It was in the early part of the afternoon that she first began to notice a dull ache on the back of her wrist. By no means are all the decisions that lead to the creation of these artificial environments made by professionals.
What is ergonomics?
What criteria define a successful match?
What if these criteria prove incompatible?
The defendants discovered a design flaw in the petrol tank of one of their models which meant it was likely to explode in a rear-end collision. Based on some supposed cost-benefit analysis, they decided it would be cheaper in the long run to pay damages for the resulting victims and injuries than to redesign the car, and decided to take no further action.
Anthropometries
Outraged by this cynical view of the economic value of human life and limb, a US jury awarded the defendants $125 million in damages. The fragmented and repetitive tasks of short-cycle industrial assembly remain an efficient enough way to produce many of the manufactured goods demanded by the consumer economy.
Human proportion: an historical perspective
Until Leonardo's day, the theory of human proportions was associated with the so-called "golden proportion" or The classical tradition briefly reasserted itself in the middle of this century in the work of the famous French architect Le Corbusier.
Ergonomics and design
The user-centred approach
This is the equivalent of the principle of the limiting user in anthropometry which we will encounter in the next chapter.). The reader who desires a more detailed mathematical treatment of the subject is referred to the Appendix.
The statistical description of human variability
The second is a quantity known as the standard deviation (SD), which is an index of the degree of variability in the population in question, i.e. in this version, percentiles are plotted against values of that dimension (or, if we calibrate the horizontal axis in standard deviations, we effectively have a plot of p against z).
Constraints and criteria
- Clearance
- Reach
- The principle of the limiting user
- Posture
- Strength
In general, it is necessary to work down through successive levels of the hierarchy before operationally useful recommendations emerge. Again, they are one-way constraints, but this time they are set by a small member of the population, e.g.
Fitting trials and the method of limits
The best possible compromise height is actually 75mm below the elbow height of the average user (ie at the mid-point of the optimum range). With the wisdom of hindsight, we can see that this necessarily follows from the shape of the normal distribution.
Anthropometric data
- Sources
- Accuracy
- Clothing corrections
- Standard anthropometric postures
- Defining the target population
The height of the seat is adjusted (or blocks are placed under the feet) until the thighs are horizontal and the lower legs are vertical (that is, the knees are bent at right angles). The seat reference point (SRP) lies at the intersection of these two planes and the median plane of the body (i.e., the plane that divides the body equally into right and left halves).
An annotated list of body dimensions .1 Stature
- Eye height Definition
- Shoulder height
- Elbow height
- Hip height
- Knuckle height Definition
- Fingertip height
- Sitting height
- Sitting eye height Definition
- Sitting shoulder height
- Sitting elbow height (also known as elbow rest height) Definition
- Thigh thickness (also known as thigh clearance) Definition
- Buttock-knee length Definition
- Buttock-popliteal length Definition
- Knee height Definition
- Popliteal height Definition
- Shoulder breadth (bideltoid)
- Shoulder breadth (biacromial)
- Hip breadth Definition
- Chest (bust) depth Definition
- Abdominal depth
- Shoulder-elbow length Definition
- Elbow-fingertip length Definition
- Upper limb length Definition
- Shoulder-grip length Definition
- Head length Definition
- Head breadth Definition
- Hand length
- Hand breadth Definition
- Foot length Definition
- Foot breadth Definition
- Span
- Elbow span Definition
Vertical distance from the seat surface to the acromion (i.e., the bony point of the shoulder). Vertical distance from the floor to the top surface of the knee (usually measured at the quadriceps muscle rather than the kneecap).
Clearance
Whole body access
And comparison of the MIL-STD-1472C figures with the data in Table 3.1 suggests that these are also based on the 95th %ile values for the dimensions in question. Minimum dimensions for passages in restricted access situations, such as tunnels and catwalks, etc., are given in Table 3.4 (quoted from Damon et al. 1966 with slight modifications, introduced for consistency with figures given elsewhere).
Circulation space
Reach—the workspace envelope
Your entire shoulder girdle is pushed forward (extended) and you begin to lean your trunk forward by flexing your hip joint and spine. Try it and you will quickly discover that it is the tendency to fall as the horizontal position of your center of gravity reaches the limit of the base of support of your feet.
