The effect of work-related sustained trapezius muscle activity on the development of neck and shoulder pain among young adults

Abstract

Objective:
This study aimed to evaluate if sustained trapezius muscle activity predicts neck and shoulder pain over a 2.5-year period.

Methods:
Forty young adults (15 hairdressers, 14 electricians, 5 students and 6 with various work) were followed during their first years of working life. Self-reported neck and shoulder pain during the last four weeks was assessed seven times over the observational period. Upper-trapezius muscle activity was measured during a full working day by bilateral surface electromyography (EMG) at baseline (winter 2006/7). Sustained trapezius muscle activity was defined as continuous muscle activity with amplitude >0.5% EMGmax lasting >4 minutes. The relative time of sustained muscle activity during the working day was calculated and further classified into low (0-29%), moderate (30-49%) and high (50-100%) level groups.

Results:
Generalized estimating equations (GEE), adjusted for time, gender, mechanical workload, control-over-work intensity, physical activity, tobacco use, and prior neck and shoulder pain, showed that participants with a high level of sustained muscle activity had a rate of neck and shoulder pain three times higher than the low level group during a 2.5-year period. The association was strongest at the same time and shortly after the EMG measurement, indicating a time-lag of ≤6 months.

Conclusion:
The results support the hypothesis that sustained trapezius muscle activity is associated with neck and shoulder pain. This association was strongest analyzing cross-sectional and short-term effects.

Full text

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Original article
Scand J Work Environ Health Online-first -article
doi:10.5271/sjweh.3357
The effect of work-related sustained trapezius muscle activity
on the development of neck and shoulder pain among young
adults
by Hanvold TN, Wærsted M, Mengshoel AM, Bjertness E, Stigum H,
Twisk J, Veiersted KB
The results indicate that sustained trapezius muscle activity with
periods of more than 4 minutes duration for more than half of the
working day, increases the rate of neck and shoulder pain in a
2.5-year period. This implicates the importance of reducing the
lengths of periods with sustained muscle activity at work.
Affiliation: National Institute of Occupational Health, PO Box 8149
Dep, 0033 Oslo, Norway. tno@stami.no
Refers to the following texts of the Journal: 1993;19(4):277-283
1993;19(4):284-290 2009;35(2):85-95 2001;27(1):41-48
Key terms: electromyography; EMG; muscle activity;
musculoskeletal disorder; sustained muscle activity; sustained
trapezius muscle activity; trapezius muscle; workload; young adult;
young worker

Scand J Work Environ Health – online first 1
Original article
Scand J Work Environ Health – online first. doi:10.5271/sjweh.3357
The effect of work-related sustained trapezius muscle activity on the
development of neck and shoulder pain among young adults
by Therese N Hanvold, MSc,1 Morten Wærsted, PhD,1 Anne Marit Mengshoel, PhD,2 Espen Bjertness,
PhD,2 Hein Stigum, PhD ,2, 3 Jos Twisk, PhD,4 Kaj Bo Veiersted, PhD 1
Hanvold TN, Wærsted M, Mengshoel AM, Bjertness E, Stigum H, Twisk J, Veiersted KB. The effect of work-related
sustained trapezius muscle activity on the development of neck and shoulder pain among young adults. Scand J
Work Environ Health – online first. doi:10.5271/sjweh.3357
Objective This study aimed to evaluate if sustained trapezius muscle activity predicts neck and shoulder pain
over a 2.5-year period.
Methods Forty young adults (15 hairdressers, 14 electricians, 5 students and 6 with various work) were fol-
lowed during their rst years of working life. Self-reported neck and shoulder pain during the last four weeks
was assessed seven times over the observational period. Upper-trapezius muscle activity was measured during a
full working day by bilateral surface electromyography (EMG) at baseline (winter 2006/7). Sustained trapezius
muscle activity was dened as continuous muscle activity with amplitude >0.5% EMGmax lasting >4 minutes.
The relative time of sustained muscle activity during the working day was calculated and further classied into
low (0–29%), moderate (30–49%) and high (50–100%) level groups.
Results Generalized estimating equations (GEE), adjusted for time, gender, mechanical workload, control-
over-work intensity, physical activity, tobacco use, and prior neck and shoulder pain, showed that participants
with a high level of sustained muscle activity had a rate of neck and shoulder pain three times higher than the
low level group during a 2.5-year period. The association was strongest at the same time and shortly after the
EMG measurement, indicating a time-lag of ≤6 months.
Conclusion The results support the hypothesis that sustained trapezius muscle activity is associated with neck
and shoulder pain. This association was strongest analyzing cross-sectional and short-term effects.
Key terms electromyography; EMG; musculoskeletal disorder; workload; young worker.
1 National Institute of Occupational Health, Oslo, Norway.
2 University of Oslo, Institute of Health and Society, Oslo, Norway.
3 Norwegian Institute of Public Health, Oslo, Norway.
4 VU University Medical Center, Amsterdam, The Netherlands.
Correspondence to: Therese Nordberg Hanvold, National Institute of Occupational Health, PO Box 8149 Dep, 0033 Oslo, Norway. [E-mail:
tno@stami.no]
Neck and shoulder disorders are a considerable health
problem in the working population with prevalence
rates of ≥30% (1, 2). Pain in this region is common also
among young adults in their rst years of working life
(3). Work-related neck and shoulder pain represents suf-
fering for the individual and is a considerable economic
challenge for society.
The etiology of work-related neck and shoulder pain
has been extensively examined, however the causes are
still unclear and regarded as complex and multifacto-
rial (4). Biomechanical and psychosocial workplace
factors as well as individual risk factors have all been
related to neck and shoulder pain (5–8). The deleteri-
ous effect of sustained activity pattern in the trapezius
muscle (which is the occurrence of continuous muscle
activation without interruptions) has been proposed as
a common mechanism for explaining neck and shoulder
pain (9). Veiersted and co-workers (10) found that a low
frequency of interruptions in trapezius muscle activ-
ity during repetitive tasks was associated with future
development of neck pain. More recent studies have
shown a correlation between trapezius muscle activa-
tion and pain. In a cross-sectional study, Szeto et al (11)
found that those ofce workers with high levels of neck
and shoulder pain had higher trapezius muscle activity
compared to ofce workers with lower pain levels. A

