Design for a Computer Mouse

Julie Eriksen

The computer mouse was selected for redesign to minimize cumulative stress disorders associated with current mouse designs (Smith 1997). These disorders arise from excessive hand extension, wrist deviation, arm abduction, and pronation required to use mouses currently available (Smith 1997). Less fortunately, mouse use with the hand placed palm down leaves users able to easily engage in ulnar/dorsiflexion deviations to control mouse position without moving their arms, leaving them prone to injury (Smith 1997). People who work in data entry who must use mouses for eight hours/day are at most risk because of frequency of use.

Computer mouses are data entry instruments used both to control curser position and for function selection. Curser movement corresponds to mouse movement, while functions are selected using the fingertips to press buttons located on top of the mouse. To move the curser, the user drags the mouse around in corresponding movements to the curser's movements, typically on a horizontal surface located next to the computer being used. Usually the mouse is controlled on top of a foam pad called a mousepad. Use of the mouse when placed to the side of the computer can lead to excess stress on the shoulder and back muscles because shoulder abduction is required (Harvey 1997). However, as mouse placement relates to space constraints, resolving this issue is quite difficult (Sanders 1993)

However, it is recommended that if at all possible mousepad design should permit mouse use on the user's lap, or directly in front of them, to minimize shoulder abduction. Mice are problematic because due to the mouse's size, a power grip is required, rendering instrument control through precision gripping, the grip used for stylus writing, impossible(Sanders 1993; Casteillo 1992).
Wrist/forearm positions used for writing with writing instruments such as pens, are inherently lower stress than those positions used with current mouses (Hedge 1997). Writing positions typically require the dorsal side of the wrist to face away from the saggital plane of the body. So for mousing activities, during which the elbow is typically flexed so that the mousing surface may be accessed comfortably, the palm must permitted to face toward the saggital plane of the body.

Consequently, the redesigned mouse's casing should permit the user mouse use with the arm arranged in positions similar to those used for writing. More significantly, this sort of wrist position prevents unhealthy ulnar/radial deviations because up-down movements resulting from deviations while the wrist is aligned in positions appropriate to writing, are not useful for mouse control, as that requires the mouse be moved from side to side. In addition, the wrist is much more flexible in extension and flexion behaviors than in ulnar and radial deviations. (Ryu 1991) Therefore, extension and flexion of the wrists may prove less harmful to the wrist for mousing activities, than deviations normally used with current designs, because of the wrist's inherently greater ability to manage flexion and extension behaviors.

Current mouses designs prove problematic because typical control placement at the distal area of the mouse requires finger control. Excessive use of the fingers can result in trigger finger. (Hedge 1997) The thumb is better adapted to repetitive use, because the thumb makes use of strong muscles originating in the palm, so function control should permit thumb control of most used functions (Sanders 1993). In addition, the palm down position required of mouse users to operate current mouses can result in users resting their palms on the mouse casing, putting users at risk for compressing nerves and tendons located close to the surface (Sanders 1993)

Because fingers one and two oppose each other, permitting precision gripping of objects, a curved gripping surface is well suited to follow the resultant arch formed in the hand through finger opposition created by hand geometry. More of the user's palm can contact the mouse's surface, permitting force transmitted through the user's hand to be distributed over a larger surface area, minimizing compression stress (Sanders 1993)

Palm down grips typical of current mouse casings often result in a pinch grip, by which opposition of the thumb and distal joints of the fingers act to grip the mouse (Hedge 1997). Placing the palm on a mouse while the elbow is flexed with dorsal side facing the floor requires the wrist engage in radical static radial deviations, because: 1) the elbow can only flex and extend along one plane; 2) the palmar side of the hand faces a similar direction as the frontal side of the elbow in the anatomical position; therefore when the frontal side of the forearm faces the transverse plane so must the palmar side of the hand. Turning palm down corresponds to a radial deviation. Prolonged use of mouses with the wrist placed in this position can cause epicondylitis and other repetitive stress disorders, and in addition, reduces grip strength (Sanders 1993).

