Ergonomic assessment indicates a concern because work includes repetitive exertions, forceful exertions, awkward postures, localized contact stress, and/or exposure to vibration or low temperatures.
Workers with pain and/or discomfort that does not resolve overnight
A significant proportion of workers report pain and/or discomfort from a specific work area or department.
Conduct an ergonomic job evaluation of worker exposure to occupational risk factors of musculoskeletal disorders.
Provide appropriate medical evaluation and treatment.
Provide ergonomics training to all levels of the organization.
Develop an ergonomics process for your company. Include information on this process in the ergonomics training.
Identify short-term and long-term workplace changes that will reduce worker exposure to the identified risk factors.
Implement the workplace changes when economically and technically feasible.
Complete follow-up and evaluation.
Use worker input during the job analysis process and to develop workplace changes.
Ergonomics job evaluation should be a process, not a program.
Involve health care providers in identifying appropriate tasks for workers with restrictions. Encourage health care providers to write specific (not general) work restrictions as appropriate.
The six-sigma process of Kaizen DMAIC (Define, Measure, Analyze, Improve, and Control) can be used.
Ergonomics process perceived as a management and/or productivity program
Neglecting to include workers in the ergonomics process
Lack of management support
Just because workers are not reporting symptoms or making suggestions for workplace improvements does not mean that job improvements are not needed to reduce the risk of workers developing a musculoskeletal disorder.
Timelines for Return to Activities of Daily Living/Work
Worker accommodation may be necessary.
This may be an ongoing process.
There may be work tasks that are appropriate in the short term and others that will be appropriate later.
This chapter discusses ergonomics and upper extremity musculoskeletal disorders of the hand and wrist. Ergonomics has been defined as a body of knowledge of human abilities, human limitations, and other human characteristics that are relevant to design. Ergonomic design is the application of this body of knowledge to the design of tools, machines, systems, tasks, jobs, and environments for safe, comfortable, and effective human use. Failures to adequately deal with these design issues can be found in the many accounts and studies of carpal tunnel syndrome (CTS), tendinitis, and epicondylitis. This chapter reviews some of the history and morbidity patterns of upper extremity musculoskeletal disorders and discusses job analysis and design. In addition, job analysis case studies are described to demonstrate application of ergonomics principles.
Upper Extremity Musculoskeletal Disorders
Upper extremity musculoskeletal disorders are disorders of the soft tissues caused by repeated exertions and movements of the body. Although these disorders can occur in nearly all tissues, the most frequently reported sites are the nerves, tendons, tendon sheaths, and muscles of the upper extremity. Other commonly used terms are as follows.
Repetitive trauma disorders
Cumulative trauma disorders
Repetitive strain injuries
Occupational cervicobrachial disorders
Regional musculoskeletal disorders
There is no consensus on which of these terms is best, and arguments can be made for all of them. Some of the common characteristics ascribed to these disorders are:
They are related to the intensity of work.
They involve both biomechanical and physiologic mechanisms.
They may occur after weeks, months, or years on the job.
They may require weeks, months, or years for recovery.
Their symptoms often are poorly localized and nonspecific.
They may have both occupational and nonoccupational causes.
Because there often is a long time between beginning work and the onset of upper extremity musculoskeletal disorders, because they are not immediately life-threatening and may go unreported, and because workers change jobs and employers, it is difficult to determine the exact morbidity patterns. Epidemiologic methods are required to isolate jobs, tools, areas, plants, or industries with excessive risk.
The occurrence of discomfort and pain does not necessarily mean that a cumulative trauma disorder has developed in a worker. Discomfort and other adverse performance effects may result from localized fatigue and be a consequence of normal work. Localized fatigue has qualities similar to those of upper extremity musculoskeletal disorders, but it tends to develop and resolve much more quickly. Persistence of symptoms from day to day or interference with activities of work or daily living may indicate something more serious than fatigue. Workers experiencing such symptoms should be evaluated by a professional health care provider.
As previously stated, musculoskeletal disorders may involve multiple personal and work-related factors. The extent that work is a primary cause of musculoskeletal disorders versus an aggravating cause is a long-standing and continuous debate among clinicians, researchers, and politicians. Regardless of their position on this issue, most clinicians consider work capacity and job demands and specify work restrictions.
