Are Ergonomic Mice Truly Ergonomic

The scientific literature has been indicating for some time 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 because 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 (7).

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. 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.

There are many computer mice on the market today that are described as being “ergonomic”. 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 with click buttons that support the entire length of the extended fingers. The theory underpinning these design features is to provide the hand with support.

Interestingly from an evolutionary point of view the anatomical structure of the human hand is no different from that in a gorilla, with the exception of the opposability of the thumb. The evolution of the human hand has provided it with intricate capabilities that are a result of the opposed thumb (6). To produce a design that places the hand in a position that the human foot adopts when walking would appear to be retrograde as the ability to carry out intricate movements will be restricted and will therefore impact negatively on such tasks as target entry of the cursor and ultimately on productivity.

Another form of ergonomic mouse includes the features previously mentioned but the mouse body is turned onto its side so that the palm is upright, the fingers extended and stacked one upon the other. Other forms of ergonomic mouse also utilise the palm in an upright position but the mouse body takes the form of an upright stick that is grasped by the fingers into the palm of the hand. Other ergonomic designs are adjustable or are produced in different sizes, to fit different sizes of hand.

Syndrome.
It should not be assumed, however, that activities associated with mousing actually cause these conditions.
In fact within the medical literature it is difficult to find reliable evidence linking repetitive movements of the hand and wrist with carpal tunnel syndrome (1). In addition the repetitive activity of typing has been associated with carpal tunnel syndrome (2) but recent research suggests that typing may actually be protective (3).
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 the ergonomic designs.

The International Labour Organization definition of Ergonomics is:
'The application of the human biological sciences in conjunction with the engineering sciences to the worker and his working environment, so as to obtain maximum satisfaction for the worker which at the same time enhances productivity'.

From this definition it would be reasonable to assume that if the available ergonomic mouse designs improve productivity then benefit would accrue. Indeed MacKenzie (4) states “the major aim of pointing device research is to develop devices that are as efficient as possible”.

The whole handed designs limit the efficient flexion and extension movement of the fingers that are an essential part of precise forms of control. This scenario is unlikely to improve productivity as the small muscles within the hand are largely responsible for the refinement and delicate control of digital movements (5).

As whole handed designs fill the whole of the hand and maintain the fingers in an extended position the use of the small muscles of the hand and fine movements are discouraged or prevented whilst gross whole arm movements are 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 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 which of themselves reduce the experience of pain (9).

The type of movements required to manipulate a computer mouse efficiently are those that are produced by the precision form of grip.
Napier (6) 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 (5).

The computer mouse can be considered to be a tool that is used to move a cursor about a screen. Flatt (5) 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 a fully extended position which stretches out the muscles.

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 (6) for maximum sensory input to influence delicate adjustments. In the precision grip the thumb is abducted in both the metacarpophalangeal and the carpometacarpal joints, whereas in the power grip both joints are adducted.

Summary
The precision grip does not involve the fingers being held in an extended position but in a position of mid flexion of the fingers and thumb without the object being in contact with the palm of the hand.

Taking into account the physiology of the precision grip described above in order to improve the speed and accuracy of mousing the design of a mouse should utilise the precision grip. In order to achieve this mouse design should allow for the mouse to be manipulated by the fingers and thumb tips and be compact enough to allow the body of the mouse to be drawn towards the palm of the hand.

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 aggravation upper limb disorders.
Many computer mice have been designed in an apparently 'Ergonomic way' however in doing so they can often restrict sections of the hand and upper limb from playing a part in the control of the device.

Conclusion:
A well designed pointing device which enables all parts of the human upper limb to work in synergy and perform the functions that it does best has the potential to outperform devices which inappropriately depend on a particular limb segment, such as the shoulder, for their entire operation.

WOODRIDGE

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Are Ergonomic Mice Truly Ergonomic