The current scientific literature contains many proposed reference levels or guidelines for gauging whether certain workplace conditions and job task demands may pose a risk of WMSDs. Although these recommendations are based on various assumptions and are subject to change with additional data, they offer a basis for making judgments about certain job risk factors. Exhibits of NIOSH investigations in the main text used several of these sources in making risk factor assessments. In these situations, special equipment and procedures were used to measure different characteristics of the job conditions and the exposure factors of consequence in rating the presence or absence of significant risk factors for WMSDs. The special equipment and procedures used for these purposes will not be described here since they go beyond the level of simple data gathering presented in this document. Instead, some general principles will be mentioned that govern the ratings of the different factors. Citations to articles describe the techniques and equipment for making actual job risk factor determinations.

Applying reference levels or guidelines is often a controversial process. NIOSH has included these references or guidelines in this primer because they have been published in the scientific literature and have been used by NIOSH in some workplaces to evaluate specific work situations. However, most have not been extensively tested to determine their usefulness to identify hazardous situations accurately. Most scientists who proposed these guidelines realized that they were based on limited data, but they were developed to meet the needs of those who must evaluate workplaces on the basis of the current knowledge.

• Work Space Features
• Manual Materials Handling (Lifting)
• Manual Materials Handling (Pushing, Pulling, and Carrying)
• Vibration (Whole Body)
• Hand-Arm Vibration
• Repetition
• Physical Energy Demands
• Thermal Stressors

Work Space Features

Steps in making judgments about the adequacy of work spaces would consist of considering (1) the physical makeup of the worker population, (2) the specific body parts involved in particular tasks, and (3) whether the workstation features are fixed or adjustable. Finding workers who do similar work but differ widely in height, weight, and other body dimensions is not uncommon. The problem is whether workstation features such as bench or desk heights, access to tools, and space clearances can comfortably fit the range of body sizes. Indeed, a problem may exist if some workers are engaged in tasks in which they are constantly bending over a work surface or stretching to reach needed parts. Seated work with insufficient leg room under work tables is a problem because workers have to adopt awkward postures. Adjustable workstation features, if present, can ease these as well as other problems posed by the type of work. As an example, Tray 6–A displays work surface heights judged suitable for standing work involving precision, light assembly, and heavy duty tasks. The range of bench heights in this case Tray 6-A is intended to accommodate all but extremely tall or extremely short workers, regardless of gender. If a work surface height is not adjustable, a platform may be used to raise a short worker, or a pedestal can raise the height of the work surface for a taller worker.

Workstation layout can accommodate body size characteristics of the workforce. Some general guidelines are as follows:

• Avoid placing needed tools or other items above shoulder height.
• Position items for the shortest arm reach to avoid overstretching while reaching up or down.
• Keep frequently used tools or items close to and in front of the body.
• Position items for taller workers so that workers do not have to bend while reaching down.
• Ensure that items to be lifted are kept between hand and shoulder height.

Tray 6–A also describes an optimum layout for seated work. Boundaries take into account the range of functional reaches for most of the working population. For tabletop work, the space is divided into primary and secondary task areas. The primary area represents the space recommended for doing usual work activities; the secondary task area is for doing occasional work activities.

Data on body dimensions and reach distances when standing and sitting for men and women are cited in the literature for different percentages of the U.S. population as well as for populations in other countries and regions of the world. The following text includes these data and discusses their importance in the design of work spaces to fit the user population:

Eastman Kodak Company [1983]. Ergonomics design for people at work, Vol. 1. New York, NY: Van Nostrand Reinhold Company.

Other references suggesting recommended workplace layouts are as follows:

Kroemer K, Kroemer H, Kroemer-Elbert K [1994]. Ergonomics—how to design for ease and efficiency. Englewoods Cliffs, NJ: Prentice-Hall.

Grandjean E [1982]. Fitting the task to man. London, England: Taylor and Francis.

Woodson WE, Tillman B, Tillman P [1992]. Human factors design handbook. 2nd ed. New York, NY: McGraw Hill, Inc.

UAW-GM Center for Health and Safety [1990]. Ergonomics handbook, 29815 John R. Road, Madison Heights, MI.