A hierarchical model has been developed for describing the relationship between worker capacities and job demands ( Fig. 139-1 ). The model structure is hierarchical and open-ended so appropriate data can be gathered to focus on determining the nature of the person’s physical capacities and job requirements. Additional layers of detail of medical and work information can be collected as needed for individual cases. Some uses of this model include (1) return to work of a patient who has experienced a work-related injury or illness, (2) job placement of a person who has a non–work-related condition from birth, disease, or injury, and (3) design of a new job to make the job accessible to the largest number of possible workers.
The overall incidence rate and prevalence of upper extremity musculoskeletal disorders are not known; however, reports from insurance companies, clinics, and work sites suggest that these disorders approach epidemic proportions and are a major cause of lost work in some settings. Studies by Zollinger and by Conn suggested that tendinitis was a major cause of lost work and insurance claims. In studies of medical clinic records for two separate military groups assigned to manual agricultural work in Great Britain, tendinitis was found to have developed in approximately 30% of workers within several months. Reed and Harcourt reported that 70 cases of tenosynovitis accounted for 0.54% of all visits at a U.S. industrial clinic located in the Midwest. Thompson and colleagues reported that 466 of 544 peritendinitis and tenosynovitis patients seen from 1941 to 1950 at a British hospital and outpatient service were manual workers and agricultural workers.
Upper extremity musculoskeletal disorders affect workers in a wide variety of occupations. Barnhart and colleagues used a cross-sectional study to examine the relationship between repetitive work and CTS among ski manufacturing workers. The jobs at the assembly plant were classified as repetitive or nonrepetitive, and health data were collected on 173 workers. Health data were collected using a self-administered questionnaire, a physical examination, and electrophysiologic testing. Three different case definitions were used to determine the presence of CTS. For workers in repetitive jobs, CTS was present in one or both hands in 15.4%, but among workers in nonrepetitive jobs, the rate was 3.1%.
A cross-sectional investigation was conducted to compare the prevalence of musculoskeletal disorders among supermarket cashiers and other supermarket workers. The ergonomic evaluation of the workstations showed that the supermarket cashiers had repetitive jobs with awkward postures. A logistic regression analysis found a significantly higher odds ratio (OR) for the rate of shoulder- and hand-related musculoskeletal disorders among the cashiers compared with other supermarket workers.
Workers from both strenuous and nonstrenuous manual jobs in a meat-processing company ( n = 715) participated in a cross-sectional study to determine the incidence of tenosynovitis or peritendinitis and epicondylitis. Nonstrenuous manual jobs were positions such as supervisors, executives, salespeople, and accountants. The strenuous manual jobs included job titles such as packer, sausage maker, and meat cutter. The rate of tenosynovitis or peritendinitis and epicondylitis was as high as 21.4 cases per 100 person-years for strenuous jobs (the rate ranged from 5.2 to 21.4 cases per 100 person-years) and as low as 0.7 cases per 100 person-years for the nonstrenuous jobs (the rate ranged from 0.7 to 1.1 cases per 100 person-years).
Hagberg and colleagues reviewed 21 studies describing the prevalence of CTS in various occupational groups. The prevalence of CTS in the different occupational groups ranged from 0.6% to 61%. The job titles with the highest rates included grinders, butchers, grocery store workers, frozen food factory workers, and platers. Jobs that require repetitive use of the hands and forceful gripping accounted for at least 50% and as much as 90% of all the CTS cases.
Latko and colleagues completed a cross-sectional study of the relationship between repetitive work and the prevalence of upper limb musculoskeletal disorders and discomfort. Repetition and other ergonomic exposure risk factors were rated on a 10-point scale where 0 corresponded to no stress and 10 corresponded to maximum stress. Three hundred fifty-two industrial workers from three companies participated in the study. Medical examinations and questionnaires were completed by all study participants. Repetition was found to be significantly associated with the prevalence of reported discomfort (OR = 1.17 per unit of repetition), tendinitis in the distal upper extremity (OR = 1.23 per unit of repetition), and symptoms consistent with CTS (OR = 1.16 per unit of repetition).