2 Scand J Work Environ Health – online first
Sustained trapezius muscle activity and neck and shoulder pain
longitudinal study on forest machine operators found
that a higher frequency of episodes with sustained tra-
pezius muscle activity lasting for ≥8 minutes was related
to future reports of neck pain (12). These studies suggest
that reduced muscle rest or sustained muscle activity
may be essential factors leading to neck and shoulder
pain. In contrast, others have proposed that either pain
leads to decreased muscle activity (13) or there is no
association between low-level trapezius muscle activity
and pain (14–17).
Since there is no consensus concerning trapezius
muscle activity and its relation to neck and shoulder
pain, we have used a longitudinal design for this study
with frequent measurements of neck and shoulder pain
to evaluate the long-term effects and time-lag, in addi-
tion to cross-sectional effects and reverse relationship,
of sustained muscle activity. We studied young adults
in order to investigate the development of work-related
pain in the rst years of working life when participants
have had no or little exposure from earlier work. This
is in contrast to most previous studies on this topic that
have investigated middle-aged or older workers. More-
over, only a few prior studies have applied a longitudinal
design.
The hypothesis of this study postulates that sustained
trapezius muscle activity pattern at work predicts neck
and shoulder pain over the following 2.5 years.
Methods
Study design
In this prospective study, 40 young adults were followed
over a 3.5-year period (2006–2009). Baseline measure-
ment occurred during the winter of 2006/2007, when a
recording of each participant’s upper-trapezius muscle
activity was taken during a full working day. The same
day, all study participants completed a questionnaire.
Over the following 2.5-year period, the participants
completed questionnaires approximately every 4th month
providing information on pain and other covariate vari-
ables. Over a 1-year period prior to baseline (2006),
questionnaires on neck and shoulder pain were collected
on three occasions and used as a covariate. The study
design is illustrated in table 1.
Study sample
The 40 participants (23 women and 17 men) were a
subsample from a cohort followed from 2002 and con-
sisting of 420 technical school students, representing
student hairdressers, student electricians, and art/media/
design students (18). Of the 420 subjects, 140 were not
contacted because they had withdrawn from the original
study or had moved away from the Oslo region. Tele-
phone contact was attempted with the remaining 280
subjects, and a total of 156 were reached and asked if
they would take part in the one-day eld registration. Of
these, 111 subjects refused due to, for example, prob-
lems obtaining permission from their employer or not
being interested, leaving 45 subjects for inclusion in the
study. Two of these cancelled their appointments, and
data of three subjects were excluded because of techni-
cal defects in the electromyography (EMG) registration,
leaving data on a total of 40 participants. The partici-
pants had different occupations at the time of the muscle
activity measurement: 15 females were hairdressers, 14
males were electricians, 5 females were students, and
3 females and 3 males had other occupations, such as
working in the retail business or kindergarten. Table 2
presents a further overview of the study group charac-
teristics. The study group (N=40) had signicantly lower
tobacco use prevalence at baseline than the remaining
cohort (N=380, P=0.03). Otherwise there were no sig-
nicant differences between the study groups and the
rest of the cohort for the variables listed in table 2.
Upper-trapezius muscle activity
Upper-trapezius muscle activity was evaluated by bilat-
eral surface EMG, recorded with 6 mm diameter, bipo-
lar electrodes (E-10-VS, Medicotest A/S, Ølstykke,
Denmark). The electrodes were placed with a 20 mm
inter-electrode distance parallel to the underlying muscle
bers in standardized positions (19, 20). The root mean
square (RMS) value was calculated for epochs of one-
eighth of a second, and the recorded noise level was
subtracted. The signal was then processed through
a rectangular moving average lter of 1.625-second
length, giving an RMS window length of approximately
1.6 seconds. The data was controlled for movement
artifacts and electromagnetic interference as described
by Hansson and co-workers (21). This controlling pro-
cedure did not identify any artifacts or interference and
therefore no parts of the recordings were excluded.
Applying a manual resistance against which the arm
was abducted 90° in the scapular plane (21), the inves-
tigator normalized the full working day EMG recording
in percent of the EMGmax of standardized maximum
muscle contractions. The mean EMG recording length
was 6 hours and 22 minutes (range 3 hour 39 minutes–8
hours 37 minutes). Sustained trapezius muscle activity
was dened as the relative time (% of time during the
full working day) with activity >0.5% EMGmax continu-
ously for >4 minutes. This duration was chosen on the
basis of a recent methodological study on the effects of
EMG data processing procedures where it was found
that 4 minutes was one of the preferable measures (22).

