Risk Factors

Ergonomic risk factors are the aspects of a job or task that impose a biomechanical stress on the worker. Ergonomic risk factors are the synergistic elements of MSD hazards. In the Health Effects section of this preamble (section V), OSHA discusses the large body of evidence supporting the finding that exposure to ergonomic risk factors in the workplace can cause or contribute to the risk of developing an MSD. This evidence, which includes thousands of epidemiological studies, laboratory studies, and extensive reviews of the existing scientific evidence by NIOSH and the National Academy of Science, shows that the following ergonomic risk factors are most likely to cause or contribute to an MSD:

  • Awkward postures
  • Cold temperatures
  • Contact stress
  • Force
  • Repetition
  • Static postures
  • Vibration

Of these risk factors, evidence in the Health Effects chapter shows that force (forceful exertions), repetition, and awkward postures, especially when occurring at high levels or in combination, are most often associated with the occurrence of MSDs. Exposure to one ergonomic risk factor may be enough to cause or contribute to a covered MSD. For example, a job task may require exertion of so much physical force that, even though the task does not involve additional risk factors such as awkward postures or repetition, an MSD is likely to occur. For example, using the hand or knee as a hammer (operating a punch press or using the knee to stretch carpet during installation) alone may expose the employee to such a degree of physical stress that the employee has a significant risk of being harmed.

However, most often ergonomic risk factors act in combination to create a hazard. The evidence in the Health Effects section shows that jobs that have multiple risk factors have a greater likelihood of causing an MSD, depending on the duration, frequency and/or magnitude of exposure to each. Thus, it is important that ergonomic risk factors be considered in light of their combined effect in causing or contributing to an MSD. This can only be achieved if the job hazard analysis and control process includes identification of all the ergonomic risk factors that may be present in a job. If they are not identified, employers will not have all the information that is needed to determine the cause of the covered MSD or understand what risk factors need to be reduced to eliminate or materially reduce the MSD hazards.

Although certain of the risk factors described above are easy to identify and it is not difficult to understand why they may be likely to create hazardous exposures, others are not as apparent or observable. Employers who already have ergonomics programs and persons who manage ergonomics programs should not have difficulty identifying risk factors in the workplace. Because these persons have training and experience, ergonomic risk factors are likely to be familiar concepts for them. Through the process of developing and implementing their ergonomics programs these persons have gained a good working knowledge of the ergonomic risk factors that are most likely to be present in their workplaces.

For those employers who are just beginning their programs and have little or no training and experience dealing with ergonomic risk factors, OSHA has tried to make the process of identifying them as workable as possible. Therefore, in the proposed rule OSHA has taken the ergonomic risk factors and the combination of risk factors most associated with the occurrence of MSDs and tried to present them in ways that those with more limited knowledge about ergonomics can readily identify. In this way, the ergonomic risk factors the proposed rule covers are presented in terms of specific and physically observable work activities and conditions. If any of these activities or conditions are present, the table in § 1910.918(c) tells employers which risk factors are likely to be relevant.

OSHA is proposing that employers use this list of physical work activities or conditions as a starting point for hazard evaluation, for several reasons. First, the list of activities and conditions is easy for employers to understand because they will be able to translate them to their own workplaces more readily than would be the case for ergonomic to risk factors. For example, "hand used as a hammer" is more easily understood than the term "contact stress," and "long reaches" graphically explains an "awkward posture" that may be a problem.

Second, the list helps employers quickly focus on the aspects of a job that are most likely to be associated with covered MSDs. At the same time, the list also identifies the risk factors that are most likely to be associated with the activities and/or conditions, which should help employers further focus their analysis. In this way the list serves as a bridge to the combinations of risk factors that studies have shown to be associated with an increased risk of developing work-related MSDs.

Third, having employers start the MSD identification and evaluation process with this list ensures that the analysis will be comprehensive. This is because the list includes the major components of work that have been associated with MSDs.

Physical work activities and conditions

The physical work activities and conditions OSHA has included in the proposed rule cover the basic physical aspects of jobs and workstations. These aspects include:

  • Physical demands of work;
  • Workplace and workstation conditions and layout;
  • Characteristics of object(s) that are handled or used; and
  • Environmental conditions.

