Overview / Introduction
The revised NIOSH equation introduced in 1991, to analyze simple lifting tasks. This equation was designed to assist in the identification of ergonomic solