By
Gregory Shaw
and
Alan Hedge
Introduction
A pilot laboratory study was undertaken in order to investigate the influence
of keyboard design and keyboard and mouse position on muscle activity of
the shoulder and wrist posture. Much ergonomic literature has focused on
the affect of computer input devices on upper body and wrist posture. Most
studies evaluate the impact of specific devices such as alternative keyboard
or mouse designs on posture, wrist deviation, and/or comfort; without evaluating
the affect of the device's position as well. Similarly, manufacturers of
computer equipment have developed ergonomic keyboards and mice for cursor
positioning but often do not provide guidance on positioning the devices
effectively. This study evaluates the interaction between the influence
of input device design and the device's placement on both wrist posture
and on muscle activity of the shoulder.
Treatments
Two keyboards were used in the investigation, a conventional flat computer
keyboard (CK) and the Microsoft Natural Touch keyboard (MS). A Microsoft
PS2 mouse was used for all cursor positioning trials. Three keyboard and
mouse position conditions were used: the desktop, a conventional flat keyboard
tray with a sliding mouse pad attached to the tray, and a negative slope
keyboard tray with an attached, adjustable position mouse pad. The specifications
for each of these conditions were as follows.
Desktop
The desktop height was 29.5 inches. The mouse was placed to the right side
of both the CK and MS keyboards. Each keyboard was placed flat on the desktop
without the use of attached height adjustment apparatus. The angle (slope)
of the key top surface as measured from the front (nearest the user) to
the back of the CK was +6 degrees (surface angled towards the user). The
cutup angle for the MS keyboard on the desktop was +8 degrees.
Conventional Keyboard Tray
The conventional keyboard tray was manufactured by Fox Bay Inc. (model:
Carpal Rest Wrist Support). The height was adjustable and was set to promote
a 90 degree elbow angle during typing for each subject. The tray included
an integral sliding mouse pad which could be slid to either side of the
tray, but was placed on the right side for this study. The height of the
mouse pad surface was 1.5 inches below the surface of the keyboard. The
cutup angle of the CK on the tray was +5 degrees. The cutup angle of the
MS on the tray was -2 degrees. It was necessary to extend the leveler under
the front of the MS keyboard during use on the conventional tray due to
interference between the keyboard and tray wrist rests.
Negative Slope Keyboard Tray and Adjustable Mouse Pad
A negative slope keyboard tray system (Proformix) was tested. This tray
was adjustable for height and slope. The adjustable mouse pad was a separate
unit (roughly the size and shape of a normal mouse pad) attached to the
tray with a swiveling arm which allowed the mouse pad to be positioned over
any section of the tray. In this study the pad was placed directly to the
right of the alphanumeric keys of each keyboard (above and covering the
numeric keypad). The mouse pad surface was approximately 2 inches above
the surface of the key tops. The mouse pad surface was not height adjustable.
The tray height was adjusted to promote a 90 degree elbow angle. The slope
was adjusted to the minimum negative angle which resulted in a cutup angle
of -2 degrees for the CK and a cutup angle of 0 degrees for the MS without
the leveler extended.
Study Design
Muscle activity of the shoulder was measured by surface EMG. The muscle groups instrumented were the trapezius and the deltoid. Electrodes were placed over the upper fibers of the trapezius in order to measure muscle activity involved in raising the scapula or shrugging of the shoulders. Electrodes were placed over both the lateral deltoid (middle) and the anterior deltoid (front) in order to measure muscle activity during arm abduction and flexion. The EMG data was gathered using Thought Technology's FlexComp hardware and ProComp software running on an Everex 386 computer.
Results
All data were analyzed using a repeated measures analysis of variance.
Shoulder Muscle Activity
Raw EMG data was converted to Root Mean Square (RMS) data by the ProComp
software. The mean RMS data was then converted to a percentage of an individual's
maximal voluntary contraction (MVC) for each muscle group individually.
This process normalized signal strength across subjects. The graphs below
contain the means for each keyboard condition and muscle group.
Trapezius muscle activity varied significantly within subjects during typing
(F1,5 = 6.74, p = 0.048). During typing, trapezius
muscle activity was not statistically significantly different among conditions,
though the graph shows a trend for lower muscle activity levels for the
trapezius muscle for both keyboard designs with the negative slope keyboard
tray.
Trapezius muscle activity also varied significantly within subjects during
mousing (F1,5 = 10.11, p = 0.025). There was a significant
interaction of mouse location (desk, tray, adjustable pad) and whether this
was used with a conventional keyboard or the MS keyboard (F2,10
= 4.09, p = 0.05). During mouse use the adjustable mouse pad led to lower
Trapezius muscle activity for both both designs. This is most likely due
to reduced upper arm abduction while using the mouse on the adjustable pad.
When used with the MS keyboard, the MS mouse required less trapezius activity
when on the desk but more when on the flat, conventional tray.
