) 797 173 818 236 Right shoulder (N) 394 61 433 121 Left shoulder (N) 404 73 394 123

Maximal isometric abduction force Right shoulder (N) 86 18 96 28 Left shoulder (N) 82 17 88 29

Endurance time of shoulder elevation (s) 130 55 130 76 Tzcr rain (s) 59 39 43 20 Tzcr mean (s) 76 37 66 35 Trms min (s) 30 20 35 23 Trms mean (s) 47 30 60 40 Average time of Tzcr and Trms, rain (s) 47 27 42 11 Average time of Tzcr and Trms, mean (s) 61 29 63 33

Tzcr = Time of 10% decrease in the Z C R in the endurance test. T rms = Time of 10% increase in the RMS in the endurance test. rain = the shortest time of the four muscles. mean = mean time of the four muscles.

E.-P. Takala, E Viikari-Juntura /Musc le force, endurance and neck-shoulder symptoms 129

(difference between normalized RMS and ZCR) (figure 4).

The relative EMG activity (TAMP%) of the individual muscles at the beginning of the endurance test against 30% of the maximal elevation force varied between 10 and 51% (mean 24.2%, SD 8.9%). There were no statistically significant differences between the trapezius and rhomboideus muscles, nor between the right and left side. The mean TAMP% of the four muscles was slightly higher for the referents than for the cases (26.8 vs. 22.4, not significant) (figure 5). The relative EMG activity at the beginning of the endurance test (each muscle and the mean of four muscles) was neither correlated with the endurance time nor with any of the times determined by EMG (figure 5).

None of the three subjects whose frequency of symptoms altered during the follow-up, had ex- tremely high or low values in any of the variables of muscular performance (figures 4 and 5).

The mean pressure pain sensitivity value of the eight points over the muscles was lower for the cases (47.5 N, SD 13.7 N, range 20-71 N) than for the referents (66.3 N, SD 8.9 N, range 49-80 N) (p = 0.0024, t-test). The same difference was seen on the point over sternal manubrium (36.5 N vs. 62.1 N, p < 0.001). The mean values on the

& (RMS, ZCR)

1.8

1.6

1.4

1.2

1.0

.8

.6

.4

.2

0 n

50 100 150 200 250

Endurance time. s

Fig. 4. Difference of normalized RMS and ZCR (zx (RMS, ZCR), mean of four muscles) at the end of the endurance test against the endurance time. The regression line (y = 0 . 5 6 x - 11.3, r = 0.84, p < 0.001) has been calculated for 17 subjects. The three subjects whose tests were stopped at 255 s were excluded from the regression, o Referents; • - Cases, no present symptoms, x - Cases with present symptoms. Arrows = the referent who later was symptomatic and two healed

c a s e s .

TAMP/,

40

30

20

10

o ° o

• eN

o I I I

. . . . . . . , . . . . . . .

20 40 60 80 100 120 140 160

Average time of fatigue, s

Fig. 5. Relative EMG activity (TAMP%) at the beginning of the endurance test against the time of detecting signs of fatigue (average of Tzcr and Trms), mean of four muscles, o Refer- ents; • - Cases, no present symptoms, x - Cases with present symptoms. Arrows = the referent who later was symptomatic

and two healed eases.

trapezius muscles were 39.5 N vs. 58.4 N (p = 0.0025) and on the rhomboideus muscles 47.5 N vs. 66.5 N (p = 0.01), but no statistically significant associations between the pressure pain sensitivity of these muscles and the other outcome variables were detected.

Discussion

The isometric strength and endurance of shoulder muscles did not differ between bank cashiers with frequent neck-shoulder symptoms and those with no symptoms. Therefore, the hypotheses that nonsymptomatic female sedentary workers have stronger and more endurable shoulder muscles than women with frequent neck and shoulder symptoms were not confirmed. Neither did the tests predict onset of or change in symptoms during the follow-up period.

Negative results in a study with 20 subjects do not prove that there is no association between neck-shoulder symptoms and muscular fitness. However, if we in a group of 20 subjects cannot separate symptomatic and nonsymptomatic subjects by means of our tests for muscular force and endurance, these tests hardly have much practical predictive value as diagnostic tools for neck- shoulder symptoms or in the selection of workers into light sedentary tasks.

130 E.-P. Takala, E Viikart-Juntura / Muscle force, endurance and neck-shoulder symptoms

The five subjects with symptoms at the time of the tests had low isometric forces, which however did not differ significantly from those of the referents. Failure to produce maximal voluntary force can be due to lack of motivation, to pain, or fear of pain (Basmajian, 1970). None of the subjects reported pain during or after the tests. All of the subjects were well motivated, and the feedback on the screen during the test seemed to increase their motivation.

