Exposure to local vibration occurs when a specific part of the body comes in contact with a vibrating object, such as a power hand tool. Exposure to whole-body vibration can occur while standing or sitting in vibrating environments or objects, such as when operating heavy-duty vehicles or large machinery.

 

Workplace conditions that can influence the presence and magnitude of the risk factors for WMSDs can include
• cold temperatures,
• insufficient pauses and rest breaks for recovery,
• machine paced work, and
• unfamiliar or unaccustomed work.

In addition to the above conditions, other aspects of organization of work may not only contribute to physical stress but psychological stress as well. Scientific research is examining work factors such as performance monitoring, incentive pay systems, or lack of control by the worker to determine whether these factors have a negative effect on the musculoskeletal system [Moon and Sauter 1996]. Another related area of research is to determine which personal, work, or societal factors contribute to acute musculoskeletal disorders developing into chronic or disabling problems.

Screening jobs for these risk factors may involve the following:

• Walk-through observational surveys of the work facilities to detect obvious risk factors
• Interviews with workers and supervisors to obtain the above information and other data not apparent in walk-through observations, such as time and workload pressures, length of rest breaks, etc.
• Use of checklists for scoring job features against a list of risk factors

   

While screening tools such as checklists have been widely and successfully used in many ergonomics programs, most have not been scientifically validated. Combining checklist observations with symptoms data offers a means of overcoming uncertainty.

Integrating efforts to identify risk factors for musculoskeletal disorders with efforts to identify common safety hazards such as slips and trips should be considered. Jobs with risk factors for musculoskeletal disorders also may have safety hazards.

Job analysis breaks a job into its various elements or actions, describes them, measures and quantifies risk factors inherent in the elements, and identifies conditions contributing to the risk factors [Putz-Anderson 1988; Keyserling et al. 1993; Grant et al. 1995; ANSI 1996].

Job analyses are usually done by persons with considerable experience and training in these areas. While most job analyses have common approaches, such as a focus on the same set of risk factors described above, no "standard" protocol exists for conducting a job analysis to assess ergonomic hazards.

Most job analyses have several common steps. A complete description of the job is obtained. Employees are often interviewed in order to determine if the way the job is done changes over time. During the job analysis, the job is divided into a number of discrete tasks. Each task is then studied to determine the specific risk factors that occur during the task. Sometimes each risk factor is evaluated in terms of its magnitude, the number of times it occurs during the task, and how long the risk factor lasts each time it occurs.

The tasks of most jobs can be described in terms of (1) the tools, equipment, and materials used to perform the job, (2) the workstation layout and physical environment, and (3) the task demands and organizational climate in which the work is performed. Job screening, as described above, provides some of these data. More definitive procedures for collecting information on these components can include the following:

• Observing the workers performing the tasks in order to furnish time-activity analysis and job or task cycle data; videotaping the workers is typically done for this purpose.
• Still photos of work postures, workstation layouts, tools, etc., to illustrate the job
  
• Workstation measurements (e.g., work surface heights, reach distances)
  
• Measuring tool handle sizes, weighing tools and parts, and measuring tool vibration and part dimensions
  
• Determining characteristics of work surfaces such as slip resistance, hardness, and surface edges
  
• Measuring exposures to heat, cold, and whole body vibration
  
• Biomechanical calculations (e.g., muscle force required to accomplish a task or the pressure put on a spinal disc based on the weight of a load lifted, pulled, or pushed)
  
• Physiological measures (e.g., oxygen consumption, heart rate)
  
• Special questionnaires, interviews, and subjective rating procedures to determine the amount of perceived exertion and the psychological factors influencing work performance
  

Exhibits 11, 12, 13, and 14 illustrate the varied approaches that NIOSH has taken in analyzing and evaluating jobs for apparent risk factors.

While a job analysis enables a person to characterize ergonomic risk factors, the question of what level or amount of exposure is harmful to the musculoskeletal system is a difficult one. Some have argued against the overuse of simple guidelines [Buckle et al. 1992; Leamon 1994], while others have recognized that, despite the limitations of current guidelines, many contain sufficiently useful information to identify potentially risky work activities [Karwowski 1993; Waters et al. 1993; Winkel et al. 1992]. While acknowledging the limitations of current knowledge, NIOSH and others conducting job analyses have used a variety of approaches to provide answers best suited for the specific workplaces under study. One approach calculates the muscle strength required to perform a certain job task and estimates the fraction of the working population that possesses the required strength. A second approach asks workers in the laboratory to judge acceptable work conditions by engaging them in tasks that impose different physical demands. A third method compares the forces generated in a part of the body when performing specific work tasks and compares it with a level believed to be harmful. Tray 6 of the Toolbox section contains references to and information about these and other approaches.