Zones of convenient reach
These define the zone of convenient reach (ZCR) for one hand, which extends laterally to the coronal plane of the body. To construct a zone of convenient reach in a horizontal plane, distance d above or below the shoulders, two semicircles of radius r are drawn, centered on the position of the shoulders.
The normal working area
When the elbow is flexed 90° and the humerus is rotated in the shoulder about its own axis, the comfortable limit of external rotation is limited to about 25°. In this case, the outer limit of the normal operating range is the extended epicycloid H1H2 formed by two simultaneous rotations.
Joint ranges
In general, sagittal plane movements of the trunk or limbs are called flexion and extension. Inward rotation of the forearm (turning the palm down) is called pronation; outward rotation (turning the palm upwards) is called supination.
Working posture
The speed of fidgeting can be used as an index of chair comfort – the less comfortable we are, the more we fidget. Such a hypothesis is in line with contemporary theories about the nature of pain (Melzack and Wall 1982).
Vision and the posture of the head and neck
When focused on infinity, or any object more than about 6 m away, the lens of the eye is completely relaxed. T is the torque around the neck; w is the weight of the head and neck; x is the distance from C7 to the center of gravity of the head and neck.
Working height
It is interesting to note that pain and spasms in the neck muscles (trapezius, sternomastoid, splenius, etc.) can lead to 'mechanical headaches' - experienced in different parts of the head and face and not uncommonly around or 'behind' the eyes (Travell 1967, Dalassio 1980, Travell and Simons 1983). Anatomists reading this will notice the proximity of the proprioceptive supply of these muscles to the spinal core of the trigeminal nerve.) The symptoms of mechanical headache and eye strain are very similar (see also Chapter 8). For a discussion of how these types of criteria can be applied to workplace and equipment sizing, see Chapter 2.
Posture and strength
In practice, in most cases, these kinds of leverage will be more important as determinants of the force you can apply in a given situation than the angle-to-torque ratios of the joints involved. Thus, they minimize the perpendicular distance (averaged over the motion cycle) of the thrust line from the knee joint, thus minimizing the amount of effort required by the knee extensor muscles to do a given amount of work on the pedals.
Fundamentals of seating
We could therefore assume that in the absence of such warnings there would be no threat of damage. There are those who believe that extensive hidden damage from "poor sitting" can occur in the absence of subjective discomfort.
The spine in standing and sitting
Forward tilt seating
Experience shows that maintaining proper balance in a forward-leaning seat is a skill that must be learned. The kneeling chair was about halfway down the list in their order of preference.
Anthropometric aspects of seat design
- Seat height (H)
- Seat depth (D)
- Seat width
- Backrest dimensions
- Backrest angle or ‘rake’ (a)
- Seat angle or ‘tilt’ (ß)
- Armrests
- Leg room
- Seat surface
- Seats for more than one
A medium or high backrest should be flat or slightly concave above the level of the lumbar cushion. This will tend to drive the buttocks forward out of the seat unless counteracted by (a) an appropriate seat pitch; (b) high friction cover; or (c) muscular effort by the subject.
The easy chair and its relatives
As a result, their bottoms slide forward until they are in danger of falling off the front of the seat. The luxurious sensation of sinking into a deeply overstuffed sofa is indicative of an absence of the support needed for long-term comfort in the sitting position.
Anthropometry of the hand
Anatomical terminology
Some common everyday actions fall between these two categories, in that the kinetic chain of the hand is on. In the basic power grip shown in Figure 5.3, the thumb wraps around the back of the fingers to provide extra stability and grip strength.
Fundamentals of handle design
However, as the need for precision increases, the thumb moves along the axis of the tool handle - providing additional control and the ability to grip power and precisely manipulate the situation. If part of the hand will pass through an opening (as in a suitcase or teacup) adequate clearance must be provided.
Biomechanics of tool design .1 Gripping and squeezing
Gripping and turning
When the handle is used as a T-wrench, the available torque (Tb) is, within reasonable limits, independent of the design of the handle. When the handle is used as a screwdriver (rotating around the A–A' axis), the force of action is no longer determined by the user's ability to generate torque, but by the ability to transmit it through the handgrip interface.
