J. Zander
MR ZANDER is a member of the faculty of the Agricultural University, Wageningen, the Netherlands. This article has been adapted from the introduction to Ergonomics in tropical agriculture and forestry: Proceedings of the Fifth Joint Ergonomic Commissions of IAAMRH, CIGR and IUFRO, Wageningen, the Netherlands May 1979, edited by J.H. van Loon, F.J. Staudt and J. Zander. The proceedings are available from the Centre for Agricultural Publishing and Documentation, Wageningen, the Netherlands.
As long as the tools and machines used by man were simple, it was possible to produce satisfactory designs by purely empirical methods. But the introduction of mechanization caused many changes in working methods and conditions. Sometimes machines make work easier and more congenial. In many cases, however, mechanization involves changes in the physical work load that are coupled with an increase of the perceptual and mental work load. As machinery becomes more complex, the empirical approach is often no longer sufficient to analyse working conditions and how to improve them.
Ergonomics ("fitting the job to the worker") is a multi-disciplinary activity having to do with the work situation. Its objective is to achieve an optimum man/task system in which a proper balance can be maintained between the worker and the working conditions. Thus it is suited to evaluating more complex work situation that result from rapidly changing technology.
Agricultural and forestry work play important roles in the sociology, culture and economics of tropical countries and deserve to be given more study in terms of ergonomics. How can the work and the worker best fit together under tropical conditions?
Ergonomics can play an important and very human role in improving the living and working conditions of people in agriculture and forestry, especially as changes occur. An ergonomic approach can contribute to:
· The design of an optimum man/task system (preventive ergonomics).
· The testing of a man/task system (curative ergonomics).
· The prediction of the load of the worker as well as the performance and safety of the system.
A continual stream of information -from the tool or the machine-is received by the working man using his senses. This is called "the perceptual load." Perception is followed by testing with the memory and then by making decisions from among the alternatives available to guide a specific operation. This process of selection is "the mental load." Finally, the output through muscular activity is "the physical load." During this process a worker will continuously observe the effects of his actions. This process is called "feedback."
Anthropometry. The data about the work-place layout, including the controls, are determined by the characteristics of the human body. Anthropometry refers to human body measurements and biochemical aspects of movements of the body and body members, as well as the working posture and the forces to be exerted upon the objects -hand tools and controls-surrounding him.
Through statistical research carried out in several countries, extensive knowledge of the: bodily dimensions of large population groups has been accumulated. When using data about sex, age and race, we must look not only at the mythical average but at the extremes of size, weight and other characteristics. There has to be sufficient space for adaptations to be made in a simple and safe way. Discussions of body measurements should include the following:
· A definition of the body dimension and how it was measured.
· The data, in tabular form, for the 5th, 50th and 95th percentiles and the standard deviation.
· Correction factors for clothing and personal equipment and other relevant reliables.
In order to realize optimum performance, it is necessary that the movements of the body members be such that a favourable load originates; fatiguing body motions have to be avoided. According to the principles of motion economy, the motions should be confined to the lowest classification with which it is possible to perform the task satisfactorily. By moving body members in harmony, simple and logical results are obtained.
Attention should be paid to the following: (a) there should be a good balance among bodily movements; (b) the amplitude, strength, speed and pace of movements should be mutually adjustable; and (c) movements with great accuracy requirements should not entail exertion of considerable muscular strength.
The working posture is largely determined by the layout and shaping of the tools, as well as by the location and displacement of the controls. The probable locations of pain or other symptoms, as a consequence of bad posture, have been described in current ergonomics literature.
There are three general designs for posture: standing, seated at all times, and shifting between seated and standing positions. The standing position is generally considered to be a "standing and walking position"; the worker is free at all times to move in various directions. If this freedom is not available or required, the work-place should be designed so that the worker either sits all the time or can move between a seated and standing position. Static muscular fatigue must be avoided.
The forces which can be exerted depend upon the muscle group, posture, the direction and strength of the forces, and the duration. In general a human being is capable of maintaining 10-15 percent of the maximum force of a muscle during a long period of time without muscle fatigue. Exercise can increase strength and endurance within limits imposed by a worker's innate physical potential. Attention should be paid to the following:
· Strength demands shall be compatible with the physical capacity of the worker.
· Muscle groups involved must be strong enough to meet the strength demands.
· If strength demands are excessive, auxiliary sources of energy should be introduced.
· Maintenance of uninterrupted tension in the same muscle for a long period of time should be avoided.
