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--> Work-Related Musculoskeletal Disorders
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--> 1 Introduction Background In May 1998 the National Institutes of Health asked the National Academy of Sciences/National Research Council to assemble a group of experts to examine the scientific literature relevant to work-related musculoskeletal disorders of the lower back, neck, and upper extremities. A steering committee was convened to design a workshop, to identify leading researchers on the topic to participate, and to prepare a report based on the workshop discussions and their own expertise. In addition, the steering committee was asked to address, to the extent possible, a set of seven questions posed by Congressman Robert Livingston on the topic of work-related musculoskeletal disorders. The steering committee includes experts in orthopedic surgery, occupational medicine, epidemiology, ergonomics, human factors, statistics, and risk analysis. This document is based on the evidence presented and discussed at the 2-day Workshop on Work-Related Musculoskeletal Injuries: Examining the Research Base, which was held on August 21 and 22, 1998, and on follow-up deliberations of the steering committee, reflecting its own expertise. We note the limitations of the project, both in terms of time constraints and sources of evidence. Although reports on the number of work-related musculoskeletal disorders vary from one data system to another, it is clear that a sizable number of individuals report disorders and lost time from work as a result of them.1 For example, the Bureau of 1 We use the World Health Organization's definition of work-related disorders (World Health Organization, 1985). It characterizes work-related disorders as multifactorial to indicate the inclusion of physical, organizational, psychosocial, and sociological risk factors. A disorder is work related when work procedures, equipment, or environment contribute significantly to the cause of the disorder. There is great variation in the diagnostic criteria for musculoskeletal disorders, ranging from clinical diagnoses based on symptoms and signs for some, to diagnoses based on structural and functional criteria for others. We note that ''disorder" is a broader category than "injury" and better captures the range of phenomena being considered.
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--> Labor Statistics (1995) has reported that in 1 year there were 705,800 cases of days away from work that resulted from overexertion or pain from repetitive motion. Estimated costs associated with lost days and compensation claims related to musculoskeletal disorders range from $13 to $20 billion annually (National Institute for Occupational Safety and Health, 1996; AFL-CIO, 1997). The multiplicity of factors that may affect reported cases—including work procedures, equipment, and environment; organizational factors; physical and psychological factors of the individual; and social factors—has led to much debate about their source, nature, and severity. In light of the ongoing debate, an extensive internal review of the epidemiological research was recently done by the National Institute for Occupational Safety and Health (Bernard, 1997). That study is part of the work that was considered by the steering committee. The charge to the steering committee, reflected in the focus of the workshop, was to examine the current state of the scientific research base relevant to the problem of work-related musculoskeletal disorders, including factors that can contribute to such disorders, and strategies for intervention to ameliorate or prevent them. Approximately 110 leading scientists were invited by the steering committee to participate in the workshop, and 66 were able to attend. The attendees represented the fields of orthopedic surgery, occupational medicine, public health, epidemiology, risk analysis and decision making, ergonomics, and human factors (see Appendix A in Part II). Several attendees presented prepared papers; many others presented oral and written responses to the papers or comments on the field of inquiry. Two criteria guided the selection of invitees: that they are involved in active research in the area and that the group, overall, represent a wide range of scientific disciplines and perspectives on the topic. In designing the workshop, the steering committee considered several approaches to framing the topics. After careful consideration, we chose not to have the presentations focus on specific parts of the body and associated musculoskeletal disorders. Rather, we organized our examination of the evidence—and the workshop discussions (see agenda, Appendix B in Part II)—to elucidate the following sets of relationships between factors that potentially contribute to musculoskeletal disorders: ( 1) biological responses of tissues (muscles, tendons, and nerves) to biomechanical stressors; (2) biomechanics of work stressors, considering both work and individual factors, as well as internal loads; (3) epidemiological perspectives on the contributions of physical factors; (4) non-biomechanical (e.g., psychological, organizational, social) factors; and (5) interventions to prevent or mitigate musculoskeletal disorders, considering the range of potentially influential factors. Our belief was that this approach would provide a framework for reviewing the science base for each set of relationships, as well as the wider interactions among the sets. This approach allowed us to take advantage of both basic and applied science and a variety of methodologies, ranging from tightly controlled laboratory studies to field observations. As a result, we considered sources of evidence that extend well beyond those provided by the epidemiological literature on which the public discussion has focused.