Sanders MS, McCormick EJ [1987]. Human factors in engineering and design. 6th ed. New York, NY: McGraw Hill.

Manual Materials Handling—Lifting

In 1981, NIOSH developed an equation [NIOSH 1981] to rate lifting tasks in terms of whether the loads were excessive. A revised version of the equation was published in 1993 [Waters et al. 1993]. The latter formula takes into account six different factors in defining a recommended weight limit (RWL) for lifting and lowering of loads. The formula is designed to assess only certain lifting and lowering tasks (e.g., standing, two-handed, smooth lifting of stable objects in unrestricted spaces). The six factors, each of which requires actual measurements or numerical ratings on a scale, are as follows:

• Horizontal location of the load relative to the body
• Vertical location of the load relative to the floor
• Vertical distance the load is moved
• Frequency and duration of the lifting activity
• Asymmetry (lifts requiring twisting or rotation of the trunk or body)
• Quality of the worker's grip on the load

The RWL probably represents a load that nearly all (i.e., 90% of the adult population) can lift for up to 8 hours without substantially increasing the risk of musculoskeletal disorders to the lower back. Comparing the actual load weight for a task with the computed RWL estimates the risk presented by the task. For loads that exceed the RWL for a task, the factors contributing most to the excess risk can be identified. This information will suggest where control measures should have their greatest benefits.

Materials describing the NIOSH lifting formula, including its scientific justification, its limitations, and its user guidance (with sample applications and computations), are available in the following document:

Waters TR, Putz-Anderson V, Garg A [1994]. Applications manual for the revised NIOSH lifting equation. DHHS (NIOSH) Publication No. 94–110, National Institute for Occupational Safety and Health, 4676 Columbia Parkway, Cincinnati, OH. (The manual is available from NTIS. For ordering information, call the NTIS Sales Desk at 703–487–4560. The NTIS order number for this document is PB94–176930LJM.)

Other models for rating lifting tasks in terms of risk for low back disorders have been developed. The University of Michigan two-dimensional and more current three-dimensional approaches estimate the amount of compressive forces on spinal discs in the low back as well as the muscle strength needed for a person to perform the lifting task in question. Load weight, lift height, hand location, and hip and joint angles for the observed lifting act are measured and serve as input to these calculations. Risk estimates are based on the percentages of the U.S. male workforce who would have the strength capacity to withstand the compressive forces that may be generated. Disc compression forces of 770 lb and greater have been identified with increasing rates of reported low back pain and thus would pose a significant hazard. The following user friendly computer software can be used to make these calculations and estimate these risks:

3D Static Strength Prediction Program, Version 3.0 [1995]. University of Michigan Software: Wolverine Tower, 3003 South State Street, Ann Arbor, MI 48109.

Other details of the three-dimensional model are found in the following:

Chaffin DB, Andersson GBJ [1991]. Occupational Biomechanics. 2nd ed. New York, NY: John Wiley & Sons Inc.

Another model offered by Marras et al., [1993; 1995] differs from both the NIOSH and Michigan formulations in requiring measurements of trunk motion in estimating lifting risks for low back disorders. A special lumbar motion monitor, worn as a back pack, is used for this purpose. For the same lifting rates, load weight and postural factors, higher peak, and average velocity measurements for trunk bending in certain directions and twisting movements will amplify the risk of low back problems. Further details about this model appear in the following two references:

Marras WS, Lavender SE, Leurgens SE, Rajulku SL, Allread WG, Fathallah FA et al. [1993]. The role of dynamic three-dimensional trunk motion in occupationally-related work-related low back disorders: the effect of workplace factors, trunk motion characteristics on risk of injury. Spine 18(5):617–628.

Marras WS, Lavender SE, Leurgens SE, Fathallah FA, Ferguson SA, Allread WG, et al. [1995]. Biomedical risk factors for occupationally-related low back disorders. Ergonomics 28(2):377–410.

