Understanding the NIOSH lifting equation

Responding is Bob Ernst, transportation management editor, J. J. Keller & Associates Inc., Neenah, WI.

According to the Bureau of Labor Statistics, back injuries are responsible for more lost work time than any other musculoskeletal injury. Moreover, of injuries occurring to the lower back, 3 out of 4 occurred while the employee was lifting. The frequency and economic impact of back injuries and disorders on the workforce are expected to increase over the next several decades as the average age of the workforce increases and medical costs go up.

NIOSH has developed a mathematical model that helps predict the risk of lifting-related injuries. The lifting equation defines a recommended weight limit (RWL) for specific lifting tasks that most workers could perform over an eight-hour day without increasing the risk of developing lower back pain.

The RWL is defined by the following equation:

RWL = LC x HM x VM x DM x AM x FM x CM

The equation is based on a model that provides a unit value for six task variables:

LC = Weight of the object lifted
HM = Horizontal distance of hands from midpoint between the ankles
VM = Vertical distance of the hands from the floor
DM = Vertical travel distance between the origin and the destination of the lift
AM = Angle of asymmetry – angular displacement of the load from the sagittal plane
FM = Average frequency rate of lifting measured in lifts/minute
CM = A value derived from lifting frequency and vertical displacement of the lift

The lifting index (LI) provides a relative estimate of the physical stress associated with a manual lifting job. The equation for the LI is:

LI = Load weight (weight of load in pounds or kilograms)/RWL

The LI can be used to estimate the relative magnitude of physical stress for a task or job. The greater the LI, the smaller the fraction of workers capable of safely sustaining the level of activity. Thus, two or more job designs could be compared.

When it comes to redesigning a task or workstation, you can test several scenarios using differing task variables before making adjustments or spending scarce resources. By varying the individual values in the formula, you can anticipate the effects of job modification prior to implementing any changes. Being able to change the causal factors theoretically allows the employer to address the largest contributing factors to ergonomic injuries more quickly.

The LI can also help prioritize ergonomic redesign. For example, a series of suspected hazardous jobs could be ranked according to the LI and a control strategy could be developed according to the rank ordering (i.e., jobs with lifting indices above 1.0 or higher would benefit the most from redesign).

The equation does have limitations, however. It only addresses two-handed lifts, for instance.

Material handling tasks should be designed to minimize the weight, range of motion and frequency of the activity, and using the equation allows you to get closer to optimal from the beginning.

Editor's note: This article represents the independent views of the author and should not be construed as a National Safety Council endorsement.