POTENTIAL SOURCES OF INJURY
TIPS FOR MAKING YOUR WORKSTATION ERGONOMIC
are a number of flaws with a conventional computer workstation which
can be a cause for injury over a period of time. This document will
identify many of these sources, and indicate adjustments which will
make a workstation more ergonomic. At right is a graphic which shows
the optimal layout for a computer workstation for healthy individuals
with no pre-existing conditions or injuries. Individuals who have been
injured, had surgery or experienced trauma which affects their flexibility
or muscle strength should consult with a professional ergonomist, occupational
therapist, kinesiologist or physical therapist. These are general guidelines
and may not apply to any specific individual, application or workstation.
There are many potential sources of injury which should
be considered that range from the input devices used and placement and
orientation of those devices to general considerations with respect
to work patterns and habits.
The conventional keyboard contains inherent flaws in its design. These
problems give rise to potential sources of injury when used for extended
periods of time (defined as uninterrupted sessions exceeding 15 minutes,
or intermittent keyboard use over a period of at least 3 hours). Some
of these sources arise from the design of the keyboard itself, e.g.
ulnar deviation, improper pronation and dorsiflexion. Others arise from
the physical size or placement of the keyboard in the workstation, which
include reaching outside your comfort zone.
For most people, there is a disparity between your shoulder width
and the width of the keyboard when your fingers are placed on the
home row of keys. To compensate for this, people bend their wrists
to create a 90 degree angle between the home row keys and their hands,
and this is called ulnar deviation. This constricts
the blood flow through your wrist and this posture also requires sustained
muscle tension which further reduces blood flow to the muscles. The
median nerve runs through the carpal tunnel in your wrist, ulnar deviation
reduces the size of this tunnel and pressure on this nerve causes
carpal tunnel syndrome.
Ideally your hands should be in alignment with your forearm, so that
a straight line can be drawn through the center of your forearm, your
wrist, and through your third (middle) finger, with no 'bend' or lateral
deviation at the wrist. The use of a thumb can also be a cause of
ulnar deviation as it is shorter than the other digits,
and as such people often rotate their hands outwards in order to permit
their thumb to reach keys. Some keyboard designs (such as the Kinesis
contoured) have a separate keywell for the thumb which eliminates
any need for rotation to reach keys. Since repetitive muscle use can
cause inflammation and pressure on the median nerve (especially with
keyboards which require higher force to press keys or more movement),
ergonomic keyboards which minimize movement and have a lighter touch
help reduce pressure on the median nerve.
The natural rotation of your hands is not flat but rather between 10
and 20 degrees from the horizontal (with the thumbs side of the hand being
higher than the side with the fifth finger), or vertical (i.e. a 'handshake'
position). When typing on a conventional keyboard (or using a conventional
mouse) an individual is required to force his hands to be flat to remain
in full contact with the input device. This is called pronation
(one pronates one's hands from a vertical, 'handshake'
position to a horizontal 'palms down' position). As the muscles required
to force one's hands into this position fatigue quickly, improper pronation
can not only lead to discomfort but also act as a potential source of
While the vertical alignment of the hands is the 'best' position as it
eliminates fatigue for most people, many will find that a pronation of
between 10 and 20 degrees (from the horizontal) will also provide almost
the same benefit without the potential difficulties in using vertical
input devices. When practical, a fully vertical input solution (i.e. both
keyboards and mice) can provide relief by eliminating pronation when other
ergonomic accommodations are insufficient - the main drawbacks of a vertical
keyboard are that you must be a touch typist (as you can't see the keys)
and if you routinely access more than the traditional alpha section of
the keyboard, you will have to move your hands from one keyboard section
or Wrist Extension
The natural position of your hands with respect to the relative vertical
position at the wrist is along the plane or below it (i.e. you want the
tips of your fingers to be at the same height as your wrist or preferably
slightly lower). When your hand rises above this plane (making the hand
signal for 'STOP'), this is called dorsiflexion or wrist
extension. This greatly reduces blood flow through the wrist
and can quickly cause pain, fatigue and numbness. Most keyboards have
a 'foot' located at the back of the keyboard which is not desirable as
it creates a positively inclined keyboarding surface. Many articulating
arms offer the option of a negative inclination, which will make the entire
work surface slope away from you, ensuring that your hands are not 'bent'
upward at the wrist.