Roquelaure and colleagues estimated the incidence of CTS in a general population to determine the proportion of cases that could be attributed to work. This was a 3-year study and was conducted in Maine and Loire regions of France among people aged 20 to 59 years old. Study participants were recruited by treating physicians after the diagnosis of CTS was made. Data were obtained from study participants using a self-administered questionnaire that contained questions regarding medical, surgical, and employment history. A total of 1168 cases (1644 wrists) were affected by CTS and included in this study. The data from this study show a higher incidence of CTS in the working than the nonworking population. In addition, the attributable fraction of work among the exposed persons (AFE) in the development of CTS was significant for blue collar workers (women, AFE = 66.6%, and men, AFE = 76.4%) and for lower grade white-collar workers (women, AFE = 60.5%). The prevalence of upper extremity musculoskeletal disorders in the working population of France’s Pays de la Loire region was assessed during the period 2002 to 2003. Study participants were recruited from the clinics of occupational physicians who had attended a training program to standardize the physical examination. Workplace exposures were assessed using a self-administered questionnaire. More than 50% of the 2685 study participants experienced nonspecific musculoskeletal symptoms in the preceding 12 months and approximately 30% experienced symptoms in the past week. In addition, the prevalence of clinically diagnosed musculoskeletal disorders was high. Approximately 13% of participating workers experienced at least one musculoskeletal disorder, and a high percentage of the cases could be classified as probably work related (95% in men and 89% in women younger than age 50, and 87% in men and 69% in women older than age 50).
The American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit values (TLVs) for hand activity level (discussed later) have been used in epidemiological studies to assess the prevalence of symptoms or disease among workers in the three exposure categories. The Italian study was conducted among 3578 blue-collar industrial workers. Workplace exposure was determined by ergonomics professionals using the ACGIH TLVs, and medical outcomes were determined by occupational physicians (baseline and 12-month follow-up) using interviews, physical examinations, and nerve conduction studies. There was a threefold increase in the risk of CTS in industrial workers who performed jobs with exposure above the ACGIH TLVs, compared with industrial workers who performed jobs with exposure below the action limit. A cross-sectional study was conducted by American researchers to assess the validity of the ACGIH TLVs with respect to symptoms and musculoskeletal disorders among 908 industrial and clerical workers. Workplace exposure was determined by ergonomics professionals, and medical outcomes were determined by health care professionals using physical examinations, self-administered questionnaires, and nerve conduction studies. In this study, symptoms did not significantly vary based on TLV category and the prevalence of symptoms, and disorders were substantial in jobs that were below the ACGIH TLV action limit (acceptable zone). However, elbow/forearm tendinitis and CTS were associated with TLV category.
Several literature reviews to examine the epidemiologic evidence of work-related musculoskeletal disorders have been completed. Bernard and colleagues from the National Institute of Occupational Safety and Health concluded that “a substantial body of credible epidemiologic research provides strong evidence of an association between MSDs (musculoskeletal disorders) and certain work-related physical factors when there are high levels of exposure and especially in combination with exposure to more than one physical factor.” Based on the peer-reviewed literature, Punnett and Wegman reported that “there is an international near-consensus that musculoskeletal disorders are causally related to occupational stressors, such as repetitive and stereotyped motions, forceful exertions, non-neutral postures, vibration, and combinations of these exposures.” Palmer and colleagues reviewed literature related to CTS and its relation to occupation. They concluded that there was reasonable and/or substantial evidence that physical workplace factors were related to the development of CTS. The National Research Council concluded that (1) there is a strong association between high levels of physical work factors and the incidence of reported pain, injury, loss of work, and disability; (2) there is strong biological evidence to believe that there is a relationship between the incidence of musculoskeletal disorders and workplace risk factors; and (3) research clearly demonstrates that interventions can reduce the incidence of musculoskeletal disorders among workers who are exposed to high levels of workplace risk factors.
The Bureau of Labor Statistics of the U.S. Department of Labor reports the incidence rate of musculoskeletal disorders involving days away from work among full-time workers ( Fig. 139-2 ). The government record keeping and reporting rules changed in 2001. Consequently, it is difficult to compare current data with historical data. The incidence rate of musculoskeletal disorders has decreased slightly between 2002 (55.3 cases/10,000 workers) and 2006 (39 cases/10,000 workers) (see Fig. 139-2 ). However, musculoskeletal disorders are a leading cause of work absences and accounted for 30% of the injuries and illnesses with days away from work in both 2005 and 2006. The estimated annual costs associated with work-related musculoskeletal disorders vary. However, experts agree that a reasonable figure is approximately $50 billion annually.