Scand J Work Environ Health – online first 3
Hanvold et al
An earlier study using similar episodes with sustained
muscle activity (23) and the distribution of our data were
also both important when choosing the exposure mea-
sure. In order to check how appropriate the choice of a
4-minute duration threshold was, a control analysis was
conducted with duration thresholds of >2, >6, >8, >10,
and >15 minutes, as well as with no demand on dura-
tion (ie, including all 1.6-second RMS windows >0.5%
EMGmax). Widely used EMG measures such as static
muscle activity (10th percentile of the amplitude distribu-
tion) (22, 24) and muscle rest (proportion of total time
with EMG activity <0.5% of EMGmax) (22, 25) were also
analyzed. For these measures, we used a 0.13-second
RMS window, which is in line with earlier studies (22,
24, 25). The EMG recording was done bilaterally and the
relative time of sustained muscle activity was calculated
for each side before a mean from the right and left EMG
recording was used in the analyses. The relative time of
sustained muscle activity during a full working day was
divided into three approximately equally sized groups:
(i) low (0–29%), (ii) moderate (30–49%), and (iii) high
(50–100%) level of sustained trapezius muscle activity.
Questionnaires
Assessments of neck and shoulder pain were obtained
approximately every 4th month over the follow-up period
(2006/7–2009), meaning that this information was given
in all seven questionnaires during the 2.5-year follow-up.
Prior neck and shoulder pain was collected from three
questionnaires over a 1-year period prior to the baseline
EMG measurement in 2006. The mean of the three neck
and shoulder pain index scores was used. Questions on
control over work intensity, mechanical workload, physi-
cal activity, and tobacco use were recorded approximately
every year, giving information in three of the seven ques-
tionnaires. This is illustrated in detail in table 1.
Neck and shoulder pain. The participants reported their
experienced neck and shoulder pain over the last four
weeks (26), reporting both pain intensity on a scale of
0–3 (no=0, little=1, moderate=2, and severe=3) and
duration in days on a scale of 1–4 (1–5=1, 6–10=2,
11–14=3, and 15–28=4). A mannequin drawing was used
to give a full understanding of the neck and shoulder
region (27). A pain index was calculated by multiplying
pain intensity (0–3) and duration (1–4), giving a pain
index ranging from 0–12. The reliability of this method
has been found to be acceptable based on testing the
correlation between the questionnaire and a medical
examination (Spearman`s correlation coefcient 0.73)
(28). Long-lasting or severe neck and shoulder pain was
dened as an index score of ≥4 (meaning that the subject
either reported little pain for >2 weeks or moderate/
severe pain for ≥6 days over the last 4 weeks).
Covariates. The participants’ working status was moni-
tored in all seven questionnaires during the follow-up
period, to detect possible changes from one profession to
another or a change from work to studying or other pos-
sible status changes. The participants were asked about
their perceived control-over-work intensity. Two ques-
tions were used to evaluate this: “Can you set your own
work pace?” and “Can you determine the lengths of your
own breaks?” The questions had 5 response alternatives
ranging from 0=never/seldom to 4=often/very often
(29). The mean of the response on these two questions
equalled the score on control-over-work intensity. A total
of 12 questions were used to assess the work-related
mechanical exposure (MI2) (30). The participants were
asked whether their work involved or required repetitive
movements (one question), precision movements (one
question), manual material handling (two questions),
vibration (one question), and body postures (seven ques-
tions) such as working with their arms elevated or their
back twisted or bent forward. The response alternatives
were: 0=nothing/hardly nothing, 1=somewhat, or 2=a
great deal. On the basis of the 12 questions, an index
was calculated ranging from 0–24 (30). One question
Table 1. Study design (2006–2009) and missing data information from 40 young adults. [Q=questionnaire; missing=number of non-
responders; x=reported values; “.”=missing by design]
2006 2006/2007 2008 2009
QprioraQ1 Q2 Q3 Q4 Q5 Q6 Q7
Neck/shoulder pain
Missing 2/10/9 0 8 5 10 17 11 6
Mechanical workload
Missing 2 . . 14 . . 10
Tobacco use
Missing 0 . . 10 . . 6
Control over work intensity
Missing 0 . . 10 . . 7
Physical activity
Missing 0 . . 10 . . 6
a The mean of three questionnaires on neck and shoulder pain to register “prior pain”.

