individual human) requirements. Both strategies are faced with limitations in technical and economic basic conditions, as long as they are represented in a true form.
The focus of level S3 is on human tasks embedded in motive-related execution procedures in combi- nation with systems analysis of workplaces. The concept of systems analysis and task analysis leads to a variety of models — the goals operations methods selection rules (GOMS) model for human – computer interaction, the work-factor-mento approach to visual inspection and, last but not least, the different systems of predetermined times for manual assembly. The analytical and constructive “power” of these models leads to a broad application in industry with millions of workplaces designed and operated according to an optimization of time consumption in highly repetitive tasks.
broadest context S7 cooperation between companies
medium context S6 structure of the
factory
narrowest context S5 structure of the
work group
subject system S4 activity system of
a person
functional means of a person S3
purpose oriented subsystems (tasks)
upper level of physical means S2
productive subsystems (sensomotorics)
lower level of physical means S1
reproductive subsystems of the body
society and social and labor politics
company and company wide
organizational measurement
work group and cooperative
processes structural levels
of work processes
procedural levels of work processes
aspects of human and work
qualification, motivation and
work content, types of work
tasks and workplaces
willfully steered organic systems and tools/
work means
autonomous organic systems
and work environment V2
goal oriented consciously regulated action
V3 motive related
activity V4 cooperative
group work V5 interaction of the
working actors
V1 sensomotoric
automatism (operations)
V6 work related political action
FIGURE 5.13 Structural and procedural levels of working processes and assigned aspects for the modeling of humans and work.
Humans in Work System Environment 5-25
The central level, level S4, concerns the human being as an individual working person: the typical approach on this level is a holistic view of human work as an entity of motivational, qualification- based, and social-interactive elements, which together result in forms or types of work with characteristic sets of work content, work demands, and work-induced stressors. The models to anticipate design results on this level are multifarious according to the specific anthropological perspective of the human: man as a personality regulating his actions in an environmental context of self-set or accepted tasks in social interaction with others or man as a person, underlying, withstanding, and influencing his workload (by a set of stressors) according to his abilities to cope with them in stress-strain concepts. Limitations of use and abuse of human resources are frequently based upon those concepts, and, thus, standards of acceptable and endurable working conditions can be designed anticipatorily for a person or a group of persons after stressor analysis.
Level S5 brings into focus the working groups and group work, which means the cooperation of indi- vidual working persons with their functions in the network of interactions to other persons that is deter- mined by division of labor, hierarchy, behavioral traits, participation in decision processes, as well as questions of communication and information transfer with respect to human relations. Mostly models of function allocation and simulation models for crew design and crew operations in specific goal-oriented and task-oriented settings are used to find out the effects of independent variables on crew performance, crew workload, crew behavior, etc. before implementing a specific design configuration.
Level S6 describes company organization with special reference to the personnel and to the industrial relations of employers’ and employees’ representatives. Economic and social aspects in the design of company structure and functions, related strongly to job design and evaluation, are combined in indus- trial engineering or operations research models for the preparation of decisions about a management of human resources. Cost-benefit analysis of design solutions and economical optimization criteria follow- ing cost-structure models, quantitative production output models or, recent quality-oriented modeling of production behavior are the tools for the prognostic or anticipatory approach. Whereas level S5 implies clearly a human-oriented approach by the macroergonomic perspective, level S6 derives design intentions and goals for level S5 or sets limitations for design efforts on this level.
Level S7 is oriented to comprehensive socio-political and societal contexts of work. Typical questions on this level deal with work regulation and standardization, work in the national economy, structural and economical components of employment, the labor market, activities of employers, and unions in socio- politics and cooperations between companies. Econometric models and growth models for national/
international economies try to anticipate the effects of political decisions in complex situations, obviously with limited success regarding the world-wide problem of unemployment in many national economies.
The application of the seven-level concept of work sciences to the problem of “anticipation” shows that the microergonomic models for predictive and prescriptive design reach from level S1 to level S3, whereas macromodels of work sciences can be assigned to levels S5 to S7. An intermediate position in between macro and micro, named meso, can be given to level S4 where the working person is seen in a reacting position to environmental conditions caused by work, as well as in an acting position in coping with the work environment and, thus, influencing work itself by individual and collective efforts.
