Office Chair Primer
Office Chair Primer
Office Chairs - An Ergonomic Primer
By David Gilkey, D.C., Ph.D, CPE
Director, Environmental & Radiological Health Sciences
Colorado State University
The office chair is the most frequently used piece of office equipment.
An ergonomically designed chair is an essential item for all computer
workstations. Experts have identified several features characteristic
of well-designed ergonomic office chairs. With increasing numbers of
computer users in the workforce, the computer office chair has received
great attention. It is presently estimated that 45 million American
workers spend some time each day using a computer and keyboard.
Approximately 30 million workers use the office chair, computer,
keyboard, and pointing devices as their primary work equipment each day,
all day, and up to 8 hours per day or more. Computer use has been
linked to several types of injuries known as “Upper Extremity Repetitive
Stress Injuries” (UE-RSI's), “Cumulative Trauma Disorders” (CTDs), or
“Work Related Musculoskeletal Disorders” (WRMSDs). RSIs, CTDs, and
WRMSDs are associated with the upper extremities (UE) or arms, forearms,
wrists, hands, and fingers as well as the neck, back and lower
extremities (LE) or legs. Common CTDs associated with computer input
devices include Carpal Tunnel Syndrome (CTS), Tendonitis, and
Tenosynovitis effecting the hands, wrists, and forearms as well as Neck
Tension Syndrome, Low Back Pain (LBP), and LE pain. The Bureau of Labor
and Statistics (BLS) reported that the incidence of such disorders has
increased 770% between 1981 and 1991 (BNA, 1995). Several studies have
clearly shown a definitive link between CTDs and computer use (NIOSH,
1997). Ergonomics is the premier science which concerns itself with
humans at work and the many aspects of the “Human Computer Interface”
(HCI). Studies clearly demonstrate a scientific basis for ergonomic
design of office chairs. The following is an overview of ergonomic
issues related to office chairs.
Definitions and Terms:
A) Office Workstation Chair – The “office chair” has taken many forms
in recent times with a wide array of workstation designs available for
users. The “standard” office chair sits upright on a base, has a seat
pan, chair back, and possibly arm rests. Modifications to these basic
elements include recumbent, sit-stand, and kneeling designs. This
primer addresses features related to the “standard” upright office
B) Seat Pan - The seat pan is where you sit. It is the seat of the
chair and provides the primary contact surface for sitting upright. The
seat pan is available in a variety of sizes, shapes, and materials.
Seat pans may be hard or soft, adjustable or fixed in position. Seat
pan depths and widths are typically between 15” to 19”.
C) Seat Height – This refers to the relative height from the floor to
the seat pan. Height ranges are typically between 15” to 20”. Seat
height may be adjustable or fixed.
D) Chair Back – The chair back supports the user from behind. Chair
back sizes range dramatically from “low-back” (~10”) to “high-back”
(~30”) chairs. Chair backs can be fairly straight upright (900) to
sloping back (1200) to reduce loads on the lumbar spine. They may be
fairly flat or curved with a built-in lumbar support or curve. Chair
backs may be hard or soft, adjustable, or fixed.
E) Armrests – Armrests are a feature preferred by some and not
appreciated by others. They are intended to support the weight of the
UEs while in the seated position. Armrests may be hard or soft,
adjustable, or fixed.
F) Chair Base – The chair base provides support for the entire body
weight. A typical office chair has a 5-post base for safety and
G) Adjustability – This refers to the changeability of the many
features of the office chair. Adjustability is an important feature
that may be designed into the chair to alter positions, size, and height
of the seat pan, chair back, and arm rests.
H) Seat Pan Tilt – This refers to the relative position of the seat
pan compared to level. If adjustable, it may be positioned positive
slope, negative slope, or level.
I) Backrest – Seat Pan Angle – This refers to the relative position of the chairback to the seat pan.
J) Waterfall Edge – This refers to the flowing downward of the front
edge of the seat pan to reduce pressure on the back of the knee.
K) Impaired Circulation – This describes the loss of normal
circulation to body parts such as UE or LE resulting from direct contact
pressure or sustained static loading.
L) Spinal Loading – This refers to the amount of force placed on the spine.
3) How Ergonomic Chairs Address These Issues:
A) The features that distinguish “Ergonomic” office chair designs
from less optimal chairs are identified below (Lueder, 1994):
1. Adjustable seat height
2. Adjustable back rest
3. Adjustable seat pan
4. Soft seat padding
5. Slightly concave seat shape
6. Seat pan front edge “waterfall”
7. The backrest tilts back easily
8. Padded lumbar support – adjustable
9. Full back support up to shoulders
10. Arm rest – short, padded, and adjustable
11. 5-Prong / leg chair base
12. Casters that roll easily
13. Seating that produces no pressure on knees
14. Preferably anti-static
15. Make all adjustments while seated
B) The Seat Pan – The seat should be slightly concave to fit the
contour of the buttock for comfort and even distribution of force over
contact areas. Many designs carry this a step further and sculpt the
pan to fit very comfortably for differing buttock shapes and comfort
preferences. The user must try (sample) the different designs to find
the most comfortable for him or her. The seat pan dimensions should be
in the range stated above (~(15” – 19”) x ~(15” – 19”)). Each user
should sit in the chair to test it for optimal fit, space, support, and
comfort. The material should be soft, padded, and durable. The front
of the pan should flow downward like a “waterfall”. This design ensures
no excessive pressure behind the knee causing impaired circulation.
