Office Chair Primer

Office Chairs - An Ergonomic Primer
By David Gilkey, D.C., Ph.D, CPE
Director, Environmental & Radiological Health Sciences
Colorado State University

1)    Introduction

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 chair.

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 stability.

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 the following:
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 needs.

Summary:

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.


References:   
Bureau of National Affairs. (1995). 770 percent increase. Occupational Safety and Health Reporter, 36, p.1794.
Helander, M. (Ed.). (1994). Handbook of Human-Computer Interaction. (4th ed.)  NY: North-Holland.
International Business Machines. (1991). Human Factors of Workstations with Visual Displays. (4th. ed.). IBM: Somers, NY.
Leuder, R. (1994). Seating, posture, and ergonomics. In Sweere, J. (Ed.) Chiropractic Family Practice. (p.21-2:1- 2:9). MD: Gatherspburg. Aspen Publications.
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.