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PHYSICAL WORKLOAD AS A RISK FACTOR FOR SYMPTOMS IN
THE NECK AND UPPER LIMBS: EXPOSURE ASSESSMENT AND ERGONOMIC INTERVENTION*
*Doctoral
dissertation presented on Friday 12th September 2003
at the the Faculty of Medicine of the University of Kuopio, Finland,
by permission of the Faculty of Medicine of the University of Kuopio,
Finnish Institute of Occupational Health, Helsinki, Finland.
|
Department of Physiology, University of Kuopio, Kuopio
Department of Physiology, Finnish Institute of Occupational Health, Finland
| Published
(Online) |
|
01
June 2004 |
©
Journal of Sports Science and Medicine (2004) 3, Suppl.5,
1 - 46
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This
review is based on the following orginal publications, which will be referred
to in the text as Studies 1-4:
1.
Korhonen, T., Ketola, R., Toivonen, R., Luukkonen, R., Häkkänen, M. and
Viikari-Juntura, E. (2003) Work-related and individual predictors for
incident neck pain among office employees working with video display unit
work. Occupational and Environmental Medicine 60, 1-8.
2.
Ketola, R., Toivonen, R., Häkkänen, M., Luukkonen, R., Takala, E-P. and
Viikari-Juntura, E. (2002) Effect of ergonomic intervention in work with
video display units. Scandinavian Journal of Work, Environment &
Health 28, 18-24.
3.
Ketola, R., Toivonen, R., Luukkonen, R., Takala, E-P. and Viikari-Juntura,
E. (2003) Expert assessment of ergonomics at video display unit (VDU)
workstation: repeatability, validity and responsiveness to changes Internetional
Archives of Occupational and Environmental Health. Submitted.
4.
Ketola, R., Toivonen, R. and Viikari-Juntura, E. (2001) Interobserver
repeatability and validity of an observation method to assess physical
loads imposed on the upper extremities. Ergonomics 44, 119-131.
| ABSTRACT |
|
The
aims of this study were to investigate work related and individual
factors as predictors of insident neck pain among video display
unit (VDU) workers, to assess the effects of an ergonomic intervention
and education on musculoskeletal symptoms, and to study the repeatability
and validity of an expert assessment method of VDU workstation ergonomics.
A method to assess the risk factors for upper limb disorders was
developed, and its validity and repeatability were studied.
The annual incidence of neck pain was 34.4%. A poor physical work
environment and placement of the keyboard were work-related factors
increasing the risk of neck pain. Among the individual factors,
female sex was a strong predictor.
The randomized intervention study included questionnaire survey,
a diary of discomfort, and ergonomic rating of the workstations.
The subjects (n=124) were allocated into three groups. The intensive
and the education groups had less musculoskeletal discomfort than
the control group at the 2-month follow-up. After the intervention,
the level of ergonomics was distinctly higher in the intensive ergonomic
group than in the education or control group.
Two experts in ergonomics analyzed and rated the ergonomics of workstations
before and after intervention. The validity of the assessment method
was rated against the technical measurements, assessment of tidiness
and space, and work chair ergonomics. The intraclass correlation
coefficient between ratings of the two experts was 0.74. Changes
in the location of the input devises and the screen, as well as
the values of tidiness and space and work chair ergonomics showed
a significant association with the ratings of both experts.
The method to assess the loads imposed on the upper limbs was validated
against the expert observations from the video, continuous recordings
of myoelectric activity of forearm muscles, and wrist posture, measured
with goniometers. Inter-observer repeatability and validity were
good or moderate.
Both intensive ergonomics approach and education in ergonomics have
effects in reducing discomfort in VDU work. In attemps to improve
the ergonomics of VDU workstation, the best result will be achieved
with cooperative palaning in which both workers and pratitioners
are actively invoved. The assessment methods for VDU work ergonomics
and upper limb load studied here can be utilized in a repeatable
manner.
KEY
WORDS: Human engineering, computer terminals, neck pain, upper
extremity, workload, risk factors, risk assessment, randomized controlled
trials, observation, video recording.
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| INTRODUCTION |
|
In
the European Union 45% of the workforce used computers for more
than a quater of their work time in 2000 (European Commission, 2001).
And since then, the proportion of computer users has increased enormously.
The increasing hours of computer use, together with poor work-rest
cycle control, have been associated with musculoskeletal discomfort
in the neck-shoulder area and upper limbs, especially in the use
of input devices, such as a keyboard or a mouse (Punnett and Bergqvist,
1997). In addition,
the use of graphics software and non-keyboard input devices, e.g.
the mouse, has increased rapidly, causing new demands for the design
of office work places. Computer use in sustained non-neutral postures,
such as neck rotation and shoulder abduction, has been identified
as a risk factor for neck-shoulder symptoms. Postural stress caused
by poor workstation ergonomics, such as inappropriate location of
the screen, keyboard or mouse, have been associated with musculoskeletal
problems (Bergqvist et al., 1995b;
Karlqvist et al., 1998;
Tittiranonda et al., 1999a).
However, the evidence of risk factors is based mainly on cross-sectional
studies.
Redesign, improvements in ergonomics, and educating the users, have
generally been recommended as solutions for the prevention of musculoskeletal
disorders in video display unit (VDU) work (Moon and Sauter, 1996).
A limited number of well-designed intervention studies with control
groups has been published to evaluate the effectiveness of ergonomic
interventions in the office environment (Westgaard and Winkel, 1997).
There is some evidence that keyboard and mouse users may experience
a reduction in upper limb and neck pain when using certain alternative
keyboards or types of mouse compared to the conventional ones (Punnett
and Bergqvist, 1997).
A training program in ergonomics, workstation adjustment, and frequent
breaks at VDU work have been shown to decrease the prevalence of
musculoskeletal disorders and discomfort (Bayeh and Smith, 1999;
Brisson et al., 1999;
Mekhora and Liston, 2000;
Aarås et al., 2001a).
The ergonomics in VDU work is determined by several factors, e.g.
layout and dimensions of the workstation as well as anthropometrics
and the personal preferences of the worker (Gerr, 2000).
The ergonomics can be estimated by using technical measurements
of the workstation and of work postures (Aarås et al., 1997;
Karlqvist, 1997;
Burgess-Limerick et al., 1999).
However, technical measurements are often time-consuming and may
not be feasible for assessing workstations for the evaluation of
an extensive ergonomic intervention. One option is to resort to
expert assessment for the evaluation. The trained eye of an expert
can quickly merge several variables, e.g. layout factors, the dimensions
of the workstation, and characteristics of the work posture, to
arrive at an overall ergonomic assessment based on a brief observation
(Gerr et al., 2000;
Moffet et al., 2002).
An expert rating is also possible without any technical equipment.
Even though an expert assessment of VDU ergonomics is a commonly
used method, not much is known about its repeatability and validity.
The high number of repetitive strain injuries in some occupations
reflects the need to identify the risk factors of these disorders.
Several methods have been developed for this purpose. The use of
checklists allows rapid screening of various physical load factors.
Other possibilities are observation at regular intervals, and continuous
observation (Keyserling et al., 1993;
Fransson-Hall et al., 1995;
Karhu et al., 1977).
Work cycles reported or presumed to be stressful are usually selected
for screening by a checklist, and the ensuing result reveals the
presence or absence of the selected physical load factors at predetermined
levels. In order to obtain a complete view of e.g. upper limb load
in various jobs, all work cycles should be identified and observed.
The objectives of the present study were to investigate risk factors
for neck pain among VDU workers, to assess the effects of an ergonomic
intervention on the level of musculoskeletal symptoms, and to study
the repeatability and validity of an assessment method of VDU ergonomics.
Furthermore, a method to assess the risk factors for upper limb
disorders was developed and its validity and repeatability were
investigated.
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| REVIEW
OF THE LITERATURE |
Risk factors for work-related musculoskeletal disorders
Work-related diseases may be caused, aggravated, accelerated, or exacerbated
by workplace exposures, and they may impair working capacity. 'Work-related
musculoskeletal disorders' are defined as disorders and diseases of
the musculoskeletal system, which have been proven or assumed to have
at least a partly work-related background (WHO, 1985).
These disorders constitute a heterogeneous group, but it is still
possible to identify some generic work risk factors, e.g. repetitive
or force-demanding tasks, awkward postures, features of workplace
design, cognitive demands, as well as organizational arrangements
and psychosocial factors (Bernard, 1997;
Salvendy, 1997).
Several conceptual models have been proposed; they describe relationships
between workload factors and musculoskeletal disorders (Winkel and
Westgaard, 1992;
Armstrong et al., 1993;
Panel on Musculoskeletal Disorders and Workplace, 2001).
According to the model generated by an expert group on work-related
musculoskeletal disorders, the load factors are organized into two
broad categories: workplace factors and personal characteristics (Figure
1).
Workplace factors include external physical loads associated with
job performance, as well as organizational and social factors. External
loading resulting from the work is transmitted through biomechanical
forces imposed on the limbs and trunk, and creates internal loading
on the body tissues and anatomical structures. Relevant biomechanical
factors include body posture, physical exertion, and movements. Biomechanical
loading is affected by individual factors such as anthropometry, strength
and skills, transmitting external loads to internal loads on anatomical
structures. When the load exceeds the mechanical tolerance or the
ability of the structure to withstand the load, tissue damage occurs.
The resulting pain, discomfort, impairment, and disability arise from
of the interaction of the workplace factors and the physical and psychological
characteristics of the individual. Organizational and social factors
at work may affect the external demands of the work and the individual's
response to the demands. The impact of the organizational and social
factors on the individual is mediated through cognitive and perceptual
mechanisms. These mechanisms vary from one individual to another (Panel
on musculoskeletal disorders and workplace, 2001).
In this thesis 'work load' has been used as a general term to describe
workplace factors that are independent of the worker (e.g. duration
of work tasks, workstation dimensions) and external exposure at work
(e.g. determinants of work posture, weights of objects). The expression
'risk factor' is used as a general term for those factors at work,
which are associated with an increased risk of musculoskeletal disorders.
These risk factors have in most cases been first observed empirically
and then confirmed by epidemiological studies.
Neck and shoulder
This literature review focuses separately on the risk factors of neck
and shoulder disorders, even though many studies focus on both areas
simultaneously. In addition, the dividing line between the neck and
shoulder area is ambiguous (Kuorinka et al., 1987;
Sluiter et al., 2001).
In Finland the 1-month prevalence of neck pain in persons aged over
30 years was 26% for men and 40% for women in a recent countrywide
population study, 'Health 2000'(Aromaa and Koskinen, 2002).
The prevalence rates of neck and shoulder pain among the workforce
(aged 25-64 years) have been 50% for men and 64% for women (Piirainen
et al., 2000).
These percentages are based on the self-reports of the persons studied,
and are without exception higher than those of clinical entities in
studies with outcomes based both on symptoms and physical examinations
(Andersen et al., 2002;
Aromaa and Koskinen, 2002).
Neck
Neck disorders are multifactorial. Several work-related factors have
an influence on their development. In the NIOSH (National Institute
of Occupational Safety and Health in the USA) review (Bernard, 1997),
it was concluded that repetitive upper limb motion and repeated neck
movements, forceful upper limb movements involving the same muscle
groups, static loading of the neck-shoulder muscles, and extreme neck
postures at work are related to neck disorders. In their comprehensive
review, Ariens et al., (2000)
reported a relationship between neck pain and neck flexion, arm force,
abducted arm posture, duration of a fixed, sedentary sitting posture,
twisting or bending of the trunk, hand-arm vibration, and ergonomic
design of the work place. In a later prospective study, Ariens et
al., found that sitting at work for more than 95% of the working time
is a risk factor for neck pain. They found also a trend for a positive
relationship between neck flexion and neck pain (Ariens et al., 2001a).
A longitudinal study by Viikari-Juntura et al., (2001)
indicated that duration of work with a hand above shoulder level was
associated with radiating neck pain.
Besides the physical job characteristics mentioned above, a relationship
has been found between neck pain and high job demands (Andersen et
al., 2002), low co-worker
support and job control, high and low skill discretion, and low job
satisfaction (Ariens et al., 2001b).
Limited possibilities to influence one's personal work situation,
low support from supervisors, and psychosocial distress have also
been found to predict neck pain in follow-up studies (Eriksen et al.,
1999; Leclerc et
al., 1999; Kaergaard
and Andersen, 2000).
Of the individual characteristics, age has been shown to predict frequent
neck pain (Leclerc et al., 1999;
Viikari-Juntura et al., 2001).
Of the health behavioral factors, smoking has been found to be a risk
factor for neck pain (Kaergaard and Andersen, 2000),
whereas evidence of the effects of physical exercise has been inconsistent
(Hildebrandt et al., 2000;
Miranda et al., 2001a).
In several studies, women have had a higher prevalence of neck disorders
(Bernard, 1997;
Leclerc et al., 1999).
This gender difference may result from the different types of jobs
of women and men. On the other hand, even when the job is the same,
men and women may perform the work in a different way. This may be
due to differences in anthropometry and the ability to generate force
(Viikari-Juntura et al., 2001).
Moreover, women may cumulate risk factors related to working conditions
and household work.
Shoulder
In the studies reviewed by van der Windt et al., (2000),
the 12-month prevalence of shoulder pain varied from 6- 40% in different
working populations. In the Mini-Finland Survey consisting of 7,217
adults (aged ≥30 years), 30% reported having had shoulder pain
during the previous month (Mäkelä et al., 1999).