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--> Discussions in each of the five topics (all but topic 4) revolved around a paper commissioned for the workshop and comments of invited discussants; a panel format was used to address the epidemiology of physical factors (topic 4), given the availability of recent reviews of literature on this topic. The next section presents a conceptual framework integrating the factors thought to be related to the occurrence of musculoskeletal disorders. We used this framework to select and organize topics covered in the workshop. Framework of Contributors to Musculoskeletal Disorders Figure 1 outlines a broad conceptual framework, indicating the roles that various work and other factors may play in the development of musculoskeletal disorders. This framework serves as a useful heuristic to examine the diverse literatures associated with musculoskeletal disorders, reflecting the role that various factors—work procedures, equipment, and environment; organizational factors; physical and psychological factors of individuals; non-work-related activities; organizational factors; and social factors—can play in their development. Its overall structure suggests the physiological pathways by which musculoskeletal disorders can occur or, conversely, can be avoided. The central physiological pathways appear within the shaded area of the figure. It shows, first, the biomechanical relationship between load and the biological response of tissue. Imposed loads of various magnitudes can change the form of tissues throughout the day due to changes in fatigue, work pattern or style, coactivation of muscle structures, etc. Loads within a tissue can produce several forms of response. If the load exceeds a mechanical tolerance or the ability of the structure to withstand the load, tissue damage will occur. For example, damage to a vertebral end plate will occur if the load borne by the spine is large enough. Other forms of response may entail such reactions as inflammation of the tissue, edema, and biochemical responses. Biomechanical studies can elucidate some of these relationships. Biomechanical loading can produce both symptomatic and asymptomatic reactions. Feedback mechanisms can influence the biomechanical loading and response relationship. For example, the symptom of pain might cause an individual to recruit his or her muscles in a different manner, thereby changing the associated loading pattern. Adaptation to a load might lead individuals to expose themselves to greater loads, which they might or might not be able to bear. Repetitive loading of a tissue might strengthen the tissue or weaken it, depending on circumstances. The symptom and adaptation portions of the model can interact with each other as well. For example, symptoms, such as swelling, can lead to tissue adaptations, such as increased lubricant production in a joint. These relationships can be described in mathematical models that distinguish external load (e.g., work exposure) from internal load (dose) and illustrate cascading events, whereby
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--> FIGURE 1 Conceptual framework of physiological pathways and factors that potentially contribute to musculoskeletal disorders.
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--> responses to loads can themselves serve as stimuli that increase or decrease the capacity for subsequent responses. The responses, symptoms, and adaptations can lead to a functional impairment. In the workplace, this might be reported as a work-related musculoskeletal disorder. If severe enough, the impairment would be considered a disability, and lost or restricted workdays would result. To the left of the shaded area in Figure 1, the framework shows environmental factors that might affect the development of musculoskeletal disorders, including work procedures, equipment, and environment; organizational factors; and social context. For example, physical work factors (reaching, close vision work, lifting heavy loads) affect the loading that is experienced by a worker's tissues and structures. Organizational factors can also influence the central mechanism. Although little studied, hypothetical pathways also exist between organizational influences and the biomechanical load-response relationship, as well as the development of symptoms. For example, time pressures to complete a task might induce carelessness in handling a particular load, with consequent tissue damage. The organizational culture can also create an incentive or a disincentive to report a musculoskeletal disorder or to claim that the impairment should be considered a disability. Social context factors, such as a lack of means to deal with psychological stress (e.g., no spousal support), might also influence what a worker reports or even the worker's physiological responses. To the right in Figure 1, the framework shows the influence of individual physical and psychological factors, as well as non-work-related activities, that might affect the development of musculoskeletal disorders. For example, psychological factors can affect a person's identification of a musculoskeletal disorder or willingness to report it or to claim that the impairment is a disability. Physical factors might involve reduced tissue tolerance due to age or gender or disease states, such as arthritis, which can affect people's biochemical response to tissue loading. This framework can accommodate the diverse literatures regarding musculoskeletal disorders by characterizing the pathways that each study addresses. For example, an epidemiological investigation might explore the pathways between the physical work environment and the reporting of impairments or the pathway between organizational factors and the reporting of symptoms. An ergonomic study might explore the pathways between work procedures and equipment and the biomechanical loads imposed on a tissue. This framework also focuses attention on the interactions among factors. For example, the combination of a particular set of work procedures and organizational factors might produce an increase in disorders that neither would alone produce. Looking at the evidence as a whole provides a sounder basis for understanding the overall dimensions of the problem of work-related musculoskeletal disorders than restricting an examination to any one factor or kind of evidence. It also places individual studies in context, by showing the factors and pathways that they do and do not address.