5.5.2 Inter-relations between Ordering Model and Work System Approach Systematic considerations are always expedient, if simple cause-and-effect chains do not exist between variables but, however, there are more than two variables in relation to each other. The analysis, evalu- ation, and design frames of a work system are arranged by the determination of system borders. The fore- closure of fields and, therefore, the creation of a system environment enables the ergonomist to acquire and process complexity with his limited intake capacity (Section 5.2.5). Envisioning the central aim of ergonomics — that is, to say the humane and economic design of work systems (Section 5.3.1) — the task to reach this aim is complex to such an extent that a focus on individual subsystems (e.g., the 5-26 Fundamentals and Assessment Tools for Occupational Ergonomics
effect of certain environmental variables on humans) is normally inalienable in research projects. Thus, the individual elements of work system approach are regarded as systems in each case and — if necessary
— are to be divided in subsystems. Different aspects of individual elements of a work system are the center of consideration at every level of ordering model. The system border is displaced in each case with the change from one level to another (Figure 5.14).
On level S1, the human is divided into physical “subsystems,” for example, the cardiovascular system, muscles and sinews, etc. The physiological systems of people and their interaction concerning the
broadest context S7 cooperation between companies
medium context S6 structure of the
factory
narrowest context S5 structure of the
work group
subject system S4 activity system of
a person
functional means of a person S3
purpose oriented subsystems (tasks)
upper level of physical means S2
productive subsystems (sensomotorics)
lower level of physical means S1
reproductive subsystems of the body
society and social and labor politics
company and company wide
organizational measurement
work group and cooperative
processes structural levels
of work processes
aspects of human and work
qualification, motivation and
work content, types of work
tasks and workplaces
willfully steered organic systems and tools/
work means
autonomous organic systems
and work environment task
systems job systems
group systems company
systems societal systems
operation systems
physiological systems work system perspectives
FIGURE 5.14 Inter-relations between ordering model and work system approach.
Humans in Work System Environment 5-27
dependence on the work environment and the work task take center stage in these considerations. On level S2 elementary physical and psychological functions are examined regarding the work person — mostly in cooperation with working equipment. From these investigations, design recommendations for the tech- nical system are derived (e.g., references to the design of displays and manual controls). The central focus of work systems exists, on the one hand, concerning the subsystems of humans (e.g., muscles and skeletal) and, on the other hand, concerning the technical system (e.g., workplace design). A connec- tion between human and technical systems is produced by the individual operations of the people.
Therefore, at level S2 a work system is considered through the perspective of an “operator,” so that at this level it can be spoken from an “operation system.” It is typical for this level of the ordering model that only subsystems of the work system element “task,” namely operations and activities, are regarded.
At level S3 the work system is not considered from the perspective of operators or individual operations.
The system border shifts in such a way that individual subtasks or activities are no longer regarded in relation to the work system element “task,” however, one or more total tasks are. Therefore, at this level
“task systems” should be taken into consideration. Questions concerning action regulation in connection with task systems are examined in relation to humans. With this, the center of attention is the humane design of the task system. Beyond that, questions of functional and temporal cooperation of humans and technique are discussed. Primarily, the goal of the utilization of human work is pursued (Section 5.4.1). Jobs are considered at level S4. These develop through the combination of many tasks. The job description usually takes place independent of the individual working person. From these descriptions the individual tasks and their cooperation in the “job system” can be derived to side the requirements of the working person and a quantified job evaluation. Requirement determinations serve, therefore, as a basis for a choice of personnel, a personnel development and for the design of remuneration. The center of attention is on the adjustment of humans to the work. On the other hand, it is the design of
“job system” that affects the personality and motivation promoting of the working person. The central focus of considerations is the adjustment of humans to the “job system.” At level S5 of the model, a work system is considered in the perspective of “group systems.” Level S5 is not similar to level S4 in terms of just one working person or only one job is considered rather a majority of people, that is, jobs. Groups are characterized by the fact that they consist of several coworkers who work together over a longer period with direct interaction. Members of a “group system” fulfill a whole task together, but to a certain degree role-differentiated. Apart from the execution of this primary group task, they also take over secondary tasks (Section 5.3.2). At level S6 such system components of the complex
“company system” are examined, which concern the human work. The system borderline shifts in such a way so that in reference to the working person all people or a large part of the staff are also included into the views of the system. Ergonomical questions of the design of the “company system” refer for example to operational work time regulations, the control of job execution processes, and the organiz- ation design. If the system is regarded as societal, the system borders of an enterprise and its staff shift all enterprises of a society or a majority of enterprises.
In the following, models and methods — to each level of the scheme — are presented exemplarily.
