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Nicole Matoushek PT MPH CSHE CEES -> Ergonomics: Traditional risk factors (June 7, 2008 4:47:45 PM)
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What is a Risk Factor?
Specific causes of Cumulative Trauma Disorders (CTDs) in the workplace are often difficult to identify because many risk factors may interact simultaneously to bring about or aggravate the condition. It also may be difficult to isolate occupational factors from non-occupational factors or leisure activities and individual susceptibility (possible hereditary traits). However, let’s start by defining what a risk factor is. A risk factor is any attribute, experience or exposure that increases the probability of the occurrence of a disease or disorder, although it is not necessarily a causal factor. Risk factors can cause, aggravate or precipitate CTDs. However, it is important to note that the mere presence of a risk factor does not necessarily mean that the employee performing the job is at an excessive risk of injury.
Dose-Response Relationship
The dose-response relationship refers to the specific outcome following an exposure to a specific agent or factor. The outcome resulting from the exposure to ergonomic risk factors refers to any adverse health effect, and may range from acute injury to long-term disease and loss of function. In general, a higher frequency and longer duration of exposure to single agent or risk factor, is found to be positively correlated with the development of a particular outcome. This relationship is true also with ergonomic factors. Hence, prolonged or repeated exposure to an ergonomic risk factor increases the likelihood of developing a musculoskeletal injury or CTD.
An exact dose-response relationship of ergonomic factors has not yet been established. However, the more risk factors that are present, the higher the level of risk is for developing consequent injury. In fact, according to the National Safety Council, it is possible that the interaction between various risk factors has a multiplicative effect instead of an additive. The multiplicative effect, in effect significantly increases the likelihood of disease development with multiple risk factor interaction. Having more than one risk factor present significantly increases the probability of incurring micro-trauma. On the other hand, if the identified risk level is low or there appears a sufficient provision of rest or recovery time, the actual risk may be minimal. Most importantly, reducing the amount of risk factors present will reduce the occurrence of CTDs.
Traditional Ergonomic Risk Factors
The recent literature identifies the following Occupational and Non-occupational Factors as potential risk factors leading to work related injuries or CTDs. The most important contributing factor to the development of CTDs is the production of local tissue fatigue and micro trauma, as the working tissues are overloaded. Sufficient blood supply is the most important factor in controlling tissue overload and fatigue. By maintaining an adequate supply of oxygen-enriched blood to working tissues, metabolic efficiency can be maintained, thus minimizing the adverse effects of fatigue and preventing excessive micro-trauma. The key to maintaining adequate blood flow to active tissues is in the balance of the relationship between work and human physiology. These risk factors increase the likelihood of developing fatigue and micro trauma because they may impair blood flow to the working tissues.
Occupational Risk Factors
These occupational risk factors increase the likelihood of developing fatigue and micro trauma because they may impair blood flow to the working tissues. Traditional occupational risk factors include: high task repetition, forceful exertions, static work, posture and mechanical pressure. The following section describes each traditional ergonomics risk factor and provides various methods for controlling the level of risk. As the level of risk for each ergonomic exposure is reduced, the likelihood of injury development is also reduced.
High Task Repetition
Most work tasks involve repetition of some degree. A task cycle is the length of time that is takes to complete one job cycle. High task repetition is an ergonomic risk factor for the development of CTDs. A job task is classified as highly repetitive if the cycle time is equal to, or less than 30 seconds, or if more than 50% of the time involves performing the same type of fundamental cycle. With high task repetition, the more rapid and frequent the muscle contractions need to be. The consequence of rapid and repeated muscle fiber activity and recruitment is less metabolic efficiency, as anaerobic metabolism becomes the primary energy source. Secondly, at lower metabolic efficiency levels, a higher degree of muscle effort is required, thus requiring greater time for physiologic recovery. If a work task is highly repetitious and recovery time is insufficient, soft tissue injuries may occur.