In the review of van der Windt (2000)
as well as in a prospective study, it was found that risk factors
for shoulder pain were related to heavy physical workload, awkward
postures, and long work experience. According to van der Windt et
al., and a follow-up study by Cassou et al., (2002)
also repetitive movements, the use of vibrating tools, and duration
of employment were associated with of shoulder pain. In a case-referent
study, a relationship has also been found between shoulder disorders
and severe upper arm flexion or abduction (>90 degrees). As the
number of work cycles with awkward postures (duration of severe flexion
or abduction is 10% or more of the work cycle) increased, also the
risk of shoulder disorders increased (Punnett et al., 2000).
Van der Windt et al., (2000)
argue that psychosocial factors are important in both the development
and persistence of shoulder problems. Job dissatisfaction, high psychosocial
demands and a poor social work environment, together with a poor personal
capacity to cope with these factors, may increase work-related stress.
Stress may cause a higher level of muscle tone and strengthen the
relation between physical load and shoulder symptoms.
Most studies have not shown any major gender difference in the prevalence
of shoulder pain, al though some discrepancy has been reported. In
a study among newspaper employees, the risk of shoulder pain was more
than twice as high for the women than for the men (Bernard et al.,
1994). Other individual
factors associated with shoulder pain are age and body mass index
(Miranda et al., 2001b).
Elbow, wrist and hand
Pain in the upper limbs is a problem in the industrialized countries
(Bernard, 1997;
Buckle, 1997; Riihimäki
and Heliövaara, 2002).
In Finland, the most common occupational disease group (for which
compensation is paid by an insurance company) is the repetitive strain
injury of the upper limb. A total of 1,488 cases were reported in
2001. The incidence rate was 6.3 cases per 10,000 employed workers.
The highest incidence rate was found in the food-processing industry,
where 94 cases per 10,000 employed workers were reported (Karjalainen
et al., 2002).
Physical risk factors that have been found to have an association
with upper limb disorders are high demands of force (Stetson et al.,
1993), repetitive
movements, non-neutral postures, cold temperature, and hand-arm vibration.
Especially combinations of these risk factors have been associated
with upper limb disorders (Silverstein et al., 1987b;
Punnett, 1998;
Muggleton et al., 1999;
Viikari-Juntura and Silverstein, 1999;
van der Windt et al., 2000).
The specific disorder that has been studied the most is the carpal
tunnel syndrome. Fewer studies have been carried out on epicondylitis,
wrist tendinitis, and hand-arm vibration syndrome.
Carpal tunnel syndrome
A combination of the risk factors (force and repetition, force and
posture) has been found in the reviews to be strongly associated with
carpal tunnel syndrome. There is also evidence that repetition and
force separately are related to the carpal tunnel syndrome (Bernard,
1997; Viikari-Juntura
and Silverstein, 1999;
Leclerc et al., 2001).
Also vibration has been associated with the carpal tunnel syndrome
even though the mechanism by which vibration contributes to the development
of the syndrome is not completely understood. Investigating the effect
of vibration alone is difficult, since it is usually associated with
the use of hand-held vibrating tools, the use of hand force, and non-neutral
postures (Hagberg, 2002).
It is also possible that a cold environment and local mechanical pressure
can increase the risk for the carpal tunnel syndrome. Individual factors
such as female gender, obesity, and older age have been found to increase
the risk for the syndrome (Viikari-Juntura and Silverstein, 1999).
Epicondylitis
Epicondylitis has been identified as a work-related disease in a number
of studies. It has been reported that the highest incidence of epicondylitis
occurs in occupations and jobs which are manually intensive and have
high work demands (e.g. meat-packing, construction work) (Kurppa et
al., 1991; Viikari-Juntura,
1995; Lewis et
al., 2002). According
to a longitudinal study by Leclerc et al., (2001)
there is an evidence of an association between forceful work and epicondylitis.
Also work tasks implying a combination of risk factors (force and
repetition, force and posture), especially at high exposure levels,
increase the risk for epicondylitis (Bernard, 1997).
The only individual factor that has been associated with epicondylitis
is age (Viikari-Juntura et al., 1991;
Leclerc et al., 2001).
Hand/wrist tendinitis
According to the literature review of Bernard et al., (1997)
there is an association between hand/wrist tendinitis and repetition,
force, and posture (each of the risk factors alone). And when they
occur in combination (e.g. highly repetitive and forceful hand/wrist
exertion), there is strong evidence of an association with hand/wrist
tendinitis. A cross-sectional study by Latko et al., (1999)
demonstrates a link between repetitive work and tendinitis (Latko
et al., 1999). Of
the individual factors, a higher risk of hand-wrist disorders has
been found among women and newly employed workers (Häkkänen et al.,
2001). The presence
of psychosomatic problems has also been shown to be a strong predictor
of wrist tendinitis (Leclerc et al., 2001).
Hand-arm vibration syndrome
People in occupations involving a high level of exposure to vibration
from tools are liable to the hand-arm vibration syndrome. The reviews
of studies on vibration have shown evidence of a clear association
between a high level of exposure to vibration and the hand-arm vibration
syndrome (Bernard, 1997;
Palmer et al., 2000).
In a follow-up study Kihlberg and Hagberg (1997)
found that low-frequency impact vibration was transmitted to the elbows
and shoulders and had an effect on those areas, whereas high-frequency
impact vibration transmitted to the hand and wrist may predominantly
cause symptoms there. Furthermore, Sakakibara and Yamada (1995)
showed that hand-arm vibration activates the sympathetic nervous system
and induces vasoconstriction in the feet even though they are not
directly exposed to vibration. However, Hagberg (2002)
concluded in his review that even though there is strong evidence
that jobs with vibrating machines or tools are associated with musculoskeletal
disorders, there is not sufficient evidence that vibration per se
would be a risk factor for musculoskeletal disorders.
Reference values for physical load factors as risk factors for neck/shoulder,
and elbow/wrist/hand disorders based on recent reviews and some original
studies are presented in Table 1.
Local mechanical pressure
Whenever there is contact between the body and external objects, mechanical
stress on tissues should be considered. Local stress can cause injury
to both the skin and underlying structures, most commonly nerves,
bursae and blood vessels. Common areas that should be considered include
the hand, palm, wrist, elbow and armpit (Kuorinka and Forcier, 1992).
Psychosocial risk factors
In addition to physical risk factors at work, also psychosocial risk
factors have been shown to be determinants of upper limb disorders.
A cross-sectional study by Devereux et al., (2002)
showed that workers highly exposed to both physical and psychosocial
risk factors in their work were more likely to report upper limb symptoms
than workers highly exposed to only one or the other. According to
the literature review by Bongers et al., (2002)
high job demands and low job control, low decision latitude, and low
social support have been shown to be related to upper limb disorders.
Low back
The 1-month prevalence of low-back pain was about 36% among the women
and 30% among the men (persons aged ≥30 years) in the Health
2000 study (Aromaa and Koskinen, 2002).
Prevalence rates among the working population (aged 25-64 years) have
been nearly identical: 34% for women and 32% for men (Piirainen et
al., 2000).
There is strong evidence that work-related risk factors, namely lifting,
whole body vibration, heavy physical work, and bending or twisting
the back are associated with an increased risk for low-back pain.
Psychosocial factors and mental stress are related to low-back pain
and affect the reporting of back injuries. The evidence for individual
factors such as height, weight, smoking, physical fitness, trunk muscle
performance, and mobility is less consistent (McDonald, 2000)
with regard to non-specific low-back pain. However, there is strong
evidence for an association between height and sciatic pain (Heliövaara,
1987).
Video display unit (VDU) work as a risk factor for musculoskeletal
disorders
The most visible feature of changing work has been the enormous increase
of computerized work in the industrialized countries. Despite the
low level of physical load, a great number of computer users have
musculoskeletal problems, especially in the neck, shoulders, wrists,
and hands. Punnett and Bergqvist (1997)
reviewed 56 epidemiological studies published on VDU work before 1997.
Most of these studies were cross-sectional, but several trends were
detected between features of computer work and musculoskeletal problems
(Punnett and Bergqvist, 1997).
Time spent in VDU work
In VDU work, visual information is presented on a screen, and the
information is handled by manual input devices like the keyboard and
mouse. All the equipment is stable in the same position on the table,
and the worker is therefore required to keep the same static posture
while working. Concentrating on the work may prevent the worker from
becoming aware of early signals of fatigue (Aarås et al., 2000).
Insufficient recovery after local muscle fatigue is believed to be
essential in the genesis of muscular pain in static work (Sjögaard
et al., 2000).
The time spent with computers has been shown to be associated with
discomfort especially in the neck-shoulder area (Karlqvist et al.,
1996; Tittiranonda
et al., 1999a;
Blatter and Bongers, 2001;
Fredriksson et al., 2002).
In a prospective study Gerr et al., (2002)
showed that for over 50% of the study participants who used computers
for over 15 hours per week reported musculoskeletal symptoms in their
first year in a new job. Jensen et al., (1998;
2002) found that
workers, who used a computer almost all the time at work, reported
more repetitive movements than those who used it less. Jensen hypothesized
that the repeated hand movements when using the keyboard and mouse
could explain the association between the symptoms and time spent
in computer work.
Screen
In VDU work, the muscular activity of the neck and shoulders resists
the gravity acting on the forward flexed head while the worker views
the screen. The bones and joints of the upper limb have to be stabilized
by the muscles to enable exact movements of the fingers and hands.
If there is no mechanical support for the forearm, the shoulder muscles
must hold the weight of the whole upper limb, and this further increases
muscle tension (Takala, 2002).
Muscle tension increases even more if the worker performs the task
in a non-neutral posture. It has been shown that computer use in sustained
non-neutral neck or shoulder postures, such as rotated neck or the
abducted shoulder is a risk factor for neck pain (Karlqvist et al.,
1998; Tittiranonda
et al., 1999a).
It has also been shown that visual discomfort and musculoskeletal
strain, particularly in the neck and shoulders, are associated with
computer screen height (Bergqvist et al., 1995a;
Villanueva et al., 1997;
Burgess-Limerick et al., 1999;
Psihogios et al., 2001).
Computer ergonomics researchers have disputed on how the computer
screen should be placed in relation to the worker's eyes. A higher
monitor placement has been associated with strenuous neck extension
caused by visual demands (Burgess-Limerick et al., 1999).
On the other hand, an extreme low location is often associated with
musculoskeletal stress caused by neck flexion (Turville et al., 1998;
Fries Svensson and Svensson, 2001).
However, the benefit of a lower placement is reduced of eye irritation,
as the open surface of the eyes is smaller and lacrymation is better
(Sotoyama et al., 1996).
Finally, the results of a field study support the midlevel (~20° viewing
angle) placement of the screen (Psihogios et al., 2001).
Mouse and keyboard
Arm or hand disorders in VDU work have been reported to be less frequent
than neck or shoulder disorders (Gerr et al., 2002).
VDU operators have nevertheless been shown to have two to nine times
higher rates of hand/wrist disorders than would have been expected
if they had done industrial work with low physical exposure (Punnett
and Bergqvist, 1997).
In most cases, VDU work includes the use of both a mouse and a keyboard.
Although the use of the mouse has increased significantly during the
past decade, knowledge of the impact of mouse use on the musculoskeletal
system is limited. Some studies indicate increased musculoskeletal
symptoms in relation to the duration of mouse use (Fogleman and Brogmus,
1995). Contrary findings regarding the effects of the duration of
mouse use on symptoms have also been described, but these results
have been challenged (Cook et al., 2000).
A typical VDU work posture described during mouse use is that the
mouse is kept away from the midline of the body, the arm is unsupported,
and the shoulder abducted and outward rotated (Karlqvist et al., 1996;
Aarås et al., 1997).
Mouse users have also been reported to adopt working postures in which
the wrist is extended and in ulnar deviation (Aarås and Ro, 1997).
These non-neutral postures have earlier been shown to be harmful,
for example in industrial work, and they are presumed to be risk factors
also in VDU work (Malchaire et al., 1996).
There is still a lack of prospective studies on mouse use and upper
extremity disorders.
In a cross-sectional study Cook et al., (2000)
found a relationship between arm abduction in mouse use and neck symptoms.
Among CAD workers, the higher prevalence of shoulder, elbow, wrist
and hand pain has been found in the hand operating the mouse compared
to the contralateral side (Jensen et al., 1998).
The authors suggested that this result was probably due to mouse use
per se, even though a causal relationship can not be verified with
a cross-sectional study design. However, working with the hands and
forearms supported and in a nearly neutral position, when using a
mouse, decreased neck, shoulder, and arm pain (Aarås et al., 1997;
Lintula et al., 2001;
Marcus et al., 2002).
The use of input devices influences the activation of the arm and
hand muscles. A computer mouse and a keyboard demand different hand-eye
coordination. For a trained typist, keyboard use requires no hand-eye
coordination and, therefore, may result in a highly automated process.
The use of a computer mouse, in contrast, requires positioning of
the mouse, and controlling the relation between the mouse and the
cursor on the screen. The computer mouse use, therefore, requires
extensive hand-eye coordination and may thus be more difficult to
automate (Ferrel et al., 2001).
Increased muscular activity has been found in the neck during the
use of the mouse compared with the use of the keyboard (Laursen et
al., 2002). Laursen
et al., demonstrated also that mental stress factors increased the
activity of the neck muscles more during the use of the mouse than
during the use of the keyboard.
It has been shown that fast repetitive finger movements in VDU work
activate co-contraction in the neck and upper limb muscles. There
is also lack of variation in the activation of muscle motor units
in the work tasks with finger clicks (Kitahara et al., 2000).
It has therefore, been suggested that the worker should limit repetitive
movements in VDU work, especially when using a mouse. In order to
decrease repetitive movements with the mouse hand, workers are commonly
guided to switch the mouse to the other hand. Unfortunately, this
might not help because contralateral activity may occur in the muscles
(Birch et al., 2000).