These models and methods are able to support a systematical proceeding with the anticipating and prospective work system design. These approaches and methods are the result of research projects, which were accomplished by the IAW and FIR at the RWTH Aachen. The exemplarily cited research results are supposed to show that the use of the work system approach is possible and reasonable on different levels of ergonomic investigation. Beyond that, the exemplarily represented models and methods make clear how the system limitations of one level shift ordering models to the next.
5.5.3 Identification of Recovery Times in Heat Work as an Example of Work System Design at Level S1
In the level S1 of the ordering model the human is subdivided into physical subsystems, for example, the cardiovascular system, muscles, and metabolic system. Subject of the level S1 is the physiological work system design, which includes all measures that have a direct influence on human physiology.
5-28 Fundamentals and Assessment Tools for Occupational Ergonomics
According to the work system approach, different influences of work environment affect a working person (Figure 5.15). These influences hardly ever appear isolated, but mostly in combination (Section 5.3.2). Concerning heat work, for example, unfavorable climatic conditions take effect in combination with hard dynamic muscle work (Luczak, 1979). In doing so, the climate conditions are allocated to environmental influences of a work system, whereas, muscle work is considered as an action resulting from the work task.
The subjects of investigation concerning heat work are the questions, which “human subsystems” are especially loaded, how these loads interact, and when “bottlenecks” arise in the human body. For example, the cardiovascular and the muscle system cooperate, because the amplitude and the frequency of respiration depend on the muscular load. In the same way, the cardiovascular and the metabolic system depend on each other due to the perspiration under heat work conditions — that is, an increasing loss of water and salt — these materials urgently have to be taken in. Therefore, interferences caused by salt deficiency (e.g., heat convulsions, which effect the cardiovascular system and, therefore, also the muscle system) can be anticipated.
At the time of publication of the following described study (Luczak, 1979), numerous scientific realiz- ations were present concerning “pure” muscle work and especially the endurance limit and the recovery time. However, combinations with environmental influences were examined only sporadically, referring to a few points of the entire load continuum.
A goal of the investigation was the obtaining of physiologically justified realizations for the determi- nation of recovery time for heat work. These realizations should cover a wide range of relevant load con- tinuums. In addition recovery times should be assigned to measurable load values.
The general objective of the investigation is based on the following partial goals:
. Development of a model for the coupling of the thermal regularization system and the cardio- vascular system
. Examination of the model on the basis of individual results from the literature
. Determination of recovery times and the superposition principles of energetic effects and climatic loads in the fast-time-simulation based on the model
s d a o
l resultingfromactivity cardiac circulatory
system
(transport system and heat balance)
spine
(carrying of loads and parts of the body)
muscles & tendons
e.g.
climate, substances
bones & joints metabolic system
(O2 and nutrition converting)
loads resulting from environmental influences
FIGURE 5.15 Physical subsystems of humans and influences on these subsystems.
Humans in Work System Environment 5-29
. Experimental evaluation of the model results concerning the recovery times at selected test conditions
In the following, physiological bases of the thermal regularization and selected results of the study are presented.
5.5.3.1 Physiological Bases of the Thermal Regulation
The core body temperature is kept constant within a wide range independent of outside climatic con- ditions. The desired value of the core body temperature is 378C. High and low outside temperatures, as well as manual labor lead to measurable deviations in the target value. The temperatures are measured by central and peripheral thermoreceptive cells.
Smaller climatic loads, warming and cooling stimuli, can be compensated by regulation of the heat transfer resistance between core areas and body surface. Larger thermal loads are compensated by increased perspiration, whereby a multiplicity of theoretical and experimental results regarding the relationship between different, locally distinguishable temperatures and the perspiration level are indicated.
The core body temperature rises during combined muscular and thermal load. The temperature can reach a steady degree, as long as heat production and heat dissipation are balanced. The circulatory system is a limiting factor of the heat dissipation and in supplying the musculature with high-energy substances. A rise of the body core temperature is usually accompanied with a rise of the heart frequency.
The heat dissipation becomes insufficient, if an increase in blood circulation of the skin is no longer possible. Other reasons for the insufficient heat dissipation are loss of water and a conversion of thermal balance (environmental temperature is more than core body temperature) under the conditions of increasing duration of work load and very high outside temperatures.
5.5.3.2 Model for the Description of the Thermal Regularization System and the Cardiovascular System
Regarding the determination of the recovery times and because of the interconnection of thermo-regu- latory and cardio-respiratory indicators it appears necessary to consider these both, several multiple con- necting physiological systems. The model describes the regulation and rhythm of the momentary heart frequency, the respiration, the blood pressure, the core temperature and the skin temperature, that is, those physiological functions, which are predominantly affected by a change in heat work and interfere in a quantitatively describable way.
