SECTION 15.35 ERGONOMICS
Last
Update: 11/03
Ergonomics is the study of
work. The goal of an ergonomics program
is to make the job fit the person, not make the person fit the job. This goal is accomplished by redesigning the
job or tool to reduce the job demands of high force, repetition, and awkward
postures. By improving the fit between
the worker and the job, we not only contribute to the well being of the worker,
we are also improving productivity.
In dealing with ergonomics, the
initial task of identifying the causes is often overshadowed by the more
difficult task of deciding on the most effective method of control or
intervention. There is seldom a simple,
single change to be made. Perfect
solutions are rarely available and design decisions often involve compromises
and trade-offs. The most efficient way
to proceed in administrating ergonomic intervention is as follows:
1.
Perform a thorough examination (job analysis), to determine
the specific problem. Interview workers
to find out what problems they are encountering while working at their
workstation.
2.
Evaluate, select, and implement the most appropriate
intervention(s), supported with training and education.
3.
Begin conservative medical management, as appropriate.
4.
Monitor progress.
Continually interview employees to find how corrective measures have
helped, as well as asking if additional steps need to be taken.
5.
Continue to adjust or refine the scope of the intervention
as needed. Fine-tune any additional
ergonomic corrections that need to be made.
A good ergonomic program will
target the following objectives.
1. Reduction
of extreme joint movement. Work
activities should ideally be performed with the joints at about the midpoint of
their range of movement. For example,
when force is being applied by the hand, the wrist should be kept straight and
the elbow bent at a right angle. All
side-to-side deviations of the wrist should be avoided. The hands should be kept in line with the
forearms. At least three methods exist
for reducing deviations of the wrist.
These include:
(1)
Altering the tool or controls by
bending the tool or handle instead of the wrist.
(2)
Moving the part in front of the worker so
the wrist can be straight.
(3)
Moving the worker and changing the position
of the worker in relation to the part.
2.
Reduction of excessive force levels. Jobs should not require the worker to exert
more than 30% of his or her maximum force for a particular muscle, in a
prolonged or repetitive way. Three
general approaches to controlling job forces are:
(1)
Reducing the force required (keep
cutting edges sharp, use weaker springs in triggers, use jigs and clamps
instead of hands to grip parts).
(2)
Spreading the force (use trigger levers rather
than single-finger push buttons and allow the worker to alternate hands).
(3)
Getting better mechanical advantage (use
stronger muscle groups and use tools with longer handles).
3.
Reduction of highly repetitive and stereotyped
movements. Since highly
repetitive and stereotyped movements contribute to CTD's (Cumulative Trauma
Disorders or referred to as MSD’s Musculoskeletal Disorders), potentially
aggravating production and design factors must be identified and altered to
reduce the repetitive levels of a work cycle.
Counter-measures include limiting the duration of continuous work or
restructuring of work methods. In
general, jobs that have a cycle time of less than 30 seconds and take at least
50% of the workers time in a given cycle should be considered as posing a risk
for CTD's. Several approaches may be
taken to reduce rates of repetition:
(1) Task
enlargement. Restructure jobs
so that each worker has a larger and more varied number of tasks to perform.
(2) Mechanization. The use of special tools with ratchet
devices or power drivers can reduce stressful repetition.
(3) Automation. Repetitive tasks are performed best by a
machine. To be cost effective, this
must typically involve a high volume, long-term production process design of
workstation.
In order to accomplish the
reduction of extreme joint movements, excessive force levels, and highly
repetitive movements, the workstation should be designed to fit the
person. Guidelines for workstations are
as follows:
An ergonomic workstation should
accommodate a vast majority of the people who work on a given job and not merely
the average. A work station that is
adjustable and was either designed or selected to fit a specific task should be
experienced as relatively comfortable by 90% to 95% of the worker population.
The workspace should also be large enough to accommodate the full range of
required movements.
To avoid static loading of
muscles, good workstation design should permit the worker to adopt several
different but equally healthy and safe postures, which still allow performance
of the job. Ideally, a worker should be
able to choose either a sitting or standing position. Sitting is best for tasks that require fine precision work, while
standing is best for tasks that require a large space to be covered or large
forces to be exerted. Arm rests and
footrests should be supplied when appropriate.
Controls, tools, and materials
should be placed between shoulder and waist height to be easily reached and
manipulated. Reaching above shoulder
level or behind the body should be avoided. All reaching should be below and in
front of the shoulder. As a general
rule, for most types of industrial jobs, the work area should be about 2-4
inches below the elbow when standing or seated in an erect posture. The work areas may be raised 2-4 inches above
the elbow for very precise or delicate work, whereas for heavy manual-assembly
jobs, the work surface should be 4-5 inches below elbow height. For jobs where the work surface is elevated
above the elbow height, adjustable pads may be provided for resting the
forearms.
Chairs that are correctly designed
for the job and the person are also important.
A chair that is inappropriate is not only a source of discomfort, which
can affect productivity, but in the long run can contribute to back, neck, and
leg problems. The seat height,
backrest, and footrest should be adjustable to accommodate 90% of the
population and the adjustments should be simple and easy to perform. The height of chairs should be adjusted so
that the thighs are horizontal, feet rest on the floor, and the arms and hands
are comfortably positioned at the keyboard or work surface. The backrest should be positioned so that it
supports the lower back and fits the curvature of the spine. People should frequently change their seated
position throughout the day.
Overall levels of lighting in work
environments should be between 300 and 500 lux, with dual level switches. Task lamps should be provided where
necessary. Indirect lighting or
parabolic fluorescent fixtures should be used in open spaces.
Screens on all video display
terminals should be positioned to minimize glare and reflections from overhead
lights, windows, and other light sources.
The display should be adjusted so that the top of the screen is slightly
below eye level. The contrast or
brightness of the screen should be placed at a comfortable level. Where it is impossible to avoid reflections
or adjust lighting, an anti-glare filter placed over the screen can be helpful.
In addition to these corrective
measures, a worker must also work smart.
A worker should change position, stand up, or stretch whenever feeling
tired. Rest the eyes occasionally
throughout the workday to alleviate eyestrain.
Finally, try to keep a soft touch on keyboards and keep the hands and
fingers relaxed.