INTERNATIONAL JOURNAL OF BEHAVIORAL MEDICINE, lf4), 354-370 Copyright 1994, Lawrence Erlbaum Associates, Inc. Psychophysiological Stress and EMG Activity of the Trapezius Muscle Ulf Lundberg, Roland Kadefors, Bo Melin, Gunnar Palmerud, Peter Hassm6n, Margareta Engstr6m, and Ingela Elfsberg Dohns Although it is generally assumed that mental stress induces muscular tension, the experimental data have, So far, been inconclusive. Likely explanations for these inconsistent findings are (a) too small subject samples in some experi- ments, (b) the use of only one type of stress stimulation, and (c) the lack of objective (physiological) measurements documenting the stress-inducing prop- erties of the experimental treatment. Furthermore, the effect of mental stress and physical load separately, versus the combined influence of physical and mental load on muscular tension, has not been investigated earlier. Therefore. the aim of the present experiment was to examine the effects of mental stress as well as of physical load, separately and in combination, on perceived stress, physiological stress responses, and on muscular tension as reflected in elec- tromyographical (EMG) activity of the trapezius muscle. Sixty two female subjects were individually exposed to mental arithmetic, the Stroop color word test (CWT), the cold pressor test, standardized test contractions (TCs). and the CWT combined with a TC. Compared to baseline, the stress session induced significant increases in systolic and diastolic blood pressure, heart rate, urinary catecholamines, salivary cortisol, and self-reported stress. Each of the two mental stress tests induced a significant increase in EMG activity. The CWT caused a rise in EMG activity also during the TC. which was significantly more UIf Lundberg, Bo Melin, Peter Hassm~n, and Margareta Engstr~m, Division of Biological Psychology, Department of Psychology, Stockholm University, Stockholm, Sweden; Roland Kadefors and Gunnar Palmerud, Lindholmen Development, Gothenburg, Sweden; Ingela Elfsberg Dohns, Kooperationens F6retagshiilsovtirdseentral, Stockholm, Sweden. This study was supported by grants from the Swedish Council for Research in the Humanities and Social Sciences and the Swedish Work Environment Fund. We are indebted to FHC-Stockholm and personnel at the Konsum and OBS stores in Stock- holm for assistance and contributions tO the data collection. Correspondence concerning this article should be addressed to UIf Lundberg, Department of Psychology, Stockholm University, S-I06 91 Stockholm, Sweden.
Transcript
INTERNATIONAL JOURNAL OF BEHAVIORAL MEDICINE, lf4), 354-370 Copyright �9 1994, Lawrence Erlbaum Associates, Inc.
Psychophysiological Stress and EMG Activity of the Trapezius Muscle
Ulf Lundberg, Roland Kadefors, Bo Melin, Gunnar Palmerud, Peter Hassm6n,
Margareta Engstr6m, and Ingela Elfsberg Dohns
Although it is generally assumed that mental stress induces muscular tension, the experimental data have, So far, been inconclusive. Likely explanations for these inconsistent findings are (a) too small subject samples in some experi- ments, (b) the use of only one type of stress stimulation, and (c) the lack of objective (physiological) measurements documenting the stress-inducing prop- erties of the experimental treatment. Furthermore, the effect of mental stress and physical load separately, versus the combined influence of physical and mental load on muscular tension, has not been investigated earlier. Therefore. the aim of the present experiment was to examine the effects of mental stress as well as of physical load, separately and in combination, on perceived stress, physiological stress responses, and on muscular tension as reflected in elec- tromyographical (EMG) activity of the trapezius muscle. Sixty two female subjects were individually exposed to mental arithmetic, the Stroop color word test (CWT), the cold pressor test, standardized test contractions (TCs). and the CWT combined with a TC. Compared to baseline, the stress session induced significant increases in systolic and diastolic blood pressure, heart rate, urinary catecholamines, salivary cortisol, and self-reported stress. Each of the two mental stress tests induced a significant increase in EMG activity. The CWT caused a rise in EMG activity also during the TC. which was significantly more
UIf Lundberg, Bo Melin, Peter Hassm~n, and Margareta Engstr~m, Division of Biological Psychology, Department of Psychology, Stockholm University, Stockholm, Sweden; Roland Kadefors and Gunnar Palmerud, Lindholmen Development, Gothenburg, Sweden; Ingela Elfsberg Dohns, Kooperationens F6retagshiilsovtirdseentral, Stockholm, Sweden.
This study was supported by grants from the Swedish Council for Research in the Humanities and Social Sciences and the Swedish Work Environment Fund.
We are indebted to FHC-Stockholm and personnel at the Konsum and OBS stores in Stock- holm for assistance and contributions tO the data collection.
Correspondence concerning this article should be addressed to UIf Lundberg, Department of Psychology, Stockholm University, S-I06 91 Stockholm, Sweden.