In terms of this model different approaches described in the literature were examined. In principle all the available approaches at the time were similar so that easiest correlation indicated by Behling (1971) has been transferred into the model. Thereby, a correction factor (Scarperi et al., 1972) was considered.
The factor contains the loss of water in the organism in a long-term test. Behling adopts two blocks, body core and body surface. A defined heat exchange takes place between these blocks. Perspiration rate, oxygen intake, and heat development are determined by a weighted sum of core and skin temperatures.
The model was compared and examined with the measured values from physiological experiments regarding its reactions to thermal and energetic-effective load both for the steady state and for the courses of the most important model parameters.
5.5.3.3 Test Run to Determine the Recovery Time Diagrams
With the successful examination of the model, the requirements were given to superimpose the load factors “hard dynamic muscle work” and “thermal environment” for defined work durations and sub- sequently to pursue the recovery time processes. The climatic conditions of the test runs were varied effectively within the range of 0 – 358C, since the model is appropriate for a temperature range, which does not include the conversion of thermal balance with the rise in the human body temperature by external thermal influences.
The energetic load was varied upto 150 W, whereby the fictitious endurance level of the model was focused at 100 W. A hard dynamic muscle work with high efficiency is presupposed, since the model 5-30 Fundamentals and Assessment Tools for Occupational Ergonomics
equations based on the investigations were accomplished with such work forms. The fictitious endurance level of 100 W corresponds to the continuous endurance level of a young, healthy, male employee. It is realistic, because only suitable employees are involved in heat work because of personal selection and adaptation processes. The values for the work duration were applied in five geometrical stages: 15, 30 min, 1, 2, and 4 h. These values appear meaningful regarding their delimitations, since a continuous work duration under 15 min represents a special case in operational practice, and a work duration over 4 h may not be accepted due to the laws of working time regulation.
5.5.3.4 Experimental Examination of the Model Results
Four test subjects were selected. These subjects differed broadly in terms of size, weight, and physical efficiency and covered, therefore, a wide range of observable continuum. Three of these persons were heat-adapted, that is, the test subjects had to generate a performance (on an ergometer) between 50 and 100 W for 2 h daily in a climatic chamber at an effective temperature of 338C.
5.5.3.5 Conclusion
Both model results and experimental results were recorded in diagrams. The diagrams indicate that the recovery time increases exponentially starting from a certain energetic load value and from a certain effective temperature. The influence of the effective temperature begins with a certain temperature level. Until this level the thermal regularization system of the human body is able to regulate the body core temperature independent from the work duration due to a reconciliation of heat production and heat dissipation.
It is evident in all test results that the four factors “effective temperature, muscular load, work dur- ation, and individual characteristics of the working person” determine the characteristic curves. The influence of the effective temperature begins, thereby, from a threshold value. This value is again sup- posed to be dependent on the energetic-effective load, that is, an endurance level for the superposition of these two types of loads. Beyond this point, the slope of the course of the curve is determined by the energetic-effective load. The recovery time increases to the power of the energetic load value and the effective temperature.
If the model results are compared with the experimental results, then it shows up that the tendency of the courses of the curves exhibits large agreements (Figure 5.16).
5.5.4 Coordination of Movements as an Example of Work System Design at Level S2
The combination of power-, speed-, and acceleration development of muscles and skeletal elements in a purposeful process with spatial and temporal coaction is denominated coordination of movements.
Working persons coordinate the movement of their extremities and their body to act on the work object indirectly and (with the help of work and operational funds) directly. Thus, they fulfill their work tasks. Powers, which are raised by individual muscles are inter-connected regarding amount and direction. Thereby, the movements of the extremities and the body are spatially and temporally controlled.
The movement of the human body and its extremities in its entirety is a complicated mechanical process. The principles of this process can quantitatively and causally are conceived by the use of biome- chanical analysis. Apart from the proposition concerning movement coordination based on a predomi- nately energetic-effective view, however, informational-mental principles are also important for the work system design. Furthermore, movements are also temporal-operative processes, that is, the adjustment quality to an optimal course concerning spatial approximation and movement economy, as well as con- cerning speed-oriented, temporal aspects is achieved by continuous iteration of movements. This process is denominated as an exercise, especially with short-cyclic-repetitive sensorimotor activities.
Selected realizations concerning biomechanic analyses, valuate-informatical processes, and exercise processes are presented in the following (Luczak, 1983).
Humans in Work System Environment 5-31