5.4 Biomechanics of tool design .1 Gripping and squeezing
5.4.3 The neutral position of the wrist
Grip strength is greatest when the wrist is in its neutral position—falling off progressively as the wrist moves away from the neutral position in any direction (i.e.
flexion, extension, radial deviation, ulnar deviation). The strength of grip is least when the wrist is flexed. This is because when the wrist is flexed, the finger flexors (which are the prime movers in the gripping action) are shortened and their capacity to generate tension is thus diminished (see Section 3.9).
For this reason alone, it would seem desirable that the handles of tools should be designed a such a way that when the tool is in use the wrist should remain as close as possible to the neutral position, since the less the strength of the gripping action in a given position, the harder the muscles will have to work to maintain a given level of gripping force. There are also other reasons.
The tendons of the various forearm muscles that act on the fingers and hand run around a variety of bony and ligamentous ‘pulleys’ where they cross the line of the wrist joint. When the wrist is in a non-neutral position, the mechanical loading on the tendons at these points of contact will be increased. (This is a matter of basic mechanics.) This increase in loading may lead to an increase in the ‘wear and tear’
Table 5.2 Handle sizes that allow the greatest force/
torque in operation.
Note: For cylindrical handles used to exert force or torque perpendicular to the axis (Fb, Tb) the diameter is not critical;
a diameter of 30–50 mm is suitable.
on the tendons which the working task entails, and to the development of conditions like tenosynovitis, carpal tunnel syndrome, and other work-related musculo-skeletal disorders attributable to over-use (see Chapter 8).
When the wrist is in its neutral position, the long axis of a cylindrical handle that is held firmly in the hand, makes an angle of 100–110° to the axis of the forearm (Figure 5.7). This is because the carpal bones in the palm are different lengths. This
Figure 5.7 The neutral position of the wrist is preserved if the axis of grip makes an angle of 100–110° with the axis of the forearm.
Figure 5.8 Incidence of repetitive strain injuries of the forearm in groups of trainee electronics assembly workers using conventional pliers (upper) and redesigned pliers (lower). Figures for tenosynovitis, carpal tunnel syndrome and epicondylitis (tennis elbow) have been lumped together. Data from Tichauer (1978).
so-called ‘natural angle of the wrist’ is seen in the traditional designs of carpenters’
saws (for example). When using such a tool, the cutting edge of the blade is parallel to the axis of thrust of the forearm when the wrist is in its neutral position. Thus the neutral position of the wrist is preserved in use (see Pheasant 1991a for a further discussion).
Tichauer (1978) describes a study of trainee electronics assembly workers who were required to use pliers extensively for twisting bundles of cables. These trainees had a particularly high incidence of work-related upper limb disorders. Tichauer argued that it was better to bend the handle of the tool than to bend the user’s wrist.
Figure 5.8 shows the incidence of these conditions in a batch of trainees using conventional pliers, as compared with trainees using the special pliers which Tichauer designed for the purpose. The results are very convincing indeed. This study is now around twenty years old and has been very widely quoted in the literature. To the best of this author’s knowledge, Tichauer’s redesigned pliers are still not on the market—
at least, I have never seen a pair, only pictures of them in books.
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Ergonomics in the office
The basic office workstation typically consists of a desk and chair at which the user will undertake:
(i) paper-based tasks, reading and writing, etc.
(ii) screen-based tasks, involving keyboard use (and other input devices).
A few years ago it was confidently predicted that the ‘paperless office’ was just around the corner—and that before long all information would be handled solely by means of electronic media. Although this goal has not yet been achieved, office work has, over the last decade or so, become steadily more screen-based, to the extent that the office workstation that does not have a visual display unit (VDU) is now a rarity.
It is widely accepted that in the interests of comfort (and the avoidance of adverse long-term effects on health, etc.):
§ For writing, the working surface (i.e. desk top) should be somewhat above the user’s elbow height, as measured in the standard upright sitting position (see sections 2.4 and 3.8). This is because, in order to write with a relaxed and natural action, the arms must be abducted and flexed somewhat at the shoulders (i.e.
raised sideways and forwards).
§ For keyboard work, the shoulders should be relaxed with the upper arms hanging freely at the sides and the forearms more or less horizontal and the wrists as far as possible in a neutral position (i.e. neither bent forwards, backwards or sideways). Thus the so-called ‘home row’ of keys (ASDFG, etc.) should be at or close to elbow height.
§ In either case, for comfortable sitting, the user’s thighs should be approximately horizontal, with the lower legs vertical and the feet resting flat on the floor. Thus the seat should be at or close to the user’s popliteal height—but preferably somewhat below it.
§ In the case of keyboard work there is an element of controversy as to whether it is preferable that the user’s trunk should be upright or reclined. We shall return to this last point in due course (see section 6.5 below).
In the UK, office furniture and workstations are the subject of two British Standards:
BS 5940 (1980) which deals with furniture, and BS 7179 (1990) which deals with visual display terminals. At the time of writing, the latter is in the process of being superseded by a European Standard BS EN 29241 (1993) which is in turn based upon the International Standard ISO 9241 (1992). In the UK, such standards are regarded as advisory but not mandatory. Screen-based office work, however, is subject to the provisions of the 1992 Health and Safety (Display Screen Equipment Regulations) which implement an EC Directive on the subject. The reader who is concerned with compliance with these regulations is referred to the relevant HSE publication (HSE 1992a), in which they are discussed in detail.