Manual Materials Handling—Pushing, Pulling, and Carrying

Men and women performing pushing, pulling, and carrying tasks under laboratory conditions have been asked to judge the maximum loads or force levels that they believe are acceptable. Varying the frequency rate as well as the push, pull, or carry distances affects these judgments. The resulting data offer a reference for (1) evaluating whether these kinds of materials handling jobs are potentially problematic, and (2) setting future design or redesign requirements for similar tasks. The procedure for making this assessment includes a number of steps. The first is to identify the particular activity in question (i.e., pushing, pulling, or carrying). For pushing and pulling tasks, the initial and sustained forces involved in handling the load are then measured, usually by a strain gauge or "fish scale." For carrying tasks, the weight of the object being carried is measured, the frequency of the activity per min is determined, and measurements are taken of the vertical distance of the hands from the floor when the object is carried. These measurements are compared with tabled values corresponding to the task and considered acceptable for 75% and 90% of both male and female populations. For most protection, NIOSH recommends using the 90% table values. Finding the measured values to exceed these table values may suggest needs for controls to reduce task risk factors. Details of this procedure and the tables for rating the conditions are contained in the following document:

Snook SH, Ciriello VM [1991]. The design of manual handling tasks: revised tables of maximum acceptable weights and forces. Ergonomics 34:1197–1213.

Vibration—Whole-Body

Work conditions that involve sitting, standing, or lying on a vibrating surface produce whole- body vibration. Excessive levels and durations of exposure to whole-body vibrations may contribute to back pain and performance problems. The International Standards Organization (ISO) and American Conference of Governmental Industrial Hygienists (ACGIH) have proposed duration limits for vibration levels to reduce these problems. These limits take into account the fact that whole-body vibrations may be transmitted along three different axes corresponding to back-to-chest, right-to-left, and foot-to-head movements and that the body is more tolerant of certain vibration frequencies than others. Procedures for measuring and analyzing vibration are complex. They require use of special equipment, such as lightweight accelerometers. Accelerometers are positioned to take concurrent readings along the three axes. These readings are taken by frequency bands with the results compared with the vibration limits proposed for various exposure times. Added details about the measurement procedure appear in the following references:

ISO (International Organization for Standardization) [1985]. Evaluation of human exposure to whole-body vibration. Geneva, Switzerland: ISO Report No. ISO–2631.

ACGIH [1996]. Threshold limit values for chemical and physical agents and biological exposure indices: whole-body vibration. Cincinnati, OH: American Conference of Governmental Industrial Hygienists, pp. 123–131.

Hand-Arm Vibration

Vibrating handtools or work pieces transmit vibrations to the holder and, depending on the vibration level and duration factors, may contribute to Raynaud's syndrome or vibration-induced white finger disorders. These disorders show a progression of symptoms beginning with occasional or intermittent numbness or blanching of the tips of a few fingers to more persistent attacks, affecting greater parts of most fingers and reducing tactile discrimination and manual dexterity. Measurements of hand-arm vibration, like whole-body vibration, are made along three axes. Accelerometers are used for these readings with the data collected and analyzed to take into account any changes in vibration hazard and frequency. Other details regarding the measurement procedures appear in the following references:

ANSI [1986]. American national standard—guide for measurement and evaluation of vibrations transmitted to the hand. New York, NY: American National Standards Institute, Inc., ANSI S3.34.

ACGIH [1996]. Threshold limit values for chemical and physical agents and biological exposure indices: hand-arm vibration. Cincinnati, OH: American Conference of Governmental Industrial Hygienists, pp. 84–87.

These references propose limiting the values for exposure to the hand for the dominant frequency of vibration in any of the three directions. Measured vibration levels found to exceed the limits shown would dictate the need for actions to reduce the intensity or duration of the exposure.

NIOSH developed a recommended standard for hand-arm vibration that is not based on exposure limits, but focuses on engineering controls, work practices, and protective clothing to minimize vibration exposures. A cornerstone of this approach is medical monitoring for early identification of any signs of hand-arm vibration disorders among exposed workers. For details, see the following document:

NIOSH [1989]. NIOSH criteria for a recommended standard: occupational exposure to hand-arm vibration. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 89–106.