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--> 2 State of the Evidence The goal of the steering committee was to examine the current state of the scientific research base relevant to the problem of work-related musculoskeletal disorders. As mentioned, we identified five major topics, each of which has been the subject of scientific examination. The resulting literature represents a wide variety of research designs, measures, apparatuses, and modes of analysis. Our representation of the science base therefore covers a wide range of theoretical and empirical approaches. For example, there are highly controlled studies of soft tissue responses that are based on work with cadavers, animal models, and human biomechanics; survey and cross-sectional studies that examine the relationship between musculoskeletal disorders and work, organizational, social, and individual factors; and experimental, quasi-experimental, and time-series studies that are designed to examine the effects of various interventions. In order to make sense of such a multifaceted body of research, we have extended our analysis beyond the traditional criteria used in epidemiological studies; we rely, instead, on five commonly accepted criteria for establishing causal linkages among factors. Following the discussion of the causal criteria, this review of the evidence is divided into five sections. The first, soft tissue responses to physical stressors, covers material that corresponds to the load and response boxes in Figure 1. The second section, work factors and biomechanics, discusses the biomechanics of the load-response relationship and examines the contributions of work procedures, equipment, and environment to this relationship. The third section covers the epidemiological
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--> evidence relating biomechanical factors at work to musculoskeletal disorders. The fourth section examines the state of the evidence regarding the contributions of the non-biomechanical factors listed in Figure 1—organizational, social, and individual. The final section presents a discussion of workplace interventions; this section covers material relevant to all factors represented in the figure. Criteria to Determine Causality Five criteria are normally considered in determining whether scientific evidence supports a causal claim as internally valid—that is, that the purported cause was uniquely responsible for the effect (Campbell and Stanley, 1966; Cook and Campbell, 1979; Cordray, 1986; Einhom and Hogarth, 1986). The first criterion, temporal ordering, requires that the cause be present before the effect is observed. It can be assessed by examining the type of control the investigator has over the timing and delivery of the causal agent in an experimental study (e.g., conducted under controlled conditions) and the course of events in an observational study (e.g., one involving systematic observation of events in the real world). The second criterion requires that the cause and effect covary. For example, when no force is applied to a tendon, it remains in a relaxed state; in the presence of the cause (a force), the tendon responds. The third criterion involves the absence of other plausible explanations for the observed effect. To the extent that confounding factors have been controlled by the design of the experiment or observation, other explanations for the observed effect are less likely. In some studies, it is possible to use random assignment of participants to conditions to control the influence of other factors, but this is not the only means for achieving control. In making a determination of whether factors other than the experimentally manipulated factor (e.g., ergonomic redesign) offer plausible explanations for the observed effect (e.g., a lower average level of sick days in the post-redesign period of observation), it is necessary to identify and test whether other plausible factors might have been operative, mimicking the effect of the target cause (e.g., change in sick leave policy, turnover in personnel). If no other such causes can be shown empirically to be responsible for the effect, it is reasonable to attribute the effect to the cause under investigation. Note that this criterion stipulates that other causal factors need to be plausible, not merely logically possible. Claims of plausibility have merit to the extent that they can be empirically supported. The fourth criterion, temporal contiguity, amplifies the first (temporal ordering). To the extent that the effect follows the cause closely in time, the plausibility that other factors are operative is reduced. For example, if a tendon reacts immediately to the presence of force, it is unlikely that other factors (e.g., gravitational pull) are responsible for the sudden elongation. On the other hand, if there is a delay between
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--> the application of force and the response that cannot be explained by the biomechanical mechanisms associated with tendon structures, other factors may be operative, weakening the strength of the causal claim. The fifth criterion is that the size of the cause is related to the size or magnitude of the effect: that is, there is a congruity between the cause and effect. More specifically, a small force or small change in the workplace ought to correspond to a small effect and a large force or major change (e.g., multiple components of the workplace are altered) ought to be accompanied by a large effect (e.g., failure of a tissue or a substantial reduction in work-related injuries). When results from experiments or quasi-experiments violate these expectations, it is necessary to examine how the effect was modified (either enhanced, in the case of a small cause that leads to a large effect, or dampened, in the case of a large cause that produces a small effect). To the extent that a compelling explanation for these anomalies cannot be provided (e.g., delays in the implementation of a workplace redesign), it is plausible to assume that other processes (not related to the