STRESS AND EMG ACTIVITY 3 5 5
pronounced than the increase induced by the CWT alone. Blood pressure responses and self-reported stress followed the same pattern as the EMG activ- ity. The results are consistent with the assumption that pSychological stress plays arole in musculoskeletal disorders by increasing muscular tension both in low-load work situations and in the absence of physical load. it is also indicated that the stress-induced increase in muscular tension is accentuated on top of a physical load.
Despite considerable ergonomic improvements in work environments in recent decades, musculoskeletal disorders have continued to be a major hea l th problem in indust r ia l ized count r ies (e.g., Haldeman, 1991; KvarnStr6m, 1983; Tellnes, I989). Low back pain and pain from the neck/shoulders and so forth are frequent problems not only in physically heavy jobs, but also in light assembly work, in office work, and among people working as cashiers at banks, post offices, and supermarkets (Barn- met, 1990). Several recent studies suggest that psychosocial factors, such as low job satisfaction and lack of autonomy and variation at work, are signifi- cantly associated with back pain and shoulder problems (review by Bongers, de Winter, Kompier, & Hildebrandt, 1993), indicating that psychological stress may be involved in the etiology of musculosketetal disorders. Neck and shoulder symptoms have also been associated with some stress-related individual characteristics, such as Type A behavior, neuroticism, and anxiety (Bru, Mykletun, & Sveback, 1993; Flodmark & Aase, 1992; Salminen, Pen- tti, & Wiekstrdm, I991).
Presently, the high incidence of myalgia-type disorders in light industrial work has no satisfactory explanation. However, it is generalty assumed that not only physical workload but also psychological stress may induce muscu- lar tension. Several studies tend to support this assumption (Svebak, Anjia, & K~rstad, 1993; Tulen, Moleman, van Steenis, & Boomsma, 1989) but the experimental data remain inconclusive (Wmrsted, Bjcrklund, & Westgaard, 1987, 1991; Weber, Fussier, O 'Han lon , Gierer , & Grandjean ,1980; Westgaard & Bjcrklund, 1987). The lack of consistent findings could be due to methodological factors, such as too small subject samples in some exper- iments, the use of only one type of stress stimulation and the lack of documented stress-inducing properties of the experimental treatment. With- out an independent verification of the individual's stress responses, the effect of the stressor on electromyographical (EMG) activity cannot be evaluated. In addition, the separate effects of mental stress and physical load compared to the combined effect of mental stress and physical load do not seem to have been investigated experimentally in earlier studies. Therefore, the present study comprising 62 women was aimed at investigating the
356 LUNDBERG ETAL.
relation between mentally induced stress, postural load, psy~hophysiological stress responses, and muscular tension as measured by EMG activity of the trapezius muscle.
METHOD
Subjects and General Procedure
Sixty two women. 18-64 years of age (M = 37.2, SD = 13.7), participated in the study; 42 of them worked as cash register operators and the other 20 were university students. The cashiers participated in the experiment in a mobile laboratory at their workplace during their regular paid working hours. The students were paid SEK 250 (about U.S. $30) or participated as part of their course requirements at Stockholm University.
Before the experiment proper, each woman was individually informed about the design of the experiment and exposed to the various stress tests to be used. During this introductory session, she was also made familiar with the equipment and the measurements, for example, her systolic blood pres- sure (SBP) and diastolic blood pressure (DBP) were recorded and test sam- ples of saliva and urine were taken. The aim of this session (lasting about 30 min) was to reduce possible novelty effects of the experimental conditions and measurement procedures. Each subject was individually exposed to one stress session (60 rain) and one baseline session (60 rain). For each individ- ual, the two sessions took place at the same time of day (between 9 a.m. and 4 p.m.) on separate days to control for the diurnal variation of the physiolog- ical variables. The baseline session was always presented last, to secure reliable baseline values. During the stress session, the subject was exposed to two mental stressors (mental arithmetic, Stroop color word test [CWT]), standardized physical loads (test contractions [TCs]), and two tests involv- ing both mental and physical components (cold presser test, TC combined with the CWT).
Experimentat Conditions
In the stress session, the subject was seated on a height-adjusted chair without arm supports and exposed to standardized postural Ioads (TCs) and the different stress tests, each separated by l-rain rest intervals as illustrated in Table 1. Each se~io~ started and ended with a TC. Before the stress tests, the subject relaxed for 5 min (pretest period). The TCs during the session were presented twice, in one case combined with a mental stressor (the Stroop CWT). The order of the tests was balanced between subjects, except for the cold presser test, which was always presented last (Table 1).