Many hand tools and machines used by agricultural and forestry workers in tropical countries are imported. This can lead to inadequate work-place layouts if no attention is paid to the anthropometric and biomechanical aspects of the users. Therefore anthropometric surveys have to be carried out in these importing countries.
Perception. The perception of information a worker receives from his work environment occurs through the senses, which are sensitive to specific impulses. The reaction to the information depends upon the sense organ that is stimulated, the strength of the stimulus and the place where the stimulus arrives.
Sight plays an important role in man/task systems, since most information enters the central nervous system via the eyes. Moreover, most actions are performed under optical control. With our eyes we can observe not only colour, light and darkness, but can also estimate in a reasonable way the direction and speed of moving objects.
The characteristics of the visual field determine the visibility of the work. The visual field can be enlarged by body motions, but these then lead to loss in the quality of the body posture as well as to increased work load and a decreased performance.
Communication between man and task, like communication between man and man, takes place through hearing, in which vibrations of pressure act as a medium. Depending on frequency, sound pressure level and duration of exposure, the effects of sound can range from annoying to damaging.
The sound pressure level of machines in agriculture and forestry has received a great deal of attention. For many machines the average sound pressure is too high. This will lead to impairment of hearing, mostly of a permanent and irreversible nature. It is now generally accepted that a constant exposure to sound pressure levels of 85 dBA or more necessitates protection of the ears to avoid damage. Protection can be achieved by technical means (quieter machines, isolation of the sound force or reduction of sound transmission) or by personal means (larger distances between the source and the worker or wearing ear protectors).
From an ergonomic point of view, scent and taste are less important since only a few actions specifically require these senses. However, workers in agriculture and forestry are being subjected to increasing amounts of air pollutants such as dust, gases and smoke. Working under dusty, dirty or smoky conditions reduces visibility, produces eye, nose and throat irritation and can induce sickness. These are generally short-term effects. Of greater concern are the long-term chronic effects, which cause illness or debility. They affect the worker's vitality and make him more susceptible to many forms of disease.
Though rather little operational information enters the central nervous system through touch, it can have much impact on a worker's health and performance. Vibrations are transmitted through the parts of the body in contact with the source of vibration - usually the hands, arms, feet or buttocks. Sometimes only the hands or arms experience the vibration; in other cases, it is the whole body.
During recent years special attention has been paid to various bodily deficiencies and infirmities related to exposure to mechanical vibrations. The effect and impact of vibrations vary with the frequency, as the human body is most sensitive to mechanical vibrations within the range of 2-6 Hz. The impact of vibrations varies from annoyance to damage to health and performance. Protection against mechanical vibrations can be achieved by technical means (reduction and isolation of the vibration source or transmission) or by personal means (cushions and seats).
Selection. A worker does not simply receive data from his sensory organs. The data are processed; the various possibilities are sorted out; and the proper response is chosen and transformed into action. The capacity of information processing is primarily determined by the capacity of the central decision mechanism: the more complex the circuits to be followed, the lower the capacity.
Given the great variety of structure and operations, it is hardly thinkable that one universal parameter could be found for the assessment of mental load. The following categories, however, may be distinguished: (a) performance measurements; (b) psychological measurements; and (c) physiological measurements.
The assessment of mental load presents a great challenge to experimental psychologists and psychophysiologists. Despite a plethora of techniques, few are of general applicability, and even those are of dubious validity. Based upon available investigations, the best results for the assessment of mental load can be expected from:
· A method involving dual tasks, wherein unused mental capacity is occupied by performing an additional task.
· A method in which the heart rate at work, in particular the relative suppression of the sinus arrhythmia, is measured in comparison with the heart during rest.
When periodic or otherwise predictable inputs are recognized and learned, the worker responds with well-practised movements and works effectively. Learning consists of easier, quicker and better reactions to the information presented. Moreover, since the worker becomes more familiar with his own particular machine than any other, there is a reduced chance for error. This will lead, indirectly, to a better output of the whole man/task system.
Simulators have been developed for studying the loading components in a man/task system under controlled conditions and for obtaining information about the ergonomic qualities of the work-place layout of a certain machine or of the effect of training. This makes it possible, with improvements in research costs, to obtain valuable data in a relatively short period.
One goal of the ergonomist must be to try to reduce the mental load in various processes in agriculture and forestry. This is possible by rejecting redundant and irrelevant information, as well as by presenting the relevant information adequately. In this way, ergonomics can contribute to effective processing of the increasing speed of information input (as a result of increasing capacity) and to a favourable work load.