suggested cause) are responsible for some or all of the effect. By applying these criteria to evaluate the credibility of scientific evidence, one need not place heavy emphasis on the types of research design that have been used in a given study. It is a disciplined way to take advantage of the research provided by a wide variety of methods and, thus, it has substantial implications for the manner in which a science base is considered. Rather than focusing on a study's design features, one considers the pattern of data for each study and its associated design on a case-by-case basis. For some studies, it is readily apparent that even the minimal causal criteria cannot be substantiated (e.g., it is impossible to establish that the cause came before the effect). For others, even when there is no conventional experimental control group (created through random assignment), observations before and after the introduction of an intervention can produce valid claims if coupled with other evidence about the change and with probes concerning the presence or absence of other plausible explanations. In complex domains, single studies can seldom provide a conclusive verification of a causal proposition. It is through replication and synthesis of evidence across studies, preferably with studies that use a variety of methods (each with different strengths and weaknesses), that causal claims gain their inferential strength. In performing such syntheses, greatest weight should be given to the evidence from studies that most completely satisfy the five criteria specified above. Poorly conceptualized and executed studies may have little to offer for assessments of causal claims. In contrast, the evidence from a few well-conceived and well-executed studies can strongly outweigh the "noise" created by a large number of studies that do not satisfy the five criteria for causality. (A similar argument was made in deliberations about the evidence related to the effectiveness of a set of complex public health programs; see Normand et al., 1995). Finally, inferential strength is gained by examining the evidence from a variety of theoretical perspectives (as well as a variety of research methods), as specified in the framework provided in Figure 1. Establishing that biological and biomechanical pro-
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--> cesses influence tissues, that these forces are present in some work environments or work-related tasks, that their presence is associated with musculoskeletal disorders, and that their influence can be reduced by workplace redesign (or other interventions) should provide a greater understanding of the evidence than can be gained by considering each factor separately. The findings presented in the rest of this section reflect the steering committee's application of the criteria to the research literature. Soft Tissue Responses to Physical Stressors Several well-established findings are supported by the papers of Rempel et al. (1998) and Ashton-Miller (1998), presented at the workshop. While certain loads can be tolerated and adapted to, all soft tissues, including muscle, tendon, ligament, fascia, synovia, cartilage, intervertebral disc, and nerve, fail when subjected to sufficient force. Data from cadaver studies provide ranges within which such failures occur, and animal models of some tissue provide support for the laboratory data. Even at levels of force clearly below the failure level, however, there is scientific evidence, from these types of studies, that tissue response to deformation can produce inflammation, failure at microscopic levels, and muscle fatigue. Injuries to muscle from single-event and repetitive contractions have been documented in humans and in animal models. Local muscle fatigue occurs at low contraction levels when maintained for long periods. Inflammatory muscle responses have been documented in humans subjected to repetitive or prolonged loading. Muscles also are affected by individual factors, such as age and level of conditioning. These effects involve not only the tissue, but also the neuromuscular control system. Ligaments and tendons also fail from single or repetitive loading. For tendons, disorders can occur at the insertion into the bone, in the tendon proper, or at the junction between tendon and muscle. Animal models have demonstrated the development of inflammatory responses in the tendon sheath and insertion areas (tendinitis and tendinosis). Cartilage is a tissue known to deteriorate when subjected to abnormal loads. Intervertebral discs, which are a special type of cartilage, fail when loaded, as fissures develop within the substance. Age, gender, and other individual factors influence these processes. Nerves subjected to tension or compression will respond with pain, dysfunction, and ultimately permanent tissue changes. There is good scientific evidence about both acute and chronic effects of nerve compression and about the effects of vibration on nerves. Critical pressure and duration values have been established for acute nerve compression, but they have not yet been established for chronic compression. It is important to note that many of the well-designed experimental studies in animals and cadavers have been successfully replicated. The applicability of their findings to humans in the workplace has been addressed by observations of comparable effects in exposed humans. On all of these highly controlled studies, the causal criteria
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--> Although a good deal is known about musculoskeletal disorders, a better understanding of the clinical courses of these disorders would be possible with improved models and measures. Better understanding of the course of these disorders would provide information that would assist in formulating strategies for tertiary intervention, by altering the clinical and economic impact of musculoskeletal disorders once they have become manifest. One potential area for research is the contribution of physical conditioning and exercise to developing human resistance and resilience.