The following table shows the physical work activities and workplace conditions that are associated with those physical aspects:

PHYSICAL ASPECTS OF JOBS AND WORKSTATIONS EXAMPLES OF PHYSICAL WORK ACTIVITIES AND CONDITIONS ASSOCIATED WITH THE PHYSICAL ASPECT
Physical demands of work
  • Exerting considerable physical effort to complete a motion
  • Doing the same motion over and over again
  • Performing motions constantly without short pauses or breaks in between
  • Maintaining same position or posture while performing tasks
  • Sitting for a long time
  • Using hand as a hammer
  • Using hands or body as a clamp to hold object while performing tasks
  • Objects or people are moved significant distances
Layout and condition of the workplace or workstation
  • Performing tasks that involve long reaches
  • Working surfaces too high or too low
  • Vibrating working surfaces, machinery or vehicles
  • Workstation edges or objects press hard into muscles or tendons
  • Horizontal reach is long
  • Vertical reach is below knees or above the shoulders
  • Floor surfaces are uneven, slippery or sloped
Characteristics of the object(s) handled
  • Using hand and power tools
  • Gloves bulky, too large or too small
  • Objects or people moved are heavy
  • Object is slippery or has no handles
Environmental Conditions
  • Cold temperatures

 

Employers who examine the job in which a covered MSD occurred to identify the physical work activities and workplace conditions and then evaluate the risk factors that OSHA has identified as potentially relevant, will be considered to be in compliance with the hazard analysis requirements of the proposed rule.

Exerting considerable force to complete a motion (forceful exertions)

It is not difficult to understand why jobs that require employees to apply a lot of physical effort may involve significant exposure to ergonomic risk factors and pose an increased risk of injury. For example, it is easy to see how much biomechanical stress employees are under when you see them grimace while trying to loosen lug nuts on an old tire, shift body weight and stance to wrench open stuck valves, or stiffen the body in order to lift a heavy or bulky object from the floor of a truck. Simply put, forceful exertions like these take more out of a person than tasks that do not require much physical effort. An easy way to confirm whether a task involves forceful exertions is to ask workers who are doing the task, or to try to do it yourself.

Performing forceful exertions requires an application of considerable contraction forces by the muscles, which causes them to fatigue rapidly. The more force that must be applied in the exertion, the more quickly the muscles will fatigue or become strained. Excessive or prolonged exposure to forceful exertions also leads to overuse of muscles and may result in muscle strain, soreness and damage. Performing forceful exertions can also irritate tendons, joints and discs, which leads to inflammation, fluid build up, and constriction of blood vessels and nerves in the area. Increased compression of nerves from the pressure imposed by inflamed tendons or muscle contractions may cause disorders of the nervous system (carpal tunnel syndrome and other nerve entrapment disorders).

Injuries related to forceful exertions can occur in any tissue or joint. As mentioned above, back injuries from overexertion are a leading cause of workplace injuries and workers' compensation cases. A number of studies also show that repeated forceful exertions of the hands and arms are associated with work-related MSDs (using tools, pinching or pushing with the fingers).

Lifting and carrying heavy objects are usually the tasks that come to mind as examples of forceful lifting tasks, but high forces are also involved in other types of jobs. These include jobs that require employees to apply pinch forces with their fingers (picking up or placing small items on an assembly line with the fingers), static forces (applying a lot of physical effort to put the last turn on a screw, pulling hard on a 30-inch wrench to loosen a bolt), and dynamic forces (tossing objects into containers). (Forceful lifting/lowering, pushing/pulling and carrying are discussed under "Manual Handling" activities and conditions below.)

Doing the same motions over and over again (repetitive motions)

Many jobs that involve repetition of the same job again and again are apparent even upon cursory observation: assembly line jobs where motions are repeated every few seconds, data processing jobs, directory assistant operators, court reporting, letter and package sorting. Repetitive motion jobs include performance of identical motions again and again, but also include repeating multiple tasks where the motions of each task are very similar and involve the same muscles and tissues.

Evidence in the Health Effects section shows a strong association between the occurrence of MSDs and jobs involving exposure to repetitive motions. The joints are most susceptible to repetitive motion injuries, especially the wrists, fingers, shoulders, and elbows. Repetitive work that is done with the foot (operating foot activated controls) or knees (climbing ladders or using a carpet kicker) may also result in an MSD.