TO. The seated subject kept her arms straight and elevated to 45 ~ of abduction in the scapular plane (the same angle for all subjects), half-way
STRESS AND EMG ACTIVITY 357
TABLE 1 Conditions and Measurements of the Stress Session
Duration
1. Arrival, urine voiding, test sample of saliva, a glass of water Application and test of equipment IEMG, SBP, DBP, HR)
2. TC during 5 see = standard for EMG 3. Pretest (resting) 5 rain
SBP, DBP, HR Self-rating
4. "CWT + T C - C W T - CWT + TC 1 . 5 - 2 - 1.Stain SBP, DBP, HR Self-rating Rest I rain
Note. CWT -- color word test. TC -- test contraction. aBalaneed order between subjects.
be tween the f rontal and sagi t ta l p lanes ( 0 b e r g , Sandsj6 , Kadefors , & Lars - son, 1992),
S t r o o p C W T . Color words pr inted in an incongruen t color , a c c o m p a - nied by a vo ice naming a third incongruent c o l o r and another voice naming a number , were presented rapid ly (about I i tem/s) on a te lev is ion screen. The sub jec t ' s task was to name the co lor o f the print of each co lo r word (see F r a n k e n h a e u s e r & 3"obansson, 1976, for deta i ls ) .
Mentalarithmetic. The subject was required to add and subtract a ser ies o f one-d ig i t numbers accord ing to cer tain rules and name the final resu l t (see F rankenhaeuse r & Lundberg , 1977). The e x p e r i m e n t e r inter-
358 LUNDBERG ET AL,
rupted and informed the subject about any errors she made. For the CWT and mental arithmetic the subject was told that a performance score would be calculated from a sound tape recorded during the experiment. However, no performance score was actually calculated.
Cold pressor. The subject put her left hand up to the wrist in cold water (+4 ~ for 1 rain.
Rest. During the rest periods the subject was asked to relax in the chair with her arms resting in her lap.
Baseline session. The subject sat comfortably and was asked to relax while listening to soft music and a relaxation training tape. Psychological and physiological measurements were obtained after 15, 30, and 60 min. EMG was not recorded during this session as no electric activity was ex- pected during reIaxation.
Measurements
Blood pressure and heart rate. SBP and DBP were measured during each of the separate conditions (Table 1) using an automatic digital blood pressure device (UA-751, A&D Company, Japan) with a triple cuff automat- ically compensating for arm size. Heart rate (HR) was also recorded during the measurement period (about 40 see).
Urinary catecholamines and cortisol. The subject emptied her blad- der immediately before each session, a urine sample was then obtained at the end of the 60-min period. The urine volume was measured and 2 ml was separated for cortisol analysis by radioimmunoassay (RIA; RIA kits avail- able from New England Nuclear). The pH of the remaining urine was ad- justed to 3.0 with 2N HCI and the sample was frozen (-18 ~ until analyzed for epinephrine and norepinephrine by HPLC (Riggin & Kissinger, 1977).
Salivarycortisol. The subject placed a cotton roll (Salivette) in the mouth for 90 sec after each session. The roll was frozen ( - t8 ~ until centrifuged and analyzed for cortisot by RIA.
Self-ratings. The subject reported successively her subjective experi- ences during each condition of the experiment on 12 6-point scales ranging from 0 (not at all) to 5 (extremely; see Table 1). A factor analysis of the 12 scales suggested (by a marked drop in eigenvalues) that data could be
STRESS AND EMG ACTIVITY 359
reduced to three factors. The first one, which was bipolar, explained 35% of the variance and was divided into: (a) Stressed (stressed, under pressure, tense) and (b) Relaxed (relaxed, tranquil, calm). The second one, explaining 21% of the variance, was named Active (active, energetic, concentrated) and the third one, also explaining 21% of the variance, was named Inert (ineffi- cient, unactivated, passive). The correlation between Stressed and Relaxed was - .63, whereas correlations between the other indices did not reach significance (r < .25).
EMG recording and signal processing. Bipolar surface electrodes were placed over the upper part of the trapezius muscle as described by 0berg, SandsjiS, and Kadefors (1990, 1992). EMG was recorded simulta- neously from the right and the left sides. The signa} was preamplified close to the electrodes, transmitted via radio telemetry to a six-channel amplifier and stored on a frequency modulated tape recorder (TEAC-R71, Tokyo, Japan). A quality control routine was applied to the signal (Arvidsson, 1982; Kadefors, Arvidsson, Herberts, & Peters6n, 1983). The signal was analyzed in sequences of 0.5 sec. The root mean square (RMS) values were calculated and used for further calculations. The mean RMS of the first TC was used as a reference value, set to I00 for each individual.
Statistical Analysis
As shown in Table l , the stress session included five l-min rest periods. As no significant differences in subjective or physiological measures were ob- tained among these rest periods, a mean value was calculated for each individual and used in the analyses. Similarly, means were calculated for the two presentations of the CWT and the two TC during the stress session.
