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3. Decrease the vertical distance that loads must be lifted
lowered and the distance which loads must
/
be pushed, pulled or carried
4. Decrease the frequency of the task or increase the number or workers performing the task
5. For pushing and pulling tasks, provide equipment that provides the least resistance so that initial
forces required to overcome inertia are as low as possible. Maintenance of mechanical assists is
very important with regards to this principle
6. For all MMH tasks, provide good hand-to-object coupling when possible, that is, tote boxes with
handles, carts with handle bars, etc
7. Decrease the duration over which the task is performed
8. Change pulling tasks to pushing tasks
47.3 The Psychophysical Approach to Designing Upper
Extremity Tasks
The primary risk factors for WRMSDs of the upper extremity are fairly well known
35
. Task-related risk
factors include posture, force, and repetition. Vibration and cold are task-related risk factors for some
disorders such as carpal tunnel syndrome. Duration of the task and rest periods are also important
since these factors affect the acceptability of task. Altering work-rest relationships can alter the
acceptability of a particular combination of posture, force, and repetition.
There are few quantitative guidelines for limits of posture, force, and repetition. Although general
guidelines suggest maintaining a neutral wrist posture and reducing the force requirements and
frequency of a task, these guidelines do not indicate acceptable levels of the variables. Once ergonomic
analyses and task redesign are done, a decision as to the acceptability of a task is difficult.
The application of the psychophysical approach to the design of UEI tasks was a response to the need
for establishing quantitative guidelines with which to assess tasks. Currently, quantitative dose-response
relationships developed with epidemiological techniques that provide relationships between individual
risk factors and their interactions and the risk of upper extremity WRMSDs do not exist. In the
absence of such relationships, psychophysical data will continue to be one option of setting task
limits. The remainder of this section will provide an overview of the current state of psychophysical
data as well as discussion of how these data are applied in the workplace.
One advantage of the psychophysical approach is that data can be developed which incorporate force,
posture, and repetition into the development of data for different durations. This is important in that this
approach allows for trade-offs between variables, that is, for some tasks, it is not always possible to
modify all factors. The psychophysical approach has been extended to study specific tasks such as
acceptable impact severity levels for an automotive trim installation.
36
Subjects performed five hand
impacts per minute on a device that simulated the process of seating push pins during door trim
panel installation, and subjects altered the impact to a level they felt could be performed without
causing injury, numbness, or pain.
47.3.1 Setting Acceptable Force and Frequency Limits
Fernandez and his colleagues have collected maximum acceptable frequency data for several types of tasks
include drilling,
37 - 40
riveting,
41,42
and tasks requiring pinch and power grasps.
43,44
In these studies,
factors such as wrist posture and duration were incorporated into the experimental protocol to
provide frequency limits for a variety of UEI tasks. In a similar study, Abu-Ali et al.
45
had subjects
control the length of rest periods for task combinations of varying wrist postures, exertion periods,
and power grip forces.
In order to use these data, one would record relevant task parameters, find the data relevant for a par-
ticular task in the database, and determine if the task is acceptable to the majority of the population, just
as with MMH data. If the task is not acceptable, then the frequency would need to be reduced, the
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