Repetition

A series of motions performed every few seconds with little variation may produce fatigue and muscle-tendon strain. If adequate recovery time is not allowed for these effects to diminish, or if the motions also involve awkward postures or forceful exertions, the risk of actual tissue damage and other musculoskeletal problems will probably increase. A task cycle time of less than 30 sec has been considered as "repetitive." Evidence that shows a link between highly repetitious actions and the development of WMSDs appears in the following reference:

Bernard BP, 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, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health.

Estimates vary as to repetition rates that may pose a hazard, because other factors, such as force and posture, also affect these determinations. One proposal for defining high risk repetition rates for different body parts is shown in the chart in Tray 6–B.

Tray 6–B. High Risk Repetition Rates by Different Body Parts

The reader is cautioned not to judge the risk of WMSDs solely on the basis of repetition. As already noted, much depends on force and the postural factors that reflect the effort intensity of each action. Admittedly, this is more difficult to measure than repetition rate. In making risk determinations, NIOSH typically supplements repetition measurements with ratings of the forces being exerted (using force gauges and subjective ratings of effort levels) and postural deviations of the body parts that may be involved (derived from time-motion analyses and other techniques). High repetitiveness when combined with high external forces and extreme postures probably represents the highest risk of WMSDs.

Physical Energy Demands

Muscular exertions to meet the physical demands of work need ample blood flow to carry oxygen to the tissues and carry away certain by-products from metabolic processes. Fatigue is experienced when the cardiovascular system cannot furnish sufficient oxygen to the muscles involved in coping with the imposed workload. Oxygen consumption measurements offer a direct means for determining the energy demands of a job. Heart rate is a less direct measurement, but heart rate reacts faster to an imposed work load. Portable direct reading instruments are available for capturing both kinds of data. Job energy demands may be determined by monitoring the oxygen consumption or heart rate of a few representative workers while they perform their usual tasks. Tables published in different sources use these measures to estimate the "heaviness" of work. The table values offer a basis for gauging whether job energy demands may be excessive and require rest or break periods to reduce fatigue, which is believed to increase a worker's risk of musculoskeletal injury.

Tables and procedures for collecting oxygen consumption and heart rate data appear in the following references:

Eastman Kodak Company [1986]. Ergonomic design for people at work. Vol. 2. New York, NY: Van Nostrand Reinhold.

Astrand P, Rodahl K [1986]. Textbook of work physiology: physiological basis of exercise. New York, NY: McGraw Hill Book Co.

NIOSH [1986]. Criteria for a recommended standard: occupational exposure to hot environments. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 86–113.

Another way for assessing the degree of physical effort is to have workers rate the perceived exertion in performing a work task. One scale especially designed for this purpose includes values with verbal reference points which range from"very, very light" to "very, very heavy" as aids to making these judgments. Ratings on such a scale have been found to correlate highly with physiological measures such as heart rate and offer an alternative to evaluating physical effort which is both convenient and inexpensive. Information about this type of scale and similar ones proposed for measuring the intensity of physical work appears in the following document:

Krawczyk S [1996]. Psychophysical methodology and the evaluation of manual materials handling and upper extremity intensive work. Chapter 6. In: Bhattacharya A, McGlothlin JD, eds. Occupational ergonomics. New York, NY: Marcel Dekker, Inc.

Thermal Stressors

Cold and hot working conditions can create added problems in assessing risk factors for WMSDs. Keeping hands warm may require gloves which, in turn, may cause workers to grip handtools more forcefully, resulting in added stress to the hands and wrists. More forceful gripping may also occur under hot conditions because sweating may increase the slipperiness of handtools. Workstation clearances should take into account workers wearing extra clothing for thermal protection in the cold. At the other extreme, hot work conditions may reduce a worker's capacity to do heavy physical work. In this situation, cardiac output needed to keep the body's temperature from rising too high limits the amount of blood that can deliver oxygen to the muscles. Fatigue buildup would be more readily experienced in these situations. NIOSH has published recommended exposure limits (RELs) for work under hot environmental conditions. These limits are provided for heat-acclimatized and nonacclimatized workers when performing tasks requiring different levels of energy expenditure. For details, see the following document:

NIOSH [1986]. NIOSH criteria for a recommended standard: occupational exposure to hot environments. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control, Public Health Service, National Institute for Occupational Safety and Health, DHHS [NIOSH] Publication No. 86–113.