Everyone has a comfort zone within which they are able
to perform repetitive activities without excessive strain. If the majority
of your tasks can be accomplished without leaving your comfort zone, your
muscles will not become as easily fatigued, and you will reduce your risk
of injury due to repeatedly stretching beyond the zone. Ideally your keyboard
and mouse should be located on a work surface within that zone, at
the same height. Most conventional keyboards are 18-19" wide,
and basic fixed-split style 'ergonomic' keyboards can be as wide as 22".
A typical mousing area is approximately 8" wide x 10" deep (to
allow for space to comfortably use the mouse without bumping against the
This means that using a conventional keyboard with a mouse requires a
26" wide work surface, located at a height just above the user's
thighs (or lap), and the work surface should be directly in front of the
user when facing the monitor. Many workstations cannot accommodate this,
and as such larger trays, or more effective devices (keyboards which take
up less space, or mice that take up less space) are an excellent option.
Ideally your elbows should be slightly higher (1") than your wrists,
and your wrists slightly higher (0.5") than the tips of your
An easy way to find a smaller keyboard (with the same size keys) is to
simply eliminate the numeric keypad. Most users rarely use the numeric
keypad, and by purchasing a keyboard which offers an embedded numeric
keypad (i.e. the right side of the alpha keys double as a numeric keypad),
you can reduce the width of the keyboard by 2"-7", depending
upon the layout. Note that many so-called ergonomic keyboards are a trade-off,
as they attempt to compensate for ulnar deviation while typing only to
force the user into reaching out of their comfort zone when mousing.
Cursor control devices (which to most of us mean mice) provide
a wide variety of potential problems arising from their use. GUIs (Graphical
User Interfaces) have become for many users the default method of interacting
with their computer - for example it is estimated that Internet users
spend 80% of their time using the mouse, and only 20% of the time using
their keyboard. The ratio can be similar with many other applications
(including presentation programs, CRM (customer relations manager) software,
and data entry) as while there are many keyboard shortcuts available,
only a small percentage of users actually use them (as the GUI requires
less memorization). The problems can be broken down into several categories:
- Gripping the Mouse - The design of most mice requires the user
to control the mouse by gripping or pinching the mouse using the flesh
under their fifth finger and the flesh under their thumb (or the edges
of the fingers themselves). The muscles that are used to accomplish
this motion were not designed for the constant and strenuous use that
mousing requires, and can lead to pain and injury. Mice which avoid
the need to grip (such as touchpads), are redesigned to change the grip
(trackballs, vertical mice or sculpted/contoured mice) or find other
ways of reducing the gripping force are helpful in eliminating this
potential source of injury.
- Death Grip - A side effect of the grip, many users continue
to maintain a viselike grip on their mouse even during periods of
inactivity (as a habit) which means that the user's hands never
get a chance to rest. In addition, most users tend to grip the mouse
harder than is required in order to increase the feeling of control
while working, thus the term death grip.
- Trigger Finger - This refers to not only the action of clicking,
but of hovering above the mouse buttons in anticipation of the next
click. Eventually you lose the functionality and flexibility of the
click finger as the tendon becomes damaged, often referred to as trigger
finger . Products like touchpads allow any finger to generate
a click, and many mice and trackballs now have multiple buttons which
can be programmed, allowing you to shift the workload. For some people,
using a footswitch to provide the clicking action is a good way of giving
your hands a rest.
- Click Force - This refers to the amount of force required
to generate a click on the mouse. A higher click force can irritate
and increase the potential for injury while using a mouse, whereas
a lighter force can help to reduce or minimize the risk. Touchpads
respond to the lightest touch, as do some of the newer style mouse
alternatives on the market and most mice manufacturers are moving
to a lighter, tactile feel to the mouse buttons.