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--> 3 Seven Questions Posed by Congressman Robert Livingston The material in Section 2 puts in perspective the state of the evidence with respect to the range of factors that could be contributing to musculoskeletal disorders and describes the extent to which there is a scientific basis for concluding that such disorders originate in the workplace and can be reduced through programmatic interventions. This section presents the steering committee's response to the seven questions posed by Representative Livingston. As our analysis has been at the level of the family of musculoskeletal disorders, our response will be provided across disorders. Question 1: What are the conditions affecting humans that are considered to be work-related musculoskeletal disorders? The musculoskeletal conditions that may be caused by (non-accidental) physical work activities include disorders of inflammation, degeneration, and physiological disruption of muscles, tendons, ligaments, nerves, synovia, and cartilage involving limbs and trunk. These entities are included in categories 353-355, 722-724, and 726-729 of the International Classification of Diseases (commonly referred to as ICD-9) (World Health Organization, 1977). Not every disorder in these categories may be caused by mechanical stressors, but all the major musculoskeletal disorders of interest are included in these groupings. Common examples are low back strain, tenosynovitis, and carpal tunnel syndrome.
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--> Question 2: What is the status of medical science with respect to the diagnosis and classification of such disorders? There is great variation in the diagnostic criteria for the many musculoskeletal disorders, ranging from clinical diagnoses based on symptoms and signs for some, to diagnoses based on structural and functional criteria for others. The diagnostic criteria used in epidemiological studies are often different from those used to make treatment decisions. This difference can lead to classification of cases for research that would be unacceptable when invasive treatment alternatives are considered, but it has little influence on the conclusion of appropriately designed epidemiological surveys. In the classification of back pain, a symptom classification is often used because the precise etiology of the painful process often cannot be identified. For specific disease entities, such as herniated disks, there are accepted diagnostic criteria that are based on clinical symptoms and signs, as well as imaging information. Similarly, there are disease entities for upper extremities for which there are accepted diagnostic criteria; for other disease entities, they are classified broadly on the basis of clinical symptoms and signs. Question 3: What is the state of scientific knowledge, characterized by the degree of certainty or lack thereof, with regard to occupational and nonoccupational activities causing such conditions? The relationships among work factors, biomechanical loads, and responses are supported by mathematical models and direct measurements. The mathematical models are widely accepted and applied to design mechanical structures in aircraft and automotive design. Direct measurements have been used to a lesser extent than modeling because they are potentially injurious to human subjects; however, when they have been used they generally support the biomechanical models. It has been shown that the load forces encountered over time in normal work activities often approach the physiological and mechanical tissue limits. Limits may be exceeded as a result of a single high force or as a result of repeated loads over time. Some tissues have a greater ability to adapt to repeated loads if there is sufficient recovery time between successive loads, while other tissues, e.g., nerves, are less able to adapt. Biomechanical loads are encountered in activities of work, daily living, and recreation. The contribution of these activities to tissue response is related to their relative duration and intensity. For most people, their main exposure is at work. There is a substantial body of epidemiological literature that shows a disproportionately high incidence of musculoskeletal disorders of all types among persons exposed to high biomechanical loads. Although there can be debate about acceptable exposure limits, there can be little disagreement about the fundamental relationship between extreme work exposures and musculoskeletal morbidity. Question 4: What is the relative contribution of any causal factors identified in the literature to the development of such conditions in (a) the general population; (b) specific industries; and (c) specific occupational groups?