Performing motions constantly without short pauses or breaks in between (inadequate recovery time)

Jobs that do not provide short pauses or breaks between motions or task cycles are often a problem because there may not be adequate time for muscles to recover from the effects of the exertion before the motion must be repeated. If there are no pauses between motions or the pauses are too short, the muscles cannot recover to the rested condition. Thus, the effects of the forces on the muscles accumulates and the muscles become fatigued and strained. The lack of adequate recovery time often occurs in jobs involving highly repetitive tasks. This happens when task cycle lengths are very short, which also means that the job involves a high number of cycle repetitions per minute. For example, some research shows that tendons and muscles in the wrists may not be able to recover where repeated task cycles are less than 5 seconds in length, that is, they are repeated more than 12 times per minute (Ex. 26-2).

Jobs involving constant muscle activity (static contractions) also may not provide adequate recovery time. These types of jobs may involve continuously holding hand tools (knife, paint brush, staple gun), which means that employees have constant exposure to static postures and low contraction forces.

The longer motions or job tasks are performed, the less likely that there will be adequate recovery time. The accumulation of exposure leads to muscle fatigue or overuse. In addition, where the intensity of exposure is greater, for example, in repetitive motion jobs that involve exposure to additional risk factors (force, awkward postures, or static postures), the increased forces required for the exertion also increase the amount of recovery time that is needed. Any part of the musculoskeletal system involved in moving the body is subject to injury where there is inadequate recovery time, and the recovery times needed vary by body part. For example, although employees may not be at high risk for forearm injury if task cycles are 25 seconds long or not repeated more than 3 times per minute, they may be at high risk of shoulder injury under this regimen.

Awkward postures, static postures, contact stress, vibration

The presence of any or all of these risk factors in a job, particularly jobs involving repetitive motion or forceful exertion, increases the force already required to perform job tasks and, therefore, increases the amount of time muscles need to recover from the exertions the task requires. If the recovery time is not adequate, the presence of these risk factors hastens the onset of fatigue and the effects associated with overuse of muscles, joints and tendons.

  • Attaching doors on the bathroom vanity assembly line
  • Capping and cupping cookies on an assembly line

Performing tasks that involve long reaches

Many job tasks involve long reaches: working overhead, putting items on a high shelf, reaching across a conveyor to put in a part or grasp an object, or bending over to reach a part in the bottom of a big supply box. These tasks expose employees to extreme awkward postures. Where long reaches are momentary and/or infrequent and the forces are low, these tasks are not a problem because there is likely to be adequate time for the body to recover between reaches. However, when long reaches are done frequently, force is involved and/or a long reach lasts more than a few seconds, the risk of harm increases.

Long reaches usually have the greatest impact on the shoulders and lower back. The shoulder is unique in its wide range of motion when compared with other joints in the body. The bony restraints are minimal, but soft tissue constrains the motion. Thus, injuries usually occur when the soft tissue is used to maintain an awkward posture and/or forceful exertion.

The back is flexed forward or extended back to extend reaches beyond the limit of the arm length. In addition, workers in repetitive jobs will often bend their back so that they can reduce the awkward shoulder posture. Bending the back forward adds the weight of the upper body to the force exerted by the back muscles and supported by the spine. Bending to the side, backwards or twisting puts the spine and back muscles in awkward postures.

Working surfaces are too high or too low

Working surfaces that are too high or too low are another way in which employees are exposed to awkward postures. Where employees must work on such surfaces for a long period, the risk of tissue damage and other MSD problems increases.

Working surfaces can be too high or too low for many employees because most working surfaces are not adjustable. For example, 30 inches is a typical height for desks, tables and other working surfaces operated from a sitting position, and 36 to 40 inches is a typical height range for working surfaces operated from a standing position. Although employees of average height may be able to work comfortably at these working surfaces, the typical heights may not work for shorter or taller employees. An assembly-line employee who is 6'5" may have to bend over significantly to assemble the parts on a conveyor that is 36 inches high, while a 5-foot employee working on a 42-inch conveyor may have to work with her elbows away from the body.

