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Ergonomic Risk Factors Article

Posted May 22 2009 11:56pm

By Nicole Matoushek, MPT, PT (printed in ADVANCE magazine for Rehabilitation Directors, March 2009)

Rehab providers in industrial medicine often operate in a gray area. It can be difficult to connect a specific workplace injury to a single job task, posture, lifestyle behavior or hereditary trait. Many elements may be at work.
But employers are becoming more aware of the causative relationship between modifiable risk factors and cumulative trauma disorders (CTDs) at work. While the mere presence of a risk factor doesn’t necessarily relate to injury risk, it may cause, aggravate or precipitate a CTD.
In fact, according to research from the National Safety Council, it’s possible that interacting risk factors have a multiplicative effect, which can significantly increase the likelihood of microtrauma and worker injury.
Ergonomic and industrial medicine professionals can play a role to reduce the number, frequency and effect of workplace hazards. Doing so requires an understanding of the difference between traditional and non-traditional (environmental) risk factors, and recognizing the steps to reduce their consequences.

When lowering the risk of cumulative trauma disorders, your main consideration is sufficient blood flow. Maintaining an adequate supply of oxygen-enriched blood to working tissues ensures metabolic efficiency, and minimizes the adverse effects of tissue overload, fatigue and microtrauma.

The key to adequate blood flow lies in the balance between work and human physiology. Attempt to mitigate the following contributors to cumulative trauma and fatigue.
Task repetition. Most work tasks involve repetition of some degree, but high task repetition is a common risk factor for CTD. High task repetition can also magnify the effects of other risk factors. Control methods are important.

A job task is highly repetitive if the cycle time is equal to or less than 30 seconds, or if more than 50 percent of the time involves performing the same fundamental cycle. Because most work cycles are determined by daily production quotas or machine pace, workers can’t always control their pace or work-rest cycle.

However, a job rotation program can reduce overall exposure, and mechanical aids can reduce contraction force and duration. Adjust machine pace and production quotas, and stress the importance of rest and stretch breaks.

Forceful exertions. Force exertions are the manual efforts required to accomplish a specific task, movement or action. The higher the magnitude of force, the higher the fatigue risk.

To control microtrauma from forceful exertions, recommend mechanical aids or power tools, mechanical lifting devices, counter balance systems, transport carts and dollies. Instruct workers to slide objects rather than carry them, and use conveyor systems for long distances. Eliminate physical barriers, reduce the weight’s lever arm, cut down the size and shape of raw and produced materials, and use adjustable height or lift tables.

Proper work area and tool design principles provide appropriate object handling, efficient work interface design and good work flow. Implement rest or stretch breaks for tissue recovery and better circulation.

Static muscle contractions. Static muscle contractions, such as holding an object for inspection, can fatigue workers at much faster rates than intermittent contractions. Higher utilization of nutrients, oxygen and energy within the working muscle lowers metabolic efficiency, inhibits blood flow, raises internal pressures and facilitates anaerobic metabolism and local muscle fatigue.

To control its effects, limit high-effort static contractions to less than 10 seconds, moderate effort contractions to less than 60 seconds, and light effort contractions to less than 4 minutes. Provide alternatives to grasping objects for extended times. Implement mechanical aids, job rotation and rest or stretch breaks.

Posture. Awkward positions overload muscles and tendons, and load joints asymmetrically, increasing joint compressive forces. Workers should avoid long periods of stressful positioning, and vary postures and rotate jobs.

Evaluate and modify a worker’s area to maintain joint range of motion in the neutral or mid-range positions, design tools that maintain neutral joint postures, and stress the need for rest and stretching.
Contact stress. Continuous use of tools, or leaning on work surfaces with hard or sharp edges, can compress and impede blood flow and nerve conduction. In addition, weight bearing on the wrist and pounding with the hand can diminish blood flow and nerve conduction in soft tissue.
Help control the microtrauma from these motions by providing tools for pounding and hammering. Better equipment and tool design ensures appropriate handle sizes and shapes. Modify sharp edges by rounding or adding padded covers, and use power or spring-loaded tools to reduce contact pressure and promote blood flow.
In addition to traditional risk factors, industrial medicine clinicians must watch for environmental factors that may contribute to work-related injuries.
To protect workers from environmental risk factors, groups such as the Occupational Safety and Health Administration and the American Conference of Governmental Industrial Hygienists have developed acceptable exposure limits and guidelines. While some may be used as legal standards, others are merely used to make recommendations for worker safety. These common environmental risk factors can put workers in harm’s way.
Vibration. Vibration is the back and forth, linear motion that an object experiences as it moves to a defined stationary point. A worker’s body can experience an amplification of the vibration’s intensity, which exacerbates the effects.
Vibration can represent a type of cumulative trauma, affecting multiple body parts and organs. The duration of exposure and the direction and intensity of the vibration are important factors to consider. In an occupational setting, workers can experience whole-body or segmental vibration.
The human body tolerates whole-body vibration better in the standing position. Women tend to experience more discomfort than men. To modify its effects, recommend suspended seating systems to isolate the vibration source. Maintain vehicle suspension systems regularly and keep tires properly inflated. Remote-control systems can operate vibration-producing processes without body contact.
Promote good working postures and neutral joint positions, and avoid lifting or bending immediately following vibration exposure. Allow a longer recovery period for exposed tissues, and provide vibration-absorbing materials, such as rubberized anti-vibration mats or gel shoe inserts.
Segmental vibration affects a specific body segment--often the upper extremity--when using power tools or industrial machinery. Control strategies include fasteners or dampening devices to reduce the “drive” of the tool; attenuation covers and personal protective equipment, such as gel packed gloves and gel shoe inserts; tool and balance maintenance; sharpening grinders and cutting tools; and increasing revolutions per minute or cycle time of a power tool.
Thermal stress. Workplace temperature can affect worker performance and increase the likelihood of cumulative trauma injuries.
Hot and cold extremes can lead to overexertion injuries and even death. Hot and humid temperatures cause excessive fatigue, while extreme cold can cause overexertion and reduced finger tactility. Temperature extremes may also require additional or less protective clothing, leading to mobility impairments and trip-and-fall hazards.
Manage cold stress with dry insulating clothing, gloves, warm air jets, radiant heaters or warming plates. Cover metal tools with thermal insulation. Reduce airflow with windshields or barriers. If cold stress can’t be controlled, recommend job rotation and warm-up periods.
With excessive heat, adequate ventilation through airflow and industrial fans can help. Clothing should be loose and made from thin, breathable materials. Job rotation and rest periods are also essential.
Sound and noise. Excessive noise can distract workers and lead to long-term damage. Implement noise control programs, hearing conservation programs (including annual hearing test), sound-absorbing barriers and ear protection can guard against excessive noise exposure.
Illumination. Lighting levels influence worker ability. Straining can lead to eye and muscle problems, while excessive brightness can be distracting.
Provide additional task lighting when high visual acuity is required. Minimize indirect and direct glare by placing the light source as far away from a worker’s visual line of sight as possible, using multiple low-level light sources and implementing light shields or reflectors. When using video display units, apply anti-glare screens or anti-reflection coatings, and change the work area to avoid indirect glare on the monitor.
When an employee misses work because of injury, families and companies suffer. A few inexpensive recommendations can make the difference between a safe workplace and a costly misfortune.

Nicole Matoushek, MPT, PT, is an ergonomics professional in industrial medicine, physical therapy and clinical management education. She is based in St. Petersburg, Fla., and can be reached at
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