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5 General Discussion CONCLUSIONS As a result of the workshop, the members arrived at the following three major conclusions. 1. There is a needfor an integrated model of the human body, its performance characteristics and limitations, and its interactions with technological systems. An integrated ergonomic model would provide a valuable too! for the development of specifications for the physical parameters of the work site based on the anthropometric, biomechanical, and interface characteristics of the operators. A valid mode} of the performance of people in technological systems in the early conceptual and design stages could result in substantial savings in terms of effort, time, and money. The development of an integrated computer model that describes human traits and limitations could prove useful to those who research basic human qualities as well. Thus, the need has both theoretical and practical implications. 2. The development of such a mode! appears to be feasible. Ad- vances in research methods and instrumentation, many of which are associated with the increasing sophistication in the use of computerized systems, have made research feasible on the many anthropometric, biomechanical, and interface details, as well as their interactions regarding human performance capabilities and limitations. The establishment of a standard protocol and nomen- ciature is essential to the integration effort. 68

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69 3. An integrated ergonomic mode! would be usefulfor guidance for research, development, and engineering applications. While current moclels indicate the usefulness of the approach, they also reflect many of the shortcomings of the diverse approaches identi- fied earlier. The approaches provide solutions to specific problems but contribute little to a generaTizable model. Typical examples in which an integrated ergonomic mode} would be very useful is in applications to computer-aided design (CAD) and engineering, which are fast evolving as major design tools. Requirements A study of the requirements for the development of a standard ergonomic reference data system (SERDS) was prepared by the National Bureau of Standards (Van Cott et al., 1978~. Although this system was never implemented, the study provides informa- tion relevant to the development of an integrated ergonomic model. The following were some of the major findings of this study: (1) A definitive survey of user needs and priorities is necessary in order to define the scope of the system. (2) Standards must be devel- oped for the definition of units, measures, measurement methods, and data reporting. (3) An assessment of alternate technologies for capturing, storing, and processing ergonomic data is needed to identify a cost-effective approach. (4) Data derived from the published ergonomics literature and from the national ergonorn~cs survey (discussed in the SERDS report) must be evaluated criti- cally. A preliminary survey of potential users at that time identified several important areas of needed research. These areas are shown in Table 5-1. Similar findings were identified at a conference on the the- ory and application of anthropometry and biomechanics in 1980 (Emsterby et al., 1982) and at a conference on space workstation human factors in 1982 (Montemerio and Cron, 1982~. Criteria for the Development of an Integrated Ergonomic Mode! Several general major criteria that should guide the develop ment of an integrated ergonomic mode! require that the mode! have the following characteristics (this list does not imply a rank- ing by importance):

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70 TABLE 5-1 Data Requirements of Potential SERDS Users Area of Need Specific Date Needed Date Application Static Basic human body Design of tools and other anthropometry dimensions as function hard goods; development of age/sex, etc. of clothing sising and tariffs Dynamic Bending and stooping Control location and anthropometry capabilities; reach operation; workspace dimensions design Strength Static and dynamic force Equipment and job design - characteristics measurements; lifting; for industrial workers pushing and pulling product portability design capabilities Physiological Aerobic and anaerobic Environmental design, job characteristics capacity; maximal heart specifications; toxicity rate; expiratory volume levels Sensory/perceptual Measures of visual and Design of controls, digital processes auditory acuity, color displays; visual and Derision auditory warning signals Tolerance to Exposure tolerance to Protection of workers and environmental physical and chemical environmental design conditions agents, e.g., tolerance to high-intensity light, noise, temperature, radiation Reaction time Simple and complex Display-control reaction time to a relationships; blade variety of stimuli stopping time Information Interpretation of symbols, Design of displays processing/ learning processes, signals, instructional cognitive functions memory materials, training devices Capabilities Anthropometric, sensory, Product and environmental of special physiological measures design populations of children, the aged, the handicapped SOURCE: Van Cott et al. (1978~.