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--> Data at the population and industry level have been collected for a variety of purposes by different groups using different methods. Some data are based on survey results, some on clinical or medical diagnoses, and some on compensation claims. In the judgment of the steering committee, it is not possible to make useful comparisons on the basis of these data. The incidence of musculoskeletal disorders in any specific occupational group can be expected to reflect the tissue loads imposed by the work, the tissue tolerances of the mix of individuals doing it, and the other activities in their lives imposing related loads. The relative contribution of the different factors in any occupational group depends on ( 1) the strength of these relationships across the ranges of individuals and activities in that group and (2) the variability of the individuals and activities. For example, gender will not be an important predictive factor if men and women do not, on average, have different tissue tolerances for the loads imposed in that occupational group or if workers in the group are overwhelmingly men or women. The relative contribution in a specific industry will, similarly, depend on the mix of individuals and tasks in it. Unfortunately, measurements of the relevant features of individuals and tasks are typically unavailable, limiting our ability to assess the relative contribution of different factors across groups. The evidence shows that the incidence of musculoskeletal disorders is higher among individuals who perform activities that exceed tissue tolerances. Generally speaking, the more that they perform those activities, the greater are their risks for such disorders. Question 5: What is the incidence of such conditions in (a) the general population; (b) specific industries; and (c) specific occupational groups? Current knowledge about the incidence of each of the conditions described in Question 1 in the general adult population is limited because (1) the conditions are clinically diagnosed, typically in doctors' offices; (2) diagnostic criteria for these conditions are not uniformly applied; (3) there are no data collection systems to capture such diagnoses in the health care system; and (4) two-thirds of the adults in the general population are employed, with variable occupational risks. Those who are not employed, including those with various chronic conditions and disabilities, are not a suitable reference population. Data on industry and occupational groups are based on a wide variety of methods that have been collected for different purposes. It is the steering committee's judgment that it is not possible to make useful comparisons on the basis of these data. Question 6: Does the literature reveal any specific guidance to prevent the development of such conditions in (a) the general population; (b) specific industries; and (c) specific occupational groups? Specific interventions can affect the reported rate of musculoskeletal disorders in specific industries and for specific occupations. Interventions can also reduce reports of musculoskeletal disorders, the presence of risk factors, and the reporting of comfort and pain associated with work. It is also clear that the effectiveness of interventions can be
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--> improved if they are tailored to specific occupations and work settings. There is a dearth of data on interventions in the general population. Question 7: What scientific questions remain unanswered, and may require further research, to determine which occupational activities in which specific industries cause or contribute to work-related musculoskeletal disorders? Looking at this web of evidence, we have reached three major conclusions: Musculoskeletal disorders are a serious national problem: estimates of costs range from $13 to $20 billion annually. These problems are caused by work and non-work activities. There are interventions that can reduce the problems. We have also identified some focused research projects, the results of which could increase the efficacy of interventions. Some of these projects would produce useful results within specific research areas; others would increase the connections among the areas. It is a strength of the science that it points to these specific opportunities (see the discussion of future research above).
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--> 4 Conclusions The steering committee has explored the complex problem of musculoskeletal disorders in the workplace. We have supplemented our professional expertise with workshop presentations, commissioned papers and other submissions, and discussions with invited workshop participants. We find very clear signals on some topics and weaker signals on others—but little in the way of contradiction. Thus, while there are many points about which we would like to know more, there is little to shake our confidence in the thrust of our conclusions, which draw on converging results from many disciplines, using many methods: There is a higher incidence of reported pain, injury, loss of work, and disability among individuals who are employed in occupations where there is a high level of exposure to physical loading than for those employed in occupations with lower levels of exposure. There is a strong biological plausibility to the relationship between the incidence of musculoskeletal disorders and the causative exposure factors in high-exposure occupational settings. Research clearly demonstrates that specific interventions can reduce the reported rate of musculoskeletal disorders for workers who perform high-risk tasks. No known single intervention is universally effective. Successful interventions require attention to individual, organizational, and job characteristics, tailoring the corrective actions to those characteristics.
