THE NIOSH EQUATION AND ITS APPLICATION
The NIOSH equation is an ergonomic assessment method for determining the maximum permissible lifting load. By taking a number of environmental and postural factors into account, this method can be used to determine the Recommended Weight Limit (RWL) in order to reduce the risk of musculoskeletal disorders (MSDs).
Let’s explore the origins, principles and calculation of this equation!
NIOSH : Origins & Use
The origins
In the 1980s, work-related low back pain reached alarming proportions, becoming one of the main causes of work stoppage. What’s more, the increase in reported cases of low back pain far outstripped the overall growth in work stoppages, underlining the urgency of the problem.
These painful, multifactorial back injuries led NIOSH (National Institute for Occupational Safety and Health) to develop a specific assessment tool: the NIOSH equation. Developed in 1981 and revised in 1991, this model is used to analyse the lifting of loads with both hands as a factor in lumbar risk.
From then on, the objective of the NIOSH equation was simple: to determine the maximum load that an operator can carry (with both hands and in a standing position) without risk of MSD.
Key facts about low back pain :
Work stoppages
Occupational diseases
In the 2002-2003 ten-year health survey, more than half of French people aged between 30 and 54 stated that they had suffered from low back pain on at least one day during the previous 12 months. Nearly half of all work stoppages due to low back pain occurred while carrying loads.
Want to find out more about low back pain? Read our article dedicated to the condition: https://moovency.com/en/focus-on-a-msd-3-low-back-pain/
The principle
Three criteria were used to develop the equation:
- Biomechanical: Maximum permissible disc compression*
- Physiological: Maximum permissible energy expenditure
- Psychophysical: Recommended weight limit (/RWL).
*Disc compression refers to the pressure exerted on the intervertebral discs (located between the vertebrae and the spinal column).
It is used to determine the RWL a s a function of various factors linked to the work environment in which the study is being carried out, and produces the Lifting Index (LI) with the following risk levels:

NIOSH : Calculation
The recommended weight limit (RWL) and the lift index (LI) are calculated from an equation comprising 7 factors.
FP x FH x FV x FD x FA x FF x FI = CMA
- FP: weight factor
- FH: horizontal factor
- FV: vertical factor
- FD: distance factor
- FA: Asymmetry factor
- FF: frequency factor
- FI: Interface factor
WEIGHT FACTOR: The Weight factor is constant and set at 23 kilos. Note that there is no difference in weight between women and men: according to NIOSH experts, 23 kilos corresponds to the maximum weight that 75% of women and 99% of men can lift in the reference posture (standing, with both hands, back straight, 75 centimetres from the ground and against the body).
HORIZONTAL FACTOR: The Horizontal factor corresponds to the horizontal distance (H, in cm) between the middle of the ankles and the projection of the hands on the ground when holding the object. This distance H can vary between 25 and 75 centimetres.
VERTICAL FACTOR: Conversely, the Vertical factor takes into account the vertical distance (V, in cm) of the hands in relation to the ground at the start of gripping the load. The distance V is between 0 (load from ground level) and 175 centimetres.
DISPLACEMENT FACTOR: The Displacement factor corresponds to the vertical displacement (D, in cm) of an object between the start and the end of the load handling. Here we refer to the ‘lifting’ of the object.
ASYMMETRY FACTOR: The Asymmetry factor represents the angular measurement of the distance the object is moved in relation to the front of the worker’s body.
FREQUENCY FACTOR: The frequency F is obtained from three pieces of information: the frequency of the lift, the operator’s posture and the continuous duration of the handling.
INTERFACE FACTOR: Finally, the Interface factor corresponds to the hand-object interface: this influences the CMA, depending on the shape of the object being handled and whether or not there are handles on the object. This will influence the gripping effort (= ability to grasp an object) which will then be classified as ‘good’, ‘acceptable’ or ‘mediocre’.

Once all these factors are known and calculated, we obtain the CMA. From this we can obtain the lifting index (LI), which can be used to interpret the results in terms of the risk of low back pain. The calculation is as follows:
LI = Weight of object (kg) / CMA (kg)
Interpretation of the results: If the index is less than 1, the manual handling task is considered to be low lumbar risk. If it is greater than 1, the task should be considered as potentially involving a risk to the lower back, especially if it is greater than 3. In this case, action is required to improve the situation.
Limits to the application of the NIOSH equation
The application of the NIOSH equation is limited to the assessment of lifting, which is only one of the many occupational risk factors associated with low back pain. As mentioned, low back pain is a multifactorial disease and other factors must be taken into account.
The NIOSH lifting equation does not apply in all cases. It cannot be applied when the person is lifting or lowering a load:
- With one hand ;
- Sitting or kneeling;
- In a confined work area;
- For more than 8 hours;
- Which is unstable;
- By carrying, pushing or pulling ;
- With wheelbarrows or shovels;
- Without ensuring a good footing on the ground (unreasonable foot/ground coupling);
- In an unfavourable environment: temperature outside the range 19-26°C or with humidity outside the range 35-50%.
KIMEA, a comprehensive scoring grid
Several MSD risk indicators are integrated into Kimea Cloud, enabling automated calculation of physical risk scores. The grids used are the ergonomics reference grids: RULA, REBA, NFX 35-109 and NIOSH.
We have set up a ‘Load Handling’ project module that enables us to assess the load handled and the daily tonnage by operators to determine whether it complies with load-carrying assessment standards.
Our load handling module integrates the NIOSH equation into the calculation and allows the various factors to be entered into the analysis of the handling task.

Why have we brought together all these analysis methods?
Simply put, to offer you a more precise and tailored result for all types of workstations. We also wanted to measure the exposure time at each risk level.
Conclusion
The revisited NIOSH equation is a valuable tool for assessing the risks associated with handling loads, taking into account various environmental and postural factors. It can be used to obtain the maximum permissible load (MPL) and to interpret the associated risks through the load lifting index (LLI).
However, the limits of its application and its exclusive approach to the lifting of loads underline the need to integrate other methods and criteria for a complete assessment of occupational risks, particularly those linked to MSDs.
Thanks to KIMEA Cloud, which integrates the NIOSH equation with other ergonomic analysis grids, it is now possible to obtain a global and accurate assessment of physical risks. By combining several approaches, it becomes easier to assess workstations and effectively reduce the risk of low back pain, so you can keep your employees in good health!
Sources :
Canadian Centre for Occupational Health and Safety (CCOHS) – NIOSH Revised Lifting Equation: https://www.cchst.ca/oshanswers/ergonomics/niosh/assessing.html
CAPSE – Maximum allowable lifting load. The revised NIOSH equation : https://osirisse.capse.fr/bdd/clients/docs/Documents/Notes_Guides/N_01_04_1995.pdf
INRS – Statistical low back pain : https://www.inrs.fr/risques/lombalgies/statistique.html
Insee – Decennial Health Survey 2002-2003 (2024) : https://www.insee.fr/fr/metadonnees/source/operation/s1381/processus-statistique