4 Scand J Work Environ Health – online first
Sustained trapezius muscle activity and neck and shoulder pain
measured physical activity level during leisure time.
The participants were asked how often they performed
activities that led to increased heart rate and shortness of
breath. The question had 7 response categories ranging
from 0=never to 6=everyday (31). The participants were
also asked about their smoking and snuff habits. If they
either were smokers or used snuff daily or occasionally
they were characterized as tobacco users.
Missing data and imputation procedures
In our study, some of the time-varying variables were not
assessed in all seven questionnaires during the follow-up
period and thus were “missing by design” (mechanical
workload, tobacco use, control over work intensity, and
physical activity). Missing data also resulted when the
participants did not answer ≥1 of the questionnaires (unit
non-responders) or did not complete single items in a
questionnaire (item non-responders). The total amount
of missing values from non-responders in each question-
naire ranged from N=2 (5%) to N=17 (42%) (table 1).
To handle the missing data, multiple imputations were
done by using a longitudinal regression model that
took into account both the clustering in data (multiple
observations per individual) and the time trends for the
whole group (32). On the basis of ve imputed datasets,
one average estimate was calculated on the associations
(33). A multiple imputation procedure of missing data
was done to reduce bias and make use of all the data
collected (34).
Data analysis and statistics
Both STATA, version 11.0 (StataCorp, College Station,
TX, USA) and the Statistical Package for Social Sci-
ences, version 18.0 (SPSS Institute, Chicago, IL, USA)
were used for the statistical analyses. Cross-sectional
differences in pain and background variables were
assessed using the independent–samples Mann-Whitney
U test. Kruskal Wallis test was used to analyze the
three levels of sustained muscle activity and pain while
Spearman`s rank correlation was used in the calculation
of the continuous muscle activity variable (0–100%) and
pain. A time-lag of pain reports approximately every
four months was employed to analyze the time that
elapses between the muscle activity exposure and neck
or shoulder pain, and generalized estimating equations
(GEE) analysis was conducted to evaluate the time-lag
and the longitudinal association between sustained
muscle activity and pain. This model takes into account
the correlation between the repeated measurements
within the individual (35, 36). A negative binomial GEE
analysis was used due to over dispersion in the discrete
outcome variable. Incidence rate ratio (IRR) with cor-
responding 95% condence intervals (95% CI) were
reported for the effect estimates. In all negative binomial
GEE analysis, an exchangeable correlations structure
was used. Because of the gender difference in effect of
pain over time, the univariate and multivariate analyses
were done for the whole group and stratied by gender.
The study had a relatively small number of subjects
(N=40). However, when incorporating the intraclass cor-
relation coefcient (rho=0.315) of the outcome variable
and the number of repeated measurements (7 repeated
measures), the effective sample size was calculated to be
N=126. This effective sample size increased the power
of the study and allowed a multivariable model with
several covariate variables. The multivariate analyses
were adjusted for time, mechanical workload, control
over work intensity, physical activity, tobacco use, prior
pain and gender.
Ethics
The Regional Committee for Medical Research Ethics
and the Norwegian Data Inspectorate approved the study.
Written consent was obtained from the participants at the
establishment of the original cohort in 2002. In addition, a
written parental consent was obtained for the participants
in the cohort who were <18 years in 2002.
Results
Pain and muscle activity at baseline
Of the 40 participants, 17 (42.5%) reported neck and
shoulder pain at baseline and, of these, 4 (10%) reported
long lasting or severe pain. No signicant difference was
found between gender and reported neck and shoulder
pain (P=0.07). Differences in pain between the four
occupational groups were found (P=0.03). Hairdressers
had the highest median pain of 1 (range 0–8). In all the
other occupational groups, more than half reported no
neck or shoulder pain (median 0, not shown in table).
Differences in muscle activity between the occupational
groups were also found (P=0.001). Hairdressers had
a median of 52% of the working day with sustained
muscle activity (range 24–91%), electricians 33% (range
17–82%), various jobs 27% (range 6–42%) and students
10% (range 2–49%).
The cross-sectional analyses showed differences
in neck and shoulder pain between the three muscle
activity groups (P<0.001). The group with high-level
muscle activity reported the highest median pain com-
pared to the low and moderate level group. The rela-
tive time of sustained muscle activity (0–100%) also
showed a significant cross-sectional correlation with
pain (Spearman`s correlation: 0.21, P<0.001). No

