Keyboard Tray Primer

Keyboard Trays - An Ergonomic Primer
By David Gilkey, D.C., Ph.D, CPE
Director, Environmental & Radiological Health Sciences
Colorado State University
 
1)    Introduction

Keyboard trays support the placement and position of keyboards for inputting data into computers.  A well-designed workstation is equipped with a keyboard tray that adjusts to the preference of the user.  Ergonomic features include adjustability for reach, height, and tilt as well as adjacent placement of the mouse or other input devices.  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 computer and keyboard as their primary work equipment each day, all day, and up to 8 hours per day or more.  Computer use has been shown to be 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.  Common UE –RSI disorders 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.  The Bureau of Labor and Statistics (BLS) reported that the incidence of such disorders has increased 770% in the past decade (BNA, 1993).  Several studies have clearly shown a definitive link between UE-RSIs and computer use (NIOSH, 1997).  Ergonomics is the premier science, which concerns itself with humans at work and the many aspects the “Human Computer Interface” (HCI).   Studies clearly demonstrate a scientific basis for ergonomic design of computer workstations.   The following is an overview of ergonomic issues related to keyboard trays.

Definitions and Terms:

A)    Keyboard Tray Performance – Performance criteria are usually based on comparing speed, error rate, fatigue, muscular strain, or other personal preferences.  Each user must decide what criterion is most important; this will help in the selection of the best keyboard tray for the desired purpose.  The data does support that users report increased comfort and less fatigue and muscular strain with ergonomically designed workstations.  User comfort is a goal of the ergonomic design for the HCI.

B)    Keyboard Trays as Part of a Workstation - The typical computer workstation includes the desk, display terminal, keyboard and tray, computer, mouse, chair, and lighting source as well as other environmental factors.  The ergonomics of the HCI includes all aspects of interaction that affect worker comfort and performance.   The relationship of the system components to human performance requires that all facets be optimized to maximize the HCI.  Keyboard trays are an important part of the HCI but must be viewed in the context of the entire work system and production goals.

C)    The Neutral Posture – The neutral posture refers to the natural, least stressful, and most comfortable position of the body and limbs.  Neutral positions minimize energy demands, provide maximum stability of the musculoskeletal system, and will assure the least amount of stress on muscles, tendons, ligaments, bones, joints, discs, blood vessels, and nerves.  Neutral positioning can be achieved while sitting, standing, or lying down.  The neutral sitting HCI position is achieved with the body upright and supported, arms at the sides relaxed, elbows flexed near 900 and the hands and wrists pointed inward.  Non-neutral positions of the hand and wrist include ulnar and radial deviation, flexion, and extension.  Supination and pronation of the hands and wrists are non-neutral positions of the forearms.  Flexion and extension of the arms are non-neutral positions.  Non-neutral positions of the upper extremities result in stress, fatigue, and strain to muscles, tendons, ligaments, blood vessels, nerves, and joints.  Postural stressors are believed to be contributors to the development of UE-RSIs.  The adjustable, well-designed keyboard trays facilitate more neutral positioning of the UE during use and thus minimize UE stress and related adverse consequences.

D)    Reach – This refers to the distance from the person to the keyboard.  More precisely, it is the distance from the neutral positioning of arms at the user’s sides outward to the keyboard.  Fixed trays or table tops to not facilitate adjustability and are likely to produce stress to the UEs for many users.

E)    Height – This refers to the placement of the keyboard tray from the floor.  Keyboard trays should be adjustable between 20” to 28” in the seated position.  Height affects the angle of the elbow to the arm and the arm to the shoulder.  It is recommended to allow flexion between (+/-) 150 from neutral or 900.  

F)    Slope – This refers to the angle from flat or level positioning parallel to the floor.  Variations in slope include positive (up-hill), negative (down-hill), or 00 angle (flat).  The positive slope requires that the nearest side of the tray is the lowest and the furthest side is the highest.  Negative slope presents the opposite configuration with the nearest side of the tray the highest and the furthest side the lowest. 

G)    Tray Size – This refers to the overall size of the keyboard tray across the work surface. The keyboard tray must accommodate keyboards of varying length and width.

H)    Mouse Pad Feature – This refers to the placement of the mouse platform adjacent or next to the keyboard to minimize reaching for mouse or trackball use.  This is an important added feature when a computer is equipped with a mouse or other portable pointing device.  This could be designed in the tray or added on.  The tray must accommodate the proper placement of the mouse or other pointing device.

I)    Adjustability – This refers to the position, placement, and configuration changeability of the tray.  Adjustability should be an option for the user.  A fixed and rigid tray is not adjustable.  An ergonomically designed tray typically has adjustability designed into it for individual user preference.

