Handbook of Industrial Automation - Richard L. Shell and Ernest L. Hall Part 16 pdf

Human Interfaces for Automated Systems 755 Table 2 Checklist for Task Analysis Activities Goals What are the important goals and supporting tasks? For every important task: Intrinsics of the task What What What What What What What What What What is the task? are the inputs and outputs for the task? is the transformation process (inputs to outputs)? are the operational procedures? are the operational patterns? are the decision points? problems need solving? planning is needed? is the terminology used for task speci®cation? is the equipment used? Task dependency and criticality What are the dependency relationships between the current task and the other tasks and systems? What are the concurrently occurring e€ects? What is the criticality of the task? Current user problems What are the current user problems in performing this task? Performance criteria What is the speed? What is the accuracy? What is the quality Task criteria What What What What What What User discretion Can the user control or determine pace? Can the user control or determine priority? Can the user control or determine procedure? Task demands What What What What What is the sequence of actions? is the frequency of actions? is the importance of actions? are the functional relationships between actions? is the availability of functions? is the ¯exibility of operations? are are are are are the the the the the physical demands? perceptual demands? cognitive demands? envirornmental demands? health and safety requirements? Adapted from Ref 5 While many di€erent task analysis techniques exist to suit the di€erent design requirements in systems, our primary focus here is on techniques that help in designing the interface The key issues involved in designing a human interface with automated equipment are assessing what will be needed to do a job (the types of information that human operators will need to understand the current system status and requirements; the types of output that human operators will have to make to control the system), and deciding how this will be provided Table 3 provides a summary of the important activities involved in the process of interface design and the corresponding task analysis technique Copyright © 2000 Marcel Dekker, Inc to use in designing this activity We present brief summaries of each of these techniques in the following sections The reader should refer to Kirwan and Ainsworth [6], or other articles on task analysis, for a detailed discussion of the di€erent task analysis techniques Hierarchical Task Analysis This enables the analyst to describe tasks in terms of operations performed by the human operator to attain speci®c goals, and ``plans'' or ``statements of conditions'' when each of a set of operations has to be carried out to attain an operating goal Goals are de®ned as ``desired states of Human Interfaces for Automated Systems 757 Figure 5 Activities involved in activity sampling tent, and actions and feedback required of the operator Once a broad list of activities and the tasks involved have been generated using either hierarchical task analysis or activity sampling, task decomposition can be used to systematically expand on the task descriptions The various steps involved in task decomposition are presented in Fig 6 Decision±Action Diagram This is one of the most commonly used tools for decision making Figure 7 is an example of a decision±action diagram [7] The decision±action diagram sequentially proceeds through a series of questions (representing decisions) and possible yes/no answers (representing actions that can be taken) The questions are represented as diamonds, and the possible alternatives are labeled on the exit lines from the diamond A thorough knowledge of the system components, and the possible outcomes of making decisions about system components is essential for constructing complete and representative decision± action diagrams Table-Top Analysis As the name implies, this is a technique through which experts knowledgeable about a system discuss speci®c system characteristics In the context of interface design, this task analysis methodology is used for checking if the information Copyright © 2000 Marcel Dekker, Inc ¯ows identi®ed during the initial task analysis and task description, is adequate for successful task completion Table-top analysis, hence, typically follows the initial hierarchical or other forms of task analysis which yield task descriptions, and provides information input for the decomposition of the tasks A number of group discussion techniques exist in practice, including the Delphi method, the group consensus approach, the nominal group technique, etc., for conducting table-top analysis, each with its own merits and demerits Walk-Through/Talk-Through Analysis These analyses involve operators and other individuals having operational experience with the system, walking and talking the analyst through observable task components of a system in real time Walk-through is normally achieved in a completely operational system or in a simulated setting or even in a mock-up setting Talk-through can be performed even without a simulation of the systemÐthe only requirements are drawing and other system speci®c documentation to enable the analysts to set system and task boundaries while performing the talk-through analysis For more information on walk-through and talk-through analyses, refer to Meister [8] 760 Mital and Pennathur Figure 7 Copyright © 2000 Marcel Dekker, Inc (continued) Human Interfaces for Automated Systems 761 Figure 7 (continued) Copyright © 2000 Marcel Dekker, Inc 762 Mital and Pennathur acterization exercise described in Sec 1.3.1.1; the checklist used for user characterization can be used for person speci®cation also One of the widely used techniques for person speci®cation is the position analysis questionnaire Broadly, position analysis questionnaires require the operator to identify for their speci®ed tasks andjobs, the information input, the mental processes, the work output, the context of the job, the relationship with other personnel in the system, and any other relevant job characteristics Using the responses from the operators, the skill content of tasks and jobs can be determined, and can help in designing personnel selection and training programs to ensure optimal human±machine interaction Ergonomics Checklists These checklists are generally used to ascertain if a particular system meets ergonomic principles and criteria Ergonomics checklists can check for subjective or objective information and can cover issues ranging from overall system design to the design of individual equipment Checklists can also range in detail from the broad ergonomic aspects to the minute detail Table 4 provides an example of a checklist for equipment operation A number of other standard checklists have also been developed by the ergonomics community Important among these are the widely used and comprehensive set of checklists for di€erent ergonomics issues by Woodson [10,11], MIL-STD 1472C [12] which covers equipment design (written primarily for military equipment, but can be used as a guide to develop checklists), EPRI NP-2360 [13] which is a checklist for maintenance activities in any large-scale system, NUREG0700 [14] which is a comprehensive checklist for control room design, the HSE checklist [15] which deals with industrial safety and human error, and the numerous checklists for CRT displays and VDUs [16,17] 1.3.1.3 Characterization of the Situation Apart from the user and the task variables that could a€ect system performance, the external environment in which the system functions can also in¯uence the human±system interaction performance Table 5 provides a representative checklist for the most commonly encountered situations for which the system analyst must obtain answers, and attempt to provide for these situations in design 1.3.2 Allocation of Functions In designing the human±machine interface, once comprehensive information about the users and the activ- Copyright © 2000 Marcel Dekker, Inc ities/tasks these users will perform is known (through the use of tools presented in the earlier sections), the speci®c activities and tasks need to be assigned either to the humans or to the machines The allocation of functions is a necessary ®rst step before any further design of the interface in the human±machine system can be carried out The need for solving the function allocation problem directly stems from the need to decide the extent of automation of manufacturing activities This is so because, in the present day manufacturing scenario, the decision to make is no longer whether or not to automate functions in manufacturing, but to what extent and how The function allocation problem is perhaps as old as the industrial revolution itself Fitts' list, conceived in 1951 (Table 6), was the ®rst major e€ort to resolve the function allocation problem However, while Fitts' list provided fundamental and generic principles that researchers still follow for studying function allocation problems, its failure to provide quantitative criteria for function allocation resulted in its having little impact on engineering design practices The development of practical and quantitative criteria for allocating functions is compounded by an important issue: unless one can describe functions in engineering terms, it is impossible to ascertain if a machine can perform the function; and, if one can describe human behavior in engineering terms, it may be possible to design a machine to do the job better (than the human) But many functions cannot be completely speci®ed in engineering (numerical) terms This implies that those functions that cannot be speci®ed in engineering terms should be allocated to humans, with the rest allocated to the machines In addition, for the practitioner, function allocation considerations have been limited due to the lack of [19]: 1 2 3 4 5 Systematic and step-by-step approaches to decision making during function allocation Systematic and concise data for addressing issues such as the capability and limitations of humans and automated equipment, and under what circumstances one option is preferable over the other Methodology for symbiotic agents such as manufacturing engineers and ergonomists, to integrate human and machine behaviors Uni®ed theory addressing domain issues such as roles, authorities, etc Integration of other decision-making criteria (such as the economics of the situation) so Human Interfaces for Automated Systems take over the function when circumstances demand it A number of approaches have been suggested in the literature for solving the function allocation problem Some of the promising approaches include function allocation criteria based on speci®c performance measures (time required to complete tasks, for example) [20±24], criteria based on comparison of capabilities and limitations of humans with particular attention given to knowledge, skills, and information sources and channels [25±34] criteria based on economics (allocate the function to the less expensive option), [21,35,36], and criteria based on safety (to the human operator in the system) [37±39] Experiments with these approaches suggest that functions that are well-proceduralized