The NIOSH lifting equation in practice

The revised NIOSH lifting equation is the most widely used tool for evaluating two-handed lifting and lowering tasks. Published by the US National Institute for Occupational Safety and Health in 1991 and explained in detail in its 1994 applications manual, it calculates a recommended weight limit (RWL) for a specific task: the weight that nearly all healthy workers could lift over a substantial period, up to eight hours, without an increased risk of developing lifting-related low back pain. Comparing the actual load with the RWL gives a lifting index that shows how risky the task is. More importantly, the equation shows which features of the task are driving the risk, so you know what to change.

The equation is used in both Canada and the US, and it is the basis of parts of ISO 11228-1. This lesson explains each part of the equation, works through a full calculation, shows how to use the results to redesign a task, and sets out the limits of the method.

The equation

The RWL is a load constant multiplied by six multipliers, each between 0 and 1, that reduce the recommended weight as the task moves away from ideal conditions.

The load constant (LC) is 23 kg. It is the maximum recommended weight under ideal conditions: a load held close to the body (25 cm out), at about knuckle height (75 cm), moved only a short vertical distance, directly in front of the body, lifted infrequently, with good handles. Every departure from these conditions lowers the RWL.

The six multipliers

Horizontal multiplier (HM): H is the horizontal distance from the midpoint between the ankles to the hands (the centre of the grip), measured in centimetres. HM equals 25 divided by H. If H is less than 25 cm, use 25 (HM = 1). If H is more than 63 cm, HM = 0, because the load is too far out to be lifted safely. This multiplier reflects the lever effect explained in Lesson 2.

Vertical multiplier (VM): V is the height of the hands above the floor, in centimetres. VM equals 1 minus 0.003 times the absolute value of V minus 75. VM is 1 at 75 cm (knuckle height) and falls for lifts lower or higher. If V is more than 175 cm, VM = 0.

Distance multiplier (DM): D is the vertical travel distance of the hands between the origin and the destination of the lift, in centimetres. DM equals 0.82 plus 4.5 divided by D. If D is less than 25 cm, use 25 (DM = 1). If D is more than 175 cm, DM = 0.

Asymmetric multiplier (AM): A is the angle of asymmetry, in degrees, between the direction the body faces at the start of the lift (the sagittal plane) and the direction of the load. AM equals 1 minus 0.0032 times A. If A is more than 135 degrees, AM = 0. Twisting while lifting reduces the recommended weight because it combines bending and rotation of the spine.

Frequency multiplier (FM): depends on how many lifts are made per minute, how long the lifting continues (up to 1 hour, 1 to 2 hours, or 2 to 8 hours), and whether the hands start below or above 75 cm (30 in). FM is read from a table. Higher frequencies and longer durations give lower multipliers.

Coupling multiplier (CM): depends on how well the hands can grip the load: good (well-designed handles or handholds), fair (less than ideal handles, or a box gripped with fingers flexed under it) or poor (no handles, bulky, slippery or awkward objects). CM is read from a table.

The duration categories refer to the length of continuous lifting followed by recovery time. Up to 1 hour assumes the lifting period is followed by a recovery period (light work) of at least 1.2 times its length; 1 to 2 hours assumes recovery of at least 0.3 times its length; otherwise use the 2 to 8 hour column.

The lifting index

The lifting index (LI) compares the actual load with the RWL:

An LI of 1.0 or less means the task is acceptable for nearly all healthy workers. An LI above 1.0 means the task poses an increased risk of lifting-related low back pain for some workers, and the goal should be to redesign it to bring the LI to 1.0 or below. NIOSH notes that many workers are at elevated risk when the LI exceeds 3.0. Research since the equation was published has found that the risk of low back pain rises as the LI increases, which supports its use as a ranking tool for setting priorities.

For each lift, calculate the RWL at the origin of the lift. If the worker must control the load at the destination, for example placing it precisely on a high shelf or setting it down gently, also calculate the RWL at the destination and use the lower of the two.

A full calculation

Note that a 12 kg (26.5 lb) case, a weight many people would consider light, produced an LI of about 2. The load was not the problem; the task design was. The multipliers show where the losses come from: the frequency and duration (FM 0.65) and the horizontal reach (HM 0.625) are the largest reductions, followed by the low starting height (VM 0.865), the vertical travel (DM 0.884) and the twist (AM 0.904).

Using the results to redesign

The power of the equation lies in what it tells you to change. Look for the smallest multipliers and ask how to raise them:

• Low HM: bring the load closer. Remove obstacles between the worker and the load, use pallets and bins that can be approached from more than one side, use turntables, reduce the size of the load, or place items at the front of shelves.
• Low VM: raise low loads and lower high ones. Use lift tables, pallet positioners, spring-loaded or self-levelling bins, and racks that keep loads between knuckle and shoulder height.
• Low DM: reduce vertical travel by keeping origin and destination at similar heights.
• Low AM: eliminate twisting by arranging the workstation so that origin and destination are in front of the worker, or provide room for the worker to step and turn the feet.
• Low FM: reduce the lift rate, add recovery breaks, rotate workers between lifting and non-lifting tasks, or use mechanical handling for part of the work.
• Low CM: provide handles or hand-hold cut-outs, use containers designed for gripping, or use grips and vacuum lifters.
• If the RWL is still too low, reduce the weight of the load: smaller cases, partial loads or two-person handling as a temporary measure, or eliminate the manual lift with conveyors, hoists or automation.

When the equation does not apply

The revised NIOSH equation is designed for specific conditions. It should not be used, or should be used with great caution and specialist advice, when:

• The lift is one-handed, or the worker is seated or kneeling.
• The worker lifts in a restricted space, such as under a low ceiling, where normal posture is impossible.
• The load is unstable, such as liquids that shift, or the lift is very fast or jerky.
• Lifting is combined with significant carrying (more than a few steps), pushing, pulling or holding, or the worker does other demanding manual work during the shift.
• The floor is slippery, or the working environment is very hot, cold or humid.
• The load is a person or an animal. Handling people requires different methods (Lesson 9).

For jobs with several different lifting tasks, NIOSH provides a composite lifting index (CLI) that combines the tasks, giving more weight to the most demanding ones. Calculating a CLI is beyond this course, but you should know it exists. Free NIOSH tools, including a calculator app, make the arithmetic quick, but you still need careful measurements of each lift.

Common measuring tips

Measure H from the midpoint of the ankles to the centre of the hands, not from the front of the body. Measure V at the hands, not at the bottom of the box. Observe and count the actual frequency over a representative period, including peaks. Use the actual weight of the heaviest loads commonly handled, not the average. For a range of loads or heights, analyse the worst common case and consider analysing several cases. Record photographs and measurements so that the analysis can be checked and repeated after changes.