Adjustability Chairs - "An Attempt to DeMystify
All Those Chair Adjustability Options"
Chair companies --- and ergonomists --- are often guilty of throwing
terms around without fully explaining the exact assumptions about why
a given adjustment or feature is going to be good for you. Here is an
attempt to link the various chair features with the supposedly good
things they will do for your body. If you think you understand why chairs
are designed the way they are, think again, and check your assumptions
against what we've written here. You may find one or two new points!
Note that this section does NOT supply specific dimensions for different
adjustments. Recommended ranges are given in documents such as the ANSI/HFES
100 standard for VDT Workstations. The standard is available from the
Human Factors and Ergonomics Society, PO Box 1369, Santa Monica, CA,
90406. And don't concentrate too much on quantity or quality of features.
Many of the features described here allow a chair to adjust to a given
posture or body size, but it is important to remember the importance
of posture change. Adjustability features should allow, rather than
inhibit, free posture change.
Feature Reason
Seat height adjustability This allows the user to adjust the
chair so that his/her feet are on the floor, or the worksurface or keyboard
is at an appropriate height, or preferably both. Pneumatic adjustability
is easier to work than mechanical adjustability.
Seat depth adjustability Achieved either by backrest in-out adjustability
or a sliding seat pan, this changes the front-to-back depth of the seat.
A shorter seat pan is necessary to allow small people to use the chair's
backrest, while a deeper one feels more stable to taller individuals.
Backrest angle adjustability This refers to changing the angle
of the backrest relative to the angle of the seat. Although this often
is done with an adjustment mechanism, it can also be achieved through
the use of flexing materials or springs in the chair shell. Backrest
angle adjustability allows the chair to support different degrees of
recline, which in turn transfers some upper-body weight to the chair
backrest and lightens the load on the lower back's intervertebral discs.
Backrest angle adjustability also increases the angle between the torso
and the thighs, which causes the lower back to curve inward. This inward
curve, called "lordosis," results in less pressure on the
discs than a flat spinal shape.
Chair recline, or tilt This changes the angle of the entire seat relative
to the floor. As with backrest angle adjustability, a reclined chair
transfers some upper-body weight to the backrest of the chair. There
are two main tilt geometries. One is column tilt, in which the chair
pivots at the top of the base post and lifts the knees slightly while
the back descends. The other is knee tilt, in which the pivot point
is forward of the post, nearer the knees. In a knee tilt chair, the
knee lift is negligible, but the back (and head) descend more than in
a column tilt chair.
Seat pan angle adjustability This generally refers to changing the
forward-back angle of the seat. It consists of a choice of fixed angle,
rather than a free-floating recline (above). Often, this feature provides
forward tilt, in which the thighs slope downward. The main purpose of
forward tilt is to open the angle between the trunk and thighs, inducing
lordosis and reducing disc pressure.
Armrests These support the arms, reducing the work of the shoulders
and possibly the upper arms. Armrests can, however, be used inappropriately
by inhibiting free motion of the arms during activities such as typing.
Height-adjustable armrests These help avoid the problems of too-high
armrests, which result in elevated shoulders and pressure on the undersides
of the elbows and forearms, and too-low armrests, which require the
worker to slump or lean to one side to use them. Height-adjustable armrests
also can keep armrests out of the way during typing or other activities
requiring free motion.
Width-adjustable armrests This kind of adjustability changes the
distance between armrests. Armrests that are close to the body can help
avoid splayed elbows, which in turn cause the wrists to bend to the
side during activities such as keying. A maintenance-adjustable mechanism
requires leaving room for the hips and therefore does not permit the
close positions that at-will adjustment allows.
Padded armrests These potentially avoid uncomfortable pressure
on the undersides of the forearms and elbows.
Lumbar support This is intended to prevent, to the extent possible,
the flattening of the lumbar spine that occurs in most people when seated.
Lumbar support is usually done through gentle curves in the backrest
shape.
Backrest height adjustability This refers to a change in height of the
lumbar support area of the chair backrest, although this feature is
often interpreted to mean a change in height of the entire backrest.
This feature accommodates preferences by different workers regarding
where and how the lumbar support curve contacts the back.
Lumbar depth adjustability This affects the size and sometimes the firmness
of the lumbar support curve in a chair's backrest. Like backrest height
adjustability, it accommodates different preferences and body shapes.
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