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.
The Sequential Lifting Index (SLI) estimates the risk associated with rotating between different tasks involving manual handling of loads with rest patterns.
For NIOSH, tasks with a SLI < 1 are likely to not represent a risk of injury due to the handling of loads for the majority of the health working population.