C) Seat Height – This must be adjustable to accommodate the
variability of leg lengths. Most ergonomically designed chairs adjust
between 16” and 25” vertically. When properly seated, the thigh should
be parallel to the floor. The seat height is the first adjustment to be
accomplished in fitting the chair to the user. The proper height
adjustment establishes the placement of the remainder of office and
computer equipment for the overall workstation layout. Some users
prefer their seat low with the thigh slanting backward and downward to
the hips. This is preferable to adjusting the seat too high, which can
cause increased pressure behind the knees and impair circulation to the
LE. In the event the seat is adjusted too high for a user to place feet
squarely on the floor a footrest may be appropriate.
D) Seat Pan Tilt – This should be adjustable 150 (+/-) from level to
suit user’s preference. Most users prefer a level seat but others
prefer a positive or negative slope for comfort or special needs. The
ergonomically designed chair accommodates users with adjustability.
C) Backrests - Ergonomic features of backrest (aka - Seatback /
Chairback) design include size, shape, and adjustability. The backrest
should be large enough to cover the entire width of the back. A minimum
of 12” is recommended for width. Seat back height preference varies
dramatically from user to user. Some users prefer chairbacks designed
for only lumbar support, these commonly range from 6” to 10” in height.
In that case, the lumbar support should be centered at L 3-4
vertebrae. A lumbar support should also have at least 4” of
adjustability to allow centering in the back. However, many ergonomic
chairs are designed with full-length chair backs that support from lower
back to the top of the shoulders. In full-length designs, backrests
should be contoured to fit the “S” shaped curves of the spine, not
entirely flat or straight.
D) Backrest - Seat Pan Angle – This important angle should be
adjustable for the individual users preference. The angle between the
seat pan and chair back should be adjustable between 600 to 1000 when
the user is seated with thighs parallel to the floor and legs properly
supported vertically. This angle permits the user to sit slightly
forward, straight up, or recline back depending on the type of computing
performed, support needed, and comfort desired.
E) Armrests - Ergonomic armrests are optional features. Individual
preference prevails in deciding to buy a chair equipped with such a
feature. Armrests, like wrist rests, aid in supporting UE weight and
thus help maintain comfort, endurance, as well as normal circulation by
decreasing static load to muscles contracted to lift and hold limb
position during computing. Muscles that contract vigorously quickly use
their energy supply and starve for more oxygen and sugar. Using
armrests reduce the amount of contraction necessary to hold the limb in
position thereby reducing the use of oxygen and energy. Armrest users
report enhanced performance including less fatigue, increased comfort,
and better endurance with sustained computing. Armrests should be
placed at least 18.5” apart and made of soft or padded material. An
ergonomically designed armrest should be adjustable vertically and not
impair circulation due to direct pressure to contact areas but
distribute that load over broad areas comfortably. Armrests should
adjust between 2” and 4” vertically to accommodate user’s preference.
F) Chair Base – An ergonomically designed chair has a solid, safe,
and stable 5-post chair base. It should be made of strong materials to
support up to five times the body weight. The chair base should also be
equipped with quality casters to permit easy maneuvering on office
floor surfaces. Specific casters are available for carpeted Vs. hard
floors. Users are recommended to purchase the appropriate caster for
the floor surface used. Do not assume that casters are universal for
all surfaces unless reported by the manufacturer.
G) Adjustability – Adjustability is the “hallmark” of ergonomics.
Chair adjustments should be easy, intuitive, and accomplished while
sitting in the chair. Leuder (1994) defined “ease of adjustability” as
1. Adjustments from the standard seated position.
2. All users can understand adjustment labels and instructions.
3. Adjustment controls are easy to find and interpret.
4. Tools are not required to make adjustments.
5. Adjustment controls provide immediate feedback.
6. The adjustment controls are logical, intuitive, and consistent.
7. A minimum amount of motion and effort is required to successfully make adjustments.
8. Adjustments may be made with one hand.
9. Adjustments are intrinsically reinforcing.
H) Spinal Loading – Protracted use of office chairs commonly cause
fatigue to the back. Ergonomic chairs are designed to support the
spine. Aspects of ergonomic design can reduce spinal loading by
properly fitting the user and providing reclined positioning. Support
to the lumbar spine is accomplished by a properly fitting backrest
adjusted to the optimum position. Reclining the backrest reduces spinal
loading. Users are advised to test the many features of ergonomic
chairs to identify the best combination of features that meet their
A) General Ergonomic Considerations:
Office chairs are the most frequently used piece of office equipment.
They are often taken for granted; yet, a properly designed office chair
is an essential piece of equipment for anyone working in a seated
posture for extended periods of time. Ergonomic designs optimize human
interaction and preserve health and well-being. Ergonomic office
chairs are a critical component of an overall workstation design.
Ergonomically designed office chairs offer a number of features to
accommodate the human body’s shape, size, capabilities, limitations, and
comfort. A well-fitted chair must be selected as though it were a
piece of clothing. Sampling the fit is recommended to ensure the
optimal user-equipment interface.
Bureau of National Affairs. (1995). 770 percent increase. Occupational Safety and Health Reporter, 36, p.1794.
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Leuder, R. (1994). Seating, posture, and ergonomics. In Sweere, J. (Ed.)
Chiropractic Family Practice. (p.21-2:1- 2:9). MD: Gatherspburg. Aspen
National Institutes for Occupational Safety and Health. (1997).
Musculoskeletal Disorders and Workplace Factors. NIOSH Pub No. 97-141.
Selan, J. (1994). The Advanced Ergonomics Manual. TX: Dallas. Advanced Ergonomics.