Workers with disorders in their mouse hand should use the keyboard
more. To get more variation in muscle activation, a selection of input
devices (e.g. including possibilities for non-hand input alternatives)
should also be available (Aarås and Ro, 1997).
Keyboard operation inherently requires repetitive hand motion in order
to depress the keys. It has been shown that keying requires ulnar
deviation and extension of the wrist and forearm pronation. Use of
the keyboard can also increase intracarpal pressure, if the wrist
deviates sufficiently from a neutral position (Punnett and Bergqvist,
1997). Several
commercially available alternative keyboard designs have been tested
with mixed results. It has been found that the split-keyboard, open
keyboard and alternative geometric keyboard have a minimal impact
on comfort, self-reported fatigue and productivity (Swanson et al.,
1997; Tittiranonda
et al., 1999b;
Zecevic et al., 2000;
Simoneau and Marklin, 2001).
Hedge et al. (1999)
have examined the effects of a downward-tilting keyboard tray on wrist
posture, seated posture, and self-assessed musculoskeletal discomfort.
They found significant improvements in wrist posture, seated posture,
and upper body discomfort among persons using the downward-tilting
keyboard compared to the conventional keyboard.
The location of the keyboard on the table might be even more important
with regard to work posture than the keyboard model. Marcus et al.,
(2002) found that
a seated posture with the keyboard low and some distance away from
the worker is associated with a lower risk of neck-shoulder and upper
limb symptoms than a posture with the keyboard at or above elbow height
and close to the worker. Summary of physical risk factors for neck/shoulder
and elbow/wrist/hand symptoms in VDU work is shown in Table
2.
Psychosocial load factors
Organizational factors of the work, such as increased work pressure,
high work speed, and lack of job security or decision-making opportunities,
as well as low possibilities for development at work, may contribute
to an increased occurrence of work-related musculoskeletal complaints
in VDU work (Tittiranonda et al., 1999a;
Seppälä, 2001).
Assessment of physical work load
Selection of job sample for assessment
In order to obtain a complete view of the work load in various jobs,
all work tasks, subtasks and cycles should be identified (Figure
2). It is important for the prevention of musculoskeletal disorders,
that repetitive work is defined and quantified. However, also force
and posture need to be assessed. Moreover, the engaged body parts
and the duration of exposure should be specified (Kilbom, 1994c).
A common feature of the different types of assessment methods is that
their use is straightforward in repetitive or monotonous jobs in which
a limited number of short cycles are repeated throughout the workday.
In such cases an assessment can easily be done using a random sample
of the work cycles. Many industrial jobs belong to this category.
If a job is highly variable and consists of tasks and subtasks, cycles
and fundamental cycles with a wide variation of contents, frequency,
and duration (e.g. job of paper cutter in Figure
2), random sampling is often not a feasible method. In such work,
work load assessment could be done for each separate task and the
mean and cumulative load can be calculated when the frequency and
duration of the different tasks are known (Winkel and Mathiassen,
1994). However, sometimes assessment of peak loads is also relevant.
Work load assessment methods
General approaches to estimate physical work load factors include
the use of job titles, workers' self-reports, checklists, interviews
and observations by trained persons, and direct measurements by some
form of instrumentation. The optimal choice of method depends on the
level of specificity and accuracy required by the study, features
of the method, and the load factors of the jobs under study (Panel
on musculoskeletal disorders and workplace, 2001).
Job title
The job titles which have been used to describe work load in many
studies give only vague or inaccurate estimations of exposure (Winkel
and Mathiassen, 1994).
They may indicate homogenous exposure for some parameters, such as
repetitiveness and force demands, while other parameters, such as
posture, may vary widely among workers in the same job (Silverstein
et al., 1987a).
In addition, individual variation in exposure can be wide with the
same person at the same job at various time points (Balogh et al.,
1999). Using job
titles as an exposure indicator involves a risk of error, and should
not be used in studies where accurate exposure levels are needed.
Self-reports of workers
The self-reports of workers are useful alternatives for evaluating
physical loads. Either with spontaneous self-reports or by completing
questionnaires, diaries or checklists, workers may report the work
load factors in their job or work environment. Self-reports permit
assessment of exposures in the past and present, and may be structured
with task-specific questions or organized to cover the job as a whole.
Self-reported data can take various forms; they may include duration,
frequency, and intensity of exposure (Panel on musculoskeletal disorders
and workplace, 2001).
In some studies, self-reports have been well in accordance with the
results of observations or direct measurements of the corresponding
exposures (Torgén et al., 1999).
However, subjective assessments are prone to be influenced by other
factors than the task or workplace investigated. The validity and
repeatability of self-reported exposure may be too low in relation
to the needs of epidemiological studies and ergonomic interventions
(Wiktorin et al., 1993;
Hansson and Westerholm, 2001).
Checklists and qualitative approaches by an expert
A trained observer can use checklists or qualitative approaches and
make notes about work load based on direct or video-assisted observation.
The documentation and description can be done simply in terms of predetermined
activities by a catalogue of action, or by recording the postures
of the upper limbs or back. With the aid of checklists, a categorical
decision can be made for each factor (i.e. presence or absence of
a load factor). A large number of checklists and qualitative approaches
have been developed in the past decades (Stetson et al., 1991;
Kilbom, 1994a; Panel
on Musculoskeletal Disorders and Workplace, 2001).
These tools allow the rapid screening of various exposure factors.
Work cycles reported by the worker or presumed to be stressful by
the experts are usually selected for screening with a checklist. Checklists
and qualitative approaches are not likely to provide sufficiently
detailed information for an effective risk assessment of the musculoskeletal
disorders.
Systematic observation methods for measuring work load
Numerous observation methods have been described in the literature,
ranging from a work place walk-through to highly detailed methods
(Corlett et al., 1979;
Armstrong et al., 1982).
The technology used in observation methods ranges from paper and pen
to complicated computerized methods (Li and Buckle, 1999).
Observation can be done either directly on the work site, or afterwards
from the video, or video recordings can be used to assist the observation.
The most common observation techniques used to characterize repetition,
posture and force level are based on either time study (continuous
observation) or work sampling (observation at regular intervals) (Fransson-Hall
et al., 1995; Fransson-Hall
et al., 1996; Karhu,
1977). Both of these
techniques require a trained observer to characterize the ergonomic
factors, and they are very time-intensive and time-consuming.
Overall evaluation of work posture and ergonomics
Many of the existing methods for assessing work load factors are used
for research purposes. The methods are often so complicated that only
researchers or well-trained analysts are able to use them. The practitioners,
on the other hand, need a tool that is fast and easy to use. The tool
should be user-friendly and flexible to accommodate the numerous and
complex tasks that the practitioners may encounter. It is also known
that practitioners prefer to use descriptive words or single numbers
to describe the load rather than define e.g. specific angles in the
body or upper limb posture (Li and Buckle, 1999).
An exposure assessment method meant to be used by the practitioners
should be able to tell whether an ergonomic intervention is necessary
for the job. The assessment method should also function as an evaluation
tool for an implemented intervention. The future exposure assessment
methods will need to combine both the experts' views and the practitioners'
needs in order to enable the development of a method that is both
practical and valid for its purpose.
Direct measurements
Work tasks vary considerably by technology, type of physical load
and loaded body parts, and the distribution and duration of the loading
(Winkel and Mathiassen, 1994).
When one compares methods, the most accurate data on physical exposure
(load level, repetition, and load duration) are gained from direct
measurements (e.g. electromyography, goniometres, and biomechanical
measurements). The costs of applying these exact methods are high,
and the methods are often limited to specific body parts, and only
a small number of persons can be measured. In order to limit the costs
and to obtain a complete view of the physical loading, a combined
exposure data from questionnaires, interviews, expert assessment,
and observation methods may be required (Juul-Kristensen et al., 2001).
Assessment of upper extremity work load
In 1994 Kilbom published a guideline, based on a literature review,
to provide assistance in the primary and secondary prevention of upper
extremity disorders associated with repetitive work (Kilbom, 1994b;
1994c). Since their
publication, the guideline and the scientific background for the articles
have been frequently cited. The risk assessment models presented by
Kilbom have influenced numerous other guidelines. A quantification
of repetitiveness has been generally used as a first step in a risk
assessment approach. The definition for repetitiveness used by Silverstein
et al., (1986)
has been incorporated in the guideline. According to it, work can
be considered to be repetitive when the cycle time is less than 30
seconds, or more than 50% of the cycle time (regardless of cycle duration)
is involved performing the same type of fundamental cycles. For guiding
practitioners Kilbom proposed more detailed definitions for the assessment
of repetition in the shoulder, elbow, wrist, and finger areas. She
defined the risk of a disorder to be high if the frequency of repetitive
movements or contraction for the shoulder is more than 2.5, for the
upper arm or elbow more than 10, for the forearm or wrist more than
10, and for the fingers more than 200 per minute. The presence of
other risk factors (high external force, high speed of motion, high
static load, extreme posture, lack of training, high work demands)
increases 'high risk'for any category to 'very high risk`. If a repetitive
work task has been identified, or an upper limb disorder has been
diagnosed, the task should be analyzed with regard to its total duration,
e.g. per day or week. According to this guideline a potential problem
arises if the task assessed as repetitive is performed for more than
60 minutes during the workday (Kilbom, 1994b).
The scientific basis for this latter value of task duration (60 min)
per day and the reference values for repetitive motions for the various
joints of the upper limb is, however, relatively weak. Yet the guideline
has proved to be useful to practitioners (Kukkonen et al., 2001).
Assessment
methods
The semi-quantitative 'Upper extremity checklist' of Keyserling et
al., (1993) was
developed as a shop-floor screening tool to determine the presence
of risk factors associated with upper limb disorders. The risk factors
to be evaluated are repetitiveness, local mechanical contact pressure,
forceful manual exertions, awkward postures, and use of hand tools.
Local mechanical contact pressure and forceful manual exertions are
classified with a dichotomous scale. The assessment of repetitiveness,
awkward postures, and use of hand tools is time-based. The observation
is ended with a calculation of the total risk score. In the evaluation
of the checklist, the classification of shop-floor representatives
and experts in ergonomics was compared. There were several disagreements
between the results of the shop-floor representatives and those of
the experts (used as a golden standard). There was a lack of consistency
between the observation of repetitiveness, pinch grip, and awkward
postures (Keyserling et al., 1993).
The validity study as a whole presented some problems: the interval
between the observations of the shop-floor representatives and the
experts in ergonomics was several months and the two observer groups
used partly different checklists. However, the checklist by Keyserling
includes all common risk factors for upper limb disorders, and the
description and classification of the factors is feasible and distinct.
The handPEO method (Portable Ergonomic Observation method) is a further
development of the PEO method (Fransson-Hall et al., 1995;
Fransson-Hall et al., 1996).
The basic PEO method is a computerized observation method which quantifies
lifting, carrying, pushing and pulling tasks and the duration, frequency,
and holding time for work postures of the upper limbs, back and neck.
In the hand PEO method, the work analysis begins with detailed interview
of the worker about the work tasks. After the interview the work is
recorded on video. In the analysis, the work operations (use of hand
tools, power tools, using the hand as a hitting tool, using the hand
for support, for manual handling, manual assembly) and the hand griping
(finger grip, whole hand grip) are registered continuously. The duration
of each separate work operation or hand grip, the sum of all holding
times during the registration period are computed. The HandPEO has
proven to be a very sensitive measure and its inter-observer repeatability
has been considered acceptable (Fransson-Hall et al., 1996).
The number of simultaneously observed parameters is fewer in the handPEO
than in the basic PEO method. For a hand-intensive job, performed
at a high rate and with a large variety of work operations, it may
still be impossible to obtain an exact result. The handPEO appears
to be a more suitable and precise tool for hand-intensive jobs than
the basic PEO method, but in practice it is a demanding and cumbersome
method for the observer.
The RULA (Rapid Upper Limb Assessment) method (McAtamney and Corlett,
1993) is designed
for the assessment of trunk and upper limb load and is meant particularly
for sedentary jobs (Hedge et al., 1999).
The range of movements for each upper body part (head, trunk, upper
and lower arm, wrist) is divided into sections that are categorized.
In addition to posture recordings, the RULA also considers the load
on the musculoskeletal system caused by static or repetitive muscle
work and force exertion, so that an action list can be produced. The
validity of the RULA method has not been reported. In addition, its
sensitivity and predictive value for quantifying the actual risk of
musculoskeletal injuries has not yet been assessed. Because the RULA
method has been planned for static sedentary jobs, its use for dynamic
industrial jobs is limited.
The OCRA method is designed to analyze repetitive upper limb movements
and the physical risk factors for upper limb work-related disorders.
Every work task involving repetitive movements is analyzed for each
worker or a group of workers. In the first stage, the work arrangements
are described and measured (distribution of work and pauses, duration
of repetitive tasks, sequences of technical actions). In the second
stage, every selected task is analyzed for repetitiveness, force,
awkward postures and movements, recovery time, and additional hazards.
In the third stage, all data gathered are combined and the OCRA index
is calculated. From the calculated OCRA value the risk is assessed
as acceptable, conditionally acceptable and not acceptable. The validity
of the OCRA method has been poorly reported. In addition, the OCRA
method is not very easy to learn or apply due to the numerous factors
to be assessed and calculated (Colombini, 1998;
Occhipinti, 1998).
