Understanding Manual Material Handling and Cart Ergonomics Standards Everywhere is the Key to Minimizing Risk of Injury
Injuries related to manual materials handling (MMH) tasks are a major concern in industrial ergonomics. The Canadian Center for Occupational Health and Safety (CCOHS) reports that “three of every four individuals whose jobs include MMH will suffer pain due to back injury at some time. Those injuries account for at about one-third of all lost work and more than one third of all compensations costs”. In addition, there are productivity losses associated with back pain; one study published by the US National Library of Medicine National Institutes of Health found that individuals working with back pain were fully productive on only about 90 percent of all workdays.
Not surprisingly, a good deal of attention has been given to the design of manual materials handling tasks to minimize the risk of injury or pain. That work forms the basis of many regulations, guidelines, and technical standards around the world. While much of that work covers lifting and carrying loads, pushing and pulling loads has also received close attention.
These push and pull guidelines may be quick, “rule of thumb” recommendations, such as those given by the Canadian Centre for Occupational Health and Safety (CCOHS) and the United States Occupational Safety and Health Administration (OSHA); essentially that the maximum cart pushing force should be about 225 newtons (50 pounds-force). However, these guidelines should be used carefully, lest they inadvertently lead to the design of high-risk tasks as a result of inaccurate measurement of forces and/or the variables that affect the specific MMH task.
As CCOHS notes, the recommended maximum force varies depending on several variables such as:
- cart design,
- distance the load is moved,
- frequency with which the load is moved,
- whether the force is starting or maintaining movement,
- gender of the person doing the work, etc.
What is a Safe Level of Push/Pull Force?
A colleague once pointed out that there is no general answer to what is the safe force level for pushing or pulling. Whenever he is asked that question, his response is “You have to do an analysis”.
International MMH Standards Examined
There are many technical guidelines that describe how one goes about doing the measurement and analysis of push and pull forces. Of these, the Liberty Mutual Manual Materials Handling Tables (LMMH) are perhaps the most commonly used guideline for determining the acceptability of push or pull forces. The criterion for an acceptable force used in the LMMH is that at least 75 percent of females can exert the required force under the given conditions. In Europe, the European Committee for Standardization or Comité Européen de Normalisation (CEN) standard EN 1005-3 sets limits for operating machinery, recommending that at least 85 percent of the adult population should be able to exert the required force in workplace settings and 99 percent of the adult population capable of exerting the required force in non-work settings.
Some of these standards have unique features well worth additional attention. For example, the International Organization for Standardization ISO 11228-2, which focuses specifically on push and pull forces, offers a very interesting definition of initial forces: “Initial forces are used to overcome the object’s inertia, when starting or changing the direction of movement”.
Careful reading of that definition clearly suggests that the forces exerted to turn a cart (change direction) while it is in motion are considered as initial forces. There is research which studies exertion patterns in nursing home environments that supports this definition and has shown that the forces used to turn a cart may be equivalent to those used to start it in motion.
Increased Push/Pull Task Frequency Means Much Higher Force Required
The result is that the frequency of exertion of initial forces may be greater than first thought. Using this definition, and assuming that turning forces are approximately equal to starting forces, turning a cart once after starting it in motion would double the frequency with which the initial force is exerted, likely reducing the maximum acceptable force recommended.
Ongoing Cart Maintenance and Consistent Ergonomic Testing Crucial to Maintain Acceptable Force Requirements
The United Kingdom’s Health and Safety Executive’s Risk Assessment of Pushing and Pulling (RAPP) tool calls out the importance of cart maintenance. An initial assessment of a cart may indicate that the required initial and sustained force requirements are within acceptable limits; however, wear and tear to the cart and its casters may result in increased force requirements. Assurance that cart push and pull forces are maintained within safe limits is a critical part of managing an ergonomics program for manual cart handling.
Considering International Industrial Ergonomic Standards for Push/Pull Tasks Ensures Best Chance of Superior MMH Ergonomics Program
In summary, although specific force recommendations may vary somewhat, there is general agreement among technical guidelines and standards regarding the approach to determining acceptable push and pull force levels. Each approach considers similar variables such as frequency of exertion, distance pushed or pulled, etc. All recommend recognizing strength differences between males and females and designing tasks so that a strong majority of women are capable of safely exerting the required forces to push or pull carts. Accommodating both women and men in this way is protective against injury for both sexes, as has been shown by Snook Tables and research.
Whatever tool is used, the design of carts and manual cart handling tasks can reduce the risk of back injuries and positively affect productivity.
Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.
Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.