A conceptual model for the pathogenesis of work-related musculoskeletal disorders has been developed. This model contains sets of cascading exposure, dose, capacity, and response variables, such that the response at one level can act as a dose at the next level. Because there are currently no dose–response data relating occupational risk factors and the incidence of upper extremity musculoskeletal disorders, systematic job analysis is necessary to identify the risk factors before one can develop interventions.
Disability Associated With Work-Related Musculoskeletal Disorders
Disability associated with work-related musculoskeletal disorders is a problem for employers, workers, society, and families. Musculoskeletal disorders are the leading cause of workplace absences. In addition, workplace disability duration of 160 days for workers with CTS in Washington State has been reported. The symptoms associated with work-related musculoskeletal disorders are often not completely resolved for many injured workers. Researchers reported that workers’ compensation claimants with upper extremity musculoskeletal disorders continued to have persistent symptoms that interfered with work (53%), sleep (44%), and home/recreation activities (64%) up to 4 years after injury. The cost to employers for disability associated with work-related musculoskeletal disorders is great. These authors estimate the direct cost of workers’ compensation costs associated with musculoskeletal disorders to be $20 billion. Less than half of work-related musculoskeletal disorders are reported, so the costs associated with disability are considerably greater.
Many conceptual models have been developed to show the interaction between the various factors that affect disability and return to work. Historically, three different perspectives have influenced the development of these models and include biomedical, biopsychosocial, and social construction. More recently, the Institute of Medicine and the World Health Organization developed integrative models that include many of the salient features of other models. The integrative models include factors relating to both the workplace and the person. The aspects that affect the workplace include external loads, organizational factors, and the social context. The aspects of the person (individual factors) identified in this model include biomechanical loading (internal loads and physiologic responses), internal tolerances (mechanical strain and fatigue), and outcomes (pain, discomfort, impairment, disability).
Several nonoccupational factors have been shown to be associated with upper extremity musculoskeletal disorders ( Box 139-1 ). However, few of the factors have been quantified as to the strength of the association. The nonoccupational or personal factors that have been related to upper extremity musculoskeletal disorders include medical conditions such as diabetes, arthritis, thyroid disease, vitamin B 6 deficiency, and pregnancy; body mass index; sex; wrist size and shape; age; general conditioning; and genetics. Recent studies suggest that both nonoccupational and occupational factors were related to the development of upper extremity musculoskeletal disorders. In a longitudinal study of industrial and clerical workers, the factors found to have the highest predictive value for identifying a person who is likely to develop upper extremity tendinitis included age older than 40, body mass index greater than 30, a symptom of shoulder or neck discomfort at baseline, a history of CTS, and a job with awkward shoulder postures. A 1-year prospective study was conducted among auto assembly workers to examine the risk factors associated with visiting a plant medical department because of an upper extremity musculoskeletal problem. The significant predictors for visiting the plant medical department included exceeding the threshold limit value for hand activity and peak force, a history of diabetes, age younger than 40, and a current diagnosis of CTS or elbow diagnosis.
Vitamin B 6 deficiency
Body mass index: weight and stature
Wrist size and shape
General conditioning: strength and aerobic conditioning
The commonly cited risk factors for upper extremity musculoskeletal disorders of the hand and wrist include ( Box 139-2 ):
Repeated and sustained exertions
Localized contact stress
Repeated and sustained exertions
Localized contact stress
Because these factors arise in the performance of work or the interaction of the worker and work equipment, these risk factors are referred to here as ergonomic factors. Although ergonomic factors are commonly cited, the data are insufficient to predict the effect of changing any one of them. The ability to predict is further complicated by the occurrence of more than one factor in a given work situation. For example, a job may be repetitive and forceful and involve occasional postural stresses and exposure to vibration. Although reducing any one of these stresses should reduce risk of upper extremity musculoskeletal disorders, the amount of risk and its significance are difficult to predict. For these reasons, any work changes to control upper extremity musculoskeletal disorders must be evaluated. Several iterations may be required to achieve the desired level of control. This section discusses each of the risk factors, how they are evaluated, some of the ways in which they can be controlled, and several case studies.
