Ergonomics Concepts
Introduction
er·go·nom·ics \,ûrg-go-‘näm-iks\The science of work. Ergonomics removes barriers to quality, productivity and safe human performance by fitting products, tasks, and environments to people.
The term ergonomics is derived from the Greek word ergos meaning “work” and nomos meaning “natural laws of” or “study of.” The profession has two major branches with considerable overlap. One discipline, the focus of this article, and sometimes referred to as “industrial ergonomics, or “physical ergonomics,” concentrates on the physical aspects of work and human capabilities such as force, posture, and repetition. A second branch, sometimes referred to as “human factors,” is oriented to the psychological aspects of work such as mental loading and decision making. The ergonomics field draws practitioners and researchers from various specialties, including engineers, safety professionals, industrial hygienists, occupational and physical therapists, and other health care specialists. The purpose/goals of workplace ergonomics applications include:- Occupational injury and illness reduction
- Workers’ compensation cost containment
- Productivity improvement
- Work quality improvement
- Turnover, absentee and presentee reduction
- Government regulation compliance
- Evaluation and control of work site risk factors
- Worker and stakeholder interviews identifying issues and potential improvements (qualitative information)
- Identification and quantification of existing work site risk conditions (quantitative information)
- Recommendation of engineering and administrative controls to reduce the identified risks
- Education of management and workers to risk conditions and improvement opportunities
Workplace Description
A typical work setting is characterized by an interaction between the following parameters:- A worker with attributes of size, strength, range of motion, intellect, education, expectations, and other physical/mental capacities.
- A work setting comprised of parts, tools, furniture, control/display panels and other physical objects.
- A work environment created by climate, lighting, noise, vibration, and other atmospheric qualities.
- A work process comprised of step-by-step interactions between the worker, work setting and environment.
Work Risk Factors
Certain workplace characteristics are associated with injury and impaired performance. These work characteristics are called risk factors and include: Task Physical Characteristics (primarily interaction between the worker and the work setting)- Posture
- Force
- Velocity/acceleration
- Repetition
- Duration
- Recovery time
- Heavy dynamic exertion
- Segmental vibration
- Heat stress
- Cold stress
- Whole body vibration
- Lighting
- Noise
Posture
Posture is the position of the body while performing work activities. Awkward posture is associated with an increased risk for injury. It is generally considered that the more a joint deviates from the neutral (natural) position, the greater the risk of injury. Posture issues can be created by work methods (bending and twisting to pick up a box; bending the wrist to assemble a part) or workplace dimensions (extended reach to obtain a part from a bin at a high location; kneeling in the storage bay of an airplane because of confined space while handling luggage). Specific postures have been associated with injury. For example: Wrist- Flexion/extension (bending up and down)
- Ulnar/radial deviation (side bending)
- Abduction/flexion (upper arm positioned out to the side or above shoulder level)
- Hands at or above shoulder height
- Flexion/extension or bending the neck forward and to the back
- Side bending as when holding a telephone receiver on the shoulder
- Bending at the waist, twisting
Force
Task forces can be viewed as the effect of an exertion on internal body tissues (e.g., compression on a spinal disc from lifting, tension within a muscle/tendon unit from a pinch grasp), or the physical characteristics associated with an object(s) external to the body (e.g., weight of a box, pressure required to activate a tool, pressure necessary to snap two pieces together). Generally, the greater the force, the greater the degree of risk. High force has been associated with risk of injury at the shoulder/neck (Berg et al., 1988), the low back (Herrin et al., 1986), and the forearm/wrist/hand (Silverstein et al., 1987). It is important to note that the relationship between force and degree of injury risk is modified by other work risk factors such as posture, acceleration/velocity, repetition, and duration. Two examples of the interrelationship of force, posture, acceleration/velocity, repetition and duration are:- A 20-pound weight lifted in a smooth, slow manner one time directly in front of the body from a 28 inch shelf to a 32 inch shelf will be much less of a risk than a 20-pound weight lifted quickly 60 times in 10 minutes from the floor to a 60 inch shelf.
