If you have ever tried to move furniture or struggled to lift heavy grocery bags out of the trunk of your car, you know that manual lifting puts lots of strain on your body. For workers who manually lift items as part of their job, and for their employers, knowing where the strain comes from can help prevent serious low back injury and its associated missed workdays and lost productivity. Recognizing the importance of lifting-related risk factors for low back injury, investigators at the National Institute for Occupational Safety and Health (NIOSH) developed the NIOSH Lifting Equation in 1981 and then expanded and released it as the Revised NIOSH Lifting Equation in 1993. The revised equation provides a proven formula for calculating a lifting index for designing safe lifting tasks to prevent work-related low back pain.
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NIOSH lifting equation is the Recommended Weight Limit (RWL), which defines the maximum acceptable weight (load) that nearly all healthy employees could lift over the course of an 8 hour shift without increasing the risk of musculoskeletal disorders (MSD) to the lower back. However, its impact on research, industry practices, and regulations has remained unclear.
The result of the NIOSH lifting equation is the recommended weight limit (RWL), a weight that 99% of males and 75% of females should be able to lift safely. Under perfect circumstances, for example, when lift- ing an object with good handles only once in a certain posture, the highest possible RWL is 51 lb.
To measure this impact, NIOSH investigators systematically reviewed published studies on the revised equation. They found that the Revised NIOSH Lifting Equation is one of the most widely used tools for designing manual lifting tasks and that several local, state, and international organizations have adopted it as a standard for safe lifting. Specifically, they reviewed 88 studies focusing on the revised equation:
· Twenty-four studies examined its criteria for defining lifting capacity.
· Twenty-eight studies examined its ability to identify low back pain.
· Twenty-three studies found it helpful in identifying the risk of low back pain among different worker populations.
· Thirteen studies found that the revised equation’s lifting index and composite lifting index helped predict the risk of low back pain. The lifting index calculates the ratio of the load lifted to the recommended weight limit, and the composite lifting index calculates this ratio for multiple lifting tasks performed one after another.
Although these results show that the revised equation has helped improve worker safety and health, more research is necessary. In particular, future studies should focus on how the lifting index and a variety of derived lifting index variables affect low back pain and other musculoskeletal disorders, according to the investigators. In addition, future studies may address incorporating additional lifting tasks and other variables such as lifting speed, degree of torso bending, gender, and age.
The NIOSH Lifting Equation is a tool used by occupational health and safety professionals to assess the manual material handling risks associated with lifting and lowering tasks in the workplace. This equation considers job task variables to determine safe lifting practices and guidelines.
The primary product of the NIOSH lifting equation is the Recommended Weight Limit (RWL), which defines the maximum acceptable weight (load) that nearly all healthy employees could lift over the course of an 8 hour shift without increasing the risk of musculoskeletal disorders (MSD) to the lower back. In addition, a Lifting Index (LI) is calculated to provide a relative estimate of the level of physical stress and MSD risk associated with the manual lifting tasks evaluated.
NIOSH Lifting Equation Outputs
Recommended Weight Limit (RWL): Answers the question… “Is this weight too heavy for the task?”
Lifting Index (LI): Answers the question… “How significant is the risk?”
A Lifting Index value of less than 1.0 indicates a nominal risk to healthy employees. A Lifting Index of 1.0 or more denotes that the task is high risk for some fraction of the population. As the LI increases, the level of low back injury risk increases correspondingly. Therefore, the goal is to design all lifting jobs to accomplish a LI of less than 1.0.
The NIOSH lifting equation always uses a load constant (LC) of 51 pounds, which represents the maximum recommended load weight to be lifted under ideal conditions. From that starting point, the equation uses several task variables expressed as coefficients or multipliers (In the equation, M = multiplier) that serve to decrease the load constant and calculate the RWL for that particular lifting task.
NIOSH Lifting Equation: LC (51) x HM x VM x DM x AM x FM x CM = RWL
Task variables needed to calculate the RWL:
· H = Horizontal location of the object relative to the body
· V = Vertical location of the object relative to the floor
· D = Distance the object is moved vertically
· A = Asymmetry angle or twisting requirement
· F = Frequency and duration of lifting activity
· C = Coupling or quality of the workers grip on the object
Lifting Index (LI): Weight ÷ RWL = LI
Additional task variables needed to calculate the LI:
· Average weight of the objects lifted
· Maximum weight of the objects lifted
The RWL and LI can be used to guide lifting task design in the following ways: 1) The individual multipliers the determine the RWL can be used to identify specific weaknesses in the design. 2) The LI can be used to estimate the relative physical stress and injury risk for a task or job. The higher the LI value, the smaller the percentage of workers capable of safely performing these job demands. Thus, injury risk of two or more job designs could be compared. 3) The LI can also be used to prioritize ergonomic redesign efforts. Jobs can be ranked by LI and a control strategy can be implemented based on a priority order of the jobs or individual lifting tasks.
The Frequency-Independent Recommended Weight Limit (FIRWL) and the Frequency-Independent Lifting Index (FILI) are additional outputs of the NIOSH lifting calculator. The FIRWL is calculated by using a frequency multiplier (FI) of 1.0 along with the other task variable multipliers. This effectively removes frequency as a variable, reflecting a weight limit for a single repetition of that task and allows equal comparison to other single repetition tasks. The Frequency-Independent Lifting Index (FILI) is calculated by dividing the weight lifted by the FIRWL. The FILI can help identify problems with infrequent lifting tasks if it exceeds the value of 1.0.