Scand J Work Environ Health – online first 5
Hanvold et al
significant differences were found between the three
groups and other background variables at baseline (see
table 2a/b).
Prior pain and muscle activity
Of the 40 participants, 23 (57.5%) had prior neck and
shoulder pain (reported in the year prior to baseline) and,
of these, 15 (65%) also reported neck and shoulder pain
at baseline, giving a signicant correlation between prior
pain and pain reported at baseline (Spearman`s correla-
tion: 0.58, P<0.001). Analyses on reverse relationship
showed a non-signicant correlation between relative
time of sustained muscle activity (0–100%) and prior
pain (Spearman’s correlation: 0.03, P=0.37).
Pain over time
Neck and shoulder pain was found to be fairly stable
over time on a group level, showing no signicant effect
of time (P=0.68). The prevalence of neck and shoulder
pain varied 26–56% during the 2.5-year period. In the
same period, between 0–17.5% reported long-lasting
or severe neck and shoulder pain. Even if the between-
subject neck and shoulder pain was found stable over
time on a group level, a large within-subject pain vari-
ability was found over the same period. Figure 1 illus-
trates this within-subject pain variability, distributed
according to the relative time with sustained trapezius
muscle activity.
Work-related exposure over time
The working status, self-reported mechanical workload
and control over own work intensity were followed for
the whole observation period. The participants working
status was rather stable over the study period. After six
months, two participants changed status (one electri-
cian became unemployed and one subject working in
kindergarten started to study). After one and two years,
ve participants changed working status (one of the
electrician and one of the subjects in other profession
started to study, one hairdresser changed occupation
and two students started to work). A small but insigni-
cant increase in the self-reported mechanical workload
score was found over the 2.5-year period [mean score
Q1=10.8, Q4=11.2, Q7=11.3 (P= 0.65)]. Control over
work intensity showed a slight, but signicant, increase
during the observation period [mean score Q1=2.0,
Q4=2.5, Q7=2.5 (P= 0.04)].
Pain and muscle activity: longitudinal analyses
Unadjusted analyses. The participants with a high level
of sustained muscle activity had a rate of neck and
shoulder pain twice that of participants with a low level
of sustained muscle activity in the long-term effect-
analysis (2.5-year follow-up period). The group with a
moderate level of sustained muscle activity showed a
tendency to have a higher rate of neck and shoulder pain
compared to the low-level group, but the results were
not statistically signicant (table 3).
Figure 2 shows the difference between the three
muscle activity groups and neck and shoulder pain
among men and women over the period. It also illus-
trates that the association between the levels of sustained
muscle activity and neck pain was stronger at the same
time and shortly after the muscle activity measurement
was done, further decreasing over time.
Adjusted analyses. In the multivariate analyses on the
2.5-year follow-up, a signicant association between
high level of muscle activity and neck and shoulder
pain was found for the whole group after adjusting for
time, mechanical workload, control over work intensity,
physical activity, tobacco use, prior pain, and gender.
Analyses on the long-term effect stratied by gender
showed that male participants with a high level of sus-
tained muscle activity had a rate of neck and shoulder
pain 6-times higher than participants with a low level
of activity. A tendency towards an association between
high sustained trapezius muscle activity and occurrence
of pain was also seen among female participants (table
3). When analyzing the effect over a 2-year follow-up
period (Q2–7), excluding the baseline pain measurement
and adjusting for prior pain in the multivariate analyses,
the results still showed a signicant difference between
the high- and low-level muscle activity group (IRR=2.53
95% CI 1.26–5.07, P<0.001). When adjusting for pain
at baseline instead of prior pain in the multivariate
analyses, however, this association disappeared in the
whole group (IRR=1.49 95% CI 0.79–2.83, P=0.21),
stratifying by gender gave a tendency of effect for men
but not women. Short-term effect analyses of the 0.5
year follow-up (Q1–2), showed a signicant difference
between the high and low muscle activity groups for
both men (IRR=9.57 95% CI 1.88–48.68, P<0.01) and
women (IRR=7.82 95% CI 2.67–22.92, P<0.001).
In all the analyses, the minimum duration of sustained
muscle activity was set at 4 minutes. Using a minimum
duration of 2, 6, 8, and 10 minutes gave similar results.
However, choosing a minimum duration of 15 minutes
or including all episodes with elevated muscle activity
regardless of the length of the episodes (>1.6 seconds)
did not result in signicant associations with neck and
shoulder pain. All the analyses were done using sustained
muscle activity divided into three groups. Small changes
in the cut-off points did not change the results in a substan-
tial way when dividing the three groups. The results were
not substantially altered either by dichotomizing the vari-