Front deltoid muscle activity varied significantly within subjects during
typing (F1,5 = 15.49, p = 0.011). There was a marginally
significant interaction of location and keyboard design (F2,10
= 3.43, p = 0.073). Front deltoid activity tended to be highest for those
typing on the desktop. This is most likely due to the need to reach forward
and up in order to reach the keyboard when it is placed on a desktop. Front
deltoid activity tended to be lowest for those typing on both keyboards
on the negative slope keyboard tray.
Front deltoid muscle activity again varied significantly within subjects
during mousing (F1,5 = 22.06, p = 0.005). There was
a significant interaction of location and keyboard design (F2,10
= 5.37, p = 0.026). Front deltoid activity was lowest for those using the
mouse on the adjustable mouse pad. The close proximity of the mouse tray
to the user allowed them to keep their arms close to their bodies while
cursor positioning. When used with the MS keyboard, the MS mouse required
less front deltoid activity when on the desk but more when on the flat,
conventional tray.
There was no significant variability within subjects for mid-deltoid activity
for typing, and there were no significant location or keyboard effects.
There was no significant variability within subjects for mid-deltoid activity
for mousing, and there were no significant location or keyboard design effects.
There was a trend for mid-deltoid activity to appear marginally higher for
mouse use on the adjustable pad, but this difference was not statistically
significant. Observation of the video tapes suggests that with the adjustable
mouse pad some users moved the mouse with their upper arm rather than 'skating'
the mouse.
Wrist Movement
Graphs of mean wrist movement (angle data) during typing (for both the right
and left hands) and during mouse use (for the right hand) follow. Wrist
extension refers to the vertical movement of the hand and ulnar deviation
to the lateral movement of the hand.
There was no significant variability within subjects for wrist extension
while typing, no effect of keyboard design, no effect of hands (right and
left), and no interactions. There was a highly significant main effect of
location while typing (F1,5 = 29.12, p = 0.000). A
number of observations can be made about wrist extension while typing. Extension
is higher for the left hand in all conditions. Extension is lowest with
the CK on the negative slope keyboard tray. Extension is highest with the
MS on the conventional keyboard tray. It is interesting to note that extension
is moderate for the CK on the desk. This is probably due to the height of
the desk relative to the chair in this study. Typists probably had to hold
their arms in front of their bodies angled up to the surface of the desk
resulting in a vertically straight wrist (i.e., a straight wrist can be
achieved with hands sloped upwards or downwards, but the hands fatigue more
rapidly when sloping upwards).
There was significant variability within subjects for ulnar deviation while
typing (F1,5 = 80.36, p = 0.000). There were highly
significant main effect of location (F1,5 = 40.19,
p = 0.000) and keyboard (F1,5 = 30.54, p = 0.003),
but no effect of hands (right and left). There were significant interactions
of location by keyboard (F2,10 = 6.62, p = 0.015) and
location by hand (F2,10 = 8.81, p = 0.006). The graph
clearly shows that the MS keyboard leads to lower ulnar deviation in all
conditions, which is expected due to the split keyboard design. Lower ulnar
deviation has been reported in a number of studies of split keyboards. However,
only the left hand ulnar deviation is significantly changed by the different
keyboard locations, and left hand ulnar deviation generally is lower than
right hand ulnar deviation.
There was significant variability within subjects for wrist extension while
mousing (F1,5 = 22.12, p = 0.005). There was a significant
main effect of location (F1,5 = 6.87, p = 0.013), and
an interaction of location by keyboard design (F2,10
= 10.58, p = 0.003). Extension during cursor positioning was highest for
the conventional tray condition, and extension was slightly worse for the
MS mouse on the tray. This seems due to users reaching down and to the right
while using the mouse on the conventional tray.
There was significant variability within subjects for ulnar deviation while
mousing (F1,5 = 12.83, p = 0.016). There was a significant
main effect of location (F1,5 = 5.94, p = 0.020). Ulnar
deviation during cursor positioning was highest when using the adjustable
mouse pad. Analysis of the video tapes suggests that this is due to the
close proximity of the pad to the center of the body and the design of the
mice that were tested. Cursor positioning requires users to orient the front
of the mouse directly in front of themselves. The shape of the MS mouse
encourages lateral bending (ulnar deviation) of the wrist in order to keep
the mouse pointed straight ahead when the upper arm is relaxed close the
the side of the body. A mouse with a different shape (e.g. an Apple mouse)
may not produce this effect.
The lowest ulnar deviation was found with using a mouse on the conventional
tray, where the mouse is to the side of the keyboard. This position required
users to hold their upper arms abducted away from their bodies which allows
them to reduce ulnar deviation of their wrist while pointing the mouse directly
in front of their bodies, though extension increases and this posture ultimately
is more fatiguing to the shoulders and arms.
Observational Data From Videotape
Each experimental trial was videotaped from the right side. The mouse use
trials were viewed in order to determine if mouse position (desk, tray,
and negative slope keyboard system) affected how users held or moved their
arms and if users rested their palms or wrists while using the mouse. The
findings for each position are summarized below.
Desk
Four users rested their palms or wrists heavily while cursor positioning,
two did not. Those who rested their hands moved their upper arms only slightly,
while those who did not rest their hands used their upper arms to move the
mouse during the trials.
