What is the NIOSH lifting equation?
In 1994, the U.S. National Institute for Occupational Safety and Health published the manual for applying the revised lifting equation, authored by Waters, Putz Anderson, and Garg, which remains the standard reference for the method. Its purpose is to estimate the risk of lower back injury associated with a specific manual lifting task and to guide workstation redesign.
The method does not establish a universal maximum weight. It calculates, for specific conditions of height, distance, rotation, frequency, and grip, how much weight is advisable to lift, and compares that value with the actual weight. The result of this comparison is the lifting index.
In Spain, the INSST technical guide for the assessment and prevention of risks related to manual handling of loads , in its September 2024 edition, includes the equation between the recommended methods for lifting, moving and placing tasks.
What it evaluates and what it does not evaluate
- Evaluate. The dorsolumbar risk associated with a manual lifting task performed with both hands, based on the geometry of the lift, its frequency and the grip.
- It allows you to analyze simple, multiple, and compound lifting tasks.
- It does not evaluate pushing, pulling or transporting loads, which have their own methods.
- It does not take into account the cumulative effect of repetitive lifting over time, nor unforeseen events such as slips, falls, or unexpected overloads.
- It is not a legal limit. Royal Decree 487/1997 and its technical guide establish the mandatory framework in Spain; the equation is an assessment tool within that framework.
When should it not be used
The official documentation precisely defines the cases in which the method is not applicable. It should not be used when there is displacement, pushing, or pulling of the load, when using trolleys or shovels, or when lifting or placing is done with one hand, with unstable objects, for more than eight hours, in a seated or kneeling position, in confined spaces, in unfavorable temperature and humidity conditions, with very rapid and abrupt movements, or with a coefficient of friction between the floor and footwear of less than 0.4.
It is also not designed for moving people or handling cold, hot, or dirty objects. If the load is handled by two or more people, the task cannot be evaluated using this method. Outside of these conditions, other tools should be used, such as ISO 11228-1 or tables of maximum forces for pushing and pulling.
Variables involved
- Load weight. Weight actually handled in the evaluated task.
- Horizontal distance. Separation between the midpoint of the grip and the midpoint between the ankles, projected onto the ground.
- Height of the load. Vertical position of the hands at the start of the lift.
- Vertical displacement. Height difference between the origin and destination of the load.
- Angle of asymmetry. Rotation of the trunk with respect to the sagittal plane during lifting.
- Frequency and duration. Number of lifts per minute and time the task is maintained.
- Grip quality. Ease of holding the load, depending on the shape of the object and the existence of handles.
How to calculate
The recommended weight limit is obtained by multiplying a load constant by seven factors, each of which takes a value between zero and one depending on how far the actual situation deviates from optimal conditions:
LPR = LC · HM · VM · DM · AM · FM · CM , where LC is the load constant, HM the horizontal distance factor, VM the height factor, DM the vertical displacement factor, AM the asymmetry factor, FM the frequency factor and CM the grip factor.
The load constant is the maximum recommended weight for a lift performed from the standard location under optimal conditions, that is, in a sagittal position, occasionally, with a good grip, and with a vertical displacement of less than 25 centimeters. Its value was set at 23 kilograms following biomechanical and physiological criteria. The specific values for each multiplier factor are obtained from the tables and expressions in the official manual, which are not reproduced in this sheet.
The lifting index is the ratio between the weight of the load lifted and the recommended weight limit calculated for those conditions.
How to interpret the lifting index
Official documentation warns that the risk function is not defined, therefore the index does not allow for a precise quantification of the degree of risk. Three indicative zones are considered:
- Index less than 1. Limited risk: most people performing the task should not experience problems.
- Index between 1 and 3. Moderate increase in risk: some people may suffer from illnesses or injuries, so the task should be redesigned or assigned with specific control.
- Index greater than 3. Marked increase in risk: the task is unacceptable from an ergonomic point of view and must be modified.
The index also serves to compare the relative severity of two tasks and prioritize redesign. It should be calculated at both the origin and destination of the survey, because the most unfavorable condition can be at either point.
Limitations and common errors
- Presenting the constant of 23 kilograms as a maximum legal weight. In Spain, the general reference value of the regulatory framework is different and its treatment corresponds to Royal Decree 487/1997 and its technical guide.
- Apply the equation to pushing, pulling, or transporting tasks, for which it is not designed.
- Evaluate only the origin of the uplift and omit the destination.
- Ignoring exclusion conditions, particularly lifting with one hand, squatting, in confined spaces or in an unfavorable thermal environment.
- Using the method without first checking that the floor and footwear provide sufficient friction.
- Confusing an index below 1 with the absence of risk or with exemption from applying organizational and training measures.
Practical example
In a distribution warehouse, order picking staff place nine-kilogram boxes from a pallet on the floor onto a shoulder-height shelf, twisting their torsos and moving at a high frequency for much of the shift.
- Data. Distances, heights, turning angle, frequency and grip quality are recorded at the origin and destination.
- Calculation. The recommended weight limit is low at the origin due to the horizontal distance and the turn, so the lifting index exceeds the value of 3 at that point.
- Measures. The pallet is lifted with a scissor lift, the load is brought closer to the body using a rotating tool that eliminates torsion, and the task is redistributed to reduce frequency.
- Re-evaluation. The new calculation places the index below 1 at both the origin and destination, and the result is documented in the job risk assessment.
Differences with other methods
- Compared to RULA and REBA, which score postures, the NIOSH equation calculates a recommended weight and a specific lifting index.
- Compared to OCRA. OCRA assesses the repetitiveness of the upper limb, not the load lifted.
- Compared to ISO 11228-1, which establishes recommended limits for lifting, lowering, and carrying loads of three kilograms or more, and considers workdays of up to twelve hours, a different scope than that of the equation.
- In contrast to tables of maximum forces, psychophysical methods of pushing and pulling work with forces, not with weights lifted.
Technical and regulatory framework
- Royal Decree 487/1997 . Minimum health and safety provisions relating to the manual handling of loads that entail back and lumbar risks.
- INSST Technical Guide . It develops the royal decree and sets out the equation between the applicable assessment methods, with their exclusion conditions.
- Law 31/1995, articles 15 and 16. Principles of preventive action and obligation to evaluate.
- Royal Decree 39/1997, article 5. Requires evaluation procedures that provide confidence in their outcome.
The equation is not incorporated as a normative text in Spain: it is a recognized technical method whose use is supported by the INSST technical guide.
