['Ergonomics']
['Lifting and Back Safety', 'Ergonomics']
04/29/2024
Copyright 2024 J. J. Keller & Associate, Inc. For re-use options please contact copyright@jjkeller.com or call 800-558-5011.
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The revised lifting equation published by the National Institute of Occupational Safety and Health (NIOSH) defines a recommended weight limit (RWL) for specific lifting tasks that most workers could perform over an 8-hour day without increasing risk of developing lower back pain.
The equation is based on a model that provides a unit value for six task variables. They are expressed as coefficients that decrease the load constant, which represents the maximum recommended load weight to be lifted under ideal conditions.
The RWL is defined by the following equation:
| RWL = LC x HM x VM x DM x AM x FM x CM |
Where:
- HM = Horizontal distance of hands from midpoint between the ankles.
- VM = Vertical distance of the hands from the floor.
- DM = Vertical travel distance between the origin and the destination of the lift.
- AM = Angle of asymmetry - angular displacement of the load from the sagittal plane.
- FM = Average frequency rate of lifting measured in lifts/minute.
- The load constant (LC) is the weight of the object lifted.
- The coupling multiplier (CM) is a value derived from a table that relates work duration to lifting frequency. It also takes into account the vertical displacement of the lift.
- The lifting index (LI) provides a relative estimate of the physical stress associated with a manual lifting job. The equation for the LI is:
- LI = load weight (weight of load in lb. or kg) / RWL.
The RWL and the LI can guide ergonomic design in several ways:
- The individual multipliers can be used to identify specific job-related problems. The relative magnitude of each multiplier indicates the relative contribution of each task factor (e.g., horizontal, vertical, frequency, etc.)
- The RWL can be used to guide the redesign of existing manual lifting jobs or to design new manual lifting jobs. For example, if the task variables are fixed, then the maximum weight of the load could be selected so as not to exceed the RWL; if the weight is fixed, then the task variables could be optimized so as not to exceed the RWL.
- The LI can be used to estimate the relative magnitude of physical stress for a task or job. The greater the LI, the smaller the fraction of workers capable of safely sustaining the level of activity. Thus, two or more job designs could be compared.
- The LI can prioritize ergonomic redesign. For example, a series of suspected hazardous jobs could be ranked according to the LI and a control strategy could be developed according to the rank ordering (i.e., jobs with lifting indices above 1.0 or higher would benefit the most from redesign).
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['Ergonomics']
['Lifting and Back Safety', 'Ergonomics']
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