Control Computer
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Eur J Appl Physiol (****) **: *** ***
O R I GI N A L A R T IC L E
Anne Katrine Blangsted Karen S gaard
Hanne Christensen Gisela Sj gaard
The effect of physical and psychosocial loads on the trapezius muscle
activity during computer keying tasks and rest periods
Accepted: 29 August 2003 / Published online: 21 October 2003
Springer-Verlag 2003
Abstract The overall aim was to investigate the e ect of Keywords Electromyography Contra lateral
psychosocial loads on trapezius muscle activity during Mental stressors
computer keying work and during short and long
breaks. In 12 female subjects, surface electromyography
(EMG) was recorded bilaterally from the upper trape-
zius muscle during a standardized one hand keying Introduction
task interspaced with short (30 s) and long (4 min)
breaks in sessions with and without a combination of A large fraction of work-related musculoskeletal disor-
cognitive and emotional stressors. Adding psychosocial ders is associated with physically repetitive and monot-
loads to the same physical work did not increase the onous work involving low level of muscle activity
activity of the trapezius muscle on either the keying or (Bernard 1997). The high prevalence of musculoskeletal
the control side, both of which remained at median and disorders in psychologically stressful but light physical
static EMG activity levels of around 5% and 2.5% of work, such as computerized data entry, has indicated
the maximal voluntary electrical activity (EMGmax), that additional mental load plays an important role
respectively. The di erence between the keying and the (Jensen 2003; Jensen et al. 2002). In a review based on
control side was signi cant; and further the control epidemiological studies it was concluded that poor work
side activity was signi cantly increased above resting station design, continuous computer use for the entire
level. During both short and long breaks, exposure to work day, and repetitive computer work such as data
psychosocial loads also did not increase the activity of entry were associated with an increased risk of devel-
the trapezius muscle either on the side of the keying or oping symptoms of musculoskeletal disorders (Punnett
the control hand. Of note is that during long breaks the and Bergqvist 1997). In addition, they pointed out a
muscle activity of the keying side as well as that of the need to investigate physical loads within the full range of
control side remained at the same level as during mental loads and vice versa. Moreover, it has been sta-
the short breaks, which was increased above resting le- ted that during low-level physical activity e.g. computer
vel. This was to be seen from the static and the median work, the responses to psychophysiological factors may
EMG activity levels as well as gap times, the overall be particularly important (Sj gaard et al. 2000). How-
mean values being: 0.4%EMGmax, 1.1%EMGmax, and ever, only few studies have investigated both physical
50% in gap time, respectively. In conclusion: psycho- and a spectrum of psychosocial factors in jobs with
social loads are not solely responsible for increased intensive computer work. It was found that repetitive
non-postural muscle activity; and increasing the dura- movements and tasks as well as psychosocial factors
tion of breaks does not per se cause muscle relaxation. such as time pressure and low possibilities for personal
development at work were associated with musculo-
skeletal symptoms among computer users (Jensen et al.
2002). The proposed mechanism behind mental stress to
increase the risk for development of musculoskeletal
disorders is related to an increase in prolonged low-level
A. K. Blangsted K. S gaard
muscle activity with continuous ring of single motor
H. Christensen G. Sj gaard
Department of Physiology, National Institute of Occupational
units (Hagg 1991; Lundberg et al. 1994; Rissen et al.
Health, Lers Parkalle 105, 2100 Copenhagen, Denmark
2000).
E-mail: ***@***.**
Psychosocial risk factors for development of muscu-
Tel.: +45-39165352
loskeletal disorders include a range of various exposures
Fax: +45-39165201
254
4. Exposure to psychosocial loads increases the activity
at the work place, such as: high mental demands, lack of
of the trapezius muscle on the side of the keying hand
in uence over the work situation, low job satisfaction,
and the control hand during both short and long
lack of support from managers and other colleagues,
breaks
monotonous work, and time pressure (Ariens et al. 2001;
Bongers et al. 1993). The demand/control model of job
strain proposed by Karasek (1979) suggests that jobs in
Methods
which the demands are high and the control is low are
particularly stressful. The model was later elaborated
Subjects
with a work place social support dimension, an imbal-
ance between external demands and the individual s re- Twelve female subjects, without earlier experience from laboratory
sources to meet those demands, as well as by an experiments, participated in the study after having provided written
imbalance between e ort and rewards (Johnson and informed consent. The mean (range) age, height and body mass
were 23.7 (19 38) years, 1.70 (1.62 1.78) m, and 62.2 (56 74) kg.