- A 45-degree neck flexion position held for one minute will be much less of a risk that a 45-degree neck flexion position held for 30 minutes.
Static Exertion
Although defined in a variety of ways, static exertion generally means the performance of a task from one postural position for an extended duration. The condition is a combination of force, posture, and duration. The degree of risk is in proportion to the combination of the magnitude of the external resistance, awkwardness of the posture, and duration.Grip
A grip is the conformity of the hand to an object accompanied by the application of exertion usually to manipulate the object. Hence, it is the combination of a force with a posture. Grips are applied to tools, parts, and other physical objects in the work setting during task performance. To generate a specific force, a pinch grip requires a much greater muscle exertion than a power grip (object in the palm of the hand). Hence, a pinch grip has a greater likelihood of creating injury. The relationship between the size of the hand and the size of the object also influences risk of injury. Grant et al. (1992) found reduced physical exertion when the handle was one cm less than the subjects’ grip diameter.Contact Stress
Two types of contact trauma are:- Local mechanical stress generated from sustained contact between the body and an external object such as the forearm against the edge of a counter.
- Local mechanical stress generated from shock impact such as using the hand to strike an object.
Bulky Clothes
Bulky clothes, used to protect the worker from cold or other physical elements, may increase the muscle effort required to perform tasks.Velocity/Acceleration
Angular velocity/angular acceleration is the speed of body part motion and the rate of change of speed of body part motion, respectively. Marras and Schoenmarklin (1991, 1993) found a mean wrist flexion/extension acceleration of 490 deg/sec sec in low risk jobs and acceleration of 820 deg/sec sec in high risk jobs. Marras et al. (1995) associated trunk lateral velocity and trunk twisting velocity with medium and high-risk occupationally-related low back disorder.Repetition
Repetition is the time quantification of a similar exertion performed during a task. A warehouse worker may lift and place on the floor three boxes per minute; an assembly worker may produce 20 units per hour. Repetitive motion has been associated with injury (Hagberg, 1981; Armstrong et al., 1982) and worker discomfort (Ulin, 1990). Generally, the greater the number of repetitions, the greater the degree of risk. However, the relationship between repetition and degree of injury risk is modified by other risk factors such as force, posture, duration, and recovery time. No specific repetition threshold value (cycles/unit of time, movements/unit of time) is associated with injury.Duration
Duration is the time quantification of exposure to a risk factor. Duration can be viewed as the minutes or hours per day the worker is exposed to a risk. Duration also can be viewed as the years of exposure to a risk factor or a job characterized by a risk factor. In general, the greater the duration of exposure to a risk factor, the greater the degree of risk. Specific duration limits/guidelines have been established for risk factors that can be isolated. These include:- Whole Body Vibration – ISO 2631, British Standards Institution No. DD 32
- Segmental Vibration – ISO/Dis 5349.2, ACGIH Threshold Limit Values for Chemical Substances and Physical Agents and Biological
- Exposure Indices
- Noise – ISO 2204, OSHA Standard 29 CFR 1910.95
- Heavy Physical Exertion/Whole body fatigue – Chaffin (1966)
Recovery time
Recovery time is time quantification of rest, performance of low stress activity, or performance of an activity that allows a strained body area to rest. Short work pauses have reduced perceived discomfort (Hagberg and Sundelin, 1986) and rest periods between exertions have reduced performance decrement (Caldwell, 1970). The recovery time needed to reduce the risk of injury increases as the duration of risk factor increases. Specific minimum recovery times for risk factors have not been established.Heavy dynamic exertion