- Reaching for the Mouse over the Keyboard - This is a bigger
problem than many may realize. Users often center themselves on the
center of the alphanumeric section of the keyboard and have to reach
over the cursor area AND the numeric keypad to reach their mouse. This
almost always takes them out of their comfort zone. Some strategies
were discussed in the keyboard section, but another method for users
who don't want to change their keyboard is to get a keyboard with a
cursor control device embedded in the base of the keyboard (a touchpad,
trackball, etc.) or an add on that sits in front of your keyboard, eliminating
the reach for the mouse without necessitating any change to your familiar
These can be the most difficult to identify, as the body is a system, and
quite versatile in adjusting to adverse situations. A pain in your lower
back may be caused by a phone which is placed at an uncomfortable distance,
or an individual may change their workstation to work at an odd angle due
to lighting conditions in their office. These guidelines will help to identify
potential sources of injury and help ensure you are arranging your workstation
(and your work habits) to maximize comfort and productivity and reduce the
chance of injury.
Work Surface Considerations
As mentioned earlier, the primary work surface (where the keyboard and
mouse are located) should be just above the legs of the user and have
a negative inclination. This should result in forearms (and hands) which
are parallel to the ground or slightly negative while working. It is very
important to have the keyboard and the mouse on the same work surface
at the same height - as there is only one comfortable height when seated
in an relaxed posture.
Your monitor should be directly in front of you when typing with the
top of viewing surface of the monitor at or below eye level. The monitor
should be located at least 12" (preferably 18") and up to 36"
from your eyes; if uncertain, place it a distance equal to the size of
the monitor (i.e. if you are using a 17" monitor, place it at least
17" from your eyes when normally seated). Screen glare can be prevented
by adjusting the screen angle and orientation of your workstation so that
you are parallel to sources of light. If this is not possible, try using
softer color schemes on your monitor or purchase an anti-glare screen.
Location of Documents (Document Holders)
Reaching for documents and twisting your body to review documents can
lead to overextension, posture problems, and eyestrain. Use an 'in-line'
document holder which is located under your monitor and fills the space
between the bottom of the monitor and the top of your keyboard. Now any
documents you have to reach are just to the left or right of your 'comfort
zone' and your eyes have a vertical corridor to work in with the monitor
at the top, active documents in the middle and keyboard and mouse at the
Adjust the height of your chair to achieve proper posture - your thighs
from the body should be close to parallel to the floor. Adjust the backrest
of your chair to provide support for the lower back. Proper posture can
avoid strain on muscles and early fatigue and is achieved by 90 degree
or higher angles at the hips and knees with feet supported by an angled
footrest (the top of your thigh where it meets your body should be about
1" higher than the top of your thigh at the knee, and your ankle
should be slightly further under the desk than your knee). Your chair
should have three finger-widths of space behind your knees when seated
properly with your back against the backrest to facilitate blood flow
and prevent cramping. Arm rests (either on the chair or mounted on the
worksurface) are useful as they will take the load from the weight of
your arms off your shoulders, removing a source of bad posture to relieve
the strain on shoulder and back muscles from supporting your arm weight.
- .Keep your shoulders relaxed and head and neck in an upright but relaxed
- Avoid long periods of repetitive activity; where possible, alternate
computer work with other tasks like filing, phone calls, etc.
- Drink water throughout the day and keep yourself well hydrated; avoid
coffee and sugared beverages as they will dehydrate you.
- Drinking regularly throughout the day will also ensure you occasionally
get up and stretch your legs (an automatic reminder few can ignore)
- Take microbreaks (brief 5-10 second breaks every 15 minutes) to shake
out hands and stretch muscles, and make sure during your coffee breaks
you get up and walk around (don't take them at your desk).
*** Thanks to Safetype.com for several of the demonstrative graphics
which were used on this page.