Hall 1988; Siegrist 1996). Therefore, in the study of
The participants reported no discomfort in the upper body regions
possible e ects of psychosocial factors on somatic re- within the week prior to testing. They were all experienced com-
sponses, such as changes in the activation of skeletal puter users and 11 of the subjects were right-hand dominant while
muscles, care must be taken to design a comprehensive one subject was left-hand dominant. They all normally operated
the keyboard with their dominant hand. Prior to the study the
combination of such risk factors. The Stroop colour
subjects were informed only of the general aims of the study and
word test is a commonly used stress test (Laursen et al. not of the psychological stress test. The latter information was
2002; Lundberg et al. 1994) and also intense memory given to the subjects after the experiment. The local ethical com-
demands has been used as standardized mental demand mittee of Copenhagen approved the study.
(Finsen et al. 2001). Nevertheless, for the present study,
psychosocial exposures that are more similar to work
Procedures
place conditions were developed representing cognitive
aspects, surveillance of the worker, low control, and lack The participants were seated on a height-adjustable chair at a table
of support. with arm supports and performed standardized computer work.
The computer work consisted of keying random numbers with the
As mentioned above, the physiological mechanisms
dominant hand using the numeric part of the keyboard. Six digits
that may be causally related to the development of
were presented to the participants on the computer screen for 4 s
musculoskeletal disorders are sustained patterns of during which time the subjects were told to key in these digits, and
muscle activation. The trapezius muscle, especially, is they were given the instructions that failing to key in all numbers as
activated in response to both physical and psychosocial well as mistakes including wrong order would be regarded as
errors. The errors were automatically counted by the software of
loads; e.g. to stabilize the shoulder girdle mechanically
the computer task program and expressed as percentage errors
when working with arms and hands as well during of total numbers six digits . These data were used as a measure
mental demands (Laursen et al. 1998a,b; Sj gaard et al. of their performance. The participants performed three sessions of
2000; Waersted et al. 1996). Further, it is of note that the computer work, i.e. an introductory session (IS), a stress session
(SS), and a control session (CS) in the order given. The order of the
decay of muscle activity is not instantaneous at the end
SS and the CS was not randomized because we expected the e ect
of an activity period and short resting periods may thus of novelty to be the least pronounced in the last session. Each
not allow for full muscle relaxation (S gaard et al. session consisted of four work periods of 3 min each separated by
2001). short breaks of 30 s. The three sessions were separated by pro-
longed breaks of approximately 8 min. The physical work was held
In the present study the subjects worked physically
constant in the three work sessions.
with only one hand performing computer keying tasks,
The participants were familiarized with the situation in the IS,
the other side serving as control. Following an intro- and in the IS as well as in the SS the experimenter was unfriendly
ductory session the same keying tasks interspaced with towards the colleagues and neutral/strict towards the participant
short and long breaks were performed with and with- (lack of support), the participant was monitored by a video camera
(surveillance of the worker), and a memory test was included
out a combination of cognitive and emotional stressors.
(cognitive aspect). In the CS the same keying task was performed,
The muscle activity patterns of the right and left trape- but the imposed stressors were, as far as possible, eliminated. The
zius muscles were recorded to test the hypotheses: experimenter was friendly and encouraging, there were no memory
tests, and the camera was turned o . The computer work was
1. During keying work the activity of the trapezius externally paced during all three sessions (low control).
muscle on the side of the keying hand is higher than The memory test consisted of recognition of words (Nilsson
the activity of the control side that remains at resting et al. 1997). The participants were presented with a list of 12
common words before the IS and SS. Each word was in view for
level
2 s. The participants were instructed to remember as many words
2. During short breaks the activity of the trapezius as possible, which they had to present to the video camera after the
muscle on the side of the keying hand is higher than IS and SS, respectively.
the activity of the control side, but during long breaks
it gets equally low
Electromyography
3. Exposure to psychosocial loads during physical work
increases the activity of the trapezius muscle on the Electromyographic signals (EMG) were recorded bilaterally from
side of the keying hand and the control hand, but the upper trapezius muscles using bipolar surface electrodes (Ag
relatively more of the latter AgCl electrodes, type 72001-K, Medicotest, Denmark). The centre
255
di erences were seen between the sessions. Similarly for
of each pair of electrodes was placed 2 cm medial to the midpoint
between the seventh cervical vertebrae and the lateral end of RPE, this value tended to be slightly lower during IS but
acromion. The inter-electrode distance was 20 mm. The EMG
no signi cant di erences were found either for the initial
signal was ampli ed, low-pass ltered (8th order Butterworth l-
value of RPE or for the change in RPE during the ses-
ter, cut-o 400 Hz), and sampled on a computer with a sampling
sion. This was true for right and left shoulders, head,
frequency of 1,024 Hz. The signals were visually checked and
high-pass ltered (cut-o 10 Hz), full-wave recti ed, and root- and eyes (Fig. 1). These data were in support of the same
mean-square converted within windows of 100 ms duration. The
physical work being performed in SS and CS.