The cardiovascular system provides oxygen and metabolites to muscle tissue. Some tasks require long-term/repetitive muscle contraction such as walking great distances, heavy carrying, and repeat lifting. As physical activity increases, muscles demand more oxygen and metabolites. The body responds by increasing the breathing rate and heart rate. When muscle demand for metabolites can not be met (metabolic energy expenditure rate exceeds the body’s energy producing and lactic acid removal rate) physical fatigue occurs. When this happens in a specific area of the body (shoulder muscle from repeat or long term shoulder abduction), it is termed localized fatigue and is characterized by tired/sore muscles. When this happens to the body in general (from long-term heavy carrying/lifting/climbing stairs), it is termed whole body fatigue and may produce a cardiovascular accident. Also, high heat from the environment can cause an increase in heart rate through body cooling mechanisms. Therefore, for a given task, metabolic stress can be influenced by environmental heat.Segmental vibration (Hand-Arm vibration)
Vibration applied to the hand can cause a vascular insufficiency of the hands/fingers (Raynaud’s disease or vibration white finger). Also, it can interfere with sensory receptor feedback leading to increased hand grip force to hold the tool. Further, a strong association has been reported between carpal tunnel syndrome and segmental vibration (Silverstein et al., 1987; Wieslander et al., 1989).Heat stress
Heat stress is the total heat load the body must accommodate. It is generated externally from environment temperature and internally from human metabolism. Excessive heat can cause heat stroke, a condition that can be life threatening or result in irreversible damage. Less serious conditions associated with excessive heat include heat exhaustion, heat cramps, and heat-related disorders (e.g., dehydration, electrolyte imbalance, loss of physical/mental work capacity).Cold stress
Cold stress is the exposure of the body to cold such that there is a lowering of the body’s deep core temperature. Systemic symptoms that a worker can present when exposed to cold include shivering, clouded consciousness, extremity pain, dilated pupils, and ventricular fibrillation. Cold can also reduce hand grip strength and coordination. As mentioned earlier in the section on Force, bulky clothes and gloves used to protect the worker from cold exposure can increase the muscle effort required to perform tasks.Whole Body Vibration
Exposure of the whole body to vibration (usually through the feet/buttocks when riding in a vehicle) has some support as a risk for injury. Boshuizen et al. (1990) found the prevalence of reported back pain to be approximately 10 percent higher in tractor drivers than in workers not exposed to vibration, and the prevalence of back pain increased with vibration dose. Dupuis (1987) reported that operators of earth-moving machines with at least 10 years of exposure to whole body vibration showed lumbar spine morphological changes earlier and more frequently than non-exposed people.Lighting
With industrialization, the trend regarding lighting has been to provide a higher lighting level. This has proven hazardous within certain work settings such as in offices in which problems with glare and eye symptoms have been associated with levels above 1000 lux (Grandjean, 1988). Barreiros and Carnide (1991) found differences in visual functions over the course of a workday among VDT operators and money changers who worked in badly lighted environments. The current recommended trend in office lighting is to have low background lighting (from 300 to 700 lux) coupled with nonglare task lighting which can be controlled with a rheostat. This is consistent with Yearout and Konz’s (1989) findings of operator preference regarding lighting. Work that requires high visual acuity and contrast sensitivity needs high levels of illumination. Fine and delicate work should be illuminated at 1,000 to 10,000 lux (Grandjean, 1988).Noise
Noise is unwanted sound. In the industrial setting, it may be continuous or intermittent and present in various ways (bang of a rifle, clatter of a pneumatic wrench, whirl of an electric motor). Exposure to noise can lead to temporary and permanent deafness, tinnitus, paracusis, or speech misperception. The louder the noise and the greater its duration, the greater the risk to hearing. Also, noise well below thresholds that cause hearing loss may interfere with the ability of some people to concentrate.Other Workplace Risks
The risk factors addressed by industrial ergonomics are a partial list of hazards present in the work setting. Others include:- Job stress
- Job invariability
- Cognitive demands
- Work organization
- Workload
- Working hours (shift work, overtime)
- Displays and control panels
- Slip and falls
- Fire
- Electrical exposures
- Chemical exposures
- Biological exposures
- Ionizing radiation
- Radio frequency/microwave radiation
Assessing the Workplace for Ergonomic Risk Conditions
Evaluation of ergonomic risk conditions generally involves two steps:- Identification of the existence of ergonomic risks