Methods for Controlling High Task Repetition
Most work rates or work cycles are determined by daily production quotas or machine pace. Hence, the worker does not always have control of the pace or work-rest cycles. High repetition of any tasks can actually magnify the effects of all the other risk factors; therefore control methods are very important. Methods to control for fatigue with highly repetitive tasks include:
· Implementing a job rotation program. Job rotation can be a way of reducing the overall exposure to a work stressor. Job rotation moves workers from one workstation to another to avoid prolonged periods performing one particular task.
· Implementing mechanical aids. The use of machinery or mechanical aids will reduce muscle contraction force and duration.
· Adjusting machine pace or production quotas. At times the best control method is to lower the production quotas or the pace of the machinery.
· Implementing rest or stretch breaks. Rests or stretch breaks will provide an opportunity for tissue recovery, and improve circulation to prevent tissue fatigue and micro-trauma.
Forceful Exertions
Force exertions are the manual efforts required to accomplish a specific work task, movement or action. Research has shown that with a higher magnitude of force, i.e., the higher the percentage of Maximal Voluntary Contraction (%MVC), the higher the associated risk of fatigue and the potential to develop a musculoskeletal disorder. The neuromusculoskeletal system recovers quickly when the magnitude or intensity of the force is low. With higher magnitudes of physical stress or force, longer recovery times are required. In some circumstances, when the physical workload is high, the required recovery time may actually exceed the actual work time.
As muscle effort increases in response to higher intensities of workloads or forceful exertions, the myofibril recruitment increases within the skeletal muscles. Consequently, the circulation to the active or working muscles diminishes, causing more rapid muscle fatigue. Forceful exertion can also magnify the effects from other risk factors, therefore controlling for forceful exertions is essential. In order to recovery completely from high intensity tasks, a longer rest interval is required.
Methods for Controlling Forceful Exertions
Methods to control tissue fatigue and micro-trauma due to forceful exertions and manual material handling include:
Implementing the use of mechanical aids or power tools. This will reduce both the duration and the effort of muscle contractions.
Utilize mechanical lifting devices or counter balance systems. Using lifting devices or aids will minimize lifting.
Utilize transport carts and dollies. Using carts and dollies will minimize lifting and carrying tasks, therefore reducing muscle effort.
Slide objects. Sliding objects instead of carrying objects reduces manual material handling. Conveyor systems can also be used if long distances are required.
Eliminate physical barriers. This control improves the biomechanical advantage by reducing the lever arm of the weight or object.
Reduce size and shape of raw and produced materials. This control method improves work efficiency and reduces the necessary muscle effort.
Utilize adjustable height or lift tables. Using a lift or scissor table reduces manual material handling and lifting.
Utilize work area and tool design principles. This will provide good object handling, efficient work interface design and good work flow to reduce muscle contraction duration and effort.
Implement rest or stretch breaks. Rests and stretch breaks will provide an opportunity for tissue recovery, and improve circulation to prevent tissue fatigue and micro-trauma.
Static Work or Static Muscle Contractions
Static muscle contractions are muscle contractions that are held continuously without interruption. An example of static work is holding an object for inspection. Static muscle contractions result in fatigue at much faster rates than muscle contractions that are intermittent. This fatigue is due to the higher utilization of nutrients, oxygen and energy within the working muscle. When higher muscle fiber recruitment is necessary to sustain a muscle contraction, as with static muscle contractions, there is a resultant lower metabolic efficiency in those working muscles. An additional consequence of static muscle contractions is that the flow of blood to those working muscles will be diminished, due to the rising internal pressures resulting within the muscle fibers from the actual contraction itself, this will in effect facilitate the rate of muscle fatigue. Consequently, anaerobic metabolism sets in, followed by the onset of local muscle fatigue.
Static muscle contractions can lead to local muscle fatigue within minutes and hours, even with short-duration and low-intensity tasks. Static muscle contraction endurance time is affected based on the intensity of the muscle contraction, or the higher the percentage of Maximum Voluntary Contraction (MVC). When the endurance time required for the static work is long, or the %MVC required is high, the risk for developing micro-trauma increases. The muscle-skeletal tissues over exert to accomplish the work.