resting EMG signal was recorded during 5 s of instructed rest with
Regarding the electrical muscle activation pattern of
visual feedback to eliminate visible EMG activity, and the resting
the trapezius muscle, signi cant di erences were seen in
EMG amplitude was quadratically subtracted from all other EMG
signals. For normalization, the EMG recorded during maximal the median and the static EMG activity levels from the
voluntary isometric contractions were used, best out of three tri- side of keying hand/arm compared with the contra lat-
als. The contraction was performed bilaterally in the position of
eral side. In all three sessions the EMG activity levels
90 shoulder exion with resistance just proximal to the elbow.
were signi cantly higher for the keying side compared
The maximal EMG amplitude (EMGmax) during the reference
with the control side (Table 1). However, gap time was
contractions was calculated as the highest mean EMG amplitude
obtained with a 1-s window moving in steps of 100 ms. The EMG equally low in both trapezius muscles during the keying
activity was recorded during the three work sessions, during the
work. During the short rest periods (30 s) EMG values
short breaks, and during the rst 4 min of the prolonged breaks.
were signi cantly lower than during the keying tasks and
The prolonged breaks were separated into eight 30-s periods for
no di erences were seen between the trapezius muscles
analysis in order to identify possible changes over time. The sig-
nals were analyzed according to two di erent procedures: (1) the of the keying side and the control side. However, the
amplitude probability distribution function (APDF) analysis, and
median EMG activity level was still signi cantly above
(2) the EMG gap analysis. The APDF of the EMG signal o ers a
resting level and the overall mean for the three sessions
simple pro le of the variation in muscle activity amplitude during
and both shoulders was 1.1 (0.7 1.5)%EMGmax; the
the time period analyzed in terms of the probability level of a
given fraction of time not being exceeded. In the present study the corresponding value for the static EMG activity level
static EMG activity level (P=0.1) and the median EMG activity was 0.4 (0.2 0.6)%EMGmax. Correspondingly, gap time
level (P=0.5) were calculated, which estimate the EMG activity
had increased signi cantly from keying to rest period
levels in terms of percentage EMGmax below which the muscle
but showed no di erence between sessions. Nevertheless,
activity remained for 10% and 50% of the recording time,
the overall mean during the short rest periods was still
respectively (Jonsson 1982). The EMG gap analysis is a method
that considers the temporal pattern of muscle activity by quanti- not 100% of time but the overall mean for all sessions
fying the numbers and duration of short pauses (EMG gaps) in
and both shoulders was only 50.3 (41.1 62.1)%. During
the EMG signal. The EMG gaps are de ned as periods with an
the long rest periods no further decrease in the EMG
EMG level below 0.5%EMGmax for at least 0.2 s (Veiersted et al.
values occurred as analyzed over the eight 30-s periods
1990).
and the mean values for the whole long rest periods were
not signi cantly di erent from the short rest periods.
Surprisingly, the gap time in the keying side trapezius
Rate of perceived exertion
muscle was statistically signi cant, although physiolog-
ically only slightly shorter during CS compared with SS.
The subjects were asked to report the rate of perceived exertion
(RPE) regarding the right and left shoulders, the eyes, and the head
before and after each work session using Borg s CR10-scale (Borg
1982).
Statistics
Comparison of sessions: Friedman repeated measures analysis of
variance on ranks, multiple comparison procedures Tukey test.
Comparison of dominant versus non-dominant trapezius and short
versus long breaks: Wilcoxon signed rank test. The level of sig-
ni cance was set at P 0.05.