3)    How Ergonomic Keyboard Trays Address These Issues:

A)    The Neutral Position – Ergonomically designed keyboards facilitate a neutral or more neutral position of the UE to minimize stress on the hands, wrists, forearms, arms, neck and shoulders during keyboard use.  Ergonomic keyboards are aimed at increasing user comfort and maximizing the HCI while diminishing the potential for UE-RSIs.  Users report increased comfort, less fatigue, and fewer musculoskeletal complaints.  Neutral positioning is a fundamental concept of ergonomics. 

B)    Reach – The reach distance is an individual preference.  However, the proper location and placement of the keyboard tray is essential in reducing stress related to keyboard use.  The further the reach, the greater the UE stress and potential for adverse strain.  Ergonomic keyboard trays are adjustable to permit placement at the optimal distance from the user given the overall design and performance goals of the workstation.   Optimal placement facilitates maximum benefit and performance from the HCI.

C)    Height – Ergonomic trays are designed to be adjusted optimally to the preference of the users.  Recommended height range 20” to 28” in the seated position can only be readily achievable for multiple users if adjustability is designed into the tray itself.   Keyboard trays placed too high result in stress to UE and increased risk of UE-RSI.  Keyboard trays too low may also increase stress but are sometimes preferred.  Proper placement of the keyboard tray will reduce excessive wrist extension or flexion and diminish stress to the soft tissues of the wrists while facilitating adequate finger reach to keys.  Adjustability of the tray can facilitate comfort to the hands and wrists thus reducing fatigue and injury potential. 

D)    Slope - Although most keyboards are designed to slope between 00 to 250 upward in the back, the adjustable tray can assure optimal slope as well as reach and height.   The ergonomic tray will be adjustable to the user’s preference.   The adjustable tray can be placed in a positive or negative slope or positioned flat.  The negative sloped keyboard allows complete neutral posture of the wrist and is becoming popular.  Some users believe this is the best design.  Negative sloping can also be achieved using an adjustable tray angled downward in the back.  The critical feature here is the ability to get close to neutral posture and minimize stress to the UE, maximize the HCI, and minimize the risk of fatigue, discomfort, and RSI development.

E)    Tray Size – The ergonomic tray should be designed to accommodate overall size of the keyboard across the work surface. It should also allow for a wrist rest if the user elects to use one.  The keyboard should fit level on the surface and not slide easily. 

J)    Mouse Platform Feature – The ergonomic tray should facilitate the placement of the mouse pad adjacent or next to the keyboard to minimize reaching for mouse, trackball, or other pointing device.  This is an important feature when a computer is equipped with a mouse or other portable pointing device.  Reach affects muscle recruitment dramatically.  The further the user must reach, the greater the potential for adverse stress and strain.  Mouse platforms designed to fit adjacent to the keyboard better allow neutral posturing and maximize the HCI while minimizing adverse outcomes.  The platform may be built-in the tray design or must be added to the existing design.   The ergonomic keyboard tray provides for the proper placement of the pointing device.

K)    Adjustability – Ergonomics is about adjustability to accommodate user’s individual preferences.   Adjustability is necessary to optimize position, placement, and configuration of the tray for each person that uses the keyboard.  It also facilitates change in the event the user wishes to vary the setting throughout the day.  Adjustability should be an ergonomic option for all users.

Summary:

A)    The ergonomic keyboard tray is superior to the fixed surface.   The fundamental principle of adjustability is a cornerstone of ergonomics and must be built into the tray to allow users to find the best positions for them to minimize risk of injury and maximize performance output.  Ergonomically designed keyboard trays are designed to be adjustable for distance, height, and slope and well as proper placement of the mouse or other input devices.

B)    Price –  ?

C)    Learning Curve – A small learning curve is to be expected.  The user must first learn to make the adjustments to the moveable keyboard tray.  Most trays have few adjustments that can be learned quickly.  Subsequent use allows the user to easily make adjustments to their keyboard trays in moments.  Adjustability allows the user to experiment and find the optimal locations and positions.  Use breeds familiarity and appreciation for this ergonomic feature. 

References:   
Bureau of National Affairs. (1995). 770 percent increase. Occupational Safety and Health Reporter, 36, p.1794.
Granjean, E. (1987). Ergonomics in Computerized Offices. Taylor Francis: NY.
Helander, M. (Ed.). (1994). Handbook of Human-Computer Interaction. (4th ed.)  NY: North-Holland.
National Institutes for Occupational Safety and Health. (1997). Musculoskeletal Disorders and Workplace Factors. NIOSH Pub No. 97-141.
Selan, J. (1997). The Advanced Ergonomics Manual. Advanced Ergonomics, Inc.: Dallas, TX.