Manual material handling (MMH) activities are often of concern when assessing job tasks for risks that have the potential to lead to musculoskeletal disorders. MMH activities usually contain "ergonomic" risk factors that may include awkward posture, repetition, excessive force and mechanical or contact stress. Several evaluation methods and guidelines exist for evaluating manual material handling tasks. However, guidelines from different sources often cause confusion for employers evaluating MMH tasks and companies that want to establish standards for their business operations.
Various guidelines have been developed by professionals in the field with the goal of providing effective methods for evaluating risks. While neither mandatory nor enforceable, they serve to provide users with tools based on the best available data. Unfortunately, conflict and confusion may arise when the basis and assumptions of the guidelines are different. This article reviews several common manual material handling evaluation methods and how they can be combined to provide a single useful "rule-of-thumb" guideline for employers.
Lifting and Lowering
Three of the principal guidelines currently in use that may be applied to lifting and lowering tasks include:
- 1991 National Institute for Occupational Safety and Health (NIOSH) Lifting Equation,
- Proposed American Conference of Governmental Industrial Hygienists (ACGIH) Lifting Tables, and
- Snook and Ciriello Psychophysical Tables (Liberty Mutual Research 1978, rev. 1991).
Comparing these guidelines reveals more differences than similarities. These differences can make it difficult to evaluate lifting and lowering activities and determine the degree of risk associated with the tasks.
The NIOSH model is based on biomechanical, physiological and epidemiological data. It incorporates the following assumptions:
- Lifting and lowering tasks have the same level of risk for low back injuries.
- The worker/floor surface coupling provides at least 0.4 (preferably 0.5) coefficient of static friction between the shoe sole and the working surface.
The NIOSH equation does not include factors to account for unpredicted conditions, such as unexpectedly heavy loads, slips or falls, and was not designed to assess tasks involving one-handed lifting, lifting while seated or kneeling, lifting in a constrained or restricted workspace, or lifting objects wider than 30 inches. Additionally, gender is not considered in the NIOSH guidelines (the "Recommended Weight Limit" determined by the equation applies to both males and females).
The ACGIH method contains many of the same assumptions as the NIOSH equation, although ACGIH lists additional assumptions, or limitations. In addition, the ACGIH method only applies to "two-handed, mono-lifting tasks within 30 degrees of the sagittal plane," (which divides the body into right and left sides). ACGIH also states that professional judgment should be used if any of the following conditions or factors exist in the lifting task:
- High-frequency lifting (>360 lifts per hour)
- Extended work shifts (lifting performed for more than 8 hours per day)
- High asymmetry (lifting more than 30 degrees away from the sagittal plane)
- One-handed lifting
- Constrained lower body posture, such as lifting while seated or kneeling
- High heat and humidity
- Lifting unstable objects (e.g. liquids with shifting center of mass)
- Poor hand coupling (i.e., lack of handles, cut-outs or other grasping points)
- Unstable footing (inability to support the body with both feet while standing)
The Snook and Ciriello tables integrate biomechanical and physiological stressors and are based on the perception of a discreet population. The tables provide capabilities in terms of a maximum acceptable weight of lift (MAWL) or maximum frequency of lift applicable to a population. This method acknowledges that there is no single maximum weight that applies to everyone, because strength and endurance vary greatly among individuals. Consequently, the best way to evaluate a safe lifting or lowering task is in terms of what percentage of the working population can be expected to perform the task without overexertion. The higher the population percentage is for a given weight, the lower the risk of injury; conversely, the lower the percentage, the higher the risk.
The Snook and Ciriello tables apply to two-handed symmetrical lifts and in situations where other tasks in the job cycle require minimal force. Above the limitations inherent to pschyophysical responses (perceived vs. physical response), this method does not address any additional limitations (e.g. coupling, floor coefficient, constrained posture) and duration of lifting and lowering is not a factor in the values. However, when the tables are applied to work in hot environments, the maximum acceptable weight should be decreased by 20 percent.
While the three methods take into account many of the same factors (hand location, frequency and twisting), the side-by-side results and therefore, the protectiveness of each method differs. NIOSH and ACGIH provide more conservative results than Snook and Ciriello in some situations. However, the Snook and Ciriello tables provide greater protection for female employees involved in lifting tasks.