permitting algorithmic analysis, and requiring little creative input, are prime candidates for automation On the other hand, functions requiring cognitive skills of a higher order, such as design, planning, monitoring, exception handling, etc., are functions that are better performed by humans The prime requirements for automating any function are the availability of a model of the activities necessary for that function, the ability to quantify that model, and a clear understanding of the associated control and information requirements Clearly, there are some functions that should be performed by machines because of: 1 2 3 4 5 Design accuracy and tolerance requirements The nature of the activity is such that it cannot be performed by humans Speed and high production volume requirements Size, force, weight, and volume requirement Hazardous nature of the activity Equally clearly, there are some activities that should be performed by humans because of: 1 2 3 4 5 6 Information-acquisition and decision-making needs Higher level skill needs such as programming Specialized manipulation, dexterity, and sensing needs Space limitations (e.g., work that must be done in narrow and con®ned spaces) Situations involving poor equipment reliability or where equipment failure could prove catastrophic Activities for which technology is lacking Copyright © 2000 Marcel Dekker, Inc 765 Mital et al [7] provide a generic methodology in the form of decision-making ¯owcharts for the systematic allocation of functions between humans and machines Figure 7, presented earlier is a part of these ¯owcharts These ¯owcharts are based on the requirements of complex decision making, on a detailed safety analysis, and on a comprehensive economic analysis of the alternatives These function allocation ¯owcharts are available for di€erent manufacturing functions such as assembly, inspection, packaging, shipping, etc., and should be consulted for a detailed analysis of the question of manufacturing function allocation 1.3.3 1.3.3.1 Information Presentation and Control The Scienti®c Basis for Information Input and Processing Reduced to a fundamental level, human interaction with automation can be said to be dependent upon the information processing ability of the human, and upon the exchange of information among the di€erent elements in a system Over the years, behavioral scientists have attempted to explain human information processing through various conceptual models and theories One such theory is the information theory [40] Information, according to information theory, is de®ned as the reduction of uncertainty Implicit in this de®nition is the tenet that events that are highly certain to occur provide little information; events that are highly unlikely to occur, on the other hand, provide more information Rather than emphasize the importance of a message in de®ning information, information theory considers the probability of occurrence of a certain event in determining if there is information worth considering For instance, the ``nosmoking'' sign that appears in airplanes before takeo€, while being an important message, does not convey much information due to the high likelihood of its appearance every time an aircraft takes o€ On the other hand, according to information theory, messages from the crew about emergency landing procedures when the plane is about to perform an emergency landing convey more information due to the small likelihood of such an event Information is measured in bits (denoted by H) One bit is de®ned as the amount of information required to decide between two equally likely alternatives When the di€erent alternatives all have the same probability, the amount of information (H) is given by H ˆ log2 N 766 where N is the number of alternatives For example, when an event only has two alternatives associated with it, and when the two alternatives are equally likely, by the above equation, the amount of information, in bits, is 1.0 When the alternatives are not equally likely (i.e., the alternatives have di€erent probabilities of occurrence), the information conveyed by an event is given by hi ˆ log2 …1=pi † where hi is the information associated with event i, and pi is the probability of occurrence of event i The average information (Hav † conveyed by a series of events having di€erent probabilities is given by ˆ Hav ˆ pi …log2 …1=pi †† where pi is the probability of the event i Just as a bit is the amount of information, redundancy is the amount of reduction in information from the maximum due to the unequal probabilities of occurrence of events Redundancy is expressed as a percentage, and is given by % Redundancy ˆ …1 À Hav =Hmax †  100 Information theory, while providing insight into measuring information, has major limitations when applied to human beings It is valid only for simple situations which can split into units of information and coded signals [41] It does not fully explain the stimulus-carrying information in situations where there are more than two alternatives, with di€erent probabilities The channel capacity theory, another theory explaining information uptake by humans, is based on the premise that human sense organs deliver a certain quantity of information to the input end of a channel, and that the output from the channel depends upon the capacity of the channel It has been determined that if the input is small, there is very little absorption of it by the channel, but that if the input rises, it reaches the threshold channel capacity, beyond which the output from the channel is no longer a linear function of the input [41] Experimental investigations have shown that humans have a large channel capacity for information conveyed to them through the spoken word than through any other medium A vocabulary of 2500 words requires a channel capacity of 34 to 42 bits per second [42] Designers must keep in mind that in this day and age of information technology, the central nervous system of humans is subjected to more information than the information channel can Copyright © 2000 Marcel Dekker, Inc Mital and Pennathur handle, and that a considerable reduction in the amount of information must be carried out before humans process the information In addition to theories such as the information theory and the channel capacity theory that explain information uptake, many conceptual models of human information processing have been proposed by researchers over the last four decades Figure 8 shows one such fundamental model (most other models contain elements of this basic model) depicting the stages involved in information processing [43] The key elements of the model are perception, memory, decision making, attention, response execution, and feedback The following is a brief discussion of each of these elements Perception may involve detection (determining whether or not a signal is present), or identi®cation and detection (involving detection and classi®cation) The theory of signal detection [43±45] through the concept of noise in signals, attempts to explain the process of perception and response to the perceived signals Four possible outcomes are recognized in signal detection theory: (1) hit (correctly concluding that there is a signal when there is one), (2) false alarm (concluding that there is a signal when, in actuality, there is none), (3) miss (concluding that there is no signal when, in actuality, there is one and (4) correction rejection (correctly concluding that there is no signal when there is none) The fundamental postulate of signal detection theory is that humans tend to make decisions based on criteria whose probabilities depend upon the probabilities of the outcomes above The probability of observing a signal, and the costs and bene®ts associated with the four possible outcomes above, determine the responses of the human to the signal The resolution of the human sensory activities (ability to separate the noise distribution from the distribution of the signal) has also been found to a€ect the signal detection capability of the human Memory, in humans, has been conceptualized as consisting of three processes, namely, sensory storage, working memory, and long-term memory [43] According to this conception, information from sensory storage must pass through working memory before it can be stored in long-term memory Sensory storage refers to the short-term memory of the stimulus Two types of short-term memory storage are well knownÐiconic storage associated with visual senses, and echoic storage associated with the auditory senses [46] Sensory storage or short-term memory has been shown to be nearly automatic requiring no sustained attention on the part of the human to retain it 768 Table 7 Mital and Pennathur Common Human Biases Humans attach more importance to early information than subsequent information Humans generally do not optimally extract information from sources Humans do not optimally assess subjective odds of alternative scenarios Humans have a tendency to become more con®dent in their decisions with more information, but do not necessarily become more accurate Humans tend to seek more information than they can absorb Humans generally treat all information as equally reliable Humans seem to have a limited ability to evaluate a maximum of more than three or four hypotheses at a time Humans tend to focus only on a few critical factors at a time and consider only a few possible choices related to these critical factors Humans tend to seek information that con®rms their choice of action than information that contradicts or discon®rms their action Human view a potential loss more seriously than a potential gain Humans tend to believe that mildly positive outcomes are more likely than mildly negative or highly positive outcomes Humans tend to believe that highly negative outcomes are less likely than mildly negative outcomes Adapted from Ref 43 tion is said to be divided (among the tasks) While much of the theoretical base for explaining performance of tasks requiring divided attention is still evolving [43,49], some guidelines for designing tasks that require divided attention are available, and are provided in Table 10 When humans maintain attention and remain vigilant to external stimuli over prolonged periods of time, attention is said to be sustained Nearly four decades of research in vigilance and vigilance decrement [50±53] has provided guidelines for improving performance in tasks requiring sustained attention (Table 11) In addition to the factors discussed above, considerable attention is being paid to the concept of mental workload (which is but an extension of divided attention) Reviews of mental workload measurement techniques are available [54±56], and should be consulted for discussions of the methodologies involved in mental workload assessment Table 8 1.3.3.2 The Scienti®c Basis for Human Control of Systems Humans respond to information and take controlling actions The controlling actions of the human are mediated through the motor system in the human body The human skeletal system, the muscles, and the nervous system help bring into play motor skills that enable the human to respond to stimuli Motor skill is defned as ``ability to use the correct muscles with the exact force necessary to perform the desired response with proper sequence and timing'' [57] In