Minimum Requirements
To avoid discomfort associated with sitting, chairs should be a proper fit for
the operator, and they should include features to help the operator avoid
discomfort associated with slouching and static postures.
Traditional Task Seating
"Traditional Task Seating," as discussed in this web site, refers to chairs
which include an adjustable seat pan tilt, a forward-tilting seat pan, and
an adjustable backrest angle. This allows the operator to adjust the chair
to support them in any posture. Varying these adjustments throughout the
day will help the operator avoid fatigue associated with sitting, and the declined and reclined postures
can be used to avoid slouching. That is
to say, a task chair should be looked at as a tool to be used to stay
comfortable while sitting. Other necessary features which are found on
most modern task chairs include an adjustable seat pan height, an adjustable
lumbar support, a waterfall front, and a five star base.
Proper Fit
Most task chairs are designed with features to accommodate people of different
shapes and sizes; however, no one chair will fit everybody. For a chair to be the proper
fit for an operator, it should include the recommended features and have the following
characteristics:
- An effective lumbar support. The contour of the backrest should fit the curvature of the lower back to help
prevent flattening of the lumbar spine. The backrest should be adjustable to allow the proper placement of the
lumbar support
- An effective seat cushion. The contour and padding of the seat cushion need to be sufficient
for the operator to use their chair comfortably. If the cushion is worn out or it prevents the operator
from properly using their back support, the chair will be less effective.
- The seat pan height range of the chair should be sufficient to support the operator in the declined, reclined,
and upright postures.
- The seat pan depth should not exceed, nor be more than two inches shorter than the popliteal-buttock
length of the operator. If the seat pan is too deep, the chair may increase pressure behind the knee
and decrease the effectiveness of the back support. If the seat pan is too shallow, it will increase pressure
on the operator's legs.
Seat Pan Height Range
Fig. 1. Task Chair Seat Pan Height Range
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When selecting a traditional task chair, the standing knee height is often used
to determine the seat pan height range of the chair. The standing knee height is the distance from the floor
to the upper edge of the operator's knee cap. This measurement is taken with the operator wearing their shoes.
With the chair in the forward tilt configuration, the center of the seat pan should reach the
standing knee height of the operator. It is important that the chair will support the operator
at this height, so one should take compression of the pneumatic cylinder and the seat cushion
into account. This means, when using their chair in the forward-tilt configuration, the operator will feel
even support along their leg, as opposed to pressure being centered on their tailbone. If the upper seat
pan height range supports the operator in the declined posture, the chair should be sufficient to support
the operator in the upright and reclined postures as well.
For individuals with a standing knee height of less than twenty-one inches, in this respect, many of the
standard task chairs will work. If the standing knee height is greater than twenty-one inches, more care is
usually needed for task chair selection; a chair with a larger pneumatic cylinder may be needed to increase
the seat pan height.
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Armrests
The armrests of a task chair should be taken into consideration as they can significantly help or
hinder the operator. The armrests should be adjustable and provide support with the operator comfortably
relaxed at the shoulder. If the armrests lead to awkward postures or interfere with positioning the chair
with respect to the video display terminal, it may be preferable to remove the armrests altogether or
consider a different chair.
Additional Features
Adjustable armrests and seat pan sliders will allow the chair to accommodate a greater number of people.
More advanced features, such as memory foams, inflatable lumbar support, bisynchronous tilt, special arm
options, and hip abduction are available; they aren't recommended by Environmental Health & Safety unless
they are deemed necessary on a case by case level. Selecting an appropriate task chair can be complex;
if you have questions, contact Environmental Health & Safety for assistance.
Task Chair Mechanisms
It is important to recognize that a number of manufacturers have incorporated significantly different designs
which also provide effective support. That is to say, there is a myriad of effective task chairs
that may not always include an independently adjustable backrest angle and seat pan tilt. They should include
features to help avoid static postures and slouching, and these features need to be effective considering the
size, shape, and needs of the individual.