The height of working surfaces can also be too high or too low when employees must use work surfaces or workstations that were not designed for the tasks being performed. For example, typical desks (30 inches high) are not designed for computer use. Even persons of average height may have to raise their elbows and shoulders to use the keyboard on their desks. This is especially true where desk chairs cannot be raised high enough to correct the problem. Even when the employee can be raised to a good height, the feet are often left dangling above the floor.

Maintaining same work positions or posture for a long period

The chief complaint people usually make when they have worked for a long time in the same position is that they feel "stiff, sore and tired." These are some of the effects that result when tasks involve static postures (driving for several hours without a break).

Static postures increase the amount of force required to do a task because, in addition to the force required to perform the task, contraction forces must be applied to hold the body in position throughout the work shift. Maintaining the same position or posture includes a variety of things. It includes holding the arms and shoulders in a non-neutral posture without moving.

The effects of maintaining the same work positions can occur in almost any joint of the body and vary depending on body location. For example, the effect on the knees and back from squatting or kneeling for 2 hours is likely to be greater than the effect on the neck and shoulders from looking up at a monitor for the same period.

Sitting for a long time

Sitting for long periods without the opportunity to stand up and move around is another way in which employees are exposed to static loading of tissues, primarily in the lumbar area of the back. It can also affect the upper back, neck and legs. The problem is exacerbated where awkward postures are also present.

Static postures. Employees may be exposed to static postures when they must sit for a prolonged period on chairs, stools or benches that do not provide adequate lumbar support, that is, either the back rest of the seat does not provide good lumbar support or there is no back rest at all. When there is no lumbar support and the back is bent forward, the muscles of the back are trying to force the lumbar region out of it natural curve (proper alignment of the vertebrae), which places pressure on the discs and reduces blood supply to the spinal tissue. The constant exertion of the contraction forces leads to muscle fatigue.

When the back muscles become sore, people tend to slouch. In this posture more force is being placed on the back and the discs. As the static loading continues, pressure continues to be applied to the membranes of the discs and they may become stressed. Stressed discs, in turn, may put pressure on blood vessels and may pinch a nerve (sciatic nerve), which results in pain.

Even where the chair has a back rest with lumbar support to help maintain the back in a neutral position, employees still may continue to be exposed to static loading because they cannot take advantage of the back rest. This may occur when the seat pan is too big or the seat is too high for the employee. Many employees respond by sitting forward, instead of against the back rest, so that their feet can be on the ground, thus pressing the spine out of the natural curve and placing pressure on the discs.

Using hand and power tools

"Using hand and power tools" to perform physical work activities does not in itself mean that employees are exposed to ergonomic risk factors that put them at risk of injury. Rather, it is a shorthand way of alerting employers that there are aspects of tool design and use that need to be checked out to see whether ergonomic risk factors may be present. These include:

  • Weight and size of tool
  • Tool handles and/or grips
  • Tool activation (repetitively, one finger)
  • Tool kickback, vibration and maintenance

Using hands or body as a clamp to hold objects while performing tasks

Sometimes this is referred to as having the worker act as a "human clamp" or "human vise." In these situations the worker usually holds the object being worked on with one hand (often in an awkward, forceful posture) while force is applied by the other hand. The hand being used as a clamp has to hold the object while resisting the forces being applied by the other hand. Using the hand as a clamp leads to muscle fatigue and inflammation of the muscles and tendons.

The strain on the muscles and tendons in the clamping hand is especially high when the task involves static postures or contact stress. Although the hand and arms are most often used as a clamp, some larger jobs require the feet, legs, hips or torso (lateral bending of the back) to support a part while work is performed.

  • Holding the head of a cow on a slippery surface while attempting to remove meat
  • Holding a small part while assembling it
  • Drilling a hole in a part that the worker has to hold
  • Using the hips or thighs to hold a part in place while working on the part

Gloves are too large, too small or too bulky

For many jobs it is necessary or appropriate for workers to wear gloves while doing their jobs. Gloves can make grasping an object more difficult by changing the friction, decreasing dexterity, and interfering with sensory feedback. This often leads to using more muscle force than would be required without gloves. Additionally, gloves can fold, wrinkle, and bunch so that pressure points are created that result in contact stress. Gloves that fit or are less bulky may help to relieve these problems. An even better solution is to eliminate the need to wear gloves.