addition, skilled performance requires adjusting to changing environmental conditions, and acting consistently from situation to situation [58] A number of di€erent types of human movements have been recognized in the literature [46] These include discrete movements (involving a single reaching movement to a target that is stationary), repetitive movements (a single movement is repeated), sequential movements Recommendations for Designing Tasks Requiring Selective Attention Use as few signal channels as possible, even if it means increasing the signal rate per channel Inform the human the relative importance of various channels for e€ective direction of attention Reduce stress levels on human so more channels can be monitored Inform the human beforehand where signals will occur in future Train the human to develop optimal scan patterns Reduce scanning requirements on the human by putting multiple visual information sources close to each other, and by making sure that multiple sources of auditory information do not mask each other Provide signal for a sucient length of time for individual to respond; where possible, provide for human control of signal rate Adapted from Ref 46 Copyright © 2000 Marcel Dekker, Inc 770 Mital and Pennathur alternative stimuli [66] Choice reaction time has been shown to be in¯uenced by a numerous factors, including the degree of compatibility between stimuli and responses, practice, presence or absence of a warning signal, the type and complexity of the movement involved in the responses, and whether or not more than one stimulus is present in the signal Movement time is defned as the time from the beginning of the response to its completion It is the time required to physically make the response to the stimulus Movements based on pivoting about the elbow have been shown to take less time, and have more accuracy, than movements based on upper-arm and shoulder action Also, it has been determined that movement time is a logarithmic function of distance of movement, when target size is a constant, and further that movement time is a logarithmic function of target size, when the distance of movement is constant This ®nding is popularly known as Fitts' law [67], and is represented as MT ˆ a ‡ b log2 …2D=W† where MT is the movement time, a and b are empirical constants dependent upon the type of movement, D is the distance of movement from start to the center of the target, and W is the width of the target Human response to stimuli is not only dependent upon the speed of the response, but also on the accuracy of the response The accuracy of the human response assumes special importance when the response has to be made in situations where there is no visual feedback (a situation referred to as ``blind positioning'') Movements that take place in a blind positioning situation have been determined to be more accurate when the target is located dead-ahead than when located on the sides Also, targets below the shoulder height and the waist level are more readily reachable than targets located above the shoulder or the head [68] The distance and speed of movement have also been found to in¯uence the accuracy of the response [69,70] 1.3.3.3 Displays Types of Displays A display is de®ned as any indirect means of presenting information Displays are generally one of the following four types: visual, auditory, tactual, and olfactory The visual and the auditory modes of displaying information are the most common Displays based on tactile and olfactory senses are mostly used for special task or user situations (e.g., for the hearing impaired) Copyright © 2000 Marcel Dekker, Inc Selecting the mode of display whether it should be visual or auditory in nature) is an important factor due to the relative advantages and disadvantages certain modes of display may have over other modes, for speci®c types of task situations (auditory mode is better than visual displays in vigilance), environment (lighting conditions), or user characteristics (person's information handling capacity) Table 12 provides general guidelines for deciding between two common modes of information presentation, namely, auditory and visual The types of displays to use to present information also depend on the type of information to present Di€erent types of information can be presented using displays when the sensing mode is indirect Information can either be dynamic or static Dynamic information is categorized by changes occurnng in time (e.g., fuel gage) Static information, Table 12 Guidelines for Deciding When to Use Visual Displays and When to Use Auditory Displays Characteristics Message characteristics Simple message Complex message Short message Long message Potential reference value of message High Low Immediacy of action requirement of message High Low Message deals with events in time Message deals with locations in space Human capability Auditory system overburdened Visual system overburdened Environmental factors Location too bright or too dark requiring signi®cant adaptation Location too noisy Adapted for Ref 71 Visual displays p p p p Auditory displays p p p p p p p p p p Human Interfaces for Automated Systems on the other hand, does not change with time (e.g., printed safety signs) A number of other types of information are also recognized in the literature Table 13 provides a list of these types along with a brief description of the characteristics of these types of information In the following sections, we discuss recommendations for the design of di€erent types of visual and auditory displays (we restrict our attention in this chapter only to these two common modes) We ®rst provide a brief discussion of the di€erent factors a€ecting human visual and auditory capabilities We then present speci®c display design issues and recommendations for these two broad types of displays Visual displays: factors affecting design Accommodation refers to the ability of the lens in the eye to focus the light rays on the retina The distance (of the target object from the eye) at which the image of the object becomes blurred, and the eye is not able to focus the image any further, is called the near point There is also a far point (in®nity, in normal vision) beyond which the eye cannot clearly focus Focal distances are measured in diopters One diopter is 1/(distance of the target in meters) Inadequate accommodation capacity of the eyes result either in nearsightedness (the far point is too close) or in farsightedness (the near point is too close) Literature recommends an average focusing distance of 800 mm at the resting position of the eye (also known as the resting accommodation) [72] Due to changes in the iris (which controls the shape of the lens), aging results in substantial receding of the near point, the far point remaining unchanged or becoming shorter Figure 9 shows how the mean near point recedes with age It is recom- 771 mended that the designer use this information when designing visual displays Visual acuity is de®ned as the ability of the eye to separate ®ne detail The minimum separable acuity refers to the smallest feature that the eye can detect Visual acuity is measured by the reciprocal of the visual angle subtended at the eye by the smallest detail that the eye can distinguish Visual angle (for angles less than 108) is given by Visual angle …in minutes† ˆ …3438H†=D where H is the height of the stimulus detail, and D is the distance from the eye, both H and D measured in the same units of distance Besides minimum separable visual acuity, there are other types of visual acuity measure, such as vernier acuity (ability to di€erentiate lateral displacements), minimum perceptible acuity (ability to detect a spot from its background), and stereoscopic acuity (ability to di€erentiate depth in a single object) In general, an individual is considered to have normal visual acuity if he or she is able to resolve a separation between two signs 1 H of arc wide Visual acuity has been found to increase with increasing levels of illumination Luckiesh and Moss [73] showed that increasing the illumination level from approximately 10 l to 100 l increased the visual acuity from 100 to 130%, and increasing the illumination level from approximately 10 l to 1000 l increased the visual acuity from 100 to 170% For provision of maximum visual acuity, it is recommended that the illumination level in the work area be 1000 l Providing adequate contrast between the object being viewed and the immediate background, and making the signs and Table 13 Commonly Found Types of Information and Their Characteristics Type of information Quantitative information Qualitative information Status information Warning and signal information Representational information Identi®cation information Alphanumeric and Symbolic information Time-phased information Adapted from Ref 46 Copyright © 2000 Marcel Dekker, Inc Characteristics Information on the quantitative value of a variable Information on the approximate value, trend, rate of change, direction of change, or other similar aspects of a changeable variable Information on the status of a system, information on a one of a limited number of conditions, and information on independent conditions of some class Information on emergency or unsafe conditions, information on presence or absence of some conditions Pictorial or graphic representations of objects, areas, or other con®gurations Information in coded form to identify static condition, situation, or object Information of verbal, numerical, and related coded information in other forms such as signs, labels, placards, instructions, etc Information about pulsed or time-phased signals Human Interfaces for Automated Systems 785 switches, and detent thumb wheels Common control devices used to transmit discrete information and requiring a large amount of force include detent levers, large hand push buttons, and foot push buttons For transmitting continuous information, the traditional control devices such as rotary knobs, multirotational knobs, thumb wheels, levers or joysticks, and small cranks, require only a small amount force to operate them On the other hand, other traditional control devices used to impart continuous information, such as handwheels, foot pedals, large levers, and large cranks, need large amounts of force to manipulate and operate In general, control selection for common controls, such as toggle switches, rocker switches, knobs, cranks, handwheels, etc., is based on operational factors such as speed, accuracy, space requirements, and ease of operation With the advent of information technology, control devices such as joysticks, trackballs, mice, touch tablets, light pens, touch screens, etc., are becoming popular devices for transmitting continuous information to the system Technology has advanced to such an extent that these modern devices demand only a small amount of physical force from the human operator Given the variety of both traditional and modern control devices in use in industry (see Fig 18 for examples of some of these control devices), it is beyond the scope of Figure 18 Copyright © 2000 Marcel Dekker, Inc this chapter to explain the design of each of these devices in detail Besides, many excellent design tables and recommendations already exist in the literature for design and selection of control devices, and are widely