Overall responses to the stress session were examined by paired t tests o f means calculated from all subjective and cardiovascular measurements dur- ing the stress and the baseline sessions, respectively. Analysis of variance (ANOVA) was not applicable as the number of measurements differed be- tween the two sessions. Furthermore, as the EMG measurements from the left and the right trapezius were highly correlated (greater than .90), means were calculated and used in the statistical analyses (ANOVA, paired t tests, correlations). Single data were lost in some conditions due to occasional artifacts, as indicated by the n values and the number of degrees of freedom.
RESULTS
Overall Responses to the Stress Session
Means and standard deviations of subjective and physiological stress mea- surements in the two sessions are summarized in Table 2.
3@0 EUNDBERG E% AI
TA:gLE 2 Means and Standard Deviations of Subjective and Physiological Stress
*Paired t test, p < .0001 for all measurements, except as noted otherwise, bPaired t test, ns. cPaired t test, p < .001.
Mean scores on the subject ive indices of Stressed (t = 22.0, DF = 6I , p < .0001) and Active (t = 13.1, DF = 61, p < .0001) were s ignif icant ly elevated in the stress session compared to baseline. Scores for Relaxed (t = 17.7, D F
= 61, p < .0001) and Inert (t = 10.7, DF = 61, p < .0001) indicated that the subjects were able to relax in the basel ine session. Mean SBP (t = 10.5, D F
= 53, p < .0001) , DBP (t = 14.4, D F = 6 1 , p < .0001) , HR (t = 8.1, D F = 61, p < .0001 ), urinary epinephrine (t = 5.49, D F = 61, p < .0001), and norepi- nephr ine (t = 3.31, D F = 61, p < .005), as well as salivary cortisol levels (t = 3.38, D F = 56, p < .005), were also s ignif icant ly higher in the stress session, whereas the increase in urinary cortisol did not reach s ignif icance (t = 1.25).
Responses to the Separate Stress Tests
Self-rat ings dur ing pretest and the stress tests are illustrated in Figure I. ANOVA revealed s igni f icant changes over condi t ions in the scores for Stressed (F = 37.0, D F = 5/305, p < .0001) and Active (F = 46.3, D F = 51305, p < .000I) . Table 3 shows that scores for Stressed and Active were signif i- cantly elevated from the basel ine session during all stress tests as well as dur ing the pretest period (self-reports were not obtained for the rest periods).
The variation in cardiovascular activity during the stress session was highly s ignif icant for SBP (F = 43.3, D F = 6 / 3 t 8 , p < .0001), DBP ( F = 57.3, D F = 61318, p < .0001 ), and HR (F = 47.8, DF = 6/366, p < .0001 ). As shown in Figure 2, SBP and DBP responses were most pronounced during mental ari thmetic and the cold pressor test, HR was most pronounced during mental ar i thmetic and the CWT. Table 3 shows that, with two except ions (HR dur ing
�84184184184184 �84
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D Pretest Mental A~ihmetiC
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361
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TASLE 3 Sgotfi~a.ce Tes~s (pa i~ f ~es~s} of ig~e~ed Sabiec~i,ve aa~ ~diavas~{a~
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e 5dNee; 3~gt~ i8;29 *~ :L52 e 7 ,1~* i2;~5i ~e
eest D F ~ 3 2
pe, ied fl~:e s t r e : se hm COmpad0n 0fS~msS [e:vel di~i~!g fl~e I C p;!:e eme! sei ~ra~eI'e ve~a~ COmbined Wi~ a m e e a i s;~fe er (<wT) Sho.ws {l~a! e~elf >~ip{} l: Ant{re (,- = t i 2; D/,: = gl~ p < 000 ) m d S re s:ed <e 992~ D F = 6 2 p < 000I ) b~ e~eaaaed when mel~ al s~ress was added {0 t;1 e: TC.