Methods for Controlling Static Muscle Efforts
Methods to control fatigue and tissue micro-trauma from static muscle contractions include:
Limit static muscle contractions based on effort level. Limit high effort static contractions to less than 10 seconds, limit moderate effort level contractions to less than 60 seconds, and limit light effort static contractions to less than 4 minutes.
Reduce or eliminate the need to hold on to or grasp objects. Provide alternatives.
Implement mechanical aids. These aids reduce the static muscle contraction force and duration of the worker.
Utilize job rotation. Job rotation programs will reduce the overall exposure of a worker to a work stressor.
Implement rest or stretch breaks. Rests and stretch breaks will provide an opportunity for tissue recovery, and improve circulation to prevent tissue fatigue and micro-trauma.
Posture / Awkward Positions
Posture refers to the position of a specific body part or joint relative to an adjacent body segment, determined by the joint connecting the two segments. Awkward postures overload muscles and tendons. In addition, awkward positions load joints in an asymmetric manner, thus increasing joint compressive forces and impose a static load on the active working muscles. Postural risk factors are positions that cause excessive strain on the joints and should be avoided, or minimized for prolonged and repeated exertions.
Methods for Controlling Poor Postures
Stressful postures should be avoided when they are repeated or maintained for long duration without recovery time. Methods to control fatigue and tissue micro-trauma from awkward or stressful postures include:
Job rotation. Job rotation programs reduce the overall exposure to a work stressor.
Provide postural variability. Position variability provides an opportunity to frequently alternate postures.
Evaluate workflow and work design. Evaluate and modify the work area to minimize awkward postures and to maintain joint range of motion in the neutral or mid-range of motion positions.
Design or utilize tools that maintain neutral joint postures. Neutral joint positions are optimal. They allow maximum blood flow and produce minimal strain.
Implement rest or stretch breaks. Rests and stretch breaks will provide an opportunity for tissue recovery, and improve circulation to prevent tissue fatigue and micro-trauma.
Mechanical Pressure or Contact Stresses
Frequent or continuous use of tools or leaning on work surfaces with hard or sharp edges, or short handles can cause direct compression against peripheral nerves and blood vessels, thus impeding blood flow and nerve conduction. In addition, weight bearing on to the wrist, such as in gymnastic sports and bicycle riding, or activities that use the hand for pounding, will result in direct contact pressure at the base of the palm, or the palmar surface of the fingers, can also cause diminished blood flow, reduced nerve conduction and over stress soft tissue, resulting in micro-trauma.
Methods for Controlling Mechanical Pressure or Contact Stresses
Methods to control for fatigue and micro trauma from mechanical pressure and contact stress include:
Provide tools for activities that require pounding or hammering. This will eliminate mechanical pressure to the hands and wrist.
Utilize equipment and tool design principles. These guidelines help to ensure that handle size and shapes are appropriate and do not have any prominences that may increase pressure to the tissues.
Evaluate equipment for sharp edges. Identify and modify sharp edges by rounding surface edges or adding padded covers.
Utilize power or spring-loaded tools. This will reduce contact pressures and promote blood flow to active tissues.
Summary
This article defined what a risk factor is and how various risk factors found in occupational settings can increase the likelihood of developing a CTD. A risk factor is an attribute, experience or exposure that increases the probability of the occurrence of a disease or disorder, although it is not necessarily a causal factor. In general, the greater the exposure is to a single risk factor or a combination of risk factors, the greater the risk of developing a CTD. In addition, the more risk factors that are present, the higher level of risk of developing consequent injury. Therefore it is imperative to identify and evaluate the risk factors that are present, then determine or assess the actual level of risk. To prevent CTDs we must modify both ends of the spectrum: reduce the rate and intensity of damage and increase the rate of tissue repair. This can be accomplished with risk factor control methods.
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