available The interested reader is referred to these design guidelines Such guidelines can be found in Sanders and McCormick [46], Woodson et al [11], Chapanis and Kinkade [91] Salvendy [92], Eastman Kodak [90], etc 1.3.3.5 Other Design Considerations in Information Presentation and Control Besides the individual design factors a€ecting the design and operation of displays and controls, there are other general considerations in display and control design that a€ect the overall e€ectiveness of the information presentation and control system as a whole We have chosen to present two such important factors They are compatibility, and grouping and location of controls Compatibility This the relationship between the expectations of the human and the input stimuli and responses of the system with which the human is interacting Any system with human users should be compatible with the human expectations In general, good compatibility will result in fewer user errors, and better Examples of common control devices 786 human and overall system performance Literature identi®es four types of compatibility [47] conceptual, movement, spatial and modality compatibilities Conceptual compatibility refers to the matching that should exist between certain forms of stimuli such as symbols, and the conceptual associations humans make with such stimuli Movement compatibility (also commonly referred to as population stereotypes) denotes the relationship between the movement of the displays and controls and the output response of the system being controlled Numerous types of movement compatibilities have been studied by researchers The most important types of movement compatibilities include the movement of a control to follow the movement of a display, the movement of a control to control the movement of a display, the movement of a control to produce a speci®c system response, and the movement of a display without any related response The common principles of movement compatibility for various types of displays and control devices are presented in Table 21 Spatial compatibility refers to the relationship that should exist between, the physical features, and arrangement, of the controls and their associated displays A good example of compatibility in physical features between the displays and the controls is the design of the function keys on a keyboard, and the corresponding labels for these function keys In a number of experiments with household stove tops, human factors researchers have demonstrated conclusively the need for physically arranging displays and the associated controls in a corresponding and compatible way Modality compatibility is a fairly new addition to the list, and refers to certain stimulus-response combinations being more compatible with some tasks than with others Principles of Control-Display Arrangement in a Workspace The physical location and arrangement of the displays and controls in a given workspace also has to be based on the human sensory capabilities, and the anthropometric, biomechanical, and other characteristics of the human user Table 22 provides general guidelines for locating controls in a workspace The ideal goal of placing each and every display and control at an optimal location and in an optimal arrangement with respect to the human user, is dicult, if not impossible, to achieve in practice A few general principles of control-display location and arrangement are useful in setting priorities and in determining tradeo€s for good design, if not the optimal Copyright © 2000 Marcel Dekker, Inc Mital and Pennathur According to the importance principle, components that are vital to system goals should be placed in convenient locations System experts determine what these vital goals are According to the frequency-ofuse principle, components that are frequently used should be placed in convenient locations According to the functional principle, components that are functionally related in the operation of the overall system should be grouped and placed together Figures 19a (before redesign) and 19b (after redesign) illustrate the use of the principle of functional grouping in the redesign of the machining controller of a Dynamite DM2400 bench-top programmable machining center According to the sequence-of-use principle, components should be arranged in the sequence in which they ®nd frequent use in the operation of the system or in the performance of a task Use of one or a combination of these principles requires that the system designer collect information about the human users involved (the user characterization step described in Sec 1.3.1 as the ®rst step in the process of solving human± machine interaction problems), the tasks involved (the task characterization step using task analysis techniques also described in Sec 1.3.1 as the second step in the process), and the environment in which the user has to perform the task (characterization of the situation, again mentioned in Sec 1.3.1 as the third step in the process) Based on extensive research, the recommendations that have been suggested for designing workspaces with various forms of displays and controls are presented in Table 23 1.4 SUMMARY This chapter presented the overall ``process'' of designing and evaluating systems involving humans and automated devices The key elements involved in this process were brie¯y described, and the essentials of these elements were presented in the form of guidelines and recommendations for practice ACKNOWLEDGMENTS We thank Mr Sampath Damodarasamy, doctoral student in industrial engineering at the University of Cincinnati, for help with the evaluation of the DYNAMYTE 2400 machine controller example, and the two ®gures he generated for this example 790 21 A Mital, A Mahajan, ML Brown, A Comparison of manual and automated assembly methods, In: Proceedings of the IIE Integrated Systems Conference Norcross, GA: Institute of Industrial Engineers, 1988, pp 206±211 22 A Mital, A Mahahjan, Impact of production volume, wage, and interest rates on economic decision making: the case of automated assembly Proceedings of the Conference of the Society for Integrated manufacturing Conference Norcross, GA: 1989, pp 558±563 23 A Mital, Manual versus ¯exible assembly: a crosscomparison of performance and cost in four di€erent countries In: M Pridham, C O'Brien, eds Production Research: Approaching the 21st Century London: Taylor & Francis, 1991 24 A Mital, Economics of ¯exible assembly automation: in¯uence of production and market factors In: HR Parsaei, A Mital eds Economics of Advanced Manufacturing Systems London: Chapman & Hall, pp 45±72, 1992 25 H Andersson, P Back, J Wirstad, Job Analysis for Training Design and EvaluationÐDescription of a Job Analysis Method for Process Industries Report no 6, Ergonomrad, Karlstad, Sweden, 1979 26 J Badaracco, The Knowledge Link Cambridge, MA: Harvard Business School Press, 1990 27 P Ehn, The Work Oriented Design of Computer Artifacts Stockholm: Arbetsmiljo, Arbelistratum, 1988 28 T Engstrom, Future assembly workÐnatural grouping In: Designing for EveryoneÐProceedings of the XIth Congress of the International Ergonomics Association, vol 2, London: Taylor & Francis, 1991, pp 1317±1319 29 AM Genaidy, T Gupta, Robot and human performance evaluation In: M Rahimi, W Karwowski, eds Human±Robot Interaction London: Taylor & Francis, 1992, pp 4±15 30 LS Bainbridge, SAR Quintanilla Developing Skills with Information Technology Chichester: John Wiley, 1989 31 CK Prahalad, G Hamel, The core competence of the corporation Harv Bus Rev 68: 79±91 32 J Rasmussen, Some Trends in Man±Machine Interface Design for Industrial Process Plants Report number Riso-M-2228 Riso National Laboratory, Roskilde, Denmark 1980 33 P Shipley The analysis of organizations as an aid for ergonomics practice In: JR Wilson, EN Corlett, eds Evaluation of Human Work: A Practical Ergonomics Methodology London: Taylor & Francis, 1995 34 J Wirstad, On knowledge structures for process operators In: LP Goodstein, HB Ansderson, SE Olsen, eds Tasks, Errors and Mental Models London: Taylor & Francis, 1988 35 A Mital, R Vinayagamoorthy, Case study: economic feasibility of a robot installation Eng Economist 32: 173±196, 1987 Copyright © 2000 Marcel Dekker, Inc Mital and Pennathur 36 A Mital, LJ George, Economic feasibility of a product line assembly: a case study Eng Economist 35: 25±38, 1989 37 BC Jiang OSH Cheng, Six severity level design for robotic cell safety In: M Rahimi, W Karwowski eds Human-Robot Interaction, 1992 38 J Hartley, Robots at Work: A Practical Guide for Engineers and Managers Bedford: IFS; Amsterdam: North-Holland, 1983 39 A Mital, LJ George, Human issues in automated (hybrid) factories In: F Aghazadeh, ed Trends in Ergonomics/Human Factors V Amsterdam: NorthHolland, 1988, pp 373±378 40 CE Shannon, W Weaver The Mathematical Theory of Communication Urbana, IL: University of Illinois Press, 1949 41 E Grandjean, Fitting the Task to the Man 4th ed London: Taylor & Francis, 1988 42 JR Pierce, JE Karlin, Reading rates and the information rate of a human channel Bell Teleph J 36: 497±516 43 C Wickens, Engineering Psychology and Human Performance, Merrill, Columbus, Ohio, 1984 44 J Swets ed Signal detection and recognition by human observers: contemporary readings Los Altos, CA: Peninsula Publishing, 1988 45 D Green, J Swets, Signal Detection Theory and Psychophysics Los Altos, CA: Peninsula Publishing, 1988 46 MS Sanders, EJ McCormick, Human Factors in Engineering and Design New York: McGraw-Hill, 1993 47 G Miller, The magical number seven, plus or minus two: some limits on our capacity for processing information Psychol Rev 63: 81±97, 1956 48 S Sternberg, High-speed scanning in human memory Science 153: 652±654, 1966 49 D Lane, Limited capacity, attention allocation, and productivity In: W Howell, E Fleishman, eds Human Performance and Productivity: Information Processing and Decision Making Hillsdale, NJ: Lawrence Erlbaum Associates, 1982 50 A Graig, Vigilance: theories and laboratory studies In: S Folkard, T Monk, eds Chichester: Wiley, 1985 51 R Parasuraman, Vigilance, monitoring, and search In: K Bo€, L Kaufmann, J Thomas eds Handbook of Perception and Human Performance: Cognitive Process and Performance New York: Wiley 1986 52 D Davies R Parasuraman, The Psychology of Vigilance London: Academic Press, 1982 53 J Warm, ed Sustaining attention in human performance Chichester: Wiley, 1984 54 FT Eggemeier, Properties of workload assessment techniques In: P hancock, N Meshkati, eds Human Mental Workload Amsterdam: North-Holland, 1988 55 N Moray, Mental workload since 1979 Int Rev Ergon 2: 123±150, 1988 Human Interfaces for Automated Systems 56 R O'Donnell, FT Eggemeier, Workload assessment methodology In: K Bo€, L Kaufman, J Thomas, eds Handbook of Perception and Human Performance, New York: Wiley, 1986 57 C Jensen, G Schultz, B Bangerter, Applied Kinesiology and Biomechanics New York: McGraw-Hill 1983 58 J Kelso, Human Motor Behavior: An Introduction Hillsdale, NJ: Lawrence Erlbaum Associates, 1982 59 J Adams, A closed-loop theory of motor learning J Motor Behav 3: 111±150, 1981 60 J Adams, Issues for a closed-loop theory of motor learning In: G Stelmach Motor Control: Issues and Trends New York: Academic Press, 1976 61 C Winstein, R Schmidt, Sensorimotor feedback In: H Holding ed Human Skills, 2nd ed