aeH:.vi!2 ibii0wed the same patte r~, d-m~ iS; SBP,(~ = 8:57, DF = 6 ] 0 p < . 0 0 0 t ) D g P ( t = 5 9 6 D F v : 6 { , < O0()[) m~dHR{, { 0 ; D F = 61, # < 0001} were e~ g~Kieat~ily ele:vi~{ d Whcg memai s~reas w ~ c~}~>
M e a n am~ dev{a~ims < f g M G meauremem i m i ~ ! in Sre SeSsion a~e m u m a e zed i~: Z a N e < i~g ~,est arid the {womemai si~eS iea~s me~mi ir flm~edC md CWT), . . . . . A ~ . . . . ~aa{n effer of memai ( F e 1232 D F = 2/I ! g ~ < 0001) arid SepaFate t that hr {g,Sfiea~dy f}0:n
p < 00{: CW'i : : t ) t - 9 p < {R~{) md tba~ EMG act vi~y d~r{ng ~ e.nta* a:eit%me:~ c w a ~{g~er ~t~a~ d~ri~ag ~he c W T ( ~ DI. 9 # < .0{ )
E iE
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120
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Menia l Ar i thmet ic a C W T
Coid Presaor T C TC~CW~
100
~ ~9~ �84 ~
75
E
7 0
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HRi
FIGURE 2
s6s
364 AL
~ABGE 4 M~ans and Sta~da~ Deviaiio~s of EM~ Aeti~ty (RM8} O~i~g !h@
gifai gest ~n~fac l i~ x~
Res~ Arithmetic C ~
aD ~30 .... ~ a g 4s~ g2,:{ a 61 g~ 5? @ 62
8 0 '
3
2 ~
s activity (trapezi s} uring mental stress
m d ~hc Nt o O p oo}<~r w ~ d t e ~ t ~(TW 1}
FigHre 4 s h e ~ s ~hat E M G act~v~t~ d~ri{~:~ h e T( combi~e~, wi~h d~e
m e ~ a [ s t r e s s o r fCWT} was s i g ~ i f i c a n d y ~e = 3 iS4. D F = 59 p < fiX)5 ~ highe~
~ha~ d~Mne '|s a i e ~ e I{ was tts~> Xm~d ~ha~ ~he m e , e a s e in E M G a c d v f f v
c a u s e d i~, ~he C W T was ~[ar~i*[ca~dy {g = 2~57~ D F = 58 O < 05~ m<}re
p r o n o u n c e d , 69 ; vs . 22%} w h e n [~ was c o m b b m d wi th dm TC ~ha~ whe~, it
w~a g iven separaIefv_ E M G ac'~ivit} a l s e i ~ e r e a s e d si~ ~ i f i c a ~ d y t~ ~ 6_87. D F
= 5~). p < O 0 0 t ) durb~g ~he co ld pre:as~r ~e~ c~-~mpared u~ rcs~ ~Fig~re 5; f h c
i nc rease t?om resl was m(~s? p r o a o t m c e d {296%) ~'~r ~he iet~ shte~ e:hict~ is ~he
hand dm~ was pm into ~he wa~er ~ = 6 5 f . D F = 54. p < {){]~} ~ Howe~ 'e r ~
h igh ly significan{" {g = 4~29o D F = 53. p < .(R}(}I ] [~crease was als~ i'~m~ld for
"~he r[fh~ s;~de (7(})1
EMG activity (trapezius) during mental and physical load
[] TC T C + CWT
O
i~ �84184
o i
F1GURE 4 Mean EMG acliviiy of the ttapeziu~ muscle dunng Ic~! contractmn (TC) ahme aad r c combined wilh Iht." Stroop color ~ord !e~,l (CWT~
8 o iti~
20 , iii i
O
FIGURE 5 lest,
EMG activity (trapezius) during cold pressor test
~i iliil
] [ ] Rest �9 Cold pressor
Mean EMG activity of the trapczius muscte during rcs! and the cold pre.~sor
3 6 5
EMG activity (trapezius) during mental and physical load
n TC
FlGURE 4 Mean EMG ac l~v~ iy o f thc trapellus rnuccle dunng lepi conlrocl~on ITCl .~lunr and TC comhlncd w ~ l h ihv Stronp color uord i r s l (CWTJ
EMG activity (trapezius)
801 during cold pressor test
Rest Cold pressor
FIGURE 5 Mean EMG activity of the trapezius muscle during resl and the cold pressor ICSL.
866 LUN~BER(3 E~ s
TABLE 5 Correlations Between Cardiovascular and EMG Activity During the Different
Conditions of the Stress Session
Condition
Pretest Mental Cold (5 rain) ...... Rest . . . . Arithmetic C W T Pressor
Note. CWT = color word test. *p < .05. **p < .Ol, ***p < .001.
Correlations Between Stress Levels and EMG Activity
Cardiovascular activity during pretest, rest, and the different stress tests was correlated with the corresponding EMG values (Table 5). Significant posi- tive correlations were found for SBP (during pretest, rest, and the CWT) and for DBP (during pretest and the cold pressor test). Only one correlation for HR reached significance (during the cold pressor test).
Scores on the subjective indices of Stressed and Active did not correlate significantly with corresponding EMG measurements. Mean EMG activity during the whole stress session was significantly correlated with the increase in norepinephrine (r = .30, D F = 54, p < .05), whereas corresponding correlations with urinary epinephrine (r = .14), cortisol (r = .17), and sali- vary cortisol (r = .15) did not reach significance.