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71 be dynamic, use a common notation system, incorporate or simulate the real world, have three-dimensional structure, be predictive, be capable of being validated, be user-friendly, be time- and cost-effective be flexible, permit rapid analysis, permit on-line documentation, be written in a standard language for transportability (use on different systems), have standardized segment and whole-body coordinate sys- tems, and ;, have graphical display capability. Standardization Standardization is a basic requirement for any kind of in- tegrating ergonomic model. Such standardization is particularly important in two key areas: common format of the input data and standard language to make models and submodels compatible, including their use on different computer systems. If this is not the case, each model or module needs a "transla- tor" that allows data exchange by software modulation. Graphics input and output should be in accordance with the International Graphics Exchange Standard Format (IGES). If certain assump- tions are made for the various data bases of submodels, these must be known to the user to make the system usable and reliable. Given the conclusions that an integrated ergonomic model and its submodels, that is, anthropometric, biomechanical, and interface models, are needed, feasible, and useful, methods for accomplishing the modeling goals need to be determined. APPROACHES TO THE DEVELOPMENT 01? AN INTEGRATED MODEL Two approaches to integrated ergonomic modeling evolved from the discussions at the workshop on which this report is based. The first approach is to develop one "supermodel~ that integrates

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72 the best qualities of all or most other models, while the second is a "modular approach, which would incorporate various existing models or submodels to develop an integrated ergonomic modeling system. The pros and cons of each approach were reviewed. Sup ermo de! Approach Some current interface models such as PLAID-TEMPOS, CAR, COMBIMAN, and Crew Chief appear to be moving in the direction of a supermode! approach. These models have been developed individually and, as a rule, are not compatible (e.g., there is no interface between COMBIMAN and SAMMIE). This incompatibility is usually a result of different data formats, de-- grees of modeling complexity, technically different computers, and different modeling theories or techniques. Furthermore, the data and assumptions in data collection may not be appropriate for specific models or sets of data. These models are similar in many of the respects that meet the criteria for integrated ergonomic models. An evaluation of these existing models regarding their potential for integration into a supermode! is needed. Modular Approach A modular approach to an integrated ergonomic modeling system is a building block process of joining compatible modules with a standard structure. This allows flexibility for the user to incorporate those aspects of the ergonomic mode! or its component modules into as simple or complex a system as desired. For exam- ple, an engineer interested} in fixed base activities on the ground might have no interest in a module that describes characteristics of reduced gravity or a module that incorporates platform motions and dynamics and their effects on the human operator. The modular approach requires that the modules fit together. They need to be designed and structured according to common principles and nomenclature. A modular approach thus requires a superstructure to make each module a true component of the general system. Thus, even a modular approach constitutes one form of an integrated ergonomic model. A flow chart illustrating a process for integrated ergonomic

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73 models was provided by Joe W. McDaniel (Figure 5-1~. It contains the following features: 1. Data bases are structured in a standardized protocol so that a new mode} could retrieve a data base or a subset of a data base. This minimizes the number of data bases required to be maintained on a system and makes it easier to update the data bases. (Researchers and mode} users could develop data to use with the mode} without having to be progr~runers or model developers.) 2. Each mode} has a translator or data exchange standard on the front end to access the required data. This feature would make the mode] less system dependent. 3. Mode! users have a library of programs to use in their specific designs that communicate indirectly through shared data bases, permitting the use of smaller computers. This also prevents obsolescence by allowing individual models or data bases to be ac- quired, replaced, or upgraded individually and as needed. In this concept the user can select items specific to a particular modeling analysis. Data and computer graphics systems should be standard- ized to allow interchange among systems. This approach is similar in concept to the workspace design analysis system proposed by Evans (1985~. One approach to the development of an integrated workspace design system based on the modular approach is shown in Figure 5-2. This system may be suitable to integrate operator analyses with existing computer design models (Evans, 19853. The sys- tem components, outlined in Table 5-2, provide a more detailed explanation of the data requirements (Evans, 1985~. DATA REQUIREMENTS Users of anthropometric and biomechanical data have tended to rely on existing data bases, adapting, inferring, and making assumptions to meet their needs. For the most part, these are limited to the measurements made on the male U.S. soldier. The civilian anthropometric data base, particularly that for women, is extremely weak. For other subsets of the population (e.g., the ag- ing and the handicapped), the data are virtually nonexistent. Yet,

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74 DATA I ~ ~ ~ | SOLUTION L RAW DATA PROCESSED DATA FUTURE DATA WORKPLACE TRANSLATOR DATA EXCHANGE STANDARD MODEL A B C USER SYSTEM 1 USER 1 ~ IGES WORK J ~ USER SYSTEM 2 CAD (a) - International Graphic Exchange Standard or Data Excharl<3e Standard 1 -{is GRAPHICS PRINTOUT FEASIBI LITY (VALIDATION: SENSITIVITY) ANALYSIS J FIGURE 5-1 Graphic display of an integrated ergonomic modeling system. SOURCE: Joe McDaniel (unpublished data).