Before ergonomic stresses can be identified, one must document what the worker does. This entails the following:
The work objective
The work standard
The work method
The Work Objective
The work objective is the reason that the job is performed, such as to put wheels on cars, to enter data into a computer, and to remove fat from hams. Often the job title will reflect the objective.
The Work Standard
The work standard is an expression of the quantity and quality of work expected in a given period. Manufacturing work standards usually are expressed in numbers of assemblies or parts. In office settings, they may be expressed in terms of key strokes, numbers of documents, transactions, or other tasks. Standards are based on the concept of a fair day’s work and should be within the work capacity of 95% of the work force. In addition to the base standard, there may be incentives or bonuses by which workers can earn additional income for working above the standard. In some cases, these are based on individual performance and in others on group performance. Work incentives also should be documented.
The Work Method
The work method is the procedure used to accomplish the work objective and is described as a sequence of steps or elements. Generally, there are many ways in which a given job can be performed; however, the work standard is based on the assumption of a “standard method.” The work standard should include a description of the standard method on which it is based. As a practical matter, the method used by the worker may, in fact, be significantly different from the standard method. These differences should be documented.
The workplace layout describes how the work equipment is arranged in the workplace. This description may be verbal, such as “worker seated at work bench,” or the description may be graphic, such as a blueprint and/or digital pictures of the work area. It should include key dimensions that affect how far the worker must reach. In some cases, dimensions will be a range because they vary from one work situation to another or because they are adjustable. There are many graphics programs available that facilitate scale drawings and dimensioning.
Work equipment is any device used to accomplish or facilitate accomplishing the work objective. Examples include presses, jigs, hoists, hand tools, document holders, and seating. In some cases, equipment is commercially available, so it may be necessary only to look in a catalog to determine exact sizes, weights, and capacities. In other cases, the equipment may be unique to the job and require complete on-the-job documentation. In some cases, the equipment may be improvised by the worker and provide insight into ways of reducing ergonomic stresses.
The materials include objects that go into the product. In assembly operations, these might include parts, lubricants, coatings, and packing materials; in clerical work, documents and information; in meat processing, pieces of meat and bags of additives. Material variations should be noted as these may affect the effort and number of motions required to get and use materials.
Sources of Information
Sources of information include the following.
Engineering drawings, equipment manuals, and catalogs
Engineering, Personnel, and Drawings
Work standards, methods, process data, and plant layouts often can be obtained from industrial, manufacturing, product, facility, and plant engineering departments. Depending on the sophistication of these data, it may be possible to complete much of the job analysis off-site. Formal job descriptions often can be obtained from personnel departments; however, these descriptions tend to emphasize worker qualifications in terms of worker attributes such as education or strength and dexterity rather than in terms of job attributes such as reach distances, forces, and work rates. In some cases, workplace drawings may need to be updated to show new equipment and processes.
An on-site inspection always should be performed to verify information obtained in job descriptions and drawings and to collect other information needed for the analysis. The on-site visit will also afford an opportunity to interview supervisors and workers. Differences between the published method and layout and the actual method and layout are common. Workers often find ways to arrange their work and perform the motions that are faster and easier than those designed by engineers. In addition, certain pieces of work equipment may have difficulties that cause workers to abandon them in favor of manual methods. Similarly, there may be differences from worker to worker owing to differences in body size, strength, and skill or to differences in work equipment. In a recent visit to a work site in which all workers were using the same kind of equipment to do the same job, the author found that some people spent most of their time watching the machines run while others spent most of their time working on them. Further investigation revealed that machines were assigned according to seniority and that low-seniority people inevitably got the old machines, which frequently jammed. The old machines required much more work than the new ones to achieve the production standard. These differences often provide insight into how the job can be simplified to reduce ergonomic stresses on the worker. Consequently, in the example just cited, a progressive maintenance program to reduce machine jams was recommended.
Supervisor and Worker Interviews
One must take care in worker and supervisor interviews to avoid suggesting responses. For example, when a person of authority asks a worker, “Doesn’t that tool hurt your hand?,” it suggests that there is something wrong with the tool. Whenever possible, questions should be asked in ways that provide choices. For example, ask “What do you like best about the tool?” followed by “What do you like the least?” Follow-up questions can be used to provide additional information. One should complete the job documentation before proceeding with the ergonomic assessment, although additional detail can be added as specific ergonomic factors are considered.
