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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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