- Quantification of the degree of ergonomic risk
Identification of ergonomic risk conditions
Several approaches are used to identify the existence of ergonomic risks. The method used depends on the managerial philosophy of the company (getting workers involved through a participatory process versus top/down process), level of analysis (one job versus company wide evaluation), and personal preference. There is no one correct approach.Quantifying Ergonomic Risk
Once the presence of risk factors is established, the degree of risk associated with those factors is evaluated. This is done through the application of analytical ergonomic tools and the utilization of specific guidelines. Ergonomic Assessment and Analysis Tools There are a variety of analytical tools, and they are usually orientated to a specific type of task or job (e.g., manual material handling) or a particular body part (e.g., wrist, low back). Analytical tools also vary greatly in their style of conclusions. They may provide job prioritization for intervention, quantification of activities associated with increased risk of injury, or recommendation for a load weight limit for lifting. The examiner determines which analytical tool is best for evaluation of the identified risks based on an understanding of the tool’s applications, strengths, and weaknesses. An analytical tool can provide an estimate of injury risk. Variation in individual physiology, history of injury, work methods, and numerous other factors influence whether a person will sustain an injury. You can download a comprehensive list of assessment and analysis tools — here’s a snapshot:
Guidelines for Evaluation of Environmental Risk Conditions
Strong associations have been developed between environmental risk conditions and worker injury. Guidelines, instead of analytical tools, have been developed by professional societies and used to determine the degree of risk. The guideline for each environment risk presents methods for measuring and evaluating the environmental condition. Control suggestions are also frequently made. Guidelines, categorized by environmental risk condition, include:- Heat Stress The American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices.
- Cold Stress ACGIH Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices.
- Segmental Vibration ISO 5349 (1986), Guide for the Measurement and the Assessment of Human Exposure to Hand Transmitted Vibration. ANSI S3.34 (1986), Guide for the Measurement and Evaluation of Human Exposure to Vibration Transmitted to the Hand. ACGIH’s Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices.
- Whole Body Vibration ISO 2631 (1974), Guide for the evaluation of human exposure to whole body vibration.
- Lighting Grandjean (1988). This is not a guideline but the text offers lighting suggestions for various work settings.
- Noise OSHA Standard 29 CFR 1910.95
Other Ergonomic Analytical Tools
Many more ergonomics tools are listed in the literature. Hagber et al. (1995) and Louhevaara (1995) provide information on several of them.Prevention and Control of Ergonomic Risk Conditions
Three types of solutions reduce the magnitude of risk factors:- Engineering controls
- Administrative controls
- Work practice controls
Engineering controls
Engineering controls involve altering the physical items in the workplace, including actions such as modifying the workstation, obtaining different equipment, or changing tools. The focus of engineering controls involves identifying the underlying stressor (risk factor of awkward posture, force, repetition, etc.) and eliminating it through changing the physical environment. For example, a video display terminal worker who sustains a shoulder/neck complaint from long-term typing may need forearm supports or a keyboard tray to reduce the long-term, static exertion of neck/shoulder muscles. Engineering controls are the preferred method of risk control because they permanently reduce or eliminate the risk.Administrative controls
Administrative controls involve altering work organization. These approaches usually are less expensive than engineering controls but are less dependable. Examples of administrative controls include:- Rotating workers
- Increasing the frequency/duration of breaks
- Assigning a second worker to assist in performing select tasks
- Ensuring proper work techniques are followed
- Conditioning workers for the physical exertion of task demands
- Enlarging job responsibilities such that the same task is not repeatedly performed
- Enacting a preventive maintenance program for mechanical and power tools and equipment
- Developing a housekeeping program
- Limiting overtime work