A research program called 'Project on Research and Intervention in
Monotonous Work' (PRIM) was initiated in Denmark in 1994 as a prospective
cohort study on work-related musculoskeletal disorders. A group-based,
task-related exposure assessment strategy was created. Monotonous,
repetitive jobs with an estimated similarity in physical exposure
were aggregated and 103 exposure groups were formed. The subjects
from the exposure groups were randomly sampled for exposure, and task-related
exposure levels were quantified by 43 single exposure items using
a real-time, video-based observation method that allowed computerized
estimates of repetitiveness, body postures, force, and velocity. In
combination with the questionnaire-based data on task distribution,
the duration of exposure was calculated at the individual level.
Some methodological problems came up in the use of this grouped exposure
assessment. Despite efforts to optimize group homogeneity, the within-group
variance was greater than the between-group variance for several variables
of shoulder posture (Fallentin et al., 2001;
Andersen et al., 2002).
A task-based exposure-assessment strategy was used successfully to
solve some of the main problems associated with the assessment of
physical exposure at work. The great within-group variance in exposure
may eventually require individual assessment of exposure.
Several comprehensive methods for assessing upper limb work load have
been established in different studies. The large number of the observation
methods using different criteria for upper extremity repetitiveness,
force and posture is a major problem making it difficult to compare
the respective data (Juul-Kristensen et al., 1997).
In addition, most of the methods are time-consuming and not feasible
to be used by the occupational health staff in the field. Several
methods are also suited only for jobs in which a limited number of
short cycles are repeated throughout the workday. There is a need
for methods that can be applied to a wide range of jobs, from highly
repetitive to more varied ones with a longer cycle duration.
Assessment
of VDU work ergonomics
The EU Council Directive 90/270/EEC of 29 May 1990 defines the
minimum safety and health requirements for work with display screen
equipment. Since then several guidelines have been published to apply
the principles of this directive at workplaces. Plenty of practical
advice is published in the Internet as well.
Methods based on the EU Council Directive
A Finnish ergonomic checklist for VDU work, 'Ergonomic Improvements
to the Computer Workstation', has been designed to be used by computer
users themselves or by experts in ergonomics. The checklist emphasizes
three items: the layout and environmental conditions of the work room,
adjustments of the workstation, and breaks during work. All questions
are dichotomous, requesting a 'yes'or 'no'answer. After 'no'answers
the worker is advised to define the problem further and to think what
to do. At the end of the checklist there are questions on working
time, visual environment, and use of spectacles. The method applies
a participatory approach and its aim is to activate the worker to
identify the ergonomic problems with the workstation and to solve
them. An English version of the checklist is available from http://www.occuphealth.fi/ergonomia/2003.
The method for the assessment of ergonomics in VDU work ('Näppärä')
is a rapid screening tool for identifying problems for further assessment
and corrective actions. The questions and observations are dichotomous,
requesting compliance or non-compliance. The items labeled as non-compliance
are subject to further actions. One output of the assessment is an
index indicating the percentage of compliance items out of all items.
This index can be used as a 'benchmark' of the level of ergonomics
in each office. The advantage of this method is that it is rapid and
simple to use, and the findings indicate clearly the points for intervention.
The method has been developed in co-operation with researchers, occupational
health practitioners, safety inspectors, and office workers (Rasa
and Ketola, 2002).
The development of the two methods mentioned above started with the
research and consultation activities of the Finnish Institute of Occupational
Health. The basis for these methods is the Council Directive 90/270/EEC.
Other methods
In Germany a project called SANUS has produced a handbook on the safety
aspects of computer work based on international norms and standards
(Burmester, 1997).
It contains several checklists for the assessment of physical and
psychological hazards in office work. Standards and legislative obligations
have been included. The presentation and language are rather technical,
and the users therefore need substantial training in using the method.
This may reduce the practical usability of this system for the prevention
of problems in office work.
A Swedish researcher group (Hansson et al., 2001)
has developed an ergonomics checklist for use as a part of the exposure
assessment in an epidemiological study of VDU users. The objective
of the checklist is to facilitate structured assessment of the background
factors at the workplace, workstation design, working techniques,
and work postures. The protocol for the first three parts of the list
is filled in during the observation at the workplace, while the work
postures are recorded on video and subsequently analyzed in the laboratory.
The items regarding workstation design, working techniques, and work
postures are classified into predefined categories. The items are
later aggregated into various 'non-optimal'conditions according to
known risk factors and the opinion of the researcher group regarding
'harmful'conditions. In the main study, data on 853 persons have been
collected and the preliminary results have been reported in swedish
(Hansson et al., 2001).
The analyzing and reporting of the study is still going on.
The OSHA in the USA serves as a checklist on the Internet (http://www.osha.gov)
to be used by employers and employees to identify, analyze, and control
hazards predisposing to musculoskeletal disorders at a computer workstation.
A printed version is also available. The checklist consists of 33
dichotomous questions. A 'no'answer indicates that there are problems
to be solved. The afore-mentioned Finnish checklist includes the same
risk factors as the OSHA checklist. The OSHA checklist does not actively
suggest searching for possible solutions.
Repeatability and validity of the assessment methods
The quality of an assessment or observation method in measuring physical
exposure depends on the repeatability and validity of the method.
An observation method is repeatable if the results are coherent when
the observations are duplicated. Intra-observer repeatability means
the number of identical results obtained in repeated observations
of the same work situation by the same observer at different points
of time. Inter-observer repeatability means the extent to which two
or more observers give identical results when observing the same work
situation. Several factors influence the repeatability of an observation
method: the basis for observation (real work situation - video document),
potential learning effect of the observers, and the statistical method
used to verify similarities between the observations (Kilbom, 1994a;
Ovretveit, 1998).
The repeatability of a method is a necessary condition for producing
valid data, but it is not a sufficient condition. Validity is the
extent to which a measure or piece of data reflects what it is supposed
to measure or give information about. In the literature, usually four
classes of validity have been mentioned: face validity, criterion
validity, content validity, and construct validity (Ovretveit, 1998).
Face validity means that the data gathering method appears
to measure what it claims to measure. A simple test of face validity
is to review the literature or to ask someone knowledgeable about
the phenomenon, if the person thinks that the measure represents the
phenomenon. For example, the definition for repetitive work (work
is defined to be repetitive when the cycle time is less than 30 seconds
or more than 50% of the cycle time involves performing the same type
of fundamental cycles) is collectively accepted and has been used
by researchers in ergonomics for the past two decades (Silverstein
et al., 1986; Keyserling
et al., 1993; Kilbom,
1994c).
Criterion validity means the degree of agreement between observations
and other, more accurate measurements or data gathering methods, or
the measure produces data which correlates with the data from another
method which is accepted as a valid measure of the phenomenon studied
(Kilbom, 1994b;
Ovretveit, 1998).
The criterion validity for posture observation has been estimated
e.g. with reference to measurements with opto-electronic systems,
inclinometers and goniometers (Keyserling, 1986a;
Keyserling, 1986b;
Keyserling et al., 1991;
Fransson-Hall et al., 1995;
Leskinen et al., 1997;
Juul-Kristensen et al., 2001).
In general, criterion validity depends on the actual work posture
in relation to the class limit, the viewing angle of the observer,
the size of the posture class categories, the number of observation
variables, the duration of observation, and the experience of the
observer (Fallentin et al., 2001;
Juul-Kristensen et al., 2001).
Content validity means that the measure comprehensively covers
all the aspects it is intended to measure (e.g. the quality of workstation
measure covers all aspects of the quality of a workstation). The content
validity of a phenomenon being measured is often linked to a conceptual
model of the factor. In the PRIM study (Fallentin et al., 2001),
several variables were used to quantify the complicated phenomenon
of repetitiveness. One was the number of movements per minute for
different joints based on guidelines suggesting threshold limits for
an acceptable number of movements (Genaidy et al., 1993;
Kilbom, 1994b).
Two other items were duration of exertion (total cycle time) and number
of exertions (fundamental cycles) per minute.
Construct validity refers to the extent to which the measurement
corresponds to theoretical constructs concerning the phenomenon under
study. The phenomenon can be expressed as a hypothetical construct
or 'mini theory'. The assessment of construct validity is mainly assessment
of the coherence and logic of the construct, with respect to all relevant
information about the phenomen.
Good validity is a basic prerequisite for the selection of a method
from several alternative methods. Validity has been tested for few
observation methods only.
Ergonomic interventions in VDU work
Intervention studies in VDU work
Evidence from the literature suggests that a number of important risk
factors, both physical and psychosocial, are associated with the development
of musculoskeletal disorders in VDU work. A wide variety of workplace
ergonomic interventions has been implemented in many different settings
to reduce these disorders (Table
3). During the past decade the study frame of randomized controlled
trials (RCT) has been recommended in intervention studies. However,
few well-designed RCTs or controlled interventions have been carried
out in the office environment (Punnett and Bergqvist, 1997).
This review of interventions in VDU work includes recently published
randomized controlled trials. In addition, some articles on the placement
and design of the mouse, keyboard and screen have been reviewed. A
summary of the intervention studies reviewed in this section is presented
in Table 3.
Ergonomic improvements and training
Aarås and colleagues have recently published a series of articles
on an intervention study on VDU work. They studied two intervention
groups and one control group of VDU workers using a prospective parallel
group design. There was no randomization of the subjects into the
intervention groups. The interventions were implemented three times
serially on a two-year time scale and consisted of a new lighting
system and workplace design, an optometric examination, and corrections
if needed. The intervention groups reported significantly improved
lighting conditions and visual comfort. Headache and shoulder pain
were also reduced in the two intervention groups. The control group
reported no improvement in any of these health outcomes (Aarås et
al., 2001b). After
3.5 years of follow-up, the control group underwent the same intervention
in terms of the new lighting system, new workplace design, and an
optometric examination and corrections if needed. This group reported
a significant reduction in visual discomfort after the intervention.
After 6 years of follow-up the control group still reported low levels
of shoulder and neck pain, and the two earlier intervention groups
continued to report visual comfort (Aarås et al., 2001b).
Nevala-Puranen et al., (2003)
carried out an intervention study among newspaper employees using
VDU at work. The study compared the effects of two different intervention
models: redesign of the VDU environment only (n=8), and redesign of
the environment and advice given on work techniques (n=9). All the
subjects selected for the interventions had musculoskeletal pain in
their forearms on at least 30 days during the past 12 months. Work
posture, viewing angle and distance to the screen, muscular activity
in the shoulder and forearm, and musculoskeletal pain were measured
before and after the 7-month intervention. A statistically significant
difference was found between the groups in the change in shoulder
flexion and the muscular activity of the right trapezius and right
extensor carpi radials between the baseline and 7-month follow-up.
The reduction of pain symptoms in the neck, shoulders and elbows was
greater in the redesign and advice group than in the redesign of the
environment only group.
In a study by Brisson et al., (1999)
a pretest-posttest design with a reference group was used with random
allocation into administrative and geographic units. The study population
was composed of 627 workers employed in a large university and in
another institution involved in university services. A six-hour training
program in ergonomics was carried out. The training focused on decreasing
postural stressors (neck twisting and flexion and wrist deviation)
through the use of accessories and adjustments of the workstation.
The measurements involved direct observation of the workstations,
a self-administered questionnaire, and a physical examination. In
both groups, measurements were performed 2 weeks before and 6 months
after the training. Improvements in postural stressors occurred more
frequently in the intervention group, and these positive changes tended
to be more frequent in workers under 40 years of age. The prevalence
of musculoskeletal disorders decreased among the workers under 40
years of age in the experimental group, from 29% to 13% determined
by questionnaire and from 19% to 3% determined by physical examination.
In the other groups, there was no significant change in the prevalence
of musculoskeletal disorders (Brisson et al., 1999).
In their non-randomized longitudinal intervention study, Bayeh and
Smith (1999) examined
the effect of training and impact of ergonomic interventions on workers'
health in VDU-intensive work. They used three levels of interventions:
1) ergonomic training and customized workstation adjustments, 2) specific
workstation adjustments, 3) acquisition of a new ergonomic chair.
Health data were gathered from 80 volunteer participants before the
intervention and, 6 and 12 months after the intervention. Reductions
in self-reported musculoskeletal discomfort were found in all three
ergonomic interventions.
Mouse design and arm support
It has been shown that mouse users have an outward rotated position
in the shoulder joint and pronation in the forearm. These postures
may impose high static loads on the upper limb and cause discomfort
and pain (Karlqvist et al., 1994).
Aarås et al. (2001a)
carried out an intervention study aimed at improving the posture of
the upper limb when using a computer mouse. They compared a newly
designed alternative mouse with a traditional mouse. They found in
an EMG study that the muscle load of the forearm was lower, and the
positions of the forearm and wrist more neutral when using the alternative
mouse compared to the traditional mouse (Aarås and Ro, 1997).
In a subsequent intervention study, half of the study participants
used the alternative mouse (Anir mouse) and the other half the traditional
mouse. After six months a significant reduction of neck, shoulder,
forearm, wrist and hand pain was reported among the participants who
used the alternative mouse. Aarås concluded that the effects of this
single intervention might be due to reduced shoulder abduction and
elimination of full forearm pronation. Later on an identical intervention
(the alternative mouse in use) was carried out using the former control
group as the intervention group (Aarås et al., 2001a).
After 6 months, also this group reported a reduction of shoulder,
forearm, wrist and hand pain.
In addition to mouse design, forearm support has been considered to
decrease muscle load in the upper limb. Aarås et al., (1998)
reported that supporting the user's whole forearm on the table top
in front of the computer seems to reduce static trapezius load. Lintula
et al., (2001) studied
the effects of added arm supports on wrist angles and musculoskeletal
strain in the neck and upper limb. The electrical activities in the
shoulder and arm muscles were studied during typing and use of the
mouse. Twenty-one women were randomized into 3 groups: arm support
for the mouse hand; arm supports for both hands; and control group.