Examples of glove use that may rise to the level of a hazard are providing inappropriate gloves for the work, or failing to consider the worker's needs when gloves are purchased, providing thick gloves for a task that requires dexterity beyond that allowed by the gloves, or providing vibration dampening gloves and expecting levels of dexterity or force exertion that are beyond the level possible with the gloves.

Manual handling (lifting/lowering, pushing/pulling and carrying)

Forceful manual handling activities are a leading cause of workplace injury and illness. Lower back MSDs from lifting account for a large percentage of all workers' compensation cases. Studies discussed in the Health Effects section indicate that employees performing manual handling tasks have a significantly higher risk of back injury where they are exposed to force, repetition and/or awkward postures in the job.

The physical work activities and conditions included on the manual handling list in the proposal are ones that are likely to be a significant problem because they are ones in which the major ergonomic risk factors associated with manual handling tasks are present: force and awkward postures/static postures. This discussion about physical work activities and conditions in manual handling tasks is organized by task (lifting, pulling). Manual handling tasks are discussed only where the physical work activities and conditions and ergonomic risk factors are likely to be a significant problem.

Objects or people are heavy (lifting, lowering, pushing, pulling, carrying)

Workers lift, lower and move items every day. The heavier the weight that has to be lifted, lowered and/or moved, the more force the worker will have to exert. The heavier the weight, the closer the contraction required of the muscles will be to their maximum capability. When muscles contract at or near their maximum, they fatigue more rapidly and the likelihood of damage to the muscle and other tissues involved in the activity increases. In most situations involving lifting, lowering and moving heavy objects or people, the predominant risk factor is force. Manual handling of heavy objects exposes employees to high forces and will usually have the greatest impact on the back. Another aspect of weight that should be considered is a sudden shift in weight. Workers are more often able to accomplish a manual handling task without injury when they are prepared. When a patient's legs suddenly buckle while they are being transferred or a load within a package or container shifts, the worker may not be physically or mentally prepared for the weight.

Lifting and Lowering

In lifting and lowering, force is the risk factor that most often needs to be addressed. Although there may be a perception that lifting is more problematic than lowering, they both require the worker to exert the forces commensurate with the weight of the object. The actual forces exerted by the worker are determined by the weight of the object. It is obvious that lifting containers weighing 25 pounds is considerably easier than those weighing 50 pounds and that more people are capable of lifting the smaller amount. Posture can play a major role in the force required when moving an object. If that object can be held or lifted closer to the body, the muscle forces required in the back are less. Bulky containers present more of a problem when being lifted than do those with the same characteristics, including weight, that are compact. Finally, the frequency with which an object is lifted or lowered and the times it must be supported may be important in determining the risk presented by the job.

  • Lifting a resident, who has little ability to assist, from the toilet to a wheelchair
  • Lifting a 150 pound package from a loading dock into a van

Pushing and Pulling

When pushing and pulling objects, the weight of the object or conveyance, including its contents, affects the force required of the worker. Often workers have to slide objects on a table or flat surface. In these cases the weight and the friction characteristics of the object and the surface are the prime determinants of the force required. Secondarily, the posture or reach may affect the degree of risk presented by the job. Where conveyances such as carts are used, the force required is generally determined by the characteristics and weight of the cart and contents. For very heavy carts, stopping and controlling the cart can sometimes be as difficult and important as pushing or pulling it to the desired location.

  • Pushing a 300 pound pump away from the paper machine or
  • Pushing a heavy cart up a sloped ramp

Carrying

For carrying the weight, distance and object characteristics affect the forces required. Often the forces are exerted statically for some period of time when carrying. Additionally, the worker's body is in motion and the stability and biomechanics of the activity may be much worse than in a simple lifting or lowering situation. Examples might be carrying heavy parts from one work area to another, carrying containers from production to a pallet or storage area, or carrying packages when delivering them to a customer.

