In 1981, the National Institute for Occupational Safety and Health (NIOSH) of the United States published a first version of the NIOSH equation, and in 1991 it published the second version with new advances.
The aim of applying this method is to achieve the Recommended Weight Limit, which is defined as the maximum weight that is recommended to be lifted in the post conditions to avoid the risk of lumbagos or back problems.
When the worker carries out several tasks involving load-lifting, a composite lifting index must be calculated to estimate the risk of that work.
If a mean of the different indices is calculated, this would give rise to a combination of effects that would not evaluate the real risk. Selecting the highest index would not take the increase in risk afforded by the rest of the tasks into account. NIOSH recommends calculating a composite lifting index (CLI).