Measurements were carried out before and after a 6-week intervention.
Wrist extension of the mouse hand, muscle activity of the trapezius
muscle, and ratings of subjective discomfort indicated that supporting
both arms was a better solution than supporting only the mouse hand.
Keyboard design and placement
Several researchers have suggested that the conventional linear keyboard
design may contribute to the development of upper limb disorders (Punnett
and Bergqvist, 1997).
The traditional keyboard requires an awkward upper limb posture: forearm
pronation, wrist ulnar deviation and extension. Tittiranonda and colleagues
carried out a 6-month randomized controlled trial evaluating the effects
of four computer keyboard models on hand pain severity, functional
hand status, and comfort. 80 computer users with musculoskeletal disorders
participated in this study during a period of 6 months. The study
subjects were randomized into three intervention groups: split ('adjustable')
keyboard; geometric keyboard ('comfort'); and 'natural' keyboard and
one conventional keyboard ('placebo') group. Compared to the placebo
keyboard group, the natural keyboard group, and to a lesser extent
also the adjustable keyboard group demonstrated an improving trend
in pain severity and hand function after 6 months of keyboard use.
A significant correlation was also found between reduction in pain
severity and satisfaction with the keyboard. However, there were no
improvements in the clinical findings from the upper limbs. The authors
concluded that keyboard users may experience a reduction in hand pain
after using an alternative geometry keyboard for several months (Tittiranonda
et al., 1999c).
Since the beginning of the 20th century the place of the keys on the
keyboard of typewriters has been in QWERTY order. Also on computer
keyboards the QWERTY order predominantes, but not without criticism.
A considerable number of inventions and studies have sought to find
a new keyboard design in which the keys are in an alternative order
(Punnett and Bergqvist, 1997).
Rempel et al., studied an alternative keyboard key-switch design.
This randomized clinical trial evaluated the effects of the key-switch
design on computer users with hand paresthesia. 20 computer users
were randomly assigned to a conventional layout keyboard group, and
to an alternative keyboard group (different key-switch design). Various
outcome measures were assessed during 12 weeks of use. The subjects
who were assigned the alternative keyboard experienced a decrease
in hand pain between weeks 6 and 12 when compared with the conventional
keyboard users. They also demonstrated an improvement in the Phalen
test time. Keyboard assignment had no significant effect on the change
in hand function or median nerve latency (Rempel et al., 1999).
Dowler et al., focused on developing a new approach to seated work
positions by changing the keyboard location. The study population
comprised of 67 office workers who used a computer as a major tool
during their work day. At first two sitting postures were defined
as 'neutral`. In the first posture the keyboard was placed on a 15°
downward-tilting tray (negative slope) and the upper arm forearm angle
was 115° . In the second 'neutral'position the keyboard was located
on a normal work desk and the upper arm - forearm angle was 90° .The
purpose was to compare the muscle tension between the two defined
working postures with the ANSI posture (American National Standard,
1988), and the users' own daily used posture. The study subjects worked
in each of the four work postures in a random order. Muscle tension
was measured by using surface EMG from the shoulder and forearm muscles
before and 30 days after every work session. It was found that muscle
load in the shoulder and forearm muscles was lowest in the first neutral
work position (keyboard at 15° downward-tilting angle, upper arm -
forearm angle 115° ) (Dowler et al., 2001).
Screen
The appropriate screen location is a subject of constant debate. Generally,
visual strain is associated with higher placement, and musculoskeletal
strain is associated with lower placement of the screen. Seeking a
resolution for the debate, Psihogios et al., (2001)
compared the results of laboratory-based monitor placement studies
to recommendations and outcomes from viewing preference and neutral
posture studies. The results showed that the screen height at the
workplace was associated with postures similar to those in the laboratory
studies. Additionally, the preferred screen location generally corresponded
to the location in which less neck discomfort was reported. There
is consistent evidence to support mid-level (~20° viewing angle) or
somewhat higher placement as a rule-of-thumb, considering the preferred
gaze angle and musculoskeletal aspects. However, optimal placement
may be lower for some individuals (e.g. those using bifocals) or for
specific tasks.
Breaks from computer work
In a study of Henning et al. (1997)
computer workers at two work sites (n = 73, n = 19) were advised to
take three 30-second and one 3-minute break from computer work each
hour, in addition to the regular rest breaks. Some workers were asked
to perform stretching exercises during the short breaks. Mood and
musculoskeletal discomfort were assessed at each work site over a
2- or 3-week baseline period and a 4- or 6-week treatment period,
respectively. Worker productivity measures were obtained from the
company records. No improvement in productivity or well-being was
found at the larger work site. At the smaller work site, productivity,
eye, leg and foot comfort all improved when the short breaks included
stretching exercises.
Methodological considerations
A common hypothesis is that ergonomic interventions at the workplaces
can reduce the incidence of musculoskeletal disorders and the disability
due to them. However, the use of randomized controlled trials in intervention
studies has been rare, and the results have been meagre. In addition,
only few of the lately reported intervention studies in VDU work have
followed the standards of reporting randomized controlled trials (Begg
et al., 1996). Most
of the studies reviewed here had a control group that was compared
to one or two groups of workers, subjected to an intervention. Some
groups have been used as their own controls. In some studies, the
control group has been established by unusual means, e.g. by two-group
time-staggered interventions.
Symptom ratings, such as pain or strain scores, changes in muscle
load or musculoskeletal symptoms, have been the primary outcome measures.
The reviewed studies have generally demonstrated positive impacts
of changes in workstation design, tool design, or training on upper
limb or neck/shoulder symptoms.
Intervention research using an RCT design is laborious to carry out
because the field setting often poses limitations. In many cases it
is difficult to establish an adequate control group. If the control
group is missing, there are many threats to the internal validity
of the study. The intervention often focuses on the persons with the
most problems (Ovretveit, 1998).
Due to the cyclical nature of musculoskeletal complaints, the workers
may temporarily show improvements in health, independent of the intervention.
Variables that affect the outcome, e.g. age, gender, work load, and
activities outside work, can be represented differently when different
populations are compared.
A random selection of the subjects to intervention and control groups
is desirable, including blinding of the subjects and researchers to
the group assignment. Such a design is generally not achievable in
ergonomic intervention research, because the group membership of subjects
is, in practice, given. However, group randomization is in many circumstances
an alternative study design. In addition, people should know that
they are taking part in an intervention, even if they do not know
whether they are in a control or in an experimental group, and this
may affect the outcome (the Hawthorne effect). Westgaard and Winkel
(1997) state that
the best way to decrease the Hawthorne effect is to arrange a non-effective
intervention for the control group, e.g., by introducing workplace
modifications that look good, but do not change the physical exposure.
Another criterion for a fair intervention is a reasonable size of
the experimental and control groups. Adequate sample sizes are needed
to show that an intervention is effective in increasing positive health
outcomes (Westgaard and Winkel, 1997;
Ovretveit, 1998).
In order to ascertain changes in ergonomics or in musculoskeletal
health, a long enough follow-up time is needed. No absolute limits
have been set, but a follow-up time of one year or at least 6 months
has been proposed (Westgaard and Winkel, 1997).
A long follow-up time makes the controlling of all confounding factors
in field conditions (e.g. changes in exposure over time, different
exposures to different workers and turnover) very demanding. Furthermore,
ethical considerations limit the possibility to perform a long intervention
in the work life. Some outcomes and impacts may take time to become
evident. However, a shorter observation time may provide valuable
information e.g. of acute physiological responses such as the development
of fatigue or discomfort.
If the purpose of an intervention is to describe the effects of a
change in physical exposure on musculoskeletal health, both exposure
and health outcome variables must be properly assessed and described.
The documentation of an intervention study should state clearly the
original purpose of the intervention, the intervention process, and
the results. The measurements used in the intervention should be presented
properly in order to facilitate their use in later research.
|
| THEORETICAL
FRAMEWORK OF THE STUDY |
An
impressive number of studies in the past decade have linked physical
and organizational factors at work to musculoskeletal signs and symptoms.
It has been evidenced that worker groups with specific work-related
factors are at an increased risk for musculoskeletal disorders (Panel
on Musculoskeletal Disorders and Workplace, 2001).
The mechanisms in the development of musculoskeletal disorders are
not yet completely understood. Several models attempting to explain
the exposure - response process have been presented.
The framework of this thesis is based on the conceptual model of Moon
and Sauter, 1996.
According to this model (Figure 3)
musculoskeletal symptoms are related to the work technology which
includes both the nature of tools, work place characteristics and
work processes. As shown in the model, work technology is linked directly
to physical demands, as illustrated by the physical connection between
the worker and the tool, to workstation ergonomics, as well as to
work organization. The link between work organization and physical
demands implies that the physical demands of work are affected by
organizational demands; for example, increased specialization leads
to increased repetition. The model also shows a link between work
organization and psychosocial strain, which in turn, influences biomechanical
strain. It is suggested that complex cognitive processes mediate the
relationship between biomechanical strain and the development of musculoskeletal
symptoms. Finally, the model shows the reciprocal effects of musculoskeletal
disorders on psychological strain and work organization (Moon and
Sauter, 1996).
In this thesis Studies 3 and 4 concentrated on the evaluation of the
physical demands and workload in VDU work and industrial work. Study
I examined the incidence of neck pain in VDU work and the Study 2
focused on the effect of ergonomic improvements on musculoskeletal
outcomes.
|
|
AIMS OF THE STUDY |
The
aims of this study were to investigate risk factors for neck pain
among VDU workers, to assess the effects of an ergonomic intervention
on the level of musculoskeletal symptoms, and to study the repeatability
and validity of an assessment method of VDU ergonomics. Furthermore,
a method to assess the risk factors for upper limb disorders in industrial
work was developed, and its validity and repeatability were investigated.
The specific aims were:
1. To investigate work-related and individual factors as predictors
for incident neck pain among office employees engaged in VDU work
(Study 1).
2. To study the effects of an intensive participatory ergonomic
intervention and education on the level of musculoskeletal discomfort
and strain, and the prevalence of pain among VDU workers (Study 2).
3. To investigate the repeatability and validity of an expert
assessment, and to determine to what extent an expert assessment of
VDU workstation ergonomics is related to certain workstation characteristics,
and responds to changes in these characteristics (Study 3).
4. To investigate the inter-observer repeatability and validity
of an assessment method to identify risk factors for upper limb disorders
(Study 4).
|
|
METHODS |
|
Study
group of VDU workers (Studies 1, 2 and 3)
Work-related and individual predictors for incident neck pain
A longitudinal study was conducted in three municipal administrative
units. Data were collected via structured questionnaires mailed
to the subjects. The source population in the study on the incidence
of neck pain consisted of all the full-time employees whose job
included VDU work for more than 4 hours per week (n=515). Altogether
416 workers participated in the baseline survey in 1998 (response
rate 81%). From the baseline respondents, the subjects of interest
were those who reported local or radiating neck pain for less than
8 days during the preceding 12 months. These subjects were classified
as 'healthy'at baseline (n=232). This cohort was studied 12 months
later, the response rate being 78% (n=180). At follow-up in 1999
the incident cases were those workers who reported local neck pain
or radiating neck pain for at least 8 days during the preceding
12 months (Figure 4). The employees
were mainly secretaries, technicians, architects, engineers and
draftspersons.
Ergonomic intervention in VDU work and expert assessment of ergonomics
The study population for the interventions was selected from among
those in the baseline group who returned the questionnaire (n=461)
on the basis of reported musculoskeletal symptoms, mouse usage and
age, according to the following criteria:
- symptoms
in the neck, shoulders, or upper limbs in at least one and at
most eight out of 11 anatomical areas during the preceding month
- mouse
use for more than 5% of the VDU working time
- age
< 61 years.
The
subjects fulfilling the inclusion criteria (n=124) were allocated
into three groups (intensive ergonomics, education in ergonomics,
and control) using stratified random sampling. The three administrational
units were used as a stratum. The success of the randomization was
checked with regard to age, sex, VDU work time, mouse usage, and
symptoms in the neck. At the beginning of the study there were 109
participants, in the 2-month follow-up 107, and in the 10-month
follow-up 102 participants. For the assessment of ergonomics, video
recordings were made and digital photographs taken of the subjects
(n=107) during their usual daily tasks (Figure
4).
Jobs observed in industry
In this study a semiquantitative, time-based method for assessing
six physical load factors was developed and validated. The jobs
observed were selected from a food-processing plant and a paper
mill. The jobs of 14 workers in 5 occupations (5 women and 9 men)
were selected for observation. There were 4 meat cutters, 3 sausage
packers, and 2 sausage sprayers in the food-processing plant, and
4 paper cutters and 1 pulp maker in the paper mill. All the workers
had at least two years' experience in their current work.
Questionnaire study among VDU workers (Study 1)
In the questionnaires, 11 work-related and 11 individual variables
were used as potential predictors for neck pain. The selection of
variables was based on our hypotheses and earlier evidence. The
following variables were selected:
Work-related factors (self-assessments and measurements by the
employees in their own offices)
- The
self-rated proportion of time used for VDU work as a percentage
of the total work time.
- Physical
work environment: lighting, temperature, quality of air, size
of the work room, and acoustic conditions.
- Ergonomics
of the workstation: work chair, work desk, screen, keyboard, mouse
and document holder.
- Viewing
distance between the eyes and the mid-point of the screen.
- Distance
between the upper edge of the screen and the horizontal eye level.
- Distance
between the g-h-point of the keyboard and the edge of the desk.