6 Scand J Work Environ Health – online first
Sustained trapezius muscle activity and neck and shoulder pain
able in a low (0–39%) and high group (40–100%) or by
using the continuous variable (0–100%) (IRR=1.01 95%
CI 1.00–1.03, P=0.01). If the EMG measure static muscle
activity was used in the analysis, no signicant associa-
tion with pain was found (IRR=1.36 95% CI 0.62–3.02,
P=0.44). Muscle rest showed borderline signicant asso-
ciation with reduced neck and shoulder pain (IRR=0.97
95% CI 0.95–0.99, P=0.05). Static muscle activity and
muscle rest were both found to be highly correlated to the
sustained muscle activity measure (Spearman`s correlation
coefcient: 0.82 and -0.79, respectively).
Discussion
Measured on a single working day, trapezius muscle
activity pattern, with sustained activity for >4-minute
episodes, was associated with neck and shoulder pain
the following 2.5 years. Participants with a high level
of sustained muscle activity (50–100%) during their
working day had a rate of neck and shoulder pain during
a 2.5-year period 3-times higher than participants with
low sustained muscle activity (<30%). When stratied
Table 2. Participant characteristics at the time of the EMG recording (Q1), divided into three levels of sustained trapezius muscle activity
(low, moderate and high).
Variables Sustained trapezius muscle activity
Relative time of a full working day (%)
All
N=40
Low (0–29)
N=14
Moderate (30–49)
N=13
High (50–100)
N=13
N % Median Range N % Median Range N % Median Range N % Median Range
Gender a
Men 17 42 8 57 5 38 4 31
Women 23 58 6 43 8 62 9 69
Working status n b
Electrician 14 35 6 43 4 31 4 31
Hairdresser 15 38 1 7 5 38 9 69
Various work 6 15 3 21 3 23 0 0
Students 5 12 4 29 1 8 0 0
Socio-economical background a
Low 8 20 1 7 3 23 4 33
Medium/high 31 80 13 93 10 77 8 67
Parental origin a
Western countries 38 95 14 100 13 93 13 93
Non-western countries 2 5 0 7 1 7 1 7
General health b
Bad/moderate 6 15 3 21 3 22 0 0
Good/very good 34 85 11 79 10 77 13 100
Tobacco use b
Yes 14 35 2 14 6 46 6 46
No 26 65 12 86 7 54 7 54
Age b (years) 22 21–25 22 21–22 22 21–25 22 21–24
Height a (cm) 170 152–193 173 164–185 174 152–183 169 155–193
Perceived stress b, c 1 0–2 1 0–1 1 0–2 1 0–2
Physical activity b, d 3 0–6 4 1–6 3 1–5 3 0–5
Control over work intensity b, e 2.5 0–4 2.5 1.5–4.0 2.5 0.0–3.5 2.0 0.5–4.0
Mechanical workload b, f 11 1–19 10 1–8 11 3–19 11 5–18
Neck/shoulder pain b, g 0 0–8 0 0–1 0 0–6 1 0–8
Prior neck/shoulder pain h, i 0 0–6 0 0–4 0 0–6 0 0–3
Sustained muscle activity b, j (%) 39 2–91 20 2–29 40 33–49 63 50–91
Muscle rest b, k (%) 16 3–60 24 13–60 16 10–26 9 3–20
Static muscle activity b, l (% EMGmax) 0.3 0.5–2.3 0.1 0.05–0.4 0.3 0.1–0.5 0.5 0.2–2.3
a Data taken from fall of 2002.
b Data from Q1 (2006/7).
c Perceived stress (never=0 to very often=4).
d Physical activity (never=0 to everyday=6).
e Control over work intensity (never=0 to very often=4).
f Mechanical workload (nothing=0 to great deal=24).
g Neck and shoulder pain (no pain=0 to severe pain=12 for 15–28 days in last 4 weeks).
h Data from three questionnaires prior to Q1.
i Mean of three neck/shoulder pain (0–12).
j The relative time (0–100% of a working day) with muscle activity >0.5% EMGmax lasting >4 minutes.
k The relative time (0–100% of a working day) with muscle activity <0.5% of EMGmax.
l The10th percentile of the amplitude distribution.