Repeated and Sustained Exertions
Repetitiveness of work is one of the most commonly cited risk factors of upper extremity musculoskeletal disorders. As early as 1927, Obolenskaja and Goljanitzki talked about hand motions per minute and per shift as factors of tendinitis. In 1934, Hammer defined repetitiveness in terms of the manipulations per hour. Later, in 1979, Kuorinka and Koskinen defined repetitiveness in terms of the number of parts handled per year. Armstrong and colleagues and Silverstein and colleagues defined repetitiveness in terms of fundamental cycles to relate physiologic measures to work measures.
A repetitive job can be defined as simply a task in which the worker performs the same acts or motions over and over again. Examples of repetitious jobs include entering data into a computer, assembling products on an assembly line, washing windows, and loading parts into a press or onto a conveyor belt ( Box 139-3 ). Sustained exertions are found whenever some worker body part must maintain the same position throughout each work cycle or for prolonged time periods. Static upper extremity positions are often identified in data entry work in which the arms are positioned over the keyboard for a large percentage of the work day or in manufacturing assembly tasks in which the worker holds the power tool in his or her hand for most of the work day.
Job documentation information often can be used to evaluate the ergonomic risk factors. Specifically, for analyzing the repetitiveness of a job, various methods can be used. For many jobs, the number of exertions per part can be calculated from the work method. The total number of exertions per unit time then can be computed from production information such as standard times or records of parts produced. For example, as part of the assembly of a door-locking mechanism, the repetition of the job was calculated as the number of elements in the work method multiplied by the number of panels per hour ( Table 139-1 ).
|Old Method||New Method|
|Production: 300 panels/hr||Production: 300 panels/hr|
|Number of exertions = 300 panels/hr × 6 exertions/panel = 1800 exertions/hr||Number of exertions = 300 panels/hr × 4 exertions/panel = 1200 exertions/hr|
The work method may or may not indicate whether the hand is in use. Consequently, one must observe the worker performing the job. The observer then can rate the repetition of the job using the 10-point scale depicted in Figure 139-3 . The use of this rating scale makes it possible to compare jobs that may have dramatically different quantities of work elements. In addition to observer ratings, workers may rate their perception of the effort required to keep up with jobs. Visual analog scales can be used as a standardized method to assess workers’ perceptions. The analyst must not suggest how the worker should respond.
If a problem exists with work-related upper limb musculoskeletal disorders and it cannot be controlled by regulation of other factors, the repetitiveness should be reduced. This may be performed by addressing the following issues:
Work standards and incentives
Quality control and maintenance
Repetitive exertions can be reduced through changes in work organization using strategies such as worker rotation, work enlargement, motion economy, and changing work standards. Although unpopular, work standards or incentive systems may need to be changed to decrease the incidence and costs associated with upper extremity musculoskeletal disorders. In some cases, extra employees may need to be added to the production line, or overtime or bonus pay may need to be eliminated or modified. In addition, seniority rules may result in some workers getting stuck with hard jobs, so the rules may need to be modified. Motion economy can be effectively used to decrease the number of exertions each cycle by modifying work layout, changing the arrangement of tools and materials, or designing computer software commands. If workers rotate among jobs that entail exertions of different muscles or joints, worker rotation can be used to reduce repetition. In addition, work enlargement also can be used to decrease repetition by combining operations that use different motion tasks or motion patterns into a new job.
The quality of parts, materials, and maintenance affects the number of movements and exertions that are required to complete a task. For example, additional motions are needed to trim edges of poorly molded parts. In addition, poorly maintained knives may increase the number of motions needed to complete a required task. Therefore, quality parts and materials and an aggressive preventive maintenance program can decrease the number of exertions required. Mechanical aids such as power tools can be used to reduce the frequency and time required to complete a job. The new method for the small assembly task described in Table 139-1 demonstrates how the use of new work equipment decreased the number of exertions of the task by a third.
