Civil Engineering Reference
In-Depth Information
routinized, that is, less predictably structured, valid estimation of both task distributions and task-
specific exposures becomes increasingly challenging.
Typically, both observational and direct measurement techniques generate highly detailed,
accurate exposure analyses for a relatively short period of elapsed time in each job. Most protocols
for these methods assume that the work is cyclical, with little variability over time, so that it is reasonable
to measure exposures for a short period and extrapolate them to the long term. But many jobs do not fit
this model: they are not comprised of work cycles, or the cycles are highly variable in their total duration
or content (the number or sequence of steps that comprise each cycle) and do not account for all of
the work performed by an individual with any given job title. For these jobs, it would be infeasible
to undertake continuous measurements for entire cycles as an exposure assessment strategy,
because either there are no cycles, or a very large number of (long) cycles would have to be recorded
in order to quantify accurately the total and average duration of exposures. With short measuring
times, the data collected are of uncertain representativeness because these time periods do not match
the duration of exposures that are thought to be relevant for the development of musculoskeletal
disorder.
A versatile alternative for estimating physical exposures is the use of data collected directly from
workers. Such reports may address both task-specific exposures within jobs and the distributions of
tasks performed by each worker. In addition to being time-efficient, self-reports permit assessment of
exposures in the past as well as the present and may be structured with task-specific questions or orga-
nized to cover the job as a whole. Some researchers have explicitly recommended a composite approach
to the analysis of nonroutine jobs, in which task-specific exposures are measured directly and the tem-
poral distribution (frequency and duration) of each task is obtained from self-report. Self-reported data
can take various forms, including duration, frequency, and intensity of exposure. In some studies, absol-
ute ratings have agreed well with observations or direct measurements of the corresponding exposures,
while others have diverged significantly, especially with use of continuous estimates or responses that
required choices among a large number of categories (e.g., Burdorf and Laan, 1991; Faucett and
Rempel, 1996; Lindstr¨m, et al., 1994; Rossignol and Baetz, 1987; Torg´n et al., 1999; Viikari-Juntura,
1996; Wiktorin et al., 1993).
Retrospective recall of occupational exposures has been frequently employed in studies of musculos-
keletal disorders, but there are few data on the reproducibility of such information. Three studies have
examined the potential for differential error (i.e., information bias) in self-reported exposure with respect
to musculoskeletal disorders with mixed results; some risk estimates were biased away from the null
value, some toward it, and others not at all (Torg
´
n et al., 1999; Viikari-Juntura, 1996; Wiktorin et al.,
1993). In the REBUS
1
study follow-up population, Toomingas et al. (1997a) found no evidence that indi-
vidual subjects systematically overrated or underrated either exposures or symptoms in the same direc-
tion. Self-reported exposures have promise, but their validity depends on the specific design of the
questions and response categories.
A variety of instrumentation methods exist for direct measurement of such dimensions as muscle force
exertion (electromyography), joint angles and motion frequency (e.g., electrogoniometry), and vibration
(accelerometers). For example, the goniometer has been used in a variety of studies of wrist posture,
including field assessments of ergonomic risk factors (Moore et al., 1991; Wells et al., 1994), comparisons
of keyboard designs (Smutz et al., 1994), and clinical trials (Ojima et al., 1991). Hansson et al. (1996)
evaluated the goniometer for use in epidemiologic studies, and Marras developed a device for measuring
the complex motion of the spine (Marras, 1992). While many consider these methods to represent col-
lectively the standard for specific exposures, each instrument measures only one exposure, and usually
only at one body part. When multiple exposures are present simultaneously and must be assessed at
1
In the original REBUS study conducted in 1969, participants were asked to complete a questionnaire regarding
health status — all selected were given a medical examination. A diagnosis of musculoskeletal disorder required
signs and symptoms. The follow-up study, conducted in 1993, asked the younger participants in the original
REBUS study to participate in a re-examination.
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