The neutral position of the wrist
When the wrist is in its neutral position, the long axis of a cylindrical handle held firmly in the hand makes an angle of 100–110° with the axis of the forearm (Figure 5.7). Thus the neutral position of the wrist is preserved in use (see Pheasant 1991a for further discussion).
The office desk
In the final analysis, an adjustable desk can be considered the best economic solution to the keyboard user's postural problems. In addition, it is extremely important that the thickness of the tabletop (and its supporting structures) should be kept as small as possible in proportion to the structural strength requirements.
The office chair
- Seat height
- The backrest
- Seat tilt
- Armrests
- The usability of adjustment controls
In order to offer the user the greatest possible variety of working positions, the angle of the backrest must be adjustable (independent of the seat). As we mentioned earlier, it is important that the 'contour' of the backrest is not exaggerated.
Visual demands of screen-based work
Document holders
In general, the object that the user looks at more often should be placed directly in front of him. If screen and source documents are referenced equally often, they should be placed on both sides straight and turned slightly inward toward each other.
The unskilled keyboard user
Providing a holder for source documents (so that they can be read at screen level and without turning the head) will greatly reduce the postural load on the neck muscles (especially in data entry, copy typing tasks, etc.). The preferred location of the document holder, relative to the screen, depends on the task.
The laptop
The keyboard (and other input devices)
Other input devices
One is that resting the wrist on the sharp edge of the desk (which you see quite commonly) can cause blunt trauma to the tissues on the front of the wrist (and especially the ulnar nerve). In addition to reducing mechanical stress on the lumbar spine, the reclining seating position (especially in combination with the supported wrist) has the additional benefit of tending to increase the total horizontal distance between the user's shoulders and the keyboard. (The disadvantage that would arise in terms of static loading is eliminated by the supported wrist.) It follows, as a matter of geometry, that the degree of wrist deviation required to maintain the alignment of the fingers on the keys will be correspondingly reduced.
The design of screen-based working tasks
Opponents of the relaxed approach argue that it tends to degenerate into a slumped position similar to that of the yuppie hump. Mostly the boundaries of these spaces will coincide with the rooms of the house.
The kitchen
Layout
The sum of the lengths of the sides of the triangle (drawn between the center fronts of the devices) should fall within certain prescribed limits. The Department of the Environment (1972) gives a combined minimum length of 3600mm and a maximum of 6600mm for the kitchen to 'leave sufficient working space and yet be reasonably compact' and also specifies that the sink-hob distance must be between 1200 and 1800 mm in length (figure 7.1).
Worktop height
Compare this situation with that of the standard office desk, which we discussed in the last chapter (section 6.1). For those of us who are less fortunate in this respect - and back problems are very common indeed - the task of washing up or preparing a meal at a sink or worktop that is too low can be quite an experience. Painful.
Storage
The bathroom
The bathtub
Consider a 95st man (seated shoulder height 645 mm) lying at the bottom of the bath. The outer height of the rim (above the floor) is, of course, generally greater than the depth of the tub (often up to 100 mm).
The handbasin
In fact, a typical bathtub depth these days is around 380 mm (15 in.), although older models are often deeper. Additional supports along the side of the bath are also desirable and, for those who are weak, a non-slip mat inside the bath is essential.
The water closet
It can reasonably be a little above knuckle height at the point where you get in (eg 800mm above the bath base), around shoulder height (eg 575mm) at the seat point and around 40mm in diameter. For a discussion of the ergonomics of the more conventional types of toilets, see McClelland and Ward.
The bedroom
In the case of non-fatal accidents, the seven highest statistical categories are responsible for about 90% of all accidents. However, we note that the shape of the triangle will be very different in different types of accidents.
Human factors in industrial safety: an overview
The catastrophic failure of complex systems
The overall responsibility for the ship's safety rested with the captain, who was in his normal place on the bridge. From a human point of view, the most striking feature of the accident was that, from his usual position on the bridge, the captain had no direct means of knowing whether the bow doors were open or not.
Everyday accidents
The appeal failed on the grounds that the fact that some form of negligence was widespread in the world of car ferries did not sanction it in any individual case. Unfortunately, it's all too common for people to look for ways to "beat" machine safety mechanisms in the interests of increased production - and that can result in serious injury or death.