Scand J Work Environ Health – online first 7
Hanvold et al
by gender, this association was signicant among men,
and a tendency was also seen among women. The asso-
ciation between sustained muscle activity and pain were
strongest at the same time and shortly after the EMG
measurement among both genders.
The strengths of this study were the use of longitu-
dinal design, frequent reports of neck and shoulder pain,
and an objective measure of vocational muscle activity.
This improved the internal validity and limited the pos-
sibility of differential misclassication. The longitudi-
nal design increased the number of total observations
[7 observations per subject (40)=280 observations],
which improved the statistical power in spite of a small
number of subjects. The small number of subjects may
nevertheless increase the uncertainty in the estimates,
ie, when analyzing the 0.5-year follow-up. The partici-
pants constituted a subsample of a cohort of 420 young
adults and analysis showed no signicant differences in
the relevant variables between the subsample and the
rest of the cohort. Thus, the participants seemed to be
representative of the cohort.
Measuring muscle activity on at least two full work-
ing days has been proposed as optimal (37, 38), but we
were only able to achieve one recording, decreasing
the reliability of this exposure measurement. The work
tasks of hairdressers and electricians, however, can be
considered as being rather similar from day to day, and
how they work is likely to be largely habitual. Only two
of the subjects changed their vocational status during
the rst six months. Repeated recordings of vocational
muscle activity during a one-year period (39) and several
recordings during one week (40) have also shown a stable
Figure 1. Individual
reports of neck and
shoulder pain related
to sustained muscle
activity over a 2.5-year
period [questionnaires
1–7 (Q1–7)] indicat-
ing median, minimum
and maximum values.
The sustained trapezius
muscle activity is meas-
ured at Q1. Gender and
occupational groups are
also illustrated.
Table 3. The univariate and multivariate generalized estimating equations (GEE) analyses a of the association between neck and shoulder
pain and sustained trapezius muscle activity (Q1–7, 2.5 years). [IRR=incidence rate ratio; 95% CI=95% confidence interval]
Neck and shoulder pain All (N=40) Men (N=17) Women (N=23)
N observations=280 N observations=119 N observations=161
IRR 95% CI P-value IRR 95% CI P-value IRR 95% CI P-value
Unadjusted analyses
Sustained muscle activity
Low level 1.00 1.00 1.00
Moderate level 1.32 0.56–3.12 0.53 2.05 0.48–8.82 0.33 0.87 0.36–2.07 0.75
High level 2.64 1.28–5.44 <0.01 3.93 1.18–13.06 0.02 1.94 0.80–4.72 0.14
Adjusted analyses
Sustained muscle activity
Low level 1.00 1.00 1.00
Moderate level 1.67 0.75–3.72 0.21 2.59 0.93–7.15 0.07 1.18 0.54–2.63 0.67
High level 2.89 1.45–5.79 <0.01 6.49 1.91–22.07 <0.01 1.95 0.93–3.66 0.08
a The multivariate analyses are adjusted for time, prior neck and shoulder pain, self-reported mechanical workload, control over work intensity, tobacco
use and physical activity during leisure time. In addition, adjustments for gender were done in the analyses of all subjects (N=40).
Low level (0-29%) Moderate level (30-49%) High level (50-100%)
(N=14) (N=13) (N=13)
Relative time with sustained trapezius muscle activity with duration of >4min (0-100%)
020 40 60 80 100
Neck and shoulder pain 0-12
0
2
4
6
8
10
12
Individual neck and shoulder pain reports Q1-7 related to sustained muscle activity (N=40)
Figure 1. Individual reports of neck and shoulder pain over a 2 ½ year period (Q1-7)
indicating median, min and max values. The sustained trapezius muscle activity is
measured at Q1. Gender and occupational groups are also illustrated.
● Electrician (M)
♦ Various jobs (M)
○ Hairdresser (F)
◊ Various jobs (F)
□ Student (F)

8 Scand J Work Environ Health – online first
Sustained trapezius muscle activity and neck and shoulder pain
within-subject motor pattern when evaluating a specic
work situation. This makes the use of muscle activity
measurements from a single working day reasonable as
representative for the individual exposure estimate.
Large within-subject variability in neck and shoul-
der pain was found while the pain on a group level was
rather stable. These ndings are consistent with a study
of healthy service-sector workers followed with monthly
complaint reports over a 3-year period (26) and among
female industrial workers (39). This uctuation in indi-
vidual pain over time shows the importance of using
a longitudinal design with frequent follow-ups when
evaluating pain association.
The relative time of sustained muscle activity was
divided in three equally sized groups (low, moderate,
and high) to evaluate a possible dose–response effect.
The limits for these three groups were determined by
the distribution within the sample and not biological
arguments. Different cut-offs or the use of a continuous
exposure variable showed similar results, indicating a
robust measure.
In our analyses, the length of muscle activity was
set at >4 minutes, however using 2, 6, 8, and 10 minutes
as the minimum duration of sustained muscle activity
gave similar results. The lack of signicant associa-
tion between duration of sustained muscle activity >15
minutes and pain was explained by the low statistical
power when only a few recordings included those long
episodes. In the study of Østensvik et al (23), the risk
of later neck pain was signicantly increased for sub-
jects with sustained muscle activity episodes lasting >8
minutes. This was partly in agreement with our data, but
our results suggest that an association to pain may be
relevant also for durations shorter than 8 minutes (ie, >2
minutes). Østensvik et al (23) also found reduced risk
for future neck pain among the subjects with sustained
muscle activity episodes of much shorter duration (ie,
<2 minutes). In accordance with these ndings, our
results show the total relative time with sustained activ-
ity including all episode durations (an almost recipro-
cal parameter to muscle rest) has no association with
pain. This may be explained by the fact that the number
of short episodes of muscle activity (and rest) lasting
<2 minutes vastly outnumbers those of longer dura-
tion. This shows that the analysis of sustained muscle
activity may include an evaluation of both very short
episodes with activity/rest, constituting a benecial
muscle activity pattern, and longer episodes that pos-
sibly have the opposite effect. The pattern of these short
and long episodes of activity may therefore be important
determinants of conditions in the muscle related to pain
development.
Sustained muscle activity has been hypothesized to
be a mechanism for pain development in low-level force-
demanding work (9, 41, 42) with focus on the possible
predictive feature of the temporal aspect of activity and
rest. The proportion of time with muscle rest (43, 44) and
the total relative time with episodes of sustained muscle
activity (45, 46) both capture the temporal aspect of mus-
cle activity. These methods, however, produce a descrip-
tion of the average time with sustained muscle activity
while disregarding the duration of activity episodes. To
our knowledge there has only been one earlier study
incorporating the duration of sustained muscle activity
episodes (23). This study showed that sustained muscle
activity of >8 minutes duration could be an important
risk factor for neck pain, which is in agreement with our
ndings. For reasons stated in the Methods section, we
chose a duration limit of >4 minutes in our main analysis.
However, analyzing our data with a duration limit of >8
0 1 2 3
Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q1 Q2 Q3 Q4 Q5 Q6 Q7
men women
Low level (0-29%) M oderate level ( 30-49%)
High leve l (50-100%)
Neck and shoulder pain index (0-12)
time
Graphs by gender
Figure 2. Neck and shoulder pain development
divided into three groups depending on level of
sustained trapezius muscle activity by gender [men
(N=17) and women (N=23)]. Univariate associations
over a 2.5-year period [questionnaires 1–7 (Q1–7)].