DISCUSSION
The most interesting finding in the present study was the significant increase in trapezius muscle tension induced by mental stress, This was demonstrated by the two mental stressors presented separately (Figure 3) and when a mental stressor (CWT) was added to the TC (Figure 4). In the latter case, a synergistic effect was indicated as the increase was significantly more pro- nounced than when the CWT was presented alone. EMG activity, perceived stress, and SBP and DBP followed the same pattern and were higher during mental arithmetic than during the CWT. It is likely that the experimenter 's mention of any errors made by the subject during mental arithmetic contrib- uted to this difference.
An association between EMG activity and blood pressure is also sug- gested on an individual level by the significant positive correlations in Table 5. The positive correlation with norepinephrine, which plays an important role in blood pressure homeostasis, is also in line with these findings. However, it is not clear whether this association represents the effect of an
STRESS AND EMG ACT~V~T~ 36~
underlying factor (e.g., general psychological arousal) or a more direct interdependence of blood pressure and muscular tension.
The magnitude of the muscular tension induced by mental stress is of course low, compared to that during heavy physical work. However, it is worth noting that the effect of mental stress on the EMG activity during the TC is significant from an ergonomic point of view. This indicates that psychologically induced stress may play a role not only in very light work, but indeed also in work that combines stress and materials handling, for instance.
According to the traditional model (Maeda, 1977), muscle disorders may be induced by insufficient blood flow through the muscle due to the high intramuscular pressure during contraction. However, this model does not explain why muscular disorders have also been reported in low-load work situations (e.g., Veiersted, Westgaard, & Andersen, in press) and why varia- tions in strength capability do not seem to predict musculoskeletal disorders (Batti6, 1989; Bigos et al., 1991; Bjetle, Hagberg, & Michaelsson, 1979, 1981; H~gg+ Suurkiila, & Kilbom 1990; Jonsson, Persson, & Kilbom, t988; Wiker, Chaffin, & Langoif 1990).
A new explanatory model for occupational myalgia suggested by H~igg (1991) is based on studies suggesting a fixed recruitment order of motor units (Henneman, Somjen, & Carpenter, 1965). Muscle fibers belonging to low threshold motor units are assumed to be recruited first during contrac- tion and to remain active until the muscle is completely relaxed. Injured "ragged red" fibers (Larsson, Bengtsson, Bodeghrd, Henriksson, & Larsson, 1988) are assumed to reflect unduly long activation without rest of muscle fibers belonging to these Iow threshold motor units.
In view of studies such as the present one, showing that mental stress induces significantly elevated muscular tension, the HSgg model implies that, for example, psychologically stressful jobs could carry a heightened risk for musculoskeletal disorders due to the additional muscular tension induced by mental stress and the low threshold motor units being kept active even in the absence of physical load. This is consistent with the fact that these disorders are common in jobs characterized by a relatively low physi- cal but a high psychological load, such as light assembly line work (cf. Lundberg, Granqvist, Hansson, Magnusson, & Wallin, 1989; Magnusson et at., 1990), repetitive data entry at video display terminals (Aronsson, Dall- n e r , & ,~,borg, 1994), cash register operation (Lannersten & Harms- Ringdahl, 1990), and so forth. Lack of unwinding during breaks at work and after work is known to be a common problem in many simple and repetitive jobs (cf. Frankenhaeuser & Johansson, 1986; Johansson, Aronsson, & Lindstrtim, 1978; Lundberg, Melin, Evans, & Holmberg, 1993; Rissler, 1977) and may further contribute to the negative health consequences. As neck and shoulder disorders are more frequent among women than men (Lagerltif, 1993), it is also of interest to speculate whether stress from the combined load of paid and unpaid work in employed women has contributed
3:88
to this sex difference (eL Frankenhaeuser et al., 1989; Lundberg, Mhrdberg, & Frankenhaeuser , 1994; Lundberg & Palm, I989; Picket ing et al., 1991).
Al though we need to learn more about the specific mechanisms l inking psychosocial factors to musculoskeletal disorders, the present f indings from a group of women varying in age and occupational background are consis- tent with the hypo the s i s that psychological stress plays a role in the develop- ment of these problems by increasing the museutar tens ion and prevent ing relaxat ion of the muscles also in the absence of physical toad. There is need for such f indings to be fol lowed up in field studies conducted in the work- place.
REFERENCES
Aronsson, G., Dallner, M., & ~.borg. C. (t994). Winners and losers from computerization: A study of the psychosocial work conditions ~ d health of Swedish State employees. Interna- tional Journal e~[ Human-Computer Interaction. 6. 17-35.
Rep. No. 52). Gothenburg, Sweden: Chalmers University of Technology. Bummer, G (1990). Review of current knowledge--musculoskeletal problems. In L. Bedinguet
& D. Berthelette (Eds.), Work with display units •9 (pp. I!3-120). North-Holland, The Netherlands: Elsevier.
Balti6, M. (1989). The reliabiti~_ of physical factors as predictors of the occurrence ofback pain reports: A prospective study within industry. Unpublished doctoral dissertation, Gothenburg University, Sweden.