Virtually all hand activities involve the exertion of force to propel or stabilize the fingers and wrist against gravity, inertia, and external loads. These forces are produced by the contractile proteins in muscles and transmitted through myofascial sheaths, tendons, bones, and ligaments. These forces require the expenditure of energy and result in elastic and viscous deformation of tissues. Some investigators have observed that the risk of upper extremity musculoskeletal disorders increases with the force of exertion. There is not yet agreement about what constitutes excessive force, and the effect of reducing force cannot be predicted. It should be safe to assume that reducing force probably will have a greater effect if the reduction is applied throughout the work cycle than if it is applied only occasionally. Steps for reducing force should be considered if upper extremity musculoskeletal disorders have been reported for a given job.
Several methods can be used to assess the job force requirements. Force requirements can be identified through observation, or one can use elaborate instrumentation to estimate the force exerted by workers. The methods that can be used to estimate job force requirements include the following:
Rankings (worker and observer)
Ratings (worker and observer)
Identification and ranking of forceful elements
The forceful elements of a job can be identified from the work methods analysis performed in the job documentation. Exertions are required to move, lift, lower, push, pull, slide, connect, secure, use a hand tool, or hold objects against gravity or against reaction forces (see examples in Box 139-4 ). In many cases, information about the tools, materials, and tasks can be used to estimate the minimum force requirements or at least to rank order tasks and tools in terms of their force requirements. For example, holding a 5-kg tool probably will require more force than holding a 2-kg tool. Similarly, all things being the same, tightening bolts to 100 N-m will require more force than to 50 N-m. In addition, moving an object will require more effort than reaching or grasping an object.
Worker Ratings of Force Requirements
Researchers have used psychophysical methods for many years to determine the acceptable weight of loads for various lifting situations. Similarly, worker ratings of the force exerted, tool torque, or tool weight can be used to assess forceful upper extremity exertions. In one study, Armstrong and colleagues described the use of worker ratings to determine the acceptable weights for tools used in an automobile trim department. In this study, workers used a visual analog scale where 0 equals too light, 5 equals just right, and 10 equals too heavy. Tools ranged in mass from 0.5 kg to 7 kg. Nearly all the tools with a mass less than 1.5 kg were rated as “just right,” whereas nearly all tools with a mass greater than 2.25 kg were rated as “too heavy.” In a laboratory study, subjects with industrial work experience drove screws into perforated sheet metal using air-powered tools with a mass of 1, 2, and 3 kg. The tools were used to drive screws at various horizontal and vertical work locations, and these tool/work location combinations were rated using the Borg rating of perceived exertion. In this study, the ratings of perceived exertion for driving screws with a tool of mass of 1 kg across all work locations were significantly lower than the ratings for the other work conditions using a tool with a mass of 2 or 3 kg.
Calculation of Task Force Requirements
In some cases, it is possible to actually estimate the task force requirements from known physical properties. The amount of finger force required to hold an object with a pinch grip with the arm hanging at the side of the body is proportional to the force causing it to slip out of the hand and inversely proportional to the coefficient of friction.
Direct Measurement of Force
Exertion forces sometimes can be measured by placing the work on a force gauge or attaching force-sensitive materials to the work object or hand. In a study of typing force, Armstrong and colleagues used force transducers under the keyboard to estimate the finger forces exerted by subjects. The instrumentation used for measuring force may require expensive equipment and considerable expertise.
Use of Electromyography to Estimate Force Requirements
Electromyography is the measurement of electrical potentials produced by contracting muscles. Electrodes can be positioned over various muscles that are used during work tasks. The force exerted during a work task can be estimated by recording the electromyogram as the subject works. The system must be calibrated each time that it is used, for each subject, and for each hand posture. Electromyography was used to estimate hand forces while employees used a knife to bone turkey thighs.
Analysis of Force Patterns
The analysis of hand force exerted during the entire work cycle can be characterized by identifying the start and stop times of work tasks and estimating or measuring the amount of force exerted by the hands (percentage of maximal voluntary contraction [MVC]) during each task. Three examples of this are shown in Figure 139-4 . These examples correspond to the examples shown in Box 139-3 . For the worker positioning the food bars, the greatest force occurs when holding and positioning several food bars. The force exerted by the window washer varies based on the stroke (up vs. down) and the task (wash, dry with squeegee, wipe spots with towel). The greatest force occurs while washing the windows and using the squeegee horizontally.