Results
The same physical work was performed during the IS
and the two sessions with (SS) and without (CS) addi-
tion of mental load. The performance was quanti ed
based on the number of errors occurring during the
keying. The number of errors presented as mean (SD)
was: 21.5 (28.9) in IS, 16.6 (26.1) in SS, and 15.0 (26.0) in Fig. 1 Rating of perceived exertion (RPE) presented as mean
CS. For some of the subjects the number of errors was values and standard error of mean for right and left shoulders,
larger in IS, but for the group of subjects as a whole no head, and eyes, before and after each of the three sessions
256
Table 1 Mean values of the EMG activity levels and gap time recorded on the trapezius (TRA) muscles on the keying side and non-keying
side. %EMGmax Percentage maximal EMG amplitude
Variable Median EMG activity Static EMG activity EMG gaps(% time)
level(%EMGmax) level(%EMGmax)
Session Keying TRA Non-key TRA Keying TRA Non-key TRA KeyingTRA Non-key TRA
Introductory session
Introductory keying 5.2 (4.4) 2.1 (1.1)* 3.3 (3.3) 1.2 (0.8)* 6.7 (22.9) 12.1 (25.8)*
Short rest 1.5 (1.7) 0.7 (0.4) 0.9 (1.5) 0.2 (0.3) 45.1 (37.4) 62.1 (28.8)
Long rest 1.3 (1.7) 0.5 (0.4) 0.8 (1.4) 0.1 (0.4) 56.8 (44.3) 72.8 (24.2)
Stress session
Keying with mental load 4.7 (4.1) 2.6 (1.9)* 3.0 (2.8) 1.7 (1.4)* 8.3 (24.8) 15.0 (29.2)
Short rest 1.0 (1.1) 1.2 (1.1) 0.5 (0.9) 0.6 (0.9) 57.4 (30.1) 48.2 (33.1)
Long rest 0.7 (0.8) 0.7 (0.6) 0.3 (0.7) 0.3 (0.5) 74.6 (35.3)** 61.3 (36.8)
Control session
Keying without mental load 4.9 (4.2) 2.8 (1.8)* 3.2 (3.0) 1.9 (1.4)* 8.2 (24.7) 11.8 (27.4)
Short rest 1.1 (1.1) 1.1 (1.1) 0.6 (0.9) 0.5 (1.0) 47.8 (35.4) 41.1 (28.7)
Long rest 1.0 (1.1) 1.0 (0.9) 0.4 (0.7) 0.5 (0.8) 50.0 (35.3) 46.3 (35.7)
*Signi cant di erence between keying and non-keying trapezius muscles
**Signi cant di erence between stress and control sessions
half of that of the side performing the keying task. In the
Discussion case of an attracted muscle disorder due to prolonged
muscle activity this implies that the muscle is not likely
In contrast to previous reports the present results did to be relieved just by performing the same task by the
not reveal di erences in the activity of the trapezius contra lateral side. On the other hand, these data do
muscle in conditions with and without exposure to a support the recommendation of regularly changing be-
combination of psychosocial risk factors. In this context tween hands in the performance of a given task, because
it should be emphasized that in the present experimental this will regularly decrease the muscle activity and may
design care was taken that the subjects performed the prevent the development of a disorder. More impor-
same amount of physical work. Also it was quanti ed tantly, however, breaks are recommended where the
that the number of errors performed during the keying worker can fully relax since this is shown to decrease
tasks did not deviate between the two sessions com- muscle activity most profoundly.
pared with and without psychosocial loads. This was Regarding discussions on the length of optimal rest-
achieved by having the subjects perform an introductory ing breaks, our nding of 30 s of rest to allow the
session before the experimental sessions to be compared. working muscle to relax to the same level as during
Further, the results on the RPE showed that the sub- 4 min of rest is of note. One concern, however, re-
jects did not feel more fatigued in their shoulders, head, mained: throughout the resting periods signi cant
or eyes in the second session. Therefore, we believe that muscle activity was seen, which ranged up to several
di erences in muscle fatigue may not have biased our percentages of the EMGmax. Furthermore, some studies
data. Since negative ndings are often not acknowl- have indicated that the absence of EMG gaps in the
edged, the literature may be biased in the number of trapezius muscle may be a risk factor for shoulder dis-
studies reporting no e ect on adding psychosocial loads orders among workers performing light manual work or
to a standardized physical work task. Also, all study
o ce work (Hagg and Astrom 1997; Veiersted et al.
designs do not allow for controlling the amount of 1993). Although gap time during the resting periods in
physical work to be the same. In contrast, in work place the present study did increase, this was as a mean value
studies the goal of increased psychosocial load in terms only to about 50% of the time. However, large inter-
of for example time pressure has been to stress the individual di erences were seen ranging from 40 to 60%.
workers to increase their productivity. When working This may lead to individual recommendation on the
with a computer mouse, time pressure and verbal ability of the workers to relax their muscles. Future
provocation resulted in increased activity in the trape- studies may reveal means for workers to increase their
zius muscle (Wahlstrom et al. 2002). Thus, increased
ability to relax muscles whenever they do not have to
muscle activity may in such a case have been caused by perform physical work. In particular the trapezius
the increased physical performance. muscle has been shown to react to EMG-based bio-
Another important nding in this study is the in- feedback in that the subjects were able to voluntarily
crease in activity of the trapezius muscle in the control decrease its activity on feedback, whereas no similar
side, i.e. the side contra lateral to the hand/arm per- pattern was recorded from other shoulder muscles
forming the keying work. This is especially of signi - (Palmerud et al. 1995).
cance because the activity of the control side was up to
257
Based on the present study it may be concluded that under the European Union research program BIOMED-2 (BHM-
98-3903).
although the trapezius muscle has been shown to re-
spond to arithmetic test performance (McNulty et al.
1994) as well as computer work (Waersted and Westg-
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