The Liberty Mutual Workplace Safety Index identifies the leading causes of the most disabling U.S. workplace injuries based on workers’ compensation claims and cost. The 2010 Index reported the 10 most common injuries and illnesses that accounted for over $45 billion in direct workers’ comp costs. Overexertion injuries, defined as caused by excessive lifting, pushing, holding, pulling or throwing, represented the largest proportion of injuries and cost American companies over $1 billion a month.
Over the last 20 years or so, companies have focused on implementing ergonomics improvement processes with the most successful companies focusing on engineering controls. These efforts have paid off as the cost (and incidence) of repetitive motion injuries (RMI) have decreased 44 percent in the last 11 years. Unfortunately, the cost of overexertion injuries has decreased only 5 percent and actually is on the rise, with an increase of about $700 million a year.
Why the difference in impact between RMIs and overexertion injuries? While it is true that applying ergonomics principles in the workplace will solve both RMIs and overexertion injuries, the investment required often is quite different.
The risk factors for RMIs and overexertion injuries are similar; both result from job tasks that exceed the limits of human capability for posture, force and frequency. However, for RMIs, the risk level can be reduced by making changes to the geometry of the workstation and equipment (bench heights, reach distances or tool modifications).
Unfortunately, the primary problem contributing to many overexertion injuries is the force required to complete the task. Here on earth, there are only two ways to address this issue: provide a mechanical device to aid the person in handling the task or reduce the force required. Both options are more difficult than simple workstation geometry problems and usually more costly.
Our challenge is clear: We need to know where to spend our time and resources.
Analyzing the problem
Since the solution to most overexertion issues generally requires financial resources, which often are limited, the challenge is to invest in the right solutions to the right problems. Fortunately, there are tools to help analyze the issues, determine priorities and determine effective countermeasures. The two most widely used methods are the NIOSH Lifting Equation, to analyze lifting and lowering, and the Liberty Mutual Push/Pull Tables, to analyze pushing and pulling. Both are available in traditional paper versions, but a quick search within a mobile app store will give you access to auto-calculators of equations.
More advanced methods to assess material handling tasks use 3-D modeling within current digital engineering software. There are ergonomics- and human factors-centered CAD packages as well; these methods provide a lot more information regarding burden. If you are ready to take your ergonomics process beyond counter-measuring risk in your existing operations, these advanced methods are great. But a significant investment in training is required. To complete this type of analysis, the operator must have substantial educational background and experience in ergonomics/human factors.
The NIOSH Lifting Equation
Twenty or so years ago, the National Institute for Occupational Safety and Health (NIOSH) commissioned a panel of research experts to develop a model to determine if a lifting task was “safe” for the majority of the work force. This resulted in the development of the NIOSH Lifting Equation, and it has been widely used by safety professionals since its creation.
The primary NIOSH Lifting Equation outputs the recommended weight limit (RWL) for a particular task, which is defined as the weight that the majority of the work force is capable of lifting safely, given their specific job geometry, frequency and duration of lifting. It is the “not-to-exceed” number for a particular two-handed lifting task, or the highest weight at which the working population (99 percent of men and 75 percent of women) safely can perform the lift. The highest possible RWL is 51 pounds, given the ideal lifting scenario (working in the comfort zone with no twisting, load in close to the body, lifting once every 5 minutes for 1 hour out of the day).
The equation considers six factors – or multipliers – to determine the RWL: horizontal distance, vertical location, travel distance, angle of asymmetry, coupling and frequency, in addition to the amount of total time spent lifting (duration). Other criteria also must be met, but the equation is valid for most two-handed lifting scenarios encountered in industry. The NIOSH Lifting Equation can be used to:
➤ Evaluate existing and proposed lifting conditions to identify job hazards and indicate when people may be working outside of their capabilities.
➤ Prioritize hazardous jobs for ergonomic intervention. Existing lifting conditions that are not acceptable can be ranked and ordered by degree of burden based on the Lifting Index. 1
➤ Highlight opportunities for reducing lifting hazards. Based on the multipliers, we can determine the easiest change with the highest impact. Understanding the multipliers can help you make decisions and answer that driving question: “How do I get the biggest bang for my buck?”
1Lifting Index is a ratio of what is actually being lifted to what the equation says you can lift (RWL). An LI greater than 1.0 means you are lifting more than what NIOSH recommends for your task. The higher the Lifting Index, the higher the likelihood of injury. Ideally, LI should be below 1.0.
PUSH/PULL/Carry Tables
To evaluate tasks that involve pushing, pulling or carrying, the most commonly used tool is the Liberty Mutual Push/Pull/Carry Tables, which are based on the psychophysical research of Stover Snook and Vincent Ciriello and funded by Liberty Mutual. By studying the relationship between stimulus and sensation (Stevens, 1974), the psychophysical approach assumes humans are able to perceive the strain on their bodies caused by a given work task and make judgments about perceived effort. Snook and Ciriello began controlled experimental research in the 1970s; they used psychophysical evaluation to develop a database for evaluating the design of manual handling tasks in terms of maximum acceptable weights and forces.