- Deviance
between the g-h-point of the keyboard and the mid-line of the
body.
- Distance
between the mid-point of the mouse and the edge of the desk.
- Deviance
between the mid-point of the mouse and the mid-line of the body.
- Breaks
during VDU work.
-
The extent to which the subjects were able to influence their
own work load in terms of amount of work and work pace.
Individual
factors
The following items were included:
1. Sex
2. Age
3. Frequency of physical exercise
4. Smoking
5. Health status
6. Mental stress
7. Mental strain
8. Depression
9. Job satisfaction
10. Time used for domestic chores
11. Time used for hobbies imposing static load on the neck-shoulder
region.
Intervention
study in VDU work (Study 2)
Intervention procedure
At baseline two experts in ergonomics collected data on the workplace
layout and dimensions before the intervention, and 2 and 10 months
after the intervention. They were blinded to the group assignment
of the study subjects. The outcome measures were musculoskeletal
discomfort collected by diary 2 and 10 months after the intervention.
In addition, pain and strain symptoms were inquired with a questionnaire
12 months after the preliminary survey, i.e. about 7-10 months after
the intervention (Figure 4).
Diary and questionnaires
The participants were asked to keep a diary on discomfort three
times during a workday: in the morning, at noon, and at the end
of the workday. The structured diary consisted of questions on discomfort
in different anatomical areas. The rating of discomfort had five
levels ranging from 1="feel good" to 5="feel very
uncomfortable". A human figure was used to define the anatomical
areas. The subjects filled out the diary for two weeks before the
intervention, at the 2-month follow-up, and at the 10-month follow-up.
The questionnaires, before the intervention and at the 10-month
follow-up, included questions on musculoskeletal strain and pain.
Interventions
In the intensive ergonomics group two physiotherapists visited the
work site of every member of the intensive ergonomics group. Potential
improvements based on the workers 'own views as well as on the physiotherapists'
observations were then discussed and carried out. The best solution
was sought first of all, by adjusting and altering the existing
furniture and work equipment. The worker was encouraged to participate
actively in the redesign and rearrangement of his or her workstation.
The workers were also advised to be aware of their work postures
and to take short pauses in their work. The ergonomics evaluation
and the implementation of the immediate changes to a workstation
took about 1.5 - 2 hours. In addition, the workers received a one-page
leaflet on general ergonomics in VDU work.
In the education group the workers attended a 1-hour training session
in ergonomics. A specialist in ergonomics instructed the workers
concerning the principles of ergonomics in VDU work.The workers
were given the same leaflet as the intensive group, and were encouraged
to evaluate their own workstation, to make changes, and to ask for
new equipment and furniture if needed. Moreover, the workers were
instructed to take short pauses and adopt relaxed working postures.
All subjects in the education group attended the training. The members
of the control group got only the one-page leaflet on ergonomics
during the study.
Assessment of ergonomics in VDU work (Study 3)
Video recordings were made of the subjects during their usual daily
tasks by the two experts in ergonomics at the baseline, and at the
2-month and 10-month follow-ups. A continuous 4-minute extract was
chosen from the video recordings of each subject at both time points
to illustrate the ergonomics of the workstation and the subject's
most common work postures. Two experts gave individually an overall
rating of ergonomics with a scale from 4=poor to 10=excellent. The
experts' assessment was based on common knowledge of musculoskeletal
risk factors in VDU work and the present, general knowledge of ergonomics.
The rating was made individually, and the experts had no written
instruction or checklists. The technical measurements of the workstation
(i.e. place of the mouse, screen, keyboard) were made at the same
time as the video recordings.
Simultaneously with the video recordings, five to ten digital photographs
were taken of each workstation and workplace. A researcher analyzed
the photographs taken at each time point in a random order (totally
216 workstations) to assess tidiness and available space at the
workplace. The researcher gave an overall rating of tidiness and
space with a scale from 4=poor to 10=exellent. For each worker,
the type of the work chair was recorded and photographs were taken
of the chairs without the worker sitting in the chair. A physiotherapist
experienced in the ergonomics of office chairs classified the chairs
with a scale from 4=poor to 10=exellent according to their ergonomic
properties (e.g. design, adjustments, sitting comfort).
Method to observe risk factors for upper limb disorders (Study
4)
Two occupational nurses (observers 1 and 2) made the observations
using the method developed in this study. The observers worked simultaneously
but independently.
Simultaneously with the observation, the working was recorded on
video, surface electrodes were used to record the electrical activity
(EMG) of the forearm muscles and the range of motion in flexion-extension,
and the radial-ulnar deviation of the wrist was measured by goniometers.
Estimations of the use of force based on the EMG recordings served
as validity criteria for the observed use of force. The angles measured
by the goniometers were used as criteria for the nonneutral postures
of the wrist. A work cycle was considered to include high grip force
if the grip force estimated by EMG exceeded the limit value of 44
N for at least 30% of the cycle time. Similarly, the cycle was considered
to involve a nonneutral wrist posture if the wrist angle was nonneutral
(>20° extension, flexion, ulnar, or radial deviation) for at
least 30% of the cycle time. Because the preliminary results suggested
a difference in the results for short and long cycles, the final
analysis of use of hand force was performed for all cycles, and
separately for short ( 30
s) and long (>30s) cycles. The validity of the two observers'
observations of repetitiveness, pinch grip and elevation of the
upper arm was assessed against the observation of the expert from
the video.
Statistical methods
Incident neck pain
Cross-tabulations and logistic regression models were used as main
methods for analysing the associations between neck pain and the
potential work-related and individual risk factors. To construct
a multivariable model, a forward selection strategy was used. The
inclusion of the variables for the first model was based on testing
the significance of the potential predictors as groups of variables,
adjusting for age, sex and VDU work time for work-related variables,
and for age and sex for individual variables. From each group of
variables, those with p<0.2 were selected for further analyses.
Based on the first steps of modelling, the physical work environment,
the distance of the keyboard among the work-related factors, and
smoking among the individual factors, were included in the further
stages of analysis. Finally, the first level interactions were tested.
The significant interactions, (sex with age and mental stress with
frequency of physical exercise), were added to the model of direct
effects.
Ergonomic interventions
When studying the effects of interventions in VDU work, one-way
analysis of variance was used to test differences in the ratings
of ergonomics between the three intervention groups. Each time point
was handled separately. In cases in which the F-test was statistically
significant, unpaired t-test for comparisons between two groups
(intensive vs. control and education vs. control) was applied. A
5 % level was considered to be statistically significant.
Musculoskeletal strain and maximal discomfort from the follow-up
questionnaire and diary were kept as continuous outcome variables
when the analysis of covariance was applied. The baseline value
of the outcome variable, the initial rating of ergonomics, and the
baseline value of work load (keyboard and mouse events) were included
in the models as covariates. Due to missing data on work load, this
modelling was carried out also without work load. The adjusted means
of the outcomes and their standard errors were calculated and the
statistical significance of the differences in adjusted means between
the groups was tested for with one-sided Dunnett's test. The two
intervention groups were contrasted against the control group.
Musculoskeletal pain from the 10-month questionnaire was handled
as binary variables, and logistic regression models were applied
when the pain was modeled to find the association between pain and
type of intervention. The baseline value of the outcome variable
was used as a confounder in these models.
Expert assessment method in VDU ergonomics
The inter-observer repeatability for the expert assessment method
was estimated by calculating the intraclass correlation coefficient
between the ratings of the two experts at the baseline. Associations
between expert assessments and workstation characteristics at baseline
were studied using the simple linear regression model. The effect
of the change in workstation characteristics during the follow-up
time on the expert assessment was studied using a linear regression
model.
Assessment method for upper limb risk factors
Inter-observer repeatability was assessed by calculating the proportion
of specific agreement and Kappa-coefficient ( ).
The validity of the observations of the observers was assessed against
the observation of the expert. For force, EMG measurements, and
for wrist posture, goniometer measurements, were also used as validity
criteria. Sensitivity, specificity and kappa coefficients were computed
for each physical load factor. In our study, sensitivity was the
probability that the observer finds a truly loading factor of a
work cycle. The specificity means that the observer finds no loading
factor when there really is none. The classification of Kappa values
was done according to the method of Landis and Koch (1977).
Values of < .40 were
regarded as poor, from .40 to .75 as moderate to good, and >.75
as excellent.
|
| RESULTS |
Risk
factors for incident neck pain (Study 1)
Of
the 180 VDU workers who had no neck pain at baseline, 62 (34.4%) developed
local or radiating neck pain during the12-month follow-up period.
The incidence of local neck pain was 13.3% (n=24) and the incidence
of the radiating pain was 14.4% (n=26). The incidence of combined
local and radiating neck pain was 6.7% (n=12).
The risk of neck pain was about two-fold for workers who rated their
physical work environment as poor, in comparison to those who rated
their work environment as good. Each item of the environment score
showed a positive association with the outcome as follows: lighting
(OR = 1.4), temperature (OR = 1.2), quality of air (OR = 1.7), size
of the work room (OR = 1.5), and acoustic conditions (OR = 1.4); none
of the items were significant alone. Also poor placement of the VDU
keyboard increased the risk of neck pain, and the women had an almost
three-fold risk of neck pain compared to the men. Current or ex-smokers
had an almost two-fold, though not significant, risk compared to the
never smokers.
Table 4 shows the multivariate
model with significant interactions. There was an interaction between
mental stress and physical exercise: the workers with a higher level
of mental stress and lower frequency of physical exercise had an almost
seven-fold risk compared to those with a lower stress level and higher
exercise frequency. The risk associated with the physical work environment
became higher, whereas that for the distance of the keyboard and smoking
turned out to be lower, as compared with the model with the direct
effects only. The interactive effects of sex and age showed that the
women had a higher risk than the men, except in the age group of 44-51
years.
Ergonomic intervention in VDU work (Study 2)
Changes in workstation ergonomics
The most common changes in the workstations detected or measured by
two blinded experts were changes in screen or keyboard height, or
the acquisition of accessories. Of the latter, wrist and forearm supports
were typically acquired in the intensive ergonomics group. Adjustments
were made to the chair or mouse location in all groups.
Changes in the rating of workstation ergonomics
The means for the ratings of ergonomics did not differ between the
groups at baseline. In the 2- and 10-month follow-up the level of
ergonomics was rated significantly higher in the intensive group than
in the education or the control group (Table
5). 6.2.3. Changes in daily ratings of discomfort.
In the 2-month follow-up the intensive group had less discomfort than
the control group in the neck, right and left neck/scapular region,
right and left shoulder/upper arm, and upper back (adjustment for
the baseline measurements of musculoskeletal discomfort, work load,
and ergonomics). As compared with the workers in the control group,
the education group had less discomfort in the neck, right neck/scapular
region, both shoulders/upper arms, and upper back. The results showed
the same trend in the 10-month follow-up, but there were no significant
differences between the groups (Table
6).
Adjusted only for the baseline measurements of discomfort and initial
ratings of ergonomics, two months after the intervention the intensive
group had less discomfort than the control group in the neck, right
neck/scapular region, right and left shoulder/upper arm, left fingers,
and upper back. As compared with the control group, the education
group had less discomfort in the neck, right neck/scapular region,
right forearm, and upper back. The results showed the same trend in
the 10-month follow-up, but there were no significant differences
between the groups.
Repeatability, validity and responsiveness to change in expert
assessment of VDU workstation ergonomics (Study 3)
The mean values (standard deviation) of the expert ratings of workstation
ergonomics at the baseline were 6.7 (0.9) and 6.8(1.1) for expert
1 and expert 2, respectively. The intraclass correlation coefficient
between the ratings of workstation ergonomics of the two experts was
0.74 at the baseline.
Workstation tidiness and space explained about 30%, and the work chair
about 15% of the expert ratings. The technical measurements had minor
effects on the assessments. Four dimensions of VDU workstations (distance
between the mouse and the front edge of the table, viewing angle to
the first text line of the screen, vertical distance between the eye
and floor, distance between g-h keys and the table front edge) showed
a statistically discernible (p<0.05) association with the assessments
of both experts. In addition, three dimensions (distance of mouse
and the point between g-h keys, viewing distance to the first text
line of the screen, vertical distance between the front edge of the
seat and keyboard table top) were significantly associated with the
assessments of either expert 1 or expert 2. However, each of these
individual technical measurements explained only 3- 7%, and at most
11% of the expert ratings.
The results of the linear regression model for responsiveness to change
are shown in Table 7. The coefficients
of determination (R2) for each of the models were relatively high,
since the baseline value was included as a predictor. However, five
VDU characteristic differences (distance between the mouse and the
front edge of the table, distance between the mouse and the point
between g-h keys, viewing angle to the first text line of the screen,
distance between g-h keys and the table front edge, tidiness and space)
had a significant effect (p<0.05) on the expert assessment at the
2-month follow-up. Partial coefficients of determination for these
differences ranged from 0.027 to 0.088.Directions of all estimates
were congruent for both experts.
Inter-observer
repeatability and validity of a method to observe risk factors for
upper limb disorders (Study 4)
The prevalence of positive findings was fairly similar for observers
1 and 2 for all physical load factors except for pinch grip and local
mechanical pressure on the left side where the prevalence was lower
for observer 2 (Table 8).
Inter-observer repeatability was good or moderate for repetitive use
of the hand, hand force, pinch grip, and elevation of the upper arm.
For nonneutral wrist posture, the proportion of specific agreement
was high although the kappa values were lower. The inter-observer
repeatability was poor for local mechanical pressure.
The validity was moderate or good for repetitive use of the hand,
use of hand force, pinch grip, and nonneutral wrist posture, when
expert observation was used as the reference standard. The validity
of elevation of the upper arm was moderate for observer 1 and poor
for observer 2. The validity was poor for local mechanical pressure.