  • Carrying several 50-pound bags of feedstock material to the basement
  • Carrying a resident of a nursing home to the bath tub

Horizontal reach is long (Distance of hands from body to grasp object to be handled)

Workers who are lifting/lowering, pushing/pulling or carrying are greatly affected by the distance that the hands are from the body during the activity. The forces required to manually move an object by the muscles in the back and shoulder are increased significantly as the load is moved away from the body. The resulting compression on bone and cushioning tissues is also significantly increased. The impact on the musculoskeletal system increases dramatically as the object or weight (center of gravity for bulky objects) is farther from the body. When moving objects or people, the distance away from the worker's body affects the forces for a lift or carry. Two characteristics of a lift requiring a long horizontal reach make it harder on the worker. The first is that the worker's own body weight must be supported and lifted in addition to the weight of the object. The second is that the torque required puts the muscles at a greater mechanical disadvantage when the objects being lifted are at a greater distance from the body joint involved. Because of the mechanical disadvantage, the predominant risk factor in these situations is force, which is increased because of the risk factor of awkward posture (long reach) present. The awkward posture involved in long reaches requires higher muscle forces to lift or move the same weight as would be necessary if the reach were shorter. The problem becomes worse when either greater weight or greater distance is required. Lifting, lowering and/or carrying items when a long horizontal reach is required will usually have the greatest impact on the shoulders, arms and back.

Lifting and Lowering

For lifting and lowering where the horizontal reach is long, force is the factor that needs to be addressed. This is usually accomplished by reducing the reaches or the weight. Examples would include reaching for a product on the far side of a conveyor, reaching to a parts supply bin that is on the far edge of the work surface, lifting a large box with a center of gravity at some distance from the body, lifting or lowering something on the far side of a barrier, placing packages on the far side of a pallet, or assisting a patient in sitting.

Pushing and Pulling

For pushing and pulling tasks, there may be reaches that are long; however, these are not usually a problem unless there is simultaneous lifting or unless the pushing and pulling direction is side to side rather than in and out. Moving objects from side to side is much less efficient than toward and away from the body.

  • Pushing a heavy box on a non-powered conveyor

Carrying

There are times when workers carry an object that cannot be rested against the body, so the arms are in a position that is similar to that of a long reach. This also happens when carrying a large box or container. When this happens the force risk factor is probably the most important, followed by the awkward and static posture risk factors.

  • Carrying a hot pack used in extruding plastic to the repair cart
  • Carrying a carboy of nitric acid

Vertical reach is below knees or above the shoulders (Distance of hands above the ground when the object is grasped or released)

Workers who are lifting/lowering, pushing/pulling or carrying must exert more effort if the vertical position of the hands (when the object is started in motion) is above or below 30" (Snook 1978, Ex. 2-26; Ayoub et al. 1978, Ex. 26-1416; Snook and Ciriello 1991, Ex. 26-1008). The forces required by the muscles in the back and shoulder are increased significantly as the hands near the floor or move above the shoulders. The NIOSH lift equation reduces the recommended lift by 22.5% if the lift occurs at or above shoulder level.

In addition to the force, the resulting compression on bone and cushioning tissues increases the likelihood of an injury. Ideally the hands are at (or slightly below) waist level when manual handling begins. Manual handling tasks that require the hands to be lower than the knees or higher than mid-torso put the worker at a biomechanical disadvantage, which requires the muscles to exert more force than if the starting point is near waist height. Low starting points require bending or squatting, which adds stress to the back and knees, respectively, due to the awkward posture. When the lifted object is below the worker's knees, he or she must bend forward, thus stretching the muscles in the back into an awkward and less efficient lifting posture. In addition, from a stooped posture the worker must lift the weight of the torso up as the object is lifted.

When an object is lifted above mid-torso heights, the thrust of the lifting force shifts from the larger/stronger muscles of the back to the smaller muscles of the shoulder. As the load is raised higher, the muscles of the shoulder become the primary movers. When material is lifted overhead, control of the lift becomes important. If the weight of the load were to suddenly shift while being lifted overhead, the resulting awkward posture, combined with the weight and distance of the load from the lower spine, could tear tendons, ligaments and muscles.

Lifting and Lowering

In lifting and lowering from or to low or high positions, awkward posture is a risk factor that often needs to be addressed. The awkward posture makes the muscles less efficient, and results in higher muscle forces than would be required if the lifting or lowering took place with the load within 10 inches of the waist.