Chichester: Wiley, 1989 62 B Bridgeman, M Kirch, A Sperling, Segregation of cognitive and motor aspects of visual information using induced motion Percept Psychophys 29: 336±342, 1981 63 S Grillner, Neurobiological bases of rhythmic motor acts in vertebrates Science 228: 143±149, 1985 64 S Klapp, W Anderson, R Berrian, Implicit speech in reading, reconsidered J Exper Psychol 100: 368±374, 1973 65 R Schmidt, Motor Control and Learning: A Behavioral Emphasis, Champaign, IL: Human Kinetics 1982 66 R Woodworth, Experimental Psychology New York: Henry Holt, 1938 67 P Fitts, The information capacity of the human motor system in controlling the amplitude of movement J Exper Psychol 47: 381±391, 1954 68 P Fitts, A study of location discrimination ability In: P Fitts, ed Psychological Research on Equipment Design Research Report 19, Army, Air Force, Aviation Psychology Program, Ohio State University, Columbus, OH, 1947 69 J Brown, E Knauft, G Rosenbaum, The accuracy of positioning reactions as a function of their direction and extent Am J Psychol 61: 167±182, 1947 70 R Schmidt, H Zelaznik, B Hawkins, J Frank, J Quinn, Jr Motor output variability: a theory for the accuracy of rapid motor acts Psychol Rev 86: 415±451, 1979 71 BH Deatherage, Auditory and other sensory forms of iformation presentation In: HP Van Cott, R Kinkade, eds Human Engineering Guide to Equipment Design, Washington, DC: Government Printing Oce, 1972 72 H Krueger, J Hessen, Obkective kontinuierliche Messung der Refraktion des Auges Biomed Tech 27: 142±147, 1982 73 H Luckiesh, FK Moss, The Science of Seeing New York: Van Nostrand, 1937 74 H Krueger, W Muller-Limmroth, Arbeiten mit dem Bildschirm-aber richtig! Bayerisches Staatsministerium fur Arbeit und Sozialordnung, Winzererstr 9, 8000 È Munuch 40, 1979 Copyright © 2000 Marcel Dekker, Inc 791 75 IBM, Human Factors of Workstations with Visual Displays IBM Human factors Center, Dept P15, Bldg 078, 5600 Cottle Road, San Jose, CA 1984 76 H Booher, Relative comprehensibility of pictorial information and printed words in proceduralized instructions Hum Factor 17: 266±277, 1975 77 A Fisk, M Scerbo, R Kobylak, Relative value of pictures and text in conveying information: performance and memory evaluations Proceedings of the Human Factors Society 30th Anual Meeting, Santa Monica, CA, 1986, pp 1269±1271 78 G Mowbray, J Gebhard, Man's senses vs information channels In: W Sinaiko ed Selected Papers on Human Factors in Design and Use of Control Systems, New York: Dover, 1961 79 J Feallock, J Southard, M Kobayashi, W Howell, Absolute judgements of colors in the gederal standards system J Appl Psychol 50: 266±272, 1966 80 M Jones, Color Coding Hum Factors: 4: 355±365, 1962 81 W Grether, C Baker, Visual presentation of information In: HP Van Cott, R Kinkade, eds Human Engineering Guide to Equipment Design Washington, DC: Government Printing Oce, 1972 82 P Muller, R Sidorsky, A Slivinske, E Alluisi, P Fitts, The Symbolic Coding of Informationa on Cathode Ray Tubes and Similar Displays TR-55-375 WrightPatterson Air Force base, OH 1955 83 P Cairney, D Seiss, Communication e€ectiveness of symbolic safety signs with di€erent user groups App Ergon 13: 91±97, 1982 84 H Heglin, NAVSHIPS Display Illumination Design Guide, vol 2 NELC-TD223 Naval Electronics Laboratory Center, San Diego, CA: 1972 85 A Mital, S Ramanan, Results of the simulation of a qualitative information display Hum Factors, 28: 341±346, 1986 86 B Mulligan, D McBride, L Goodman, A Design Guide for Non-Speech Auditory Displays SR-84-1 Naval Aerospace Medical Research Laboratory, Pensacola, FL 1984 87 SA Mudd, The scaling and experimental investigation of four dimensions of pure tone and their use in an audiovisual monitoring problem Unpublished doctoral dissertation, Purdue University, Lafayatte, IN 1961 88 JCR Licklider, Audio Warning Signals for Air Force Weapon Systems TR-60-814, USAF, Wright Air Development Division, Wright-Patterson Air Force Base, OH, 1961 89 JV Bradley, Desirable Control-Display Relationship For Moving-Scale Instruments TR-54-423, USAF, Wright Air Development Center, Wright-Patterson Air Force base, OH, 1954 90 Eastman Kodak Company, 1983, Ergonomic Design for People at Work Belmont, CA: Lifetime Learning Publications, 1983 792 91 A Chapanis, R Kinkade, Design of controls In: HP Van Cott, R Kinkade eds Human Engineering Guide to Equipment Design Washington, DC: Government Printing Oce 1972 92 G Salvendy, Handbook of Human Factors and Ergonomics New York: John Wiley & Sons 1997 93 MJ Warrick, Direction of movement in the use of control knobs to position visual indicators In: PM Fitts, ed Psychological Research on Equipment Design 94 J Brebner, B Sandow, The e€ect of scale side on popuation stereotype Ergonomics 19: 471±580, 1976 95 H Petropoulos, J Brebner, Stereotypes for direction-ofmovement of rotary controls associated with linear displays: the e€ect of scale presence and position, of poiter direction, and distances between the controls and the display Ergonomics, 24: 143±151, 1981 96 DH Holding, Direction of motion relationships between controls and displays in di€erent planes, J Appl Psychol 41: 93±97, 1957 97 C Worringham, D Beringer, Operator orientation and compatibility in visual-motor task performance Ergonomics, 32: 387±400, 1989 98 HP Van Cott, R Kinkade, Human Engineering Guide to Equipment Design Washington, DC: Government Printing Oce, 1972 RECOMMENDED READING LIST A Guide to Task Analysis (B Kirwan, LK Ainsworth, eds.) London: Taylor & Francis, 1992 Applied Ergonomics Handbook (B Shakel, ed.) London: Butterworth Scienti®c, 1982 Copyright © 2000 Marcel Dekker, Inc Mital and Pennathur Barnes RM Motion and Time Study: Design and Measurement of Work New York: John Wiley & Sons, 1980 Booth PA An Introduction to Human±Computer Interaction Hove and London: Lawrence Erlbaum Associates, 1989 Eastman Kodal Company Ergonomic Design for People at Work, London: Lifetime Learning Publications, 1983 Evaluation of Human Work: A Practical Ergonomics Methodology (JR Wilson, EN Corlett, eds.) London: Taylor & Francis, 1995 Grandjean E Fitting the Task to the Man 4th ed London: Taylor & Francis, 1988 Handbook of Human Factors and Ergonomics (G Salvendy, ed.) New York: John Wiley & Sons, 1997 Helander M Handbook of Human±Computer Interaction Amsterdam: North-Holland, 1988 Human Engineering Guide to Equipment Design (HP Van Cott, R Kinkade, eds.), Washington, DC: Government Pringtin Of®ce, 1972 Nielsen J Coordinating User Interfaces for Consistency New York: Academic Press, NY, 1989 Ravden S, Johnson G Evaluating Usability of Human±Computer Interfaces New York: Ellis Horwood, 1989 Sanders MS, McCormick EJ Human Factors in Engineering and Design New York: McGraw-Hill, 1993 Woodson, WE, Tillman B, Tillman P Human Factors Design Handbook: Information and Guidelines for the Design Systems, Facilities, Equipment, and Products for Human Use New York: McGraw-Hill, 1991 Chapter 9.2 Workstation Design Christin Shoaf and Ashraf M Genaidy University of Cincinnati, Cincinnati, Ohio 2.1 INTRODUCTION repetitive jobs While this chapter is intended to provide general principles and guidelines for ergonomic workstation design, detailed speci®cations are available from several sources [4±6] Workstation design, including consideration of work methods, can be used to address several problems facing the contemporary workplace With the spread of video display terminal (VDT) use in the workplace, cumulative trauma disorders are being reported with increasing frequency [1] This new technology has increased the incidence of health disorders due to its physical requirements manifested in terms of repetitive motion and static constrained posture demands [2] Workstation design principles can be used to lessen the stress demands imposed by these postures and motions and therefore reduce the risk of injury Secondly, as companies continue to cut costs and strive to achieve more with fewer people, workstation design can also be used as an e€ective tool to optimize human e€ectiveness, thus resulting in increased eciency and productivity In a competitive industrial environment with health treatment, litigation and disability costs all rising, workstation design has become a signi®cant factor not only in determining the health of the employee but the success of the business as well When considering the design of workstations, work methods, tools and handles, three factors account for the majority of ergonomic problems across a variety of industries Therefore, the design principles guiding the biomechanical solution of these problems is based on the control of these factors The three general methods [3] of reducing stress requirements are the reduction of extreme joint movement, excessive forces, and highly 2.2 Safe work results when the job ®ts the worker The contemporary workplace is composed of an increasing number of women, elderly, and minorities Although recommended workstation dimensions based on anthropometric data are available [5], this data may adequately describe the varied population in today's work environment Therefore, it is very important that workstations be designed to allow the maximum degree of ¯exibility in order to accommodate the contemporary worker population The ideal work situation is to alternate between sitting and standing at regular intervals [7] Frequently changing body postures serves to minimize the discomfort and fatigue associated with maintaining the same posture for a long period of time However, if a job cannot be designed to include tasks which include both sitting and standing postures, the seated position is preferable as it provides: Stability required for tasks with high visual and motor control requirements Less energy consumption than standing Less stress on the leg joints 793 Copyright © 2000 Marcel Dekker, Inc PHYSICAL LAYOUT CONSIDERATIONS 794 Shoaf and Genaidy Lower hydrostatic pressure on leg circulation [5, 7] 2.2.1 Chair Design Guidelines Prolonged work in the seated position can result in pain, fatigue, or injury in the lower back, shoulders, legs, arms, and neck However, careful consideration to chair design can reduce the likelihood of these problems Table 1 provides a list of chair parameters 2.2.2 Height of Work Table/Activity The work table should be adjustable to allow for work to be performed in the seated or standing position or to accommodate various seated tasks Easy adjustability ensures that a large population of workers can be accommodated and awkward postures can be avoided For most jobs, the work area should be designed around elbow height when standing or sitting in an erect posture For precise work, the working height should be 2±4 in above the elbow; for heavy manual work, the working height should be about 4±5 in below the elbow [3,6] 2.2.3 Materials, Controls, Tools, and Equipment All materials, tools, and equipment should be easily accessible to the worker to prevent awkward postures All reaching should be below and in front of the shoulder and frequent work should be kept in the area that can be conveniently reached by the sweep of the arm with the upper arm hanging in a natural position at the side of the trunk [3] Table 1 Chair Design Parameters Parameter Backrests Height Footrests Seat pan Arm rests Casters Requirements Should be adjustable for height and angle of tilt, and