Sensitivity and specificity were relatively high for both observers
with regard to use of the hand force and pinch grip. Also nonneutral
wrist posture showed a high sensitivity and specificity for observer
1. Sensitivity was high and specificity low for both observers for
repetitive use of the hand, and for observer 2 for nonneutral wrist
posture. In contrast, for elevation of the upper arm and local mechanical
pressure, sensitivity was low and specificity high for both observers.
When use of hand force was validated against force estimations by
EMG, the validity was poor for all observed cycles. For the long cycles
(>30 s), the validity was moderate for the right hand and poor
for the left hand. For the short cycles, the validity was poor for
both hands.
Sensitivity of the observations was high but specificity was low.
When the observations of wrist postures were validated against goniometer
data, the validity was poor. The sensitivity of the observations was
again high, but specificity was low (Table
8).
|
| DISCUSSION |
|
Main findings and comparison to earlier studies
7.1.1.
Risk factors for incident neck pain
The cohort study among office employees engaged in VDU work showed
that incident neck pain was associated with both work-related and
individual factors. An inappropriate physical work environment and
poor VDU-related ergonomics, together with individual factors, such
as gender, predicted neck pain. In addition, the employees with
higher mental stress and less physical exercise had an especially
high risk.
Poor placement of the keyboard was a predictor for neck pain. This
finding is supported by the study of Aarås (1997)
who found that supporting the forearms on the table top in front
of the operator reduced significantly the load on both right and
left trapezius. Also, the review of Bergqvist (1995b)
and the study of Tittiranonda et al., (1999a)
give evidence of associations between various aspects of keyboard
use and symptoms in the neck-shoulder area and upper limbs.
Most of the evidence concerning the placement of the mouse has been
related to hand/wrist disorders (Punnett and Bergqvist, 1997).
Only few studies have reported an association between mouse location
and neck pain (Aarås and Ro, 1997;
Karlqvist et al., 1998).
In the present study the placement of the mouse was not a significant
risk factor either. The workers did not perform very mouse-intensive
work: in the questionnaire the respondents reported to have used
the mouse only 28% of the VDU working time, which might affect the
result strongly.
High location of the computer screen (<20° below the horizontal
line of vision) had a tendency for being a risk factor for neck
pain. Visual discomfort and musculoskeletal strain, particularly
in the neck and shoulders, have been shown to be associated with
screen height (Bergqvist and Knave, 1994;
Villanueva et al., 1996).
Among the subjects with presbyopia, a higher monitor placement has
been associated with neck extension caused by visual demands when
using bifocals. On the other hand, an extremely low location is
often associated with musculoskeletal stress caused by neck flexion
(Turville et al., 1998;
Fries Svensson and Svensson, 2001).
However, the benefit of lower placement is a reduction of eye irritation,
when the open surface of the eyes is smaller and the lachrymation
is better. A recent field study on relatively young (mean = 37 years)
who did not use bifocals supports the midlevel placement (~20° viewing
angle) (Psihogios et al., 2001).
According to the used criterion, the midlevel location or any lower
placement was regarded as acceptable. This criterion was thought
to be feasible for our study, as the subjects were older (mean age=
47 years), used commonly bifocals, and therefore may have benefited
from a relatively lower location of the screen.
The physical work environment was a significant predictor in our
study. This variable included five aspects: lighting, temperature,
quality of air, size of the work room, and acoustic conditions of
the work environment. The mean of the five components was calculated
to represent the status of the physical work environment. However,
also each component individually showed a positive association with
the outcome. It has been suggested that especially lighting conditions
are important for the reduction of visual discomfort in VDU work.
Visual discomfort, in turn, has correlated highly with neck pain
(Aarås et al., 2001b).
Of the thermal conditions in VDU work, draught has been reported
to be a problem in connection with discomfort in the neck shoulder
region (Fanger and Christensen, 1986).
The quality of indoor air was also associated with neck pain in
our study.
The risk for neck pain was significantly higher for the women than
for the men. This is in agreement with earlier studies. Woman's
smaller stature and lower strength of the shoulder muscles have
been suggested to partly explain the sex difference (Mäkelä et al.,
1999). In VDU
work, gender differences have been found, for example, in the use
of computer mouse. Women work with a higher relative musculoskeletal
load, and apply, for instance, higher forces to the mouse, and use
a greater range of motion, than do men (Wahlström et al., 2000).
On the other hand, female sex may entail risk factors which were
not measured in the study (Mergler, 1999).
In our study, the different types of work tasks as such may be one
explanation for the effect of sex on the results. The work tasks
of the women in our study were more monotonous, such as assisting
and secretarial tasks.
Ergonomic intervention in VDU work
The intensive and the education groups had less musculoskeletal
discomfort than the control group at the 2-month follow-up. However,
the long-term effects in discomfort, strain or pain were not seen
at the 10-month follow-up. After the intervention, the level of
ergonomics based on a blind assessment, rated by two researchers,
was distinctly higher in the intensive ergonomics group than in
the education or control group. Furthermore, most changes in workstation
dimensions and accessories took place in the intensive ergonomics
group. This suggests that the changes made to the workstations had
a positive impact on ergonomics. The scale used in the assessments
was from 4 to 10. Already in the beginning, the ergonomic situation
in most workstations was satisfactory, and this might fade out the
contrast in the workstation before and after interventions, thus
making it invisible on the scale from 4 to 10.
The modifications in workstation ergonomics included mainly adjustments
of the screen, mouse, keyboard, forearm supports, and chair. These
modifications changed the postures and movements of the head, neck
and shoulder/upper arm. Since the positive effects were seen primarily
in the shoulder/upper arm, neck and upper back area, it is possible
that the effects were brought about by these changes.
Most of the amendments were done already before the 2-month follow-up,
but a part of the ergonomic improvements were implemented out after
the follow-up. In the interventions done in actual workplaces, it
is not possible to blind the participants, so they know that they
are in the intervention group. Therefore the placebo effect can
not be totally avoided.
Mekhora and Liston (2000)
modified VDU workstations to comply with the dimensions calculated
by a computer application based on the anthropometry of the workers.
It was assumed that changes in workstation dimensions could help
to reduce the discomfort level of the participants by changing their
work postures. Gerr et al., (2000)
on their part found that work postures were not greatly affected
by workstation dimensions. They pointed out that a large proportion
of computer users do not work in so-called neutral postures. People,
while sitting, use a range of different postures. Feelings of discomfort
or fatigue also modify the sitting posture. Good workstation design
supports a good posture and helps the workers to vary their postures
during VDU work. In our study we utilised a participatory approach
and personal guidance to take into account the individual preferences
of workers and changes in their work tasks.
In a controlled intervention study, Brisson et al., (1999)
found a training program in ergonomics to be an effective tool to
improve the ergonomics of VDU workers' workstations. Also another
intervention of showed training to be useful in optimizing the ergonomics
in VDU work (Menozzi et al., 1999).
In our study the level of ergonomics did not differ between the
education and the control group. The short 1-hour training may not
have been sufficient to activate the workers to improve their workstation
ergonomics. Workers may need concrete help and guidance, as in the
intensive group, to plan and implement changes in their workstations.
Although there were only some improvements in the level of ergonomics
in the education group, this group reported less discomfort than
the control group. It is possible that the workers had adopted better
working techniques or had found other ways to better organize their
work, or took more pauses. Frequent short breaks from VDU work have
been shown to reduce musculoskeletal discomfort and other complaints
(Henning et al., 1997).
This study concentrated only on physical ergonomics. However, in
order to make ergonomic interventions more effective, also psychosocial
and organizational factors deserve deeper attention.
Expert assessment of the ergonomics of the VDU workstation
The ergonomics in VDU work is defined by several factors in workstation
layout and dimensions, as well as the personal preferences of the
worker (Marcus and Gerr, 1996;
Bildt et al., 2001).
In recent studies the main issues of VDU ergonomics have dealt with
workstation arrangements, posture of the upper limbs, support for
the forearms, line-of-sight angle, and sitting posture (Karlqvist
et al., 1994;
Aarås et al., 1997;
Burgess-Limerick et al., 1999;
Gerr and Letz, 2000;
Hedge et al., 1999;
Lintula et al., 2001;
Marklin and Simoneau, 2001).
The level of ergonomics can be estimated by using technical workstation
measurements. However, the use of technical measures is often time-consuming.
One option is to use an expert assessment for the overall assessment
of ergonomics. To investigate the validity of the overall expert
assessment we chose a group of essential workstation characteristics
to represent ergonomics in VDU work.
Eleven of these characteristics were single measures of the workstation
or the work posture (location of input devices and screen, sitting
height). They are correlated with each other and they may have an
effect on various aspects of the work posture. For example, the
location of the keyboard and the mouse are dependent on each other.
The design of the keyboard affects the location of the mouse; moreover,
the location of the mouse and sitting height affect the shoulder
and arm posture. The technical measurements had minor effects on
the expert assessments.
Two of the characteristics were ratings of the tidiness and space
of the workplace and the ergonomics of the work chair, which were
the most important explanatory factors for the expert assessment.
This result was anticipated, since the space available is a basic
element of the ergonomics of a workstation (council directive 90/270/EEC).
The contents of the general European Union regulations have obviously
determined well the concept of good ergonomics for the experts.
An independent researcher assessed the tidiness and space. On the
basis of photographs only, without a deeper understanding of the
work tasks, it is difficult to assess whether there is enough space
at the workstation. The experts had accsess to video extracts showing
more about the tasks than what was seen in a single photograph.
According to the European Union directive of ergonomic requirements
in VDU work, a good chair is adjustable and stable and allows the
worker to change his/her posture easily (council directive 90/270/EEC).
In modern offices almost all the work chairs comply with these minimum
requirements. The ease of adjusting the chair and sitting comfort
are still different in different types of chairs. The classification
of the work chairs in our study was made according to their ergonomic
properties without seeing seated worker. The ergonomics of the work
chair had a strong impact on the assessments of both experts.
The changes in single work characteristics in the 2-month period
were relatively small. Still, many of these changes showed a significant
association with the ratings. For example, moving the mouse location
towards the keyboard (i.e. shortening the distance between the mouse
and the g-h keys) during the follow-up time had a positive effect
on the expert ratings. Likewise, moving the mouse and the keyboard
away from the front edge of the table, increasing the line-of-sight
angle to the first row of the screen, and replacing the chair by
one with better ergonomic properties, had all positive effects on
the expert ratings. These changes may bring along a more neutral
neck and shoulder position, a possibility to support the forearm
on the table top, and a well-supported comfortable sitting posture.
The ergonomics rating at the 2-month follow up was also related
to improved tidiness and better spatial arrangements, which further
emphasize the importance of general order and functionality at the
workstation.
A method to observe risk factors for upper limb disorders
A semiquantitative, time-based method to assess the presence of
commonly agreed risk factors of upper limb disorders (Figure
3) was developed and validated. In the validation study, inter-observer
repeatability was found to be acceptable for five of the six physical
load factors. Observations of repetitive use of the hand, use of
hand force, pinch grip, nonneutral wrist posture, and elevation
of the upper arm showed moderate or good validity when expert observations
were used as a reference standard. When observations were validated
against force estimations (EMG) and wrist goniometer data, the validity
was poor. The criteria for the threshold values for the intensity
levels of the physical load factors were derived from the literature.
The 1/3 limit for durations of risk factors was chosen arbitrarily
except for the duration of nonneutral wrist postures (Keyserling
et al., 1993).
The prevalence of positive findings estimated by EMG for the use
of hand force was 80% for the right hand and 66% for the left hand.
The observers and the expert found a considerably lower prevalence
of high-force cycles. The observers as well as the expert clearly
underestimated the use of hand force. Assessments of the use of
force are made by observing for the movements and actions of a worker,
i.e. looking for indirect evidence of the use of force. It is obvious
that there are force production situations, e.g. static postures,
in which such evidence is hard to see.
In our study a work cycle was classified as a high hand-force cycle
if the worker handled objects >4.5 kg for more than one-third
of the cycle time. When force estimations by EMG were used as a
reference standard for the use of hand force, the kappa-values showed
poor validity. Training for the observation was done against the
expert observations, i.e. EMG measurements were not utilized at
all in the training. In addition, the problem with the EMG measurements
was that they were scaled to represent momentary usage of hand force
during work even though the observation was estimated as a mean
force during the cycle. In the PRIM study, a scale from 1-5 was
used according to the percentage of maximum muscular strength. The
estimates were based on the estimated external force in combination
with the actual positions of the wrist joint. The inter-observer
reliability was found to be satisfactory (Fallentin et al., 2001).
Juul-Kristensen et al., (2001)
used the same observation criteria in her study of force demands
in repetitive work and showed that the estimated peak force demand
corresponded well with the measured peak EMG-level.
Both observers underestimated nonneutral wrist postures when the
goniometer measurements were used as a reference standard, and this
resulted in a poor validity of this item. Few previous studies have
compared observed wrist postures with goniometer measurements in
field conditions. In the study of Juul-Kristensen et al., (2001)
the differences between observed wrist postures and goniometer measurements
were small. A common difficulty in her and our study was that specifying
the zero-point (reference position) was difficult. For instance,
during gripping (both pinch and power grip) the metacarpals extend
in relation to the forearm. If the sensor of the goniometer is placed
on a metacarpal, gripping will always result in extension of the
wrist.
Moreover, forearm rotation causes zero drift errors and cross-talk
in goniometer measurements, which was not taken into consideration
in this study (Buchholz and Wellman, 1997).