Tray
On the conventional tray only two users rested their palms or wrists, these
individuals did not move their upper arms significantly. The remaining four
individuals did not rest their arms on the tray and did move their arms
significantly while moving the mouse.
Negative Slope Keyboard System - Adjustable Mouse Pad
Four individuals rested their palms and navigated the mouse from the elbow
while 2 individuals did not rest their palms and moved their upper arms
significantly to work the mouse.
These observations may indicate that users manipulate the mouse in one of
two ways: either resting their arm on the surface of the pad and moving
the mouse with small elbow and hand movements or not resting the arm and
moving the mouse with upper arm movements. It also appears that mouse users
are more likely to rest their arms on higher platforms such as desktops
or the adjustable mouse pad, than they are on low platforms like the conventional
tray.
Summary of Results
The results presented here should be regarded as preliminary and not definitive.
The frequently significant variability among subjects suggests the need
for a larger study with more subjects. However, the present results show
the following trends:
Shoulder Muscle Activity:
Wrist Movement - Typing
Wrist Movement - Mouse Use
Observational Data
Conclusions
Typing
The MS keyboard led to lower ulnar deviation during typing compared to the
conventional keyboard. Other researchers have found similar results with
the MS keyboard and other fixed-angle split keyboards. The MS keyboard also
produced fairly low extension values when placed on the desktop (L=7 degrees,
R=-1 degrees ), but did not fair well when placed on the traditional tray
(L=20 degrees, R=10 degrees) or the negative slope keyboard tray (L=15 degrees,
R=7 degrees). It is possible that the MS keyboard may have produced lower
extension values on the negative slope keyboard tray if the tray had been
angled away from the user at a steeper angle. The angle used in this study
was the minimum negative slope possible with the tray. Although placement
of the MS keyboard on a desktop provides some ergonomic benefit by reducing
ulnar deviation, the height of a desktop requires typists to hold their
arms in front of their bodies, which leads to increased neck and shoulder
muscle activity and fatigue. The EMG data from this study show that both
trapezius and front deltoid muscle activity were higher for the desktop
conditions compared to the negative slope keyboard tray conditions. Desktop
keyboard placement also causes many users to lean forward towards the desk,
thereby not relaxing back in the chair and hence not benefiting from the
back support of the chair.
The conventional keyboard on the negative slope keyboard tray produced the
lowest wrist extension (L=2 degrees, R=-2 degrees) of all six conditions.
The negative slope keyboard tray was found to reduce wrist extension values
in previous research by this laboratory as well as by other research groups.
The negative slope keyboard tray did not reduce ulnar deviation in this
study when compared to the CK on a conventional tray or on the desktop.
The negative slope keyboard tray did lead to the lowest EMG activity for
both the trapezius and front deltoid muscles.
Cursor Positioning
Wrist posture was strongly affected by the three position conditions used
in this study. The conventional keyboard tray with the mouse pad on the
right side of the tray led to much higher wrist extension values than the
other two conditions. As was previously mentioned, the high extension values
are probably due to users having to reach down and out to the right in order
to the reach the mouse on the conventional tray. Reaching down to a flat
platform would require the hand to work with the wrist extended.
The adjustable mouse pad led to more ulnar deviation values than the two
other conditions, which is probably due to the close proximity to the adjustable
mouse pad to the user's body. With the mouse design tested, pointing the
mouse straight ahead with relaxed arms requires users to ulnar deviate their
wrists in order to manipulate the mouse. The desktop fared the best as far
as wrist posture was concerned, but this also produced higher neck and shoulder
strain. Different mouse designs should be tested.
Shoulder muscle activity was also strongly affected by the three mouse position
conditions. The trends are the same as for typing. The adjustable mouse
pad led to much lower trapezius and front deltoid activity, there was no
significant difference for mid-deltoid activity. Shoulder muscle activity
generally was lower for the adjustable mouse pad position because this position
allows users to hold their arm close to their body instead of reaching up
to the desk or down to the side for the conventional tray.
Implications
The results of this small pilot study illustrate the challenges involved
in designing comfortable and safe computer workstations. The MS keyboard
works quite well to reduce ulnar deviation at desktop heights but may not
work as well when placed in a position that helps reduce muscle activity
of the shoulders (on the negative slope keyboard tray for example). During
both typing and cursor positioning the negative slope keyboard tray and
adjustable mouse pad work well to improve wrist extension and generally
reduce shoulder muscle activity. During cursor positioning ulnar deviation
was higher for the adjustable mouse pad, but this may be a consequence of
the mouse designs that were tested.
This study illustrates the point that good equipment can go a long way towards
eliminating many postural risks associated with computer use but, in some
cases, equipment alone can not eliminate all of the problems with computer
workstation design. Most researchers involved in computer workstation design
and evaluation agree that a combination of individual workstation evaluation,
good equipment design, and training are the best ways to improve comfort
and reduce the likelihood of computer related injuries.
June 3, 1997