Ergonomic injuries
Some ergonomic injuries are 'accidents' in the technical sense of the word as defined above. The risk is equally well recognized in the poultry processing industry, to the extent that, at least in these industries, such injuries must be considered 'foreseeable' in any conceivable sense of the word.
Back injury at work
In the vast majority of cases, back problems should therefore be considered a work-related condition. Over the years, the incidence for the two occupational categories varies, until in the oldest (>50) age group there is a difference in incidence of approximately two to one.
Lifting and Handling
Workspace layout
Thus, the power of the lifting action drops rapidly as a function of the distance of the load from the body – and the weight that can be safely handled becomes correspondingly smaller (see Figure 8.7). In both cases, lifting power is reduced and so is the weight that can be safely handled.
The load
Fixed horizontal handles on carts, carts, etc., must be at this level; but vertical handles will often be a better solution, allowing the user to find their own level. In general, the heaviest and/or most used items should be stored in the most accessible positions (see section 5.1).
Weight limits
The US National Institute of Occupational Safety and Health has published an influential set of guidelines specifically addressing symmetrical two-handed lifting performed directly in front of the body (NIOSH 1981). I have extensively discussed the theoretical and practical strengths and weaknesses of the NIOSH guidelines elsewhere (Pheasant 1991a).
Work-related upper limb disorders
On the varieties of RSI/WRULD
In some cases, there will be a change in the temperature of the affected limb, indicating a disturbance in blood flow. Initially, it occurs towards the end of the working day and decreases in the evenings and on weekends.
Over-use injuries to process workers
The magnitude of the forces that a person must use in the task in question is also important. Conveyor belt workers are also prone to overuse muscle and soft tissue injuries in the neck and shoulder area.
Keyboard injuries
Or the user may place her wrists on the edge of the table (probably again because it is too high compared to the seat) and thus work with her wrists in extension, again leading to an increased static load on the forearm extensor muscles . Some of them are attributed to migration and genetic mixing of hitherto distinct ethnic groups; others in more complex historical processes, which over the past century or so have led to an almost worldwide increase in stature, which is called the "secular trend."
Sex differences
Both the layman and the human-scientist want to know about the residual of the distribution. The only dimension of the limb that is proportionally greater in women is the buttock-to-knee length – this is due to differences in the shape of male and female buttocks.
Ethnic differences
Bergman's rule states that the body size of the species increases as the average temperature of the habitat decreases. Allen's rule states that the relative size of exposed body parts decreases as temperature decreases.
Growth and development
In the case of children, one can only get a very rough estimate of the rate at which changes are occurring. If body shape and composition were the same everywhere, we would expect body weight to increase with the cube of growth (since weight is directly proportional to volume, assuming constant density).
The secular trend
The remarkable secular Japanese trend appears to be associated with an increase in relative leg length—as the data of Tanner et al. Is the continuation or not of the secular trend important in terms of practical ergonomics?
Social class and occupation
It is difficult to predict the likely magnitude of these changes, as the effects of the secular trend are confounded with those of the aging process itself and demographic changes in the age structure of the population (see below). The physical content of the occupation itself can also exert a training effect—or, perhaps more importantly, a detraining effect in the case of sedentary lifestyles.
Ageing
The first table in this chapter (Table 10.1) is for the UK adult population. The Dutch (Table 10.8) are the tallest people in Europe (and the tallest included in the current database) for males by a fairly fair margin.
Notes on sources of data
The data for Polish industrial workers in Table 10.10 probably represent the shorter limit for European populations (or something very close to it). Asian populations (the tables are even shorter, the shortest population in the present collection being the sample of male Indian industrial workers reported by Gite and Yadav (1989), which is given in Table 10.14.
British adults (Tables 10.1–10.6)
American adults (Table 10.11) are slightly taller and slightly heavier than their UK peers, although in practical terms the difference will be relatively small. The data from Table 10.12 for male Brazilian industrial workers places them at the bottom of the European population scale.
Adult populations of other countries (Tables 10.7–10.16)
Infants (Tables 10.17–10.21)
Children and youths (Tables 10.22–10.38)
The stature data from this standard differs somewhat from that of the DES survey on which the current estimates are based. The reader is invited to scale up the current estimates using the stature data given in the standard, should she deem it appropriate to do so.