provide continuous lumbar region support; should be independent from the seat pan Should be adjustable Should be provided and adjustable Front edge should roll forward to prevent compression of the leg Should be provided when feasible Provide ``safety'' caster chairs (these do not roll easily with no weight in the chair) when lateral movements are required within the work area Copyright © 2000 Marcel Dekker, Inc 2.2.4 Lighting Adequate overhead lighting should be provided for all work areas Task-speci®c localized lighting should also be provided if detailed work is required Where documents are read, illumination levels of 500±700 l are recommended Inadequate lighting may cause employees to work in awkward postures 2.3 WORK METHOD CONSIDERATIONS Although physical workstation design is the primary mechanism contributing to a healthful workplace, work method considerations can be employed as a temporary solution when workstation design changes are not immediately possible, or can be used as a complement to the design changes Training in how to perform the work task as well as how to use all tools and equipment is mandatory for new employees and should be available as needed to experienced employees Self pacing of work, especially for new employees, is recommended to alleviate mental and physical stresses Also, frequent rest breaks should be allowed In addition to these measures, the design of the work situation can be altered to lessen stress e€ects Highly repetitive jobs may be automated or a job consisting of few repeated tasks can be enlarged by combining varying tasks Job enrichment is another job redesign technique which may be employed to increase job satisfaction Job enrichment increases the amount of control and meaningfulness the employee experiences When job enlargement or job enrichment is not feasible, rotating job tasks among employees is an alternative method of relieving stress due to repetition 2.4 VIDEO DISPLAY TERMINAL GUIDELINES The VDT work environment has become commonplace in recent years and has provoked a signi®cant amount of discomfort and health complaints Visual problems as well as musculoskeletal injuries are two frequently reported concerns which can be lessened by workstation design and work methods changes Visual fatigue can result from viewing objects on the VDT screen at a close range for an extended period of time as well as from excessive re¯ected glare A brightly lit oce environment, often found in the conventional oce setting, can create a risk in VDT work as screen re¯ections occur Several measures, including Workstation Design reorienting the VDT screen, selective removal of light sources or use of partitions or blinds, can aid in controlling light in VDT work areas [8] If these solutions are not feasible, a micro®lament mesh ®lter can be ®tted over the screen or a parabolic lighting ®xture (louver) can be installed below a conventional ¯uorescent ®xture to reduce screen glare Rest breaks can also be used to combat visual fatigue The National Institute of Occupational Safety and Health (NIOSH) [9] recommends, as a minimum, a break should be taken after 2 hr of continuous VDT work In order to ensure adequate employee visual capacity to perform VDT work, NIOSH [9] advocates visual testing before beginning VDT work and periodically thereafter The second frequent complaint among VDT workers is musculoskeletal discomfort Early NIOSH studies report a prevalence rate exceeding 75% for the ``occasional'' experience of back, neck, and shoulder discomfort among VDT users [10,11] As VDT work is stationary and sedentary, operators most often remain seated in ®xed, sometimes awkward, postures for extended periods of time Consequently, joint forces and static loads can be increased to levels causing discomfort For example, elevation of the arms to reach the keyboard may aggravate neck and shoulder pain Proper workstation design is the ®rst step necessary to improve the VDT work environment As previously stated, adjustability is paramount in good workstation design This is also true for the VDT workstation Some of the most important VDT workstation features are [12,13]: Movable keyboards with adjustable height that allow the operator's arms to be approximately parallel to the ¯oor Adjustable backrest to support the lower back Adjustable height and depth of the chair seat Swivel chair with ®ve-point base and casters Screen between 1 and 2 ft away; middle of screen slightly below eye level; characters large and sharp enough to read easily; brightness and contrast controls; adjustable terminal height and tilt; glareproof surface; no visible ¯icker of characters Indirect general lighting 200-500 l, moderate brightness Direct, adjustable task lighting Feet resting ®rmly on the ¯oor, footrest for shorter operators; thighs approximately parallel to the ¯oor Adequate work-table space for a document holder approximately the same distance as the screen Copyright © 2000 Marcel Dekker, Inc 795 Additional ventilation or air-conditioning where required to compensate for the heat generated by many VDTs operating in a work space VDT cables positioned and secured to prevent tripping In addition to proper workstation design, consideration to work methods is also required to discourage the health risks associated with VDT operation First, operators should be aware of ergonomic principles and then trained to adjust their own workstations Secondly, even with good workstation design, physical stress can result due to the prolonged postures demanded by many VDT tasks Frequent rest breaks can alleviate some of this stress Therefore, VDT operators should periodically change positions or stretch during long assignments Walking around during breaks or performing simple stretching exercises can also be bene®cial Lee et al [14] provide an excellent review of physical exercise programs recommended for VDT operators 2.5 SUMMARY Careful consideration of workstation design, including work methods, should be given whenever the physical work setting changes and should continually be reevaluated to ensure proper person±environment ®t Attention to workstation design can serve as an e€ective tool in the prevention of work-related health disorders as well as for increasing employee productivity Consequently, both of these outcomes will result in higher company pro®t By consulting current design guidelines, training employees in equipment use as well as in basic ergonomic principles and encouraging employee feedback, an e€ective workstation environment can be realized REFERENCES 1 EL Greene Cumulative trauma disorders on the rise Med Trib July 26: 1990 2 EB Chapnik, CM Gross Evaluation, oce improvements can reduce VDT operator Occupat Health Safety 56:7, 1987 3 V Putz-Anderson Cumulative Trauma Disorders A Manual for Musculoskeletal Diseases of the Upper Limbs London: Taylor & Francis, 1988, pp 85±103 4 American National Standard for Human Factors Engineering of Visual Display Terminal Workstations, 796 5 6 7 8 9 Shoaf and Genaidy ANSI/HFS 100-1988 Santa Monica, CA: Human Factors Society, 1988 DB Chan, GB Andersson Occupational Biomechanics New York: John Wiley Sons, 1991 E Grandjean Fitting the task to the man Philadelphia, PA, Taylor & Francis, 1988 R Carson Ergonomically Designed Chairs Adjust to Individual Demands Occupat Health Safety June: 71±75, 1993 SL Sauter, TM Schnorr Occupational health aspects of work with video display terminals In: WN Rom, ed Environmental and Occupational Medicine Boston, Toronto, London: Little Brown and Company, 1992 BL Johnson, JM Melius A review of NIOSH's VDT studies and recommendations Presented at Work with Copyright © 2000 Marcel Dekker, Inc 10 11 12 13 14 Display Units International Conference, Stockholm, Sweden, May, 1996 SL Sauter, MS Gottlieb, KC Jones, VN Dodson Job and health implications of VDT use: initial results of the Wisconsin-NIOSH study Commun Assoc Comput Machinery 26: 1982, pp 284±294 MJ Smith, BGF Cohen, LW Stammerjohn An investigation of health complaints and stress in video display operations Hum Factors 23: 1981 M Sullivan Video display health concerns AAOHN J 37:7, 1989, pp 254±257 JP Shield Video display terminals and occupational health Prof Safety Dec: 1990, pp 17±19 K Lee, N Swanson, S Sauter, R Wickstrom, A Waikar, M Magnum A review of physical exercises recommended for VDT operators Appl Ergon 23:6, 1992, pp 387±408 Chapter 9.3 Physical Strength Assessment in Ergonomics Sean Gallagher National Institute for Occupational Safety and Health, Pittsburgh, Pennsylvania J Steven Moore The University of Texas Health Center, Tyler, Texas Terrence J Stobbe West Virginia University, Morgantown, West Virginia James D McGlothlin Purdue University, West Lafayette, Indiana Amit Bhattacharya University of Cincinnati, Cincinnati, Ohio 3.1 INTRODUCTION applied to job design so that ``hard'' jobs are changed into jobs the are within the physical strength capability of most people Thus, since human physical strength is important, it is necessary to ®nd ways to quantify it through testing This chapter is about human physical strength testing Its purpose is not to recommend any particular type of testing, but rather to describe the types of testing that are available, and the uses to which strength testing has been put It is up to individual users of the strength testing to decide which testing technique is the most appropriate for his or her particular application This chapter discusses four types of strength testing: isometric, isoinertial, psychophysical, and isokinetic Humankind's interest in the measurement of human physical strength probably dates to the ®rst humans At that time, life was truly a struggle in which the ®ttest survived To a great extent, ®ttest meant strongest It is perhaps ironic that in a modern civilized world, children still emphasize the relative importance of physical size and strength in determining the status hierarchy within a group It is equally ironic that current interest in human physical strength comes from 1970s±1980s vintage research which demonstrated that persons with adequate physical strength were less likely to be injured on physically demanding jobs Survival in many modern workplaces may still be a case of survival of the strongest There is, however, a ¯ip side to this issue If persons with limited strength are likely to be injured on ``hard'' jobs, what we know about physical strength can be 3.1.1 Before describing the di€erent types of strength measurement, the term strength must be de®ned and the 797 Copyright © 2000 Marcel Dekker, Inc Human Strength 798 concept of strength measurement must be explained Strength is de®ned as the capacity to produce force or torque with a voluntary muscle contraction Maximum strength is de®ned as the capacity to produce force or torque with a maximum voluntary muscle contraction [1,2] These de®nitions have some key words which must be explained A voluntary muscle contraction is ``voluntary.'' When a person's physical strength is measured, only the voluntary e€ort the person is willing to put forth at the time is measured Thus, when we test a person's maximum strength, we do not measure their maximum; we measure some smaller number that represents what they are comfortable expressing at the time with the existing equipment and environmental conditions It is interesting to note that