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