Scand J Work Environ Health – online first 9
Hanvold et al
minutes would not haved changed our conclusions.
An increased rate of neck and shoulder pain among
subjects with a high level of sustained muscle activity was
found both with the use of pain at baseline, prior pain,
and longitudinal analysis adjusting for prior pain. This
was consistent with other cross-sectional and longitudinal
studies, which showed that workers with or contracting
neck and shoulder pain had lower frequency of interrup-
tions of sustained muscular activity (47), less muscle rest
(43, 44), higher static activity level (48), or more bursts
of muscle activity (46), compared to healthy controls.
Our results are in contrast to ndings from other
studies showing no difference in EMG activity patterns
between workers with and without pain (14–16). These
discrepancies may be partly explained by methodological
differences. Recording time differences, the use of labora-
tory recordings with work simulation versus eld regis-
trations, and differences in EMG data reduction methods
made it difcult to compare our ndings with earlier
studies (14–16). To facilitate a comparison, an analysis
of static muscle activity was conducted and showed no
association with neck and shoulder pain. This is also in
accordance with ndings in an earlier study (49). Static
muscle activity is one of the EMG measures that has been
extensively studied. However, it disregards the temporal
pattern of the activity, and it has been debated that the
lack of association with neck pain may be due to the
importance of the temporal aspect of the exposure (50).
The results showing that the pain evaluated during
the last four weeks at baseline was related to sustained
muscle activity at the same time-point, indicate that pain
and sustained muscle activity were reciprocally linked,
as explained by Travell (51) and later rened as the
“vicious cycle hypothesis” (52). This hypothesis does
not seem to explain the initiation of pain, but merely the
sustainment of the problem. On the basis of this study it
was, however, not possible to determine which comes
rst or where the circle begins. When adjusting for prior
pain in the multivariate analysis, signicant associations
suggest that there is an independent effect of sustained
muscle activity on later neck and shoulder pain. The
results were somewhat altered when adjusting for pain
at baseline instead of prior pain in the multivariate
analyses where the association disappeared in the whole
group. This nding and the strong association with neck
and shoulder pain in the cross-sectional and short-term
analysis (0.5 year follow-up) indicates that a time-lag of
≤6 months from exposure to effect may be reasonable.
Our study showed gender differences in the associa-
tion between muscle activity and pain. Among the male
participants, we found a signicantly higher rate of neck
and shoulder pain for the group with a high level of
sustained muscle activity. For women, there was only a
tendency towards association between sustained muscle
activity and pain. Some claim womens’ higher vulner-
ability explains gender differences in pain reports (53),
but a recent longitudinal study supports our ndings,
reporting that men had a higher risk of musculoskeletal
symptoms than women after equal exposure (54). The
women in our study also had a higher pain level from
the start, giving them less room for a pain increase com-
pared to the men. It is however important to be aware
that sustained trapezius muscle activity may be inu-
enced by individual muscular tension or postural motor
habits as well as the external mechanical workload. The
gender difference may also be explained by the occu-
pational differences between the genders, since most
of the females were hairdressers and most of the male
participants were electricians. Analyzing hairdressers
and electricians separately also showed the same result,
indicating that the gender difference could be partly
explained by the occupational status. However, because
of the co-linearity, it was hard to draw conclusions on
both gender and occupation.
In conclusion the results strengthen the hypothesis
that sustained trapezius muscle activity is associated
with neck and shoulder pain. With considerations to the
methodological limitations, our study implicates that
sustained muscle activity episodes lasting >4 minutes for
more than half of the working day increases the rate of
neck and shoulder pain. This association was strongest
when analyzing cross-sectional and short-term effects.
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Received for publication: 23 October 2012