researchers have experimented with startling persons being tested (for example by setting o€ a starter's pistol behind them during a test) and have found signi®cant increases in measured strength [3] It has been hypothesized that the lower strength displayed by persons during normal testing provides a margin of safety against overloading and damaging muscle tissue It is also true that the test equipment and the tested person's familiarity with the process will in¯uence their ``voluntary'' strength output This is particularly true of the interface between the tested person and the test equipment A poorly designed interface will induce localized tissue pressures which vary from uncomfortable to painful In this situation, you are measuring voluntary discomfort toleranceÐnot strength It is important for strength researchers to keep the ``voluntary'' nature of their data in mind when they are designing their equipment and protocols The de®nition of strength also speaks of force or torque Strength researchers and users of strength data must also understand this distinction We commonly use the terms ``muscle force'' and ``muscle strength'' to describe the strength phenomenon Technically, this is incorrect For most human movements and force exertions, there is actually a group of individual muscles (a functional muscle group) which work together to produce the observable output In complicated exertions, there are a number of functional muscle groups working together to produce the measured output Elbow ¯exion strength, for example, is the result of the combined e€orts of the biceps brachii, brachialis, and the brachioradialis, and a squat lift is the result of the combined e€orts of the legs, back, and arms In elbow ¯exion, each individual muscle's contribution to the functional muscle group's output depends on the posture of the arm Copyright © 2000 Marcel Dekker, Inc Gallagher et al being measured Thus, when we measure elbow ¯exion strength, we are measuring the strength of the elbow ¯exor muscle group, not the strength of any individual muscle Furthermore, we are measuring (recording) the force created by the functional muscle group(s) against the interface between the person and the equipment (a set of handles for example) Consider the elbow ¯exion measurement depicted in Fig 1 The force generated by the elbow ¯exor muscle group is shown by Fm This force acts through lever arm a In so doing, it creates a torque about the elbow joint equal to Fm a The measured force (Q, R, or S) will depend upon how far (b, c, or d) the interface (force cu€) is from the elbow Assuming that the exertion is static (nothing moves) in this example, the measured force (on the gage) will equal the elbow ¯exor torque divided by the distance that the gage's associated force cu€ is from the elbowjoint That is, Q ˆ …Fm a†=b …1† R ˆ …Fm a†=c …2† S ˆ …Fm a†=d …3† or or As we move the interface (force cu€) from the elbow to the hand, the measured force will decrease This example highlights four points First, ``muscular strength is what is measured by an instrument'' [4] Second, people publishing/using strength data must report/understand in detail how the measurements were done Third, the di€erences in published strengths of the various body parts may be due to di€erences in the measurement methods and locations Fourth, interface locations selected using anthropometric criteria will result in more consistent results across the population measured [5] In summary, a record of a person's strength describes what the instrumentation measured when the person voluntarily produced a muscle contraction in a speci®c set of circumstances with a speci®c interface and instrumentation 3.1.2 Purposes of Strength Measurement in Ergonomics There are a number of reasons people may want to collect human strength data One of the most common is collecting population strength data which can be used to build an anthropometric database; create 800 Gallagher et al demanding job and wishes to hire an individual with strength sucient for the task This employer decides to base his employment decision on a strength test given to a group of applicants Naturally, he selects the applicant with the highest strength score to perform the job The employer may have hired the strongest job applicant; however, what this employer must understand is that he may not have decreased the risk of injury to his employee if the demands of his job still exceed this individual's maximum voluntary strength capacity This example should make it clear that only through knowing both about the person's capabilities and the job demands might worker selection protect workers from WMSDs The second issue that must be considered when worker selection is to be implemented is that of the test's predictive value The predictive value of a test is a measure of its ability to determine who is at risk of future WMSD [6] In the case of job-related strengthtesting, the predictive value appears to hold only when testing individuals for jobs where high risk is known (that is, for jobs known to possess high strength demands) Strength testing does not appear to predict the risk of injury or disease to an individual when job demands are low or moderate It should be clear from the preceding arguments that worker selection procedures are not the preferred method of reducing the risk of WMSDs, and are not to be applied indiscriminately in the workplace Instead, care must be exercised to ensure that these strength testing procedures are applied only in select circumstances This procedure appears only to be e€ective when jobs are known to entail high strength demands, and only when the worker's strength is evaluated in the context of the high strength elements of a job However, if attention is paid to these limitations, worker selection can be an e€ective tool to decrease the risk of WMSDs 3.1.2.2 Job Design The use of physical strength assessment in ergonomics is not limited to its role in worker selection, it can also be used for the purpose of job design Job design has been a primary focus of the psychophysical method of determining acceptable weights and forces Rather than determining individual worker strength capabilities and comparing these to job demands, the psychophysical method attempts to determine workloads that are ``acceptable'' (a submaximal strength assessment) for populations of workers Once the acceptable work- Copyright © 2000 Marcel Dekker, Inc loads for a population are determined, the job or task is designed to accommodate the vast majority of the population For example, a lifting task might be designed by selecting a weight that is acceptable to 75% of females and 90% of males The use of strength assessment for job design has been shown to be an e€ective method of controlling WMSDs It has been estimated that proper design of manual tasks using psychophysical strength assessment might reduce the risk of back injuries by up to 33% [12] 3.1.3 Purpose of This Chapter Muscular strength is a complicated function which can vary greatly depending on the methods of assessment As a result, there is often a great deal of confusion and misunderstanding of the appropriate uses of strength testing in ergonomics It is not uncommon to see these techniques misapplied by persons who are not thoroughly familiar with the caveats and limitations inherent with various strength assessment procedures The purposes of this chapter are: (1) to familiarize the reader with the four most common techniques of strength assessment used in ergonomics (isometric, isoinertial, psychophysical, and isokinetic); and (2) to describe the proper applications of these techniques in the attempt to control WMSDs in the workplace This chapter contains four parts, one for each of the four strength measurement techniques listed above Each part describes the strength measurement technique and reviews the relevant published data Equipment considerations and testing protocols are described, and the utility of the tests in the context of ergonomics are also evaluated Finally, each part concludes with a discussion of the measurement technique with regard to the Criteria for Physical Assessment in Worker Selection [6] In this discussion, each measurement technique is subjected to the following set of questions: 1 2 3 4 5 Is it safe to administer? Does it give reliable, quantitative values? Is it related to speci®c job requirements? Is it practical? Does it predict risk of future injury or illness? It is hoped that this part of the chapter will provide a resource that can be used to better understand and properly apply these strength assessment techniques in the e€ort to reduce the risk of WMSDs Physical Strength Assessment in Ergonomics 3.2 3.2.1 PART I: ISOMETRIC STRENGTH Introduction and De®nition Isometric strength is de®ned as the capacity to produce force or torque with a voluntary isometric [muscle(s) maintain(s) a constant length] contraction The key thing to understand about this type of contraction and strength measurement is that there is no body movement during the measurement period The tested person's body angles and posture remain the same throughout the test Isometric strength has historically been the one most studied and measured It is probably the easiest to measure and the easiest to understand Some strength researchers feel that isometric strength data may be dicult to apply to some ``real life'' situations because in most real circumstances people are movingÐthey are not static Other researchers counter that it is equally dicult to determine the speed of movement of a person group of persons doing a job (each moves in their own unique manner and at their own speed across the links and joints of the body) Thus, dynamic strength test data collected on persons moving at a di€erent speed and/or in a di€erent posture from the ``real world'' condition will be just as hard to apply In truth, neither is betterÐthey are different measurements and both researchers and users should collect/use data which they understand and which ®ts their application 3.2.2 801 The following procedures are generally used in this biomechanical analysis technique First, workers are observed (and usually photographed or videotaped) during the performance of physically demanding tasks For each task, the posture of the torso and the extremities are documented at the time of peak exertion The postures are then recreated using a computerized software package, which calculates the load moments produced at various joints of the body during the performance of the task The values obtained during this analysis are then compared to population norms for isometric strength obtained from a population of industrial workers In this manner, the model can estimate the proportion of the population capable of performing the exertion, as well as the predicted compression forces acting on the lumbar disks resulting from the task Figure 2 shows an example of the workplace analysis necessary for this type of approach Direct observations of the worker performing the task provide the necessary data For example, the load magnitude and direction must be known (in this case a 200 N load acting downward), the size of the worker, the postural angles of the body (obtained from photographs or videotape), and whether the task requires one or two hands Furthermore, the analysis