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75 I N PUTS Percent , Population Tool of Interest Characteristics ~ / / Work - Workspace Data Structure, Workspace Geometry, and Commands DESIGNER 1~ ~ OUTPUT{; I _ . . space Dimensions \ I Ta k \ Demands \ \ \. 1 Reach Envelop: l \ , Interface Among Human Performance Models and Workspace Geometry Stren)\\ Profile \ \ Time \ Predi 7ions 1 / Fatigue Al lowances - Endurance _ _ Methods Analysis Strength Prediction Reach Analysis Data Base Interface , ~ t_ EXISTING W - I\4etabo!;c NIT PI T Opera tor Graphic FIGURE 5-2 System structure for an integrated computer-aided workspace design system. SOURCE: Evans (1985~. Reproduced with permission.

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76 TABLE 5-2 System Components for a CAD Approach to the Design of Manual Workspaces System Components Implementation Approaches User dialogue interface System modes (states) Design modes Operator performance prediction models Design data bases System input and output Menu-based command language Display windows showing parameters and options Default--task entry Optione--workepace entry, object definition, operator definition, task evaluation Prelirn~nary design--pro~rides design guidelines with incomplete workspace or task information Single exertion, posture prediction and biomechanical analysis Repeated trials--biomechanical and physiological effects on lifting Time prediction based on MTM-2 get and place elements Static data bases--eystem files of generic operator, workspace, or task data Dynamic data bases--user defined files, which vary with the application, and the stage of design System defaults for posture, gender, and analysis modes Task input syntax similar to current process descriptions Output in graphic format--workspace and operator three-dimensional graphics; two-dimensional graphs and charts of analysis results Output formatted to comply with designer-mated preferences SOURCE: Evans (1985~. these populations must be accommodated in the design of hun- dreds of products and workspaces. For example, clothiers, pattern makers, and other product designers have a need for anthropomet- ric and biodynamic models. It is difficult to determine whether data from different sources are comparable. The names used for the same dimension can vary from study to study, and measurements with the same name can be entirely different as a result of the use of differing landmarks or measuring techniques (Garrett and Kennedy, 1971~.

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77 In general, anthropometric measurements are static and pos- ture-dependent. While a static subject facilitates measurement, the technique creates a data set that describes the body only in relatively artificial poses and provides no direct information on how body size and shape change with motion. Changes in size and shape of the proximal ends of body segments are particularly pronounced as a result of muscle dynamics. In adclition, body shape data have generally been inferred from traditional anthro- pometric body size data and have not been quantified. Most body dimensions are measured independently of each other. While tra- ditional anthropometric analysis has a solid statistical foundation based on normally distributed variables, a comparable statistical methodology must be established for three-~rnensional models. When a person ~ viewed in the lateral plane, for example, mea- surements for stature, shoulder height, and length of leg are all taken from different measurement positions, with no established relationships among them. Therefore, it is difficult to generate a three-~unensional model from these molated measurements in a systematic manner without making artistic assumptions regarding these relationships. Loadings on internal structures in the body change signifi- cantly under dynamic conditions (Marras et al., 1984~. There is a need to measure the dynamic loadings in viva, however, since most models involving ergonomic analysm of activities have been based on static conditions. Three-dunensional models are needed that describe the acute as well as cumulative wear and tear in a joint caused by the dynamic motion of the body and the synergistic action of internal forces (e.g., muscles, ligaments, and pressures). Current methods such as electromyography and disc pressure mea- surements are questionable under true dynamic conditions. New transducers are needed based on noninvasive measure- ments such as ultrasound to identify this wear and tear. These devices must be capable of producing quantitative data regarding the load components. Dynamic biomechanical models are needed that do more than simply describe the position of the body or the body components. Optimization techniques may provide a useful approach.