NIOSH recommends the use of the NIOSH lifting equation as one useful approach in both the design of new lifting tasks and in the evaluation of existing lifting tasks [Waters et al. 1993; Waters et al. 1994]. Other assessment tools are also available for evaluating such tasks [Chaffin and Andersson 1991; Marras et al. 1993, 1995; Hidalgo et al. 1995]. Population data depicting human strength capacities can be helpful in designing and evaluating jobs [Snook and Ciriello 1991]. Tables indicating standing and seated height and reach distances that can accommodate various proportions of the worker population [Kroemer and Kroemer-Elbert 1994] can also be helpful. Comparing job analysis results with such references can yield estimates of the percentage of the population that may be especially affected by these job conditions. In some NIOSH evaluations, efforts have been made to duplicate the specific stresses observed in the job to calculate forces on joints and limbs and to arrive at risk determinations [Habes and Grant, in press]. Computerized 2- and 3-dimensional biomechanical models can predict the percentage of males and females capable of exerting static forces in certain postures [Chaffin and Andersson 1991]. Westgaard and Winkel [1996, p. 87] recently summarized the strengths and weaknesses of current guidelines by concluding that "at present, guidelines to prevent musculoskeletal disorders can only give directions, not absolute limits." These authors believe the best guidelines must consider the level, duration, and frequency of exposure.

Table 4 presents the reference levels or limiting conditions used by NIOSH to rate risk factors of consequence to the musculoskeletal problems under investigation. (For the scientific justification of each guideline or approach, the reader is referred to the references indicated in Table 4.) In some instances these determinations were based on more than one rating procedure. For example, judgments of problematic lifting conditions in many NIOSH investigations have been derived both from use of the NIOSH lifting equation [Waters et al. 1993; Waters et al. 1994] as well as the Michigan computerized 2- and 3-dimensional analyses [Chaffin and Andersson 1991].

The entries in Table 4 are offered as illustrative examples of reference levels or guidelines. The actual risk to each worker depends not only on the current level of exposure to risk factors, but also on their physical capability, their past medical history, concurrent nonwork exposures, and many other factors. These reference levels have varying degrees of scientific justification. Each was useful in a specific NIOSH workplace investigation aimed at reducing WMSDs.

In Exhibits 11 to 14, certain job tasks were targeted for more intensive analysis to verify the existence of risk factors for musculoskeletal disorders.

• In Exhibits 11 and 12, finding cases of musculoskeletal disorders prompted the followup analysis.
• In Exhibit 14, complaints of musculoskeletal discomfort, established through questionnaires, were the basis for sorting out possible work-related causes.
• The physical demands or risk factors of the job described in Exhibit 13, even without medical or symptom data, presented strong risk implications for potential WMSDs, thus triggering the analysis.

These three scenarios offer a basis for setting priorities for undertaking risk factor analyses and implementing control measures. Specifically, jobs associated with cases of musculoskeletal problems deserve the highest consideration in followup efforts to identify risk factors and implement control actions. Jobs in which current cases have been identified should receive immediate attention, followed by those in which past records have noted a high incidence or severity of WMSDs despite the lack of current cases. Priority for job analysis and intervention should be given to those jobs in which most people are affected or in which work method changes are going to be taking place anyway.

Jobs associated with worker complaints of fatigue and discomfort should be ranked next in deciding needs for followup job analysis and possible interventions.

Finally, where screening efforts suggest the presence of significant risk factors for musculoskeletal disorders, more detailed job analyses should be done to assess the problem potential. Ratings of high or extreme levels of risk factors, especially occurring in combination, may indicate a need for control actions. While appearing last in the priority order, taking steps to reduce apparent risk factors for musculoskeletal disorders is a preventative approach.

Table 5 summarizes the priority considerations in deciding about the need for job analyses and consequent control interventions for addressing WMSDs.

THIS PAGE WAS LAST UPDATED ON SEPTEMBER 11, 1997