Bigos, S., BarriO, M., Spengter, D., Fisher, L., Fordyee, W., Hansson, T., Naehemson, A., & Wt~rtley, M. (1991). A prospective study of perceptions and psychosocial factors affecting the report of back injury. Spine. 16, 1-6.
Bjelle. A., Hagberg. A.. & Michaelsson, G. (f979). Clinical and ergonomie factors in prolonged shottlder pain among industrial workers. Scandinavian Journal of Worf~ Envfronment and Health, 5, 205-210.
Bjelle. A., Hagberg, A, & Michaelsson, G. (1981). Occupational and individual factors in acute shoulder-neck disorders among industrial workers. British Journal of industrial Medicine, 38. 356-363.
Bongers, P. M., de Winter, C. R., Kompier, M. A..I., & Hildebrandt, V. H. (1993). PsyehosoeiaI factors at work and museuluskeletal disea~. Scandinavian Journal qf Work Environment. & Health, 19, 297-312.
Bru, E., Myktetun, R. J., & Svebak, S. (1993). Neurotieism, extroversion, anxiety, and type A behaviour as mediators of neck, shoulder and lower back pain in female hospital staff. Persrmality and Individual Differences 15. 485-492.
Flodmark. B. T., & Aase, G. (1992). Museulosketelal symptoms and type A behaviour in bh[e collar workers. British Journal of Industrial Medicine, 40. 683--687.
Frankenhaeuser, M., & Johansson, G. (1976). Task demand as reflected in eatr excretion and heart rate. Journtzt rrfHuman Stress, 2( I ), ! 5-23.
Frankenhaeuser. M., & Johansson, G. (1986). Stress at work: psychobiological and psychosociat aspects. International Review of AFplied Psychology. 35, 287-299,
Frankenhaeuser, M., & Lundberg, U. (1977). The influence of cognitive set on performance and arousat under different noise loads. Mt~tivation and Emotion, 1. 1.39-149,
Frankenhaeuser. M., Lundberg, U., Fredrikson, M,, Melin, B., Tuomisto, M., Myrsteu, A.-L., Hedmam M., 8ergman-Losman, B., & Wallin, L. (1989). Stress on and off the job as related
STRESS AND EMG ACTIVITY 3 6 9
to sex and occupational status in white-collar workers. Journal of Organizational Behavior, I0, 32 f-346.
Hiigg, G. M. (1991). Static work loads and occupational myalgia---a new explanation model. In P. A. Anderson, D. I. Hobart, & J. V. Danoff (Eds.), Electrnmyographical Kinesiology (pp, 141 - 144). No rth-Holland, The Netherlands: Elsevier.
H~igg, G. M,, Suurkitla, J., & Kilbom, A, (1990). Prediktorer for belastningsbesvtir i skuldra/nacke. En longitudinell studie pA kvinnliga mont0rer [Predictors of musculoskeletal problems in neck/shoulder. A longitudinal study of female assembly workers]. Arbete och Hiitsa, 10.
Haldeman, S. (1991). Presidential address, North American Spine Society: Failure of the pathology model to pmdic! back pain. Spine, I5, 7f8--724.
Henneman, E., Somjen, G., & Carpenter, D. O. (I965). Excitability and inhibitlbility of moto- neurons of different sizes. Journal of Neurophysiofogy, 28. 599---620.
Johansson, G., Aronsson, G., & Lindstrrm, B. (1978). Social psychological and neuroendocrine stress reactions in highly mechanized work. Ergonomics, 21, 583-599.
Jonsson, B. G., Persson, J., & Kilbom, A. (1988). Disorders of the ceruieo-brachial region among female worke, s in the electronics industry--a two year follow up. International Journal of lndustriat Ergonomy, 3. 1-12.
Kadefors, R., Arvidsson, A., Herberts, P., & Peters~n, I. (1983), EMG data processing in ergonomics, with emphasis on studies of shoulder muscle load. In J. E. Desmedt (Ed.), Computer-aided electromyography. Progress r clinical neurophysiology (Vol. 10, pp. 273- 286l. Basel, Switzerland: Karger.
Kvarnstr0m, S. (1983). Occurrence of musculoskeletai disorders in a manufacturing industry, with special attention to occupational shoulder disorders. Scandinavian Journal of Rehabili- tation Medicine, 20(Suppl. 8), I - 113-
Lagerl0f, E. (1993). OECD working par~ on the role of women in economy. Panel of experts on women, work and health: National report Sweden. Ministry of Health and Social Affairs. Ds. 1993:98.
Lannersten, L.. & Harms-Ringdahl, K. (1990). Neck and shoulder muscle activity during work with different cash register systems. Ergonomics, 33, 49-65.