Action. In order for a man to work, to move about, or even just to stay alive in a relaxed position, there must be energy available for his body to use. The energy required must have previously entered the body in the form of food. In its simplest terms, the body is an engine that provides power from the locomotor system, using oxygen that enters through the respiratory system to burn up the fuel entering through the digestive system.
When the rate of energy expenditure is increased by working harder, breathing becomes more rapid and deeper in order to increase the oxygen intake. The heart rate will increase since it is necessary for the blood to transport more oxygen away from the lungs to the muscles, and to move excess heat away from the working muscles to be dispersed around the body. This excess heat will also cause the body temperature to rise.
The working capacity of an individual is limited and determined primarily by the capacity for supplying sufficient oxygen and for taking in sufficient food. Other influential factors are health, fitness and nutrition. The mechanical efficiency of human labour can vary from 0-20 percent, but the maximum efficiency is seldom realized, as some muscles are permanently overloaded. The highest possible output is obtained from rhythmic movements, when a muscle or group of muscles is being stressed and relaxed in unison. Little is known about the physiological and pathological effects of the cumulative fatigue that arises after many successive days of hard work, nor is it possible yet to define the limits of the physical activity to which a worker is capable of rising in an emergency.
For the assessment of physical load, well-functioning parameters are available: energy consumption, ventilation rate, heart rate, and body temperature. Based upon the various physiological parameters, scaling of the physical load is possible. These results can then be applied to: design of the work-places; comparison of tools and machines; comparison of working methods; improvement of working methods; and the calculation of rest allowances.
Various surveys around the world have been made to determine the physiological parameters in agriculture and forestry, but little is known about the rates of work activities in tropical countries. Therefore physiological surveys in these countries are clearly needed. Although mechanization can lighten many of the traditional tasks, work in agriculture and forestry is likely to remain such that much physical energy will be expended during the course of a working day.
Climate. We cannot expect the same kind of performance under an unfavourable climate as we can under a good climate. Comfort depends not only upon the relative humidity, temperature and velocity of the air, but also on the subjective judgement of the climate as determined by the body's heat balance.
Physiological reactions to heat stress include an increase in the heart rate, body temperature, recovery time and sweat secretion. The general trend of all studies of the working environment is clear: atmospheric conditions interfering with normal or constant body temperature reduce mental and physical working capacity. In essence, the core temperature of the body should not increase above 38°C in prolonged daily exposure to heat and intense physical work. Moreover, the diet-in particular, the intake of fluids and salt-must be adequate to replace losses. Weight losses due to dehydration should not exceed 2-3 percent of the body weight.
Safety. Human beings at work are very liable to make mistakes, leading to accidents or injuries to themselves or others, to equipment damage, or to annoying holdups. The incidence of errors can be reduced by applying the principles of ergonomics. In general, safety is the joint responsibility of the designer, the manufacturer, owners, users, and safety officials. In every case the application of "common sense" is one of the most effective factors in ensuring freedom from hazards.
The reduction of accident hazards in tropical forestry and agriculture may be analysed by studying two things: (a) the number of accidents, injuries or fatalities in relation to equipment use; and (b) the primary causes of such accidents, including considerations of the hazards most likely to cause injury or death.
It is undeniable that work in agriculture and forestry is now more hazardous than it used to be. There are a number of circumstances peculiar to these areas which contribute to what is recognized to be a high accident rate:
· Varying terrain, ground and weather conditions.
· Seasonal peaks demanding sustained effort and extra hours.
· Isolation of workers who, when in difficulty, have no help within call.
· Inadequate training in the use of equipment.
There are some positive principles which, if followed, will generally lead to the safe use of equipment: (a) adequate training in using machines and equipment; (b) using equipment which is free from fatigue- and stress-inducing features; (c) clear identification of controls and instruments so that misuse is eliminated; (d) easily reachable and operable "emergency stop" facilities; and (e) a guarantee that equipment cannot be set in motion accidentally.
Design measures need to be backed up by adequate safety education and supervision, but better safety design does have one inherent advantage over periodic education and supervision: it makes the equipment inherently safe, independent of the user.
In the future, full attention must be given to the human and environmental aspects of living and working conditions in tropical countries. This is the first prerequisite for creating those conditions in which human talents can be most effectively utilized for promoting better forestry and agriculture.
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