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78 Analysm of Muscle and Joint Dynamics Three-~nnensional models of the body are useful for accu- rate descriptions of joint loadings. Therefore, more information regarding the position and the line of action of agon~t-antagonist pairs of muscles is needed. Several mode! approaches using these data have been described by Schultz and Andermon (1981~. These models determine the compression and lateral and anterior shear components of stress on joints and replace the simple compression estimates that are currently used. Current methods for determining the strain in muscles and joints measure the net output of all actions involved. This net output, however, is the sum total of a number of individual muscle efforts. For example, in elbow flexion, in which both extensors and flexors are active, the combined torques around the elbow joint partially nullify each other. The result shows the net joint loading, but does not measure the forces that are contributed by the individual muscles. The problem involved is of practical im- portance because under the combined torques, for example, those created by concurrent contraction of flexor and extensor muscles, the intermediate body joint may be overloaded. This cannot be predicted from the net result, which ~ the only information that current methodologies yield. In addition, gross estimations are required for the lever arms of muscles acting around body articulations. The geometry of these muscle attachments with respect to their lever arms may be quite different for different people. This produces uncertainty about the actual torques developed around joints, the strength to be exerted by the muscles involved, and the loading on the intermediate joints. Most information about human muscle strength capabilities assumes a 1-9 constant force field condition. We know little about the effects of higher or lower constant and transitory force fields on the ability to exert muscle strength. It is difficult to determine the muscle strength that is available, for example, in the reduced gravity of space or in an airplane that flies a path that generates accelerations on the pilot. While there have been a few isolated experiments (e.g., Kroemer, 1974), there have been no systematic measurements made under controlled laboratory conditions. There is a need to evaluate existing systems for measuring the properties of the biomechanical system and to develop a system

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79 for measuring the mechanical impedance of the body during the performance of normal activities. Then it ~ necessary to mea- sure the extent of impedance modulation as a function of the task being performed and to correlate these data with other measur- able variables such as the electromyogram activity of antagonist muscles. Body Segments and Effects of Mamma Data are needed to develop models of specific body segments such as head and neck, arm and hand, and leg and foot, beyond those required for a total body model. An an example, a total body model describing the behavior in an impact situation does not usually require specific information about the biodynamic charac- teristics of the wrist-finger subsystem. This specific information also would be useful for the design of controls to be operated with small motions of the wrist and fingers, as in high-performance aircraft under loading in excess of normal gravity. Available information on the susceptibility of the body to sin- gle (acute) or cumulative trauma ~ limited. Individual excursions or positions in body articulation that occur in activities such as force exertion or the direction of certain motions may not pro- duce immediate trauma, but the injuries may be cumulative. We know that accompanying conditions such as temperature may in- fluence the occurrence of certain cumulative trauma items. It is not known, however, how the combination of these, that is, the magnitude of excursion, directions of excursions, and accompany- ing force or torque generations, together may generate cumulative trauma injuries to tendons or tendon sheaths or impingement on nerves. Although the phenomenon is known, the conditions under which it may occur are not fully understood. Bone and Fink D~nam;ce Stressed bones behave differently when anisotropic assume tions are made. The knowledge gained from bone modeling will apply to link models of the human body. The geometrically com- plex features of bone should be included in these link models since stress within a joint would most certainly change as the geometric characteristics of interacting bone surface areas change. Studies of human body linkages, which are basic to the majority of human

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80 body analogs, require the precise location of specific skeletal land- marks. Current methods in anthropometry can only approximate the location of these landmarks in a three-dimensional system be- cause of the varying thickness of overlying tissue. Finally, link models of the human body cannot assume that the spine is a rigid link, but must acknowledge its flexibility. Finite-element models of the an~sotropic features of bone may provide useful data for the development of a complete index of bone characteristics of the body. Typically, models are needed to describe the characteristics of the spinal column and of body joints in general (Hakim and King, 1979~. Motivation and Fatigne Factors such as motivation and fatigue play important roles in human biomechanical actions. The control that the operator exerts over muscles because of transitory motivation or fatigue are biasing factors that have been largely ignored in biomechanical modeling. We know that when motivation is present, people are capable of exerting force which far exceeds that predicted by most biomechanical models. The effects of fatigue on muscular perfor- mance have not been quantified. Some of the recent literature has also indicated that when workers are subjected to unexpected loadings they are at a greater risk of musculoskeletal injury. There is a lack of suitable theory and experimental data regarding inter- nal nervous control with respect to feed-forward generated in the brain and to the rearrangement of CNS motor signals according to the feedback that is received (Kroemer et al., 1986~. Consequently, quantification of the effects of learning and adaptation and of psy- chomotor behavior while a person is fatigued is difficult since the internal processes in the central nervous system are not readily accessible. However, much of this adaptive behavior manifests itself in changes in the mechanical parameters of the biological system, specifically the unpedance (i.e., mechanical stiffness and effective viscosity) about the joints. These quantities are under voluntary control (e.g., elbow stiffness may be changed by a factor of 100 or more) and dramatically influence the behavior of the biomechanical system and its response to external loads.