  • Picking up a 35-pound spool of yarn from a peg above shoulder height
  • Picking a 40-pound item from a 60" high shelf in a grocery warehouse
  • Lifting a 50 pound motor off a pallet

Pushing and Pulling

When pushing or pulling objects, the height of hands affects the amount of force needed. When the hands are slightly above waist height, the worker gets the most from the muscles. As the hands are moved lower or higher, the worker's posture becomes more awkward and requires more force from the muscles.

  • Pushing a cart with the hands above mid chest height
  • Pulling a wooden pallet across the floor

Carrying

Carrying an object combines the static loading of the muscles with the loading caused by the awkward vertical position of the load. The combination of static and awkward postures greatly increases the fatigue on the muscles. Maintaining a stooped posture to carry a load places strain on the muscles of the back and shoulder as well as the spinal discs. Not only is the back supporting the weight of the object, but also the weight of the upper body. Carrying loads above shoulder height cannot be maintained for prolonged periods of time because the shoulder muscles will fatigue. The exception is when the weight of the load is rested on the skeletal system and the arms merely balance the weight (carrying objects on the head, carrying trays of food on the shoulder).

  • Carrying large, bulky boxes of machine parts where the worker is unable to carry the box with a horizontal hold
  • Carrying a large piece of furniture down steps

Objects or people are moved significant distance (pushing, pulling, carrying)

In producing products or even services it is often necessary to move objects or people. This may be done by a worker pushing, pulling or carrying the item. Almost invariably this involves forceful exertions. The method of movement, the force required, and the distance to be moved are the important aspects of the job that will determine the presence of MSD hazards. The higher the force required and the longer the distance to be moved, the more likely it is that the job will present a problem. Force is the predominant risk factor when objects are moved, and it can be mitigated by using carts or other conveyances. This type of job is most likely to have adverse affects on the back, shoulders and arms.

Lifting and Lowering

Lifting and lowering is usually involved in a job of this type when the object is to be carried. For the lifting and lowering part of the job, the discussion of "objects or people moved are heavy," above, should be consulted. The carry part of the task involves force and static postures. The weight of the object and the distance affect the force required and the time spent in static and forceful postures, respectively. Carrying puts the body in a dynamic activity where the stability is less than when the body is stationary. Examples of movement distances that might rise to the level of a hazard are moving a patient from the bed to the bath, lifting a tire from the floor to above the head, or carrying a heavy part from a pallet to a workstation.

Pushing and Pulling

When pushing or pulling an object for a significant distance, the forces required and the distance moved are the important aspects of the job. If a cart or conveyance is used, the force to push or pull it is almost always the risk factor of concern. Sometimes large or heavy objects are moved by sliding them across the floor. This usually involves high forces and is better done in other ways such as using a cart or powered mover.

  • Pushing a cart of restaurant supplies from the delivery truck to the restaurant
  • Pushing a patient on a gurney to physical therapy.

Carrying

Once again, the weight of the object and the distance it must be carried are the important factors. The effect of these on the worker can be reduced by providing some form of conveyance.

  • Carrying trash cans to the garbage truck
  • Carrying water bottles to the cooler

Bending or twisting during manual handling

Bending or twisting while manual handling creates an awkward posture and changes the way forces are distributed in the spine. When the spine is in its natural position, forces are directed along the bony structure and distributed into the tissue as the spine curves. However, bending and twisting redirects the forces, placing more compressive and shear forces on the discs. Psychophysical studies have reported that there is a decrease in the maximum acceptable weight of lift (MAWL) in the range of 8% to 22% where twisting of the torso is involved (Garg and Badger 1986, Ex. 26-121; Mital and Fard 1986, Ex. 26-182; Garg and Banaag 1988, Ex. 26-951). Experiments by Adams et al. (1980, Ex. 26-701) indicate that combined bending and twisting of the spine reduces the tissue tolerance of the intervertebral discs, predisposing them to rupture.

When an object to be lifted is below the worker's knees, he or she must bend forward, thus stretching the muscles in the back into an awkward and less efficient lifting posture. In addition, from a stooped posture the worker must lift the weight of the torso up as the object is lifted. Lifting from a stooped posture also creates a situation where the worker can accelerate the torso as they lift.