Programs for increasing muscular fitness have been developed for the prevention and control of neck-shoulder symptoms. Good results have been reported in one study describing rehabilitation of office workers with neck-shoulder symptoms (Dy- rssen et al., 1989). A trial among workers in light assembly and packing tasks failed to show significant differences in neck-shoulder discomfort scores between the women who participated or those who did not participate in an exercise program (Silverstein et al., 1988). The results of Kilbom et al. (Kilbom et al., 1986; Kilbom and Persson, 1987; Jonsson et al., 1988), which suggest that good muscular fitness does not protect the workers in light tasks from developing neck-shoulder symptoms, and the results of this study suggest that the basis for such fitness programs is vague.

We do not know what the limiting factor is for endurance in isometric elevation of the shoulders, i.e. which muscle is the most fatigable. None of the endurance variables measured in this study showed a significant difference between the groups.

An increase in EMG amplitude and a decrease of the mean power frequency have both been correlated with muscle fatigue (e.g. Kadefors et al. 1978; Hagberg, 1981a, b; Petrofsky et al., 1982). The zero crossing technique is an alternative to the mean power frequency but easier to compute (H~igg, 1981; H~igg et al., 1987). In this study, however, the best correlation with the endurance time and EMG parameters was obtained by combining changes both in RMS and ZCR. The question why some people tolerate fatigue, while others develop symptoms, remains unanswered.

There was a great variation in the relative EMG activity (TAMP%) of different muscles at 30% of maximal force. The mean of the TAMP% within each subject was less than 30% in all but two subjects. The upper trapezius and levator scapulae

are the main muscles participating in the elevation of the shoulder (Basmajian, 1978, pp. 189-192, Bateman, 1978, pp. 99-113; De Freitas et al., 1979, 1980; Schiildt and Harms-Ringdahl, 1988). As the upper trapezius is the largest and nearest muscle below the electrodes over this muscle, the activities recorded should mainly originate from this muscle. The location of the electrodes over the rhomboideus / tho rac ic erector spinae muscle should pick up signals from several muscles (Schiildt, 1988). In the elevation test the activities of these muscles were not significantly different from those of the trapezius, which finding is in concordance with that of Schiildt and Harms- Ringdahl (1988). The rhomboideus major muscles are mainly active in the retraction of the shoulders (De Freitas et al., 1979, 1980). The relatively high activities over rhombo ideus / tho rac i c erector spinae muscles may describe the stabilizing activity of these muscles on the thoracic spine in shoulder elevation and abduction. There is no explanation for the finding that the referents pro- duced a higher average TAMP% in the muscles recorded against 30% of maximum external shoulder elevation force.

A hyperbolic correlation between the endurance time and the relative activity of the muscles has been described (Rohmert, 1960; Bj/Srkstrn and Jonsson, 1977; Sato et al., 1984; Kahn and Monod, 1989). The referents with a higher TAMP% should therefore have had a shorter endurance time than the cases. Rohmert stated in 1973 (Rohmert, 1973) that ' there is no difference in the results as regarded groups of muscles used, or from worker to worker, if the holding forces are related to the individual maximum forces'. Later, differences between the muscle groups have been described, handgrip muscles being more fatigable than neck extensor muscles (Petrofsky and Phillips, 1982), and shoulder abductors being more fatigable than elbow flexors or knee extensors (Sato et al., 1984). In this study, no correlation between the relative muscle activity and any of the endurance time parameters was found (figure 5). Thus, the fact that the referents had a higher average TAMP% than the cases can hardly explain the lack of significant differences in the endurance times.

Pressure pain sensitivity was not associated ~ith muscle force, endurance, or changes in EMG ac-

E.-P. Takala, E Viikari-Juntura / Muscle force, endurance and neck-shoulder symptoms 131

tivity. However, this was the only variable separating cases and referents in this study. The cases had low pressure pain thresholds on the sternal bone, also. Therefore, a general tenderness may be associated with proneness to report symptoms.

We studied several muscle physiological variables in this study, but none of them was significantly associated with reported neck and shoulder symptoms. Neither was electromyographic evi- dence of muscle fatigue found in a study of assembly workers despite reported fatigue in shoulder muscles at the end of the work day (Christensen, 1986). Psychosocial factors may be more significant than physiological factors for the reporting of neck and shoulder symptoms in light sedentary work (Linton and Kamwendo, 1989). Yet, even in light work some subjects may over-strain their musculature either due to their individual working techniques (Kilbom et al., 1986; Kilbom and Per- sson, 1987; Melin, 1987; Jonsson et al., 1988) or by generating muscle tension additional to postural load (Westgaard and Bjorklund, 1987).

Acknowledgements

We thank Ms Marja Miettinen, MSc, and Mr Markku Rautanen for their assistance in the laboratory tests. This study was financially supported by the Finnish Work Environment Fund.

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