Larsson, E. E., Bengtsson, A., BodegSrd. L., Henriksson, K. G., & Larsson, J. (t988). Muscle changes in work-related chronic myalgia. Aeta Orthopedia Scandinavica, 59, 552-556.
Lundberg, U., Granqvist, M., Hansson, T.. Magnusson, M., & Watlin, L. (1989), Psychological and physiological stress responses during repetitive work at an assembly line. Work & Stress, 3, I43-153.
Lundberg, U., M~rdherg. B., & Frankenhaeuser, M. (1994). The total workload of male and female white eoltar workers as related to age occupational level, and number of children. Scandinavian Journal of P.~yeh olt~gy, 35, 315-327.
Lundberg, U., Melin. B., Evans, G. W., & Holmberg, L. (1993L Physiological deactivation after two contrasting tasks at a video display terminal: Learning vs repetitive data entry. Ergonom- ics, 36, 601-611.
Lundberg, U., & Palm, K. (1989). Workload and eatecholamine excretion in parents of pre- school children. Work & Stress, 3, 255-260.
Maeda. K, (1977). Occupational cervicobraehial disorder and its causative factors, Journal o] Human Ergoh~gy.. 6. 193-202.
Magnusson, M., Granqvist, M., Jonson, R., Linddl, V,, Lundberg, U., Wallin, L., & Hansson, T. ( t 990). The loads on the lumbar spine during work at an assembly line. The risk for fatigue injuries of vertebral bodies. Spine. 15, 774-779.
Oberg, T., Sandsjt~, L., & Kadefors, R. (t990). EMG mean power frequency in non-fatigued trapezius muscle. European Journal of Applied Physi,91ogy, 61, 362-369.
Oberg, T., Sandsjg, L., & Kadefors, R. (1992). Arm movement and EMG mean power frequency in the trapezius muscle: A comparison between surface and intramuscular recording teeh-
work-related myalgia of the trapezlus muscle. European Journal of Applied Physialog); 65, 251-257.
Picketing, T. G., James, G. D., Schnal[, P~ L., Sehlussel, Y. R., Pieper, C. E, Gorin, W-, & Karasek, R, A. (1991). Occupational stress and btood pressure. Studies in working men and women. In M. Frankenhaeuser, U. Luadlaerg, & M. Chesney (EdS.) Women, work, and health. Stres.r and opportunities (pp. 17 i-186). New York: PIennm
Riggln, R. M., & Kissinger, P. T- (1977}. Determination of cateeholamines in urine by rever~- phase liquid chromatography with electrochemical detection. Analytical Chemistry, 49, 2109-21II.
gissler, A. (1977). Stress reactions at work and after work during a period of quantitative overload. Ergoaomic.~. 20, 577-5B0.
Salminen, J. J., Pentti, J., & Wickstr~m, G. (1991). Tenderness and pain in neck and shout, dots in relation to type A behaviour. Scandinavian Journal of Rheumatology, 20, 34~---350+
Svebak, S., Alajia, R., & Kgrstad, S. I. (1993). Task-induced electromyographic activation in fibromyalgia subjects and controls. Scandinavian Journal of Rheumatology. 22. 124-130.
Tellnes, G. { 1989). Days lost by sickness certification. Scandinavian Journal of Primary Health Care, 7, 245-251.
"I'ulen, J. H. M., Moleman, P., van Sleenis, H. G., & Boomsma, F. (t989}. Characterization of stress reactions to the Slroop color word test. Pharmacology. Biachemistu,, & Behavior, 32, 9-15.
Veiersted, K. B., Westgaard, R. H., & Andersen, P. (1993). EIeetr~myographic evaluation of muscular work pattern as a predictor of trapezius myalgia. Scandinavian Journal of Work, Environment and Health, 19, 284-290.
W~ersted, M., Bjerklund, R., & Westgaard~ R. { 1987}. Get, oration of muscle tension to a demand of continuing attention. In B. Knave & P.-G. Wideb~ck (Eds.), Wort~ with di,~play units $6. Amsterdam, The Netherlands: Taylor & Francis.
Waersted, M., Bj~rklund, R., & We~gaard, R. (I991). Sho~tldet m~sele tension induced by two VDU-based tasks of different complexity: Ergonomics. 34. I37-150.
Weber, A., Fussier, C-, O'Hanlon, J. F., Gierer, R., & Grandjean, 1~. (1980). Psychophysiological effects of repetilive tasks. Ergonomics, 23, 1033-1646.
Westgttard, R., & BjerkIund, R. (1987}. Generation of muscle tension addilional to posture muscle load. Ergonomics, 30. 911-923.
Wiker. S. F,, Chaffin, D. B., & LangoIf, G. 19. (199~). Shoulder pastural fatigue and discomfort. International Journal of Industrial Ergonomics, 5. 133-146.