Marras and Granata (1995, Ex. 26-1383, and 1997b, Ex. 26-169) found that increased velocity and acceleration in trunk lateral bending and twisting result in measurable increases in both compressive and shear forces experienced by the intervertebral discs.

Lifting and Lowering

In lifting and lowering, awkward posture is the risk factor that most often needs to be addressed. The awkward posture makes the muscles less efficient and results in higher forces than would be required if the lift or lower were ±10 inches from the waist.

  • Moving 30 pound motors from a workstation to a conveyor perpendicular (90) to the workstation
  • Moving a patient from the bed to a wheelchair
  • Loading luggage into the cargo hold of an airplane

Object is bulky, slippery or has no handles (lifting, lowering, carrying)

Lack of good hand holds or good coupling between the hand and the object can result in higher grasp forces, higher other hand/arm forces, higher back forces, or the adoption of awkward postures to secure a stable relationship with the load. The predominant risk factors involved are force and awkward postures, which usually affect the back, hands, wrists and fingers.

Lifting and Lowering

When lifting and lowering an item in which the coupling is poor, the worker has to adapt. Sometimes this involves having the hands or center of gravity of the load at considerable distance from the body, which increases the forces required of the back in awkward postures. Sometimes the hands have to bend around the box corners, resulting in considerable force being exerted in an awkward posture. Bulky loads cause the worker to bend the back more. Open boxes with poor coupling may be picked up with pinch grips on the tops of the box sides, which results in high forces and an ineffective grip.

  • Lifting a 40-pound fuel pump out of a tank of mineral oil,
  • Lifting wet watermelons out of a box (which requires the worker to use excessive grip force)
  • Lifting a patient with little ability to assist out of bed

Pushing and Pulling

Hand forces will tend to be higher when pushing or pulling bulky items or those that have poor coupling

  • Pushing a large box of potatoes in a product warehouse

Carrying

The problems of carrying an object with poor coupling or that is bulky are very similar to those involved in lifting and lowering. These problems are exacerbated by the static loading required when carrying any distance.

  • Carrying a keg of beer
  • Carrying machined parts to a degreaser
  • Carrying a side of beef

Floor surfaces are uneven, slippery or sloped

Surfaces that are not level require the worker to compensate by placing the body in an awkward posture. When the spine is in its natural position, forces are directed along the bony structure and distributed into the tissue as the spine curves. However, awkward postures both redirect the forces, placing more compressive and shear forces on the discs and placing the muscle in a less efficient position. In addition, to move an object manually, the forces exerted by the feet need to be resisted by the forces that push back from the floor. When the floor is slippery or sloped, the worker must expend more energy resisting the natural tendency for the feet to slip. If the load should shift while the worker is on an uneven, slippery or sloped surface, an injury becomes more likely. Poor floor conditions can affect the footing and the ease of movement of carts. Force is the risk factor that is usually exacerbated by poor floor surfaces and the back is the usual location of MSDs that are brought on by problems of floor surfaces. Lack of good footing will result in added stress on the postural muscles and other tissues.

Lifting and Lowering

In lifting and lowering, awkward posture is the risk factor that most often needs to be addressed. The awkward posture makes the muscles less efficient and results in higher forces. The higher forces lead to fatigue and inflammation.

  • Shoveling grain
  • Lifting bags of laundry from a wet floor

Pushing and Pulling

Pushing or pulling on an uneven, slippery, or sloped surface can result in a sudden increase in the force needed to move or stop an object. The increase in force alone can tear muscles or strain tendons enough to cause an injury. When the increase in force occurs when the body is in an awkward posture due to the surface, then a muscle or tendon strain is more likely, due to the inefficient position of the muscles.

  • Pushing a laundry hamper across a wet floor
  • Pushing a file cabinet on a carpeted floor
  • Pushing a wheelchair through gravel
  • Pushing a cart on a cracked concrete floor

Carrying

Carrying an object while walking on uneven, slippery or sloped surfaces causes the body to continually shift to accommodate the changing working surface.

  • Carrying boxes of metal scraps down steps
  •  Carrying boxes of paper up a ramp into the computer room