requires accurate measurement of the load center relative to the ankles and the low back A computer analysis program Workplace Assessment When a worker is called upon to perform a physically demanding lifting task moments (or torques) are produced about various joints of the body by the external load [13] Often these moments are augmented by the force of gravity acting on the mass of various body segments For example, in a biceps curl exercise, the moment produced by the forearm ¯exors must counteract the moment of the weight held in the hands, as well as the moment caused by gravity acting on the center of mass of the forearm In order to successfully perform the task, the muscles responsible for moving the joint must develop a greater moment than that imposed by the combined moment of the external load and body segment It should be clear that for each joint of the body, there exists a limit to the strength that can be produced by the muscle to move ever increasing external loads This concept has formed the basis of isometric muscle strength prediction modelling [13] Copyright © 2000 Marcel Dekker, Inc Figure 2 Postural data required for analysis of joint moment strengths using the isometric technique 802 Gallagher et al can be used to calculate the strength requirements for the task, and the percentage of workers who would be likely to have sucient strength capabilities to perform it Results of this particular analysis indicate that the muscles at the hip are most stressed, with 83% of men having the necessary capabilities but on slightly more than half of women would have the necessary strength in this region These results can then be used as the basis for determining those workers who have adequate strength for the job However, such results can also the used as ammunition for recommending changes in job design [13] 3.2.3 Isometric Testing Protocol The basic testing protocol for isometric strength testing was developed by Caldwell et al [1] and published in an AIHA ergonomics guide by Chan [2] The protocol outlined herein includes additional information determined by researchers since that time When conducting isometric testing, there are a number of factors that must be considered and controlled (if possible) to avoid biased results These factors include the equipment used to make the measurements, the instructions given to the person tested, the duration of the measurement period, the person's posture during the test, the length of the rest period between trials, the number of trials a person is given for each test, the tested person's physical state at the time of testing, the type of postural control used during the tests, and the environmental conditions during the test 3.2.4 Test Duration The length of an isometric strength test can impact the result in two ways If it is too long, the subject will fatigue and their strength score will decline If it is too short, the subject will not reach their maximum force level before the test is terminated The existing AIHA Guide suggests a 4 sec test with the score being the average strength displayed during seconds 2±4 The appropriate 3 sec period can be determined as follows If the measuring equipment has the capability, collect strength data by having the person begin their contraction with the equipment monitoring the force until some preselected threshold is reached (usually 20± 30% below the expected maximum force for the person and posture), have the equipment wait 1 sec, and then have the equipment average the displayed force for the next 3 sec This is easily done with computerized systems Copyright © 2000 Marcel Dekker, Inc If the equipment does not have the above capability, then have the person tested begin the test and gradually increase their force over a 1 sec period The force should be measured and averaged over the next 3 sec In complex whole body tests where multiple functional muscle groups are involved, it may take persons a few seconds to reach their maximum Under these conditions, the data collector must adjust the premeasurement time interval accordingly, and they must carefully monitor the progress of the testing to insure that they are fact measuring the maximal force during the 3 sec period 3.2.5 Instructions The instructions to the person tested should be factual, include no emotional appeals, and be the same for all persons in a given test group This is most reliably accomplished with standardized written instruction, since the test administrator's feelings about the testee or the desired outcome may become evident during verbal instruction The following additional factors should also be considered The purpose of the test, the use of the test results, the test procedures, and the test equipment should be thoroughly explained to the persons tested Generally, the anonymity of the persons tested is maintained, but if names may be released, the tested person's written permission must be obtained Any risks inherent to the testing procedure should be explained to the persons tested, and an informed consent document should be provided to, and signed by, all participating persons All test participants should be volunteers Rewards, performance goals, encouragement during the test (for example, ``pull, pull, pull, you can do it,'' etc.), spectators, between person competition, and unusual noises will all a€ect the outcome of the tests and must be avoided Feedback to the tested person should be positive and qualitative Feedback should not be provided during the test exertion, but may be provided after a trial or test is complete No quantitative results should be provided during the testing period because they may change the person's incentive and thus their test result To the tested person, a 4 sec maximal exertion seems to take a long time During the test, feedback in the form of a slow four count or some other tester±testee agreed-upon manner should be provided so the tested person knows how much longer a test will last Physical Strength Assessment in Ergonomics 3.2.6 Rest Period Length Persons undergoing isometric strength testing will generally be performing a series of tests, with a number of trials for each test Under these conditions, a person could develop localized muscle fatigue, and this must be avoided, since it will result in underestimating strength Studies by Schanne [14] and Stobbe [5] have shown that a minimum rest period of 2 min between trials of a given test or between tests is adequate to prevent localized muscle fatigue The data collector must be alert for signs of fatigue, such as a drop in strength scores as a test progresses The person tested must be encouraged to report any symptoms of fatigue and the rest periods should be adjusted accordingly Whenever possible, successive tests should not stress the same muscle groups 3.2.7 Number of Trials for Each Test The test±retest variability for this type of testing is about 10% It is higher for people with limited experience with either isometric testing or with forceful physical exertion in general In addition, these people will often require a series of trials of a test to reach their maximum The use of a single trial of a test will generally underestimate a person's maximum strength, and may underestimate it by more than 50% A twotrial protocol results in less of an underestimate, but it may still exceed 30% [15] For this reason, the preferred approach to determining the number of trials for each test is to make the choice on the basis of performance Begin by having the subject perform two trials of the test The two scores are then compared and if they are within 10% of each other the highest of the two values is used as the estimate of the person's maximal strength, and you proceed to the next test If the two values di€er by more than 10%, additional trials of the same test are performed until the two largest values are within 10% of each other Using this approach, Stobbe and Plummer averaged 2.43 trials per test across 67 subjects performing an average of 30 di€erent strength tests [15] In any case, a minimum of two trials is needed for each test 3.2.8 When to Give Tests A person's measured strength is, for a variety of reasons, somewhat variable It will not be constant over time, nor over a workday However, in the absence of speci®c muscle strength training, it should remain Copyright © 2000 Marcel Dekker, Inc 803 within a relatively narrow range It is generally higher at the beginning of a workday than at the end The fatigue-induced strength decrement will vary from person to person and will depend on the nature of the work done during the day A person who performs repetitive lifting tasks all day can be expected to have a large lifting strength decrement over a workday, whereas a sedentary worker should have little or no decrement Based on these results, the fairest evaluation of a person's maximum strength can be done at the beginning of, or at least early in, a workday 3.2.9 Test Posture Measured strength is highly posture dependent Even small changes in the body angles of persons being tested and/or changes in the direction of force application can result in large changes in measured strength When collecting strength data, a researcher should ®rst determine what type of data is sought, and then one or more strength tests which will provide that speci®c type of data should be designed If, for example, the test is being done to determine whether people are physically ®t for a job, the test posture should emulate, to the extent possible, the posture required on the job Once the test posture has been determined, the researcher must ensure that the same posture is used on each trial of the test The researcher must monitor the test to ensure that the person's posture does not change during the test If these things are not done, the test results will be erratic, and may seriously overestimate or underestimate the person's actual maximal strength 3.2.10 Restraint Systems Restraint systems are generally used either to con®ne a person to the desired test posture, or to isolate some part of the tested person's body so that a speci®c muscle group (or groups) can be tested (see Fig 3) In addition, restraint systems help to assure that all persons participating in a given study will be performing the same test The type and location of restraint system used can have a major impact on test results Similarly, the lack of a restraint system can allow the posture to vary or allow the use of the wrong or additional muscle groups, both of which will impact test results Any restraint system used should be comfortable, it should be padded in a manner that prevents local tissue stress concentrations during the test; it should be positioned so that the correct muscle group(s) and ... if size (large and small) and color (black and white) were orthogonal dimensions of coding, then each of the possible codes namely, large-black, large-white, small-black, and small-white, would... Grandjean E Fitting the Task to the Man 4th ed London: Taylor & Francis, 1988 Handbook of Human Factors and Ergonomics (G Salvendy, ed.) New York: John Wiley & Sons, 1997 Helander M Handbook of. .. Stereotypes for direction-ofmovement of rotary controls associated with linear displays: the e€ect of scale presence and position, of poiter direction, and distances between the controls and the display