Although different nonneutral wrist postures differ with regard
to their effects on the wrist, e.g. carpal tunnel pressure, all
of them have been considered less desirable than the neutral posture
(Viikari-Juntura and Silverstein, 1999).
We did not differentiate between the direction of wrist deviation
in our criterion for nonneutral wrist posture and used only one
threshold value of 20° . In some studies, different reference values
have been used for the different directions of deviated postures.
Deviated postures of the wrist occur at work in combination rather
than alone. It is therefore difficult for the observer to estimate
each of the different postures in real work situations.
Methodological aspects
Study population and participation rates
As regards the validity of the results in the study on incident
neck pain (Study 1), the crucial question would be related to a
possible bias caused by a low participation rate. The drop-out rates
in various longitudinal studies of musculoskeletal disorders have
ranged from 7%-57% (Bildt et al., 2001).
The response rates in our study were 81%in the baseline survey and
78%in the follow-up, corresponding to drop-out rates of 19%-22%.
All in all, our response rates are among the highest ones in longitudinal
studies, resulting in an overall participation rate of 63%. The
non-respondents to the follow-up questionnaire did not differ from
the respondents with regard to most explanatory variables. However,
the respondents seemed to be more stressed than the non-respondents.
In the ergonomic intervention study (Study 2) 124 subjects fulfilled
the inclusion criteria. Fifteen subjects were not able or declined
to participate. Thus, at the beginning of the study there were 109
participants. In the 2-month follow-up there were 107 participants,
and in the 10-month follow-up 102. The primary reasons for dropping
out were long sick leaves or leaving the job. The dropout rates
can be considered low.
In the validation study of the exposure observation method (Study
4) a total of 127 work cycles of 14 workers in five occupations
were studied, one to four workers representing one occupation. The
cycles of a specific job are likely to be fairly similar, which
may have helped the observers in their assessment and made the inter-observer
repeatability and validity higher than they would have been had
all cycles been from different jobs. The limited range of jobs also
limited the variability in our data.
Study design
The study population for this prospective study was the entire population
of those full-time working employees whose job included VDU work
for more than 4 hours per week (n=515). Altogether 416 workers participated
in the baseline survey in 1998 (81%). Of the baseline respondents,
the subjects of interest were those who reported local or radiating
neck pain for less than 8 days during the preceding 12 months. These
subjects were classified as 'healthy' at baseline (n=232). This
cohort was studied 12 months later, the response rate being 78%
(n=180). At follow-up in 1999 the incident cases were those who
reported local neck pain or radiating neck pain for at least 8 days
during the preceding 12 months. The strength of the study was that
all three groups (intensive ergonomics, education in ergonomics,
control) were comparable as regards demographic characteristics
and occupational factors measured at the baseline. The subjects
were chosen to the three groups by individual randomization using
the administrative unit as a stratum. Hence cultural differences
between the units were controlled for. On the other hand, it was
practically impossible to prevent personal interaction between the
groups. The changes in workstation dimensions, and the slight improvement
in ergonomics in the control group may be due to contamination by
information from the intensive or education group, or may simply
be a continuous development of workplaces. The observed effect of
the intervention may therefore be an underestimation of the true
effect.
Moreover, technical problems resulting in loss of work load data
for a third of the subjects, might have weakened the power of our
analysis. The loss of measurements was, however, evenly distributed
among the groups.
The main purpose of the ergonomic intervention of the present study
was to activate VDU workers to identify problems in their own workstation
and to find ergonomic solutions themselves. The role of the physiotherapists
during the participatory process was to work as facilitators rather
than actors.
Health outcomes
In first study the workload factors and workstation dimensions were
used as explanatory variables and neck pain as the outcome. Pain
is an unpleasant sensory and emotional experience in one or more
parts of the body. Pain is always subjective. Many people report
pain in the absence of tissue damage or any likely physiological
cause. When the question is about musculoskeletal pain, it is often
widely spread and not easy to locate.
Hence, it is difficult, if not impossible, to measure pain objectively.
Self-reported symptoms collected with questionnaires have been the
outcome in the majority of epidemiological studies on musculoskeletal
disorders. The criteria for the duration and localization of the
pain have varied in different studies. Standardized questionnaires
such as the Nordic Questionnaire have been developed in order to
facilitate comparison between studies. Nordic questionnaires have
been widely used and can be considered as an international standard.
In the present study, a slightly modified version of the Nordic
Questionnaire was used.
When defining the incidence of a symptom, such as neck pain, one
has to consider which cases are truly incident cases. For the identification
of a symptom-free study population, a relevant time -period without
symptoms before the occurrence of a new episode of pain has to be
defined. No consensus of the optimal length of such a time period
exists in the literature. A commonly used symptom-free time-period
has been 12 months. The reporting of symptoms in the past year has
proven to be more reliable than reports of recent symptoms (e.g.
in the past month). In general, the Nordic questionnaire has high
repeatability and sensitivity and, hence, it is a highly utilizable
tool in screening and surveillance (Palmer, 2000).
In the ergonomic intervention study, pain and strain symptoms during
the preceding 3 months and daily discomfort were used as outcomes.
Sensations of discomfort and strain are more reversible than pain.
It is a common hypothesis, as yet unproven, that discomfort and
strain are predecessors of pain.
Self-assessment of work load factors in VDU work
Most of the questions concerning pain, workload factors or workstation
dimensions in this dissertation have been validated. The measures
specific to VDUs, such as location of the screen, keyboard and mouse,
were based on the measurements done by the subjects themselves.
This might be a source of error if there were low agreement between
the dimensions based on the measurements of the study subjects and
those of the professional ergonomists. An earlier validation study
has found good agreement between self-reported locations and direct
measurements (Karlqvist et al., 1996).
However, the keyboard and the mouse are used in parallel, their
placements being dependent on each other. The design of the keyboard
affects the location of the mouse, and the location of the mouse
affects the shoulder and arm posture. For example, mouse users may
benefit from a shorter keyboard without a number pad (Tittiranonda
et al., 1999c).
It should also be noted that the actual work posture is not exclusively
affected by these workstation dimensions (Gerr et al., 2000).
The physical work environment included five aspects: lighting, temperature,
quality of the air, size of the workroom, and acoustics of the work
environment. For each subject. the mean of the five components was
calculated to represent the status of the physical work environment.
The variables of the physical work environment were self-reported.
Although this assessment preceded incident neck pain, there is a
possibility of an error, if those who in the follow-up reported
neck pain had a different perception of their work environment at
baseline.
The various risk factors for musculoskeletal pain have depended
on the duration of VDU work. The analyses were adjusted for the
proportion of the total working time spent at the computer. The
time used for VDU work was measured as the self-reported proportion
of total working time during the preceding month. In a study among
newspaper workers it was found that the workers overestimated their
time working with the VDU when compared with that based on observation
(Bernard et al., 1994).
However, these validations concerned typing only, whereas in our
study the definition for VDU work was use of the keyboard or other
input or control device, including short periods of thinking and
checking the results on the screen. The preliminary results of our
own validation among a sample of workers support the findings of
Bernard et al., in that the workers tended to overestimate their
VDU working time.
Expert assessment
The inter-examiner repeatability between the two experts in assessing
the level of ergonomics in VDU work, and in industry, was good;
this may be due to the fact that in both studies (1 and 4) both
experts had the same educational background. Therefore they obviously
shared a similar idea of ergonomics and risk factors for musculoskelatal
disorders already at the start of the study. The training period
converged the experts' assessments further. Thus, when carrying
out the study, they obviously based their assessments on the same
criteria even if the personal weighting of single criteria might
have been different.
The inter-observer repeatability for the studied VDU ergonomic assessment
was high and the ratings of the experts correlated with the work
place characteristics. There is no golden standard for the validation
of an assessment of the ergonomics in VDU work. The values of the
technical measures can be questioned. However, in practical ergonomics,
an expert assessment is probably more useful than time-consuming
measurements and sophisticated calculations.
When
validating the observation method to assess the physical loads imposed
on the upper limbs, force estimations by EMG served as a reference
standard for the use of hand force. In the results, kappa-values
showed poor validity. Two factors can explain these contradictory
results. Firstly, the training for observation was done against
the expert observations, i.e. direct measurements of EMG were not
utilised at all in the training. Secondly, the problem with the
EMG measurements was that they were scaled to represent momentary
use of hand force during work. A biomechanical model of the hand
would have provided a more reasonable estimate of hand force, taking
into account various factors, e.g. anatomical differences, different
postures of the hand and forearm, and the non-linear relation between
the myoelectric activity of a muscle and force production.
Both observers underestimated nonneutral wrist posturs when the
goniometer measurements were used as a reference standard, and this
resulted in a poor validity of this item. The use of the wrist goniometer
in field conditions entails several problems. For instance, during
gripping (both pinch and power grip) the metacarpals extend in relation
to the forearm. If the sensor of the goniometer is placed on a metacarpal,
gripping will always result in extension of the wrist. Moreover,
forearm rotation causes zero drift errors and cross-talk in goniometer
measurements, which was not taken into consideration in this study.
This method, validated in industry, combines some features of checklists
and continuous observation methods. Continuous observation is needed
to make a decision on the fulfilment of the time aspect of the criteria.
Contrary to many other continuous methods, there is no need to assess
the level of a risk factor during the observation. Instead, the
entire cycle is observed, after which a dichotomous estimation (above
or below reference level) is given. This simplifies the continuous
observation and decreases the labriousness of the method.
| CONCLUSIONS |
- In
the prevention of neck disorders in office work with a high
frequency of VDU tasks, attention should be given to the
work environment in general, as well as to the more specific
aspects of VDU workstation lay-out. In addition, our study
gave further evidence that physical exercise may help prevent
neck disorders among sedentary employees.
- Both
the intensive ergonomics approach and education in ergonomics
reduce discomfort in VDU work. One way to improve the level
of physical ergonomics in VDU workplaces is to initiate
a co-operative action in which both workers and occupational
health practitioners are actively involved.
- The
inter-observer repeatability for the studied method was
high, and the ratings of the experts correlated with the
work place characteristics. There is no golden standard
for validating the assessment of the ergonomics in VDU work.
The values of the technical measures can be questioned.
In practical ergonomics, an expert assessment is probably
more usable than time-consuming measurements and sophisticated
calculations.
- Inter-observer
repeatability and validity were acceptable in the semiquantitative,
time-based method developed to assess the presence of the
most commonly occuring risk factors of upper limb disorders.
This observation method is meant to be used by health and
safety professionals or engineers in actual workplaces.
The used reference values for the proportional duration
of some physical load factors need further consideration.
Studies should be carried out to assess the limits that
best differentiate between safe and hazardous jobs.
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| ACKNOWLEDGMENTS |
This
study was carried out at the Finnish Institute of Occupational
Health, Musculoskeletal Research Unit of the Department of Physiology.
I thank Professor Jorma Rantanen, Director General, for creating
an enthusiastic atmosphere for research, and Professor Juhani
Ilmarinen, Director of the Department of Physiology, for placing
the facilities of the Institute at my disposal.
I wish to express my warmest thanks to Professor Eira Viikari-Juntura
from the Göteborg University and my supervisor from the Finnish
Institute of Occupational Health for her stimulating guidance
in scientific work and writing. Despite her many duties, she
has always been available. Her encouragement and friendship
during the later stages of the study have been of utmost value.
I would like to express my gratitude to my supervisor Professor
Veikko Louhevaara from the University of Kuopio and the Finnish
Institute of Occupational Health for his advice and valuable
comments. His encouraging attitude has been important for me
during these years.
I am deeply grateful to Risto Toivonen, my co-author, for his
guidance in statistics, for helping me with computer programs,
and for all the stimulating discussions, hard work and friendship.
I thank whole-heartedly also my other co-authors: Marketta Häkkänen
for her friendship and inpiring study ideas and for making the
work fun and enjoyable, Tellervo Korhonen, my dear " fellow
student" for her contribution and collaboration, Esa-Pekka
Takala, Head of the Musculoskeletal Research Unit, for his support
and help me with the literature gathering; special thanks go
to Ritva Luukkonen for her expert advise on statistics.
I extend my sincere thanks to my colleagues and friends Helena
Hanhinen, Ritva Kukkonen, Ella Kylmäaho and Sirpa Rauas-Huuhtanen
for their expert work with the data collection and for carrying
out the intervention.
I thank Chris Jensen, PhD, and Docent Anneli Pekkarinen, the
official reviewers of this study, for their most valuable comments.
Many of my colleagues at the Finnish Institute of Occupational
Health have kindly contributed to my work. I wish to express
my thanks particularly to the "Dissertation club"
and Martti Launis, Jouni Lehtelä and Sirpa Lusa for many intresting
discussions and for sharing with me their knowledge of ergonomics.
Warm thanks are due to Merja Vuorisalo for the numerous practical
arrangements during the study. I also wish to thank Terttu Kaustia
for her advice in correcting the language.
Finally, my deepfelt thanks go to my late mother for her constant
support and help during these years, and for taking care of
the many everyday needs of my family. I would not have made
this without the encouraging support of my beloved family: Salla,
Eliisa, Inari, Sampsa and Mika.
And last but not least, I thank my dear husband Eino for his
constructive comments and criticism of my work and his encouragement
throughout all years together.
The Finnish Work Environment Fund is acknowledged for funding
this study.
Vihti, September 2003
Ritva Ketola
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| AUTHOR
BIOGRAPHY |
RITVA KETOLA
Employment: Senior researcher
Finnish Institute of Occupational Health, Helsinki
Degrees: PhD, Occupational Physiotherapist
Research interests: Ergonomics, musculoskeletal health
Email: ritva.ketola@ttl.fi
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