An Ergonomics Approach to Computer Mouse Design.
DXT Ergonomic Mouse
Stephen Bowden, Ergonomist.
The purpose of a computer mouse is to allow the user to complete accurate and productive movements of the cursor on screen.
The scientific literature has been indicating for nearly 20 years that the shape and size of computer input devices should take advantage of the fine motor control of the hand for their operation. The rationale for this suggestion is that the small muscle groups and joints in the fingers are densely represented in the human motor and sensory cortex and have higher information processing abilities than other body parts (2).
DXT Ergonomic Mouse 2
A total of about one third of the motor cortex is dedicated to the control of the fine movement of the hand. A further third of the cortex controls the rest of the arm, the whole of the leg and half of the body. The final third controls the face, tongue and voice control (3). It would be assumed therefore that ergonomically designed mice would enable the thumb and fingers to freely manipulate the mouse. This does not appear to be the situation in practice.
The International Ergonomics Association defines ergonomics as follows:
Ergonomics or (human factors) is the scientific discipline concerned with the understanding of interactions among humans and other elements of a system, and the profession that applies theory, principles, data and methods to design in order to optimise human wellbeing and overall system performance.
From this definition it would be reasonable to assume that if the available ergonomic mouse designs improve performance/productivity. Indeed MacKenzie (5) states “the major aim of pointing device research is to develop devices that are as efficient as possible” – in yet computer users that utilise whole handed forms of ergonomic mice are experiencing reduced efficiency.
Many of the computer mice that are designed to support the whole hand are referred to as being whole handed in shape and tend to focus the control of movement from the hand towards the shoulder joint. This design limits the efficient flexion and extension movement of the fingers that are an essential part of precise cursor control. This scenario is unlikely to improve productivity as the small muscles within the hand are largely responsible for the refinement and delicate control (4).
As whole handed designs fill the whole of the hand and maintain the fingers in a fixed extended position the use of the small muscles of the hand and fine movements are discouraged or prevented. Gross whole arm movements are then encouraged with the consequence of producing less precise movement and greater loads being placed on the shoulder muscles with the consequence of a greater loading on the shoulder joint.
The disadvantage of this emphasis on movement towards the shoulder is that muscles will be brought into play that are not suited to performing precise forms of movement with the result that productivity and comfort will be impaired.
The upper limb has evolved to be highly dexterous as well as being strong. In order to deliver this wide ranging ability each part of the upper limb has developed its own purpose and function. The larger muscle groups that operate the wrist, elbow, and shoulder are adapted for power and a larger range of movement. The smaller muscle groups that operate the fingers and thumb have more dexterity. When all the parts work in synergy, movement range and dexterity can both be maximised and normal patterns of human hand and arm movement can be utilised.
Ergonomic computer mice can be seen to be designed differently from standard computer mice and have taken various forms. The most common features of ergonomic mice include a sloped palm encompassing shape that supports the entire length of the hand and fingers. The theory underpinning these design features is to provide the hand with support and to eliminate pinch forces that are thought to be harmful.
MacKenzie (5) showed that these forms of upright /vertical mice have shown to have lower performance levels than a standard mouse.
Interestingly the American Conference of Industrial Hygienists (12) showed the threshold at which damage appears to occur is 10 Nm which is just over 1kgm (9). The force to click a mouse button is generally of the order up to 75g which is 1/13 or 0.075 of the threshold force required to cause damage.
There is a developing understanding within ergonomics that optimal loading is required – not off loading or reducing the load to the lowest level to prevent tissue damage. The old saying of “use it or lose it” applies here. Off load tissues and they weaken – load them properly and they will strengthen.
From a design point of view the anatomical structure of the human hand is not different from that of a gorilla, with the exception of opposability of the thumb and fore finger. The design of the human hand has provided it with intricate capabilities that are a result of the opposed thumb (5).
The type of movements required to manipulate a computer mouse efficiently are those that are produced by the precision form of grip. Napier (4) showed that all forms of grip can be grouped as either power or precision grips, with a third category combining elements of both.
Power grip is thought to have developed early in humans and consists of a prehensile movement in which the object is grasped by the fingers and pressed against palm. This is a powerful movement with little skill involved. Precision grip is thought to be the most recent adaptation of the evolving human hand. It is an accurate prehensile action in which the object is held away from the palm between thumb and fingertips (4).
A benefit of the precision grip is that it utilises the sensory surface of the finger pads – the most sensitive area is the central whorl or loop of the fingertips (5) for maximum sensory input to influence delicate adjustments.
There appears to be emerging evidence, therefore, that many of the ergonomic shapes/designs have emerged for the wrong reasons although this does not mean that there will not be any ergonomic benefit emerging from some of the designs as, for example, individuals who have damaged nerves and associated weakness or paralysis may only be able use a form of mouse that is whole handed.
Another example may be if an individual has rheumatoid arthritis where their joints can be very painful indeed when it can be seen that the greater the clicking force is then the more difficult it will be to use. Having said that the build up in muscle strength can be titrated by changing to the other hand just before fatigue sets in within the forearm musculature or pain is noticed or increases above their normal baseline
The computer mouse can be considered to be a tool that is used to move a cursor about a screen. Flatt (10) has indicated that the size and shape of tool handles should be such that the digital joints are near mid-flexion so that tool retention is high and the muscles are only partially stretched. The whole handed form of computer mice that are designed to support the fingers do not allow the fingers to be in a mid- flexion position but are retained in an extended position.
Taking account of the scientific literature that design of computer input devices should take advantage of the fine motor control of the hand for their operation the design of a computer mouse should utilise the precision grip.
In order to achieve this concept design should allow for the mouse to be manipulated by the fingers and thumb tips of either hand and be compact enough to allow the body of the mouse to be drawn towards the palm of the hand.
Such a design will not only take advantage of the fine motor control of the hand but will also utilise the rich sensory nerve endings that populate the tips of the fingers. These nerve endings are less likely to be used efficiently when a mouse design promotes full contact of the palmar surface of the fingers and thumb as is the case with a range of ergonomic mice currently on the market.
A design which allows precision movement by the fingers will not restrict the control of movement to the hand but will enable the user to choose to spread the postural loading throughout the limb thus increasing the potential to increase both speed and accuracy whilst potentially reducing the risk of the aggravating upper limb disorders.
The DXT Ergonomic Mouse allows the hand to adopt the position of function/rest. In this position the muscles of the hand and forearm are in a mid ‘position’ or at rest and are ready for movement.
Summary and Conclusion
The DXT Ergonomic Mouse enables neutral postures to be adopted and for all parts of the human upper limb to work in synergy by utilising the precision form of grip. The DXT Mouse has the potential to outperform devices which inappropriately depend on a particular limb segment, such as the shoulder, for their entire operation.
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