This radial deviation affected pinch grip is particularly weak, because the twisted position of the wrist circumvents use of arm muscles to control the mouse (Sanders 1993). Consequently, only a limited amount of surface area -- that of the thumb and finger tips, can be used to transmit force from the fingers to the mouse, so undue stress on the fingers and thumb can result.(Sanders 1993) However, mouses may have a distinct advantage over stylus because of their size. The mouses's size should permit, if designed properly, for the user to use many surfaces of their hand to control the mouses, rather than just the finger and thumb tips, unlike styluses. Therefore, the redesigned mouse should not only permit users to contact the mouse with more touching surfaces, but should permit the user to grip the mouse with their wrist in a less radially deviated position.
Therefore, with criteria set up to permit optimal interface of the user's hand with the mouse, plans for a redesigned mouse will be outlined below. Sketches of the redesigned mouse and associated diagrams are located in the Appendix (# refers to diagram number to refer to in the Appendix ).

Gripping surfaces located on the side of the mouses will be curved to permit the fingers to cup the side of the mouse. (
#2) This hand orientation on redesigned mouse should result in the distal palm will be making good contact with the curved gripping surface, object contact typical of power grips. (Sanders 1993) The shelf formed varies from one to two inches, to permit both adjustability to varying hand size and maximized power grip of the mouse (#3) (Hedge 1997). In addition, the mouse will be covered with a somewhat rough material, to increase friction between the user's hand and the mouse, to reduce strength needed to manipulate the mouse(#2) (Kawai 1995)

The redesigned mouse will feature controls located on the top of the mouse and on the front sides(
#1,#2). The top button permits thumb control of functions, and is located where the thumb will rest naturally on the mouse. The forward buttons permit finger control, and are located in the approximate position the fingers will rest naturally when using the mouse. The narrow tear drop shape ranges from 2.5 -3.5 inches across the axis formed between the thumb and finger controls, to permit adjustability to hand size and to maximize grip strength (Sanders 1997). Finger buttons located on each side of the mouse's tip permits both left and right handed people to use the mouse. The thumb control's position is centrally located so that handiness doesn't affect thumb control of the mouse -- both types can rest their thumb in approximately the same place on the mouse. Controls will be rounded and covered with very firm foam, similarly to the optimized contours of seat cushions, to permit good adjustability to a variety of finger and thumb shapes and sizes operating the mouse controls (Hedge 1997)

Because curvature of the shelf/gripping surface of the mouse follows that of the arch formed by the hand, the user is permitted to contact the mouse's surface with a good percentage of the palmar side of their fingers and main hand.(
#5) Consequently, the force required to move the mouse is minimized, reducing muscle fatigue(Sanders 1993). Because the curvature of the mouse is smooth and flatter than the contours of the palm, it should not contact the hollow of the palm, so risk of compressing this sensitive area of the hand is minimized (Sanders 1993)

The more narrow precision grip and less radially deviated wrist position afforded by the new mouse design consequently permits better control of the mouse by permitting use of palm muscles such as those controlling the thumb (
#4)(Sanders 1993). Use of these strong muscles rather than the fingers, as with a stylus, can risk of finger fatigue, by reducing force transmission through the fingers. Specifically, the more straightened wrist position afforded permits the user to more readily transmit force from their arm muscles down through their palm (Sanders 1993). Therefore, the user can rely on the more powerful, larger muscles of their arms to supply the primary force required to push the mouse around for curser control, further reducing risk of hand fatigue. (Hedge 1997) Secondary fine control of mouse position can be supplied by fine movements of the wrist and palm, similar to writing, without finger fatigue associated with internal precision grips used for stylus writing (Hedge 1997)

Finally, the broad base of the teardrop shaped mouse provides a good broad surface for the palm of the hand to contact the mouse. (
#1) Thereby, force from contacting the mouse can be spread out all over the hand, minimizing compression stress (Sanders 1993).

In conclusion, the redesigned mouse, for reasons cited above, should permit better mechanical interfacing between the user's body and the curser control, than current mouse designs.

Sanders, M. et al. Human Factors in Engineering and Design. 371,383-412. 1993

Hedge, Alan 325/651 class notes. 1997

Kawai, S. et al. "Effects of varied Surface Conditions on Regulation of Grip Force During Holding Tasks Using a Precision Grip" Japanese Journal of Physical Fitness and Sports Medicine 44(5). 519-538. 1995

Smith, W. et al. "Ergonomic Test of Two Hand-Contoured Mice" Global Ergonomic Technologies, Inc. press release. 1997

Ryu, J. et al. "Wrist Joint Motion" Biomechanics of the Wrist Joint. 27-60. 1991.

Harvey, R. et al. "Surface Electromyography and Mouse Use Position. Ergonomics (40) Aug. 781-9. 1997
Castiello, U. et al. "Does the Type of Prehension Influence the Kinematics of Reaching?" Behav Brain Research. (50) 1-2. 7-15. 1992