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--> Research can ( 1 ) provide a better understanding of the mechanisms that underlie the established relationships between causal factors and outcomes so that workers who are at risk can be identified and interventions undertaken before problems develop; (2) consider the influence of multiple factors (mechanical, work, social, etc.) on symptoms, injury, reporting, and disability; (3) provide more information about the relationship between incremental change in load and incremental biological response as a basis for defining the most efficient interventions; (4) improve the caliber of measurements for risk factors, outcome variables, and injury data collection and reporting systems; and (5) provide better understanding of the clinical course of these disorders. By and large, the controversies that we observed reflect the usual disputatiousness of science, which advances when speculative challenges lead to new and clarifying results. One feature of the discourse around musculoskeletal disorders is that it sometimes involves individuals from one discipline (or subdiscipline) who reject entirely the legitimacy of research from another. The steering committee understands the claims made by these often forceful advocates of particular research ideologies. However, we respect the contributions of properly designed research conducted by the variety of disciplines needed for the topic. The steering committee's task has been to examine the state of the evidence. As such, we have tried to assess the plausible ranges of effects for the various factors that have been studied systematically. We have, however, deliberately avoided providing recommendations for action for three reasons: The risk of musculoskeletal disorders depends on the interaction of person and task, as does the effectiveness of options for reducing those risks. A full specification would require much more detailed treatment of person-task combinations than is possible here. We have, instead, focused on the scientific principles that should guide the prediction and prevention of problems. We have not reviewed the full range of consequences of musculoskeletal disorders and interventions related to them. For example, we have not evaluated the effects of ergonomics programs on employee productivity, turnover, and morale. Nor have we examined the effects of musculoskeletal disorders on the economic and psychological well-being of injured individuals and their families. Rational decision making must consider the full set of relevant consequences. Rational decision making also depends on the relative importance attached to the different consequences. Different people and institutions will have different values and different opportunities for action, at the governmental, employer, and individual levels.
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--> References Aaras, A. 1994 The impact of ergonomic interventions on individual health and corporate prosperity in a telecommunications environment. Ergonomics 37(10): 1679-1696. AFL-CIO 1997 Stop the Pain. Washington, DC: AFL-CIO. Armstrong, T. 1985 Mechanical considerations of skin in work. American Journal of Industrial Medicine 8:463-472. Armstrong, T., J. Foulke, B. Martin, J. Gerson, and D. Rempel 1994 Investigation of applied forces in alphanumeric keyboard work. American Industrial Hygiene Association Journal 55(1):30-35. Ashton-Miller, J.A. 1998 Response of Muscle and Tendon to Injury and Overuse. Paper prepared for the Steering Committee for the Workshop on Work-Related Musculoskeletal Injuries. Bernard, B.P., ed. 1997 Musculoskeletal Disorders and Workplace Factors : A Critical Review of Epidemiologic Evidence for Work-Related Musculoskeletal Disorders of the Neck, Upper Extremity, and Low Back. Cincinnati, OH: U.S. Department of Health and Human Services. Bongers, P.M., C.R. de Winter, M.A.J. Kompier, and V.H. Hildebrandt 1993 Psychosocial factors at work and musculoskeletal disease. Scandinavian Journal of Work and Environmental Health 19:297-312. Bureau of Labor Statistics 1995 Workplace Injuries and Illnesses in 1994. USDL 95-508. Washington, DC: U.S. Department of Labor. Campbell, D.T., and J.C. Stanley 1966 Experimental and Quasi-Experimental Designs for Research. Chicago, IL: Rand McNally. Chaffin, D., and G. Andersson 1990 Occupational Biomechanics. Second Edition. New York: John Wiley.
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