Pushing, Pulling and Carrying
Pushing, pulling and carrying generally do not receive the same amount of attention when assessing manual material handling tasks as other MMH activities. This may be due to the lack of research that has been directed at pushing, pulling and carrying. As a result, comparable conflicts arise when considering guidelines for pushing, pulling and carrying; however, these conflicts are due to the lack of applicable guidelines as opposed to differences between various methods.
The Push and Pull Tables published by Snook and Ciriello principally apply to carts and devices with horizontally oriented handles (e.g., gurneys, laundry hampers or food delivery trolleys). For these types of devices, the tables clearly illustrate the variables that affect worker capabilities: handle height, push/pull distances and frequency, as well as initial and sustainable forces. However, they do not apply well to other objects and materials that are pushed or pulled, such as wheelbarrows, hose lines (firefighters), electrical wires and the variety of products that might be pulled off an assembly line.
In order to develop "rule of thumb" guidelines for pushing and pulling, a variety of other guidelines were considered:
- Ergonomics Design for People at Work, Rodgers et. al., 1986
- Occupational Biomechanics, Chaffin and Anderson, 1984
- Rodgers Fatigue Model in Occupational Medicine Journal, 1992
- NIOSH Publication #97-117, Elements of Ergonomics Program: A Primer Based on Workplace Evaluations of Musculoskeletal Disorders
The values in the table below show recommended upper limits of forces for horizontal pushing and pulling. It is important to note that these forces are not the same as the weight of objects being pushed and pulled. To obtain push and pull forces, force gauges are used. A bathroom scale or a fish scale can be used to determine push and pull forces, respectively, but with less accuracy than force gauges.
The force required for handling any wheeled device involves several components: starting (initial), stopping, turning and maintaining its motion (i.e., sustained). The Maximum Push/Pull Force represents the maximum force that should be generated while completing any of these actions.
Limiting Factors in Push/Pull
There are certain conditions that should be examined in order to modify the above limits:
- Floors that are in poor shape (cracked, etched, uneven) make operating carts and tanks more difficult. The remedy for this situation is to use a cart with bigger wheels or to reduce the load.
- Transitions from hard surfaces (e.g. concrete or tile) to soft surfaces (e.g. carpet) will increase the forces. Push and pull forces should be measured on both types of surfaces.
- If ramps are used, the limits must be lowered.
- Complicated maneuvering calls for a considerably lower limit, particularly where there is not enough space to get one's body weight behind the cart/tank.
- Loads should not be transported more than 200 feet. Where there is a need to move heavier loads over longer distances, using a powered truck or conveyor system should be considered.
What a person can carry safely is often much less than what they can lift. If a person is required to carry an object frequently or over distances of more than a couple steps, the safe material handling capacity is reduced. Carrying tasks over 30 feet should be eliminated by utilizing carts or alternative methods.
By evaluating available guidelines and choosing which are most applicable to a given task, evaluators can satisfactorily identify risk in a job or task and develop reasonable and usable standards for their ergonomics initiatives or programs. The rule-of-thumb guidelines and charts presented above simplify the evaluation methods available for manual material handling tasks, however, they should not be used if a more detailed evaluation of a job task is required. Evaluators using these guidelines should understand the limitations and assumptions of each method in order to apply them properly.
Barbara A. Faville has over 18 years' experience in industrial hygiene, occupational health and safety, and ergonomics. She has performed comprehensive industrial hygiene and ergonomic assessments, surveys of domestic and international facilities, and laboratory analysis. Faville has developed and participated on ergonomics teams aimed at reducing risk factors and has developed and administered a model office ergonomics program, including training. She is well versed in current local, state and federal regulations pertaining to industrial hygiene and safety compliance, including the Occupational Safety and Health Administration (OSHA) and Washington Industrial Safety and Health Act (WISHA).
Chris C. Shulenberger has over 25 years' experience applying ergonomics in the fields of occupational health, safety, risk management, and workers' compensation. As Clayton's director of ergonomics, he is responsible for developing and managing the Ergonomics Practice Group, which oversees all client ergonomics projects nationwide including work site assessments, program development and implementation, and training. Shulenberger has particular expertise in six key areas: ergonomics program development; comprehensive ergonomics evaluations; disability management; product design and evaluation; employer, manager and team training; and litigation support.