As with every type of analysis and experimentation, there are advantages and disadvantages. However, researchers were able to identify a significant number of data points to allow users to determine a maximum acceptable initial and sustained force for a given task. As with the NIOSH Lifting Equation, a few variables must be known in order to determine the acceptability of a specific push, pull or carry scenario, including vertical height of the hands, the distance the object is carried and the frequency of the push/pull/carry scenario over time. In general, aim to design manual tasks for greater than 75 percent of the female working population to provide the best protection from manual handling injuries.
These tables and the NIOSH Lifting Equation will help you design manual handling jobs so the largest percentage of your facility’s populations can complete a task with minimal risk of injury. It is not appropriate to use these tools to determine whether male or female workers can perform certain jobs and then place them accordingly.
Addressing the Problem
Once the problem is identified and understood, it must be addressed. Options range from no- and low-cost solutions to higher-cost solutions. While there is no right or wrong solution, seek out those that are the most practical and cost-effective, and that offer the highest degree of control possible.
Lifting countermeasures – Clearly, you will not be very popular if your only solution to lifting problems is to make everything lighter. Good solutions for lifting tasks must address how effectively the worker can access the load being lifted. Focus on the vertical location, horizontal location, travel distance or the amount of twisting during the lift.
➤ Provide flow racks, rather than standard shelves, to automatically move materials closer to the point of retrieval.
➤ Use diverter bars to push material to the edge of a conveyor to reduce the reach when grasping material.
➤ Improve layout to provide walking clearances around part pallets and prevent extended reaching across the width of pallets.
➤ Utilize lift tables (spring-loaded or powered) to bring working heights to comfortable levels, but keep cost in mind. Sometimes, simply fabricating a small stand to raise up pallets is as effective as purchasing a height adjustable or rotating lift table.
➤ Move a product an extra foot away from the operator. This will promote a more neutral posture, as it forces the worker to take an extra “positioning step” to retrieve the product.
➤ More costly engineering controls, such as mechanical lifting systems, also may be an option. These systems take time and resources to install properly. Ensure that you have full operator and engineering involvement to consider the tasks, work flow and burden. When designed and installed appropriately, these types of systems reduce worker burden and do not add task time.
If you cannot change the physical nature of a job, consider reducing the frequency or duration of a lifting task. For example, implement a job rotation schedule for employees who are exposed to high-frequency or long-duration tasks during an 8-hour shift; the rotation could reduce their exposure by 2 to 4 hours. However, it is important to understand the ergonomic risk associated with each operation and tasks to help create and manage the job rotation schedule effectively.
Pushing and pulling countermeasures – Transferring carts typically involves pushing and pulling forces that approach acceptable limits. In addition to total weight of materials, consider the cart design (handle location, caster or wheel size and material) and the condition of the floor surface.
Selecting casters of an optimal design may seem costly, but they are efficient and have a long service life. When choosing a caster, consider:
➤ Diameter – At least 8 inches.
➤ Wheel swivel – The swivel should be in the front for pulled carts and in the back for pushed carts.
➤ Composition of the floor surface – It is easier to push loads on flat, hard surfaces.
➤ Wheel tread – A crowned tread is easier to move than a flat, wide tread.
In industries where there are hundreds of material movers for conveyance, it is not easy (or cheap) to replace casters and improve carts; the price per caster can range from $50 to over $100.
Other alternatives include powered cart movers, which attach to the cart and eliminate the pain and strain of manually moving heavy loads. They are easy to use, efficient and more economical than improving all carts in a facility.
Clearly, it is easier to improve the efficiency of carts and casters than it is to retrofit an unleveled or unmaintained floor. Sometimes, you have to dance on the floor you’ve got. A hard, dry surface certainly is best when pushing and pulling loads. But not all floors are perfect, and an unmaintained or cracked floor significantly can increase push/pull force requirements.
Obviously, workers should not be required to push carts over a barrier, a curb or stairs. Install ramps when there is a gradient change, avoiding slopes greater than 5 percent. For inclines of more than 30 feet, install a flat stage to allow for worker rest.
Points to Remember
An effective and sustainable strategy for reducing overexertion injuries requires a continuous improvement mindset and process. Take the time to analyze your material handling issues, measure risk, prioritize tasks and implement cost-effective countermeasures.
An effective solution cannot be implemented without first understanding the problem. Effectively quantifying risk helps you define your problem and a problem well defined is half solved. When searching for solutions, never underestimate the power of the simplest countermeasure. It’s true that we all want to win with a home run. But, we may have to start with a few base hits. Use the tools described in this article to find those opportunities and to score your own home run.
James Mallon, CPE, is a vice president with Humantech, which delivers practical solutions that impact safety, quality and productivity. Christy Lotz is a managing consultant and ergonomics engineer for Humantech, overseeing large-scale ergonomic initiatives in the manufacturing and health care industries and helping organizations build internal ergonomic expertise. For additional information, visit http://www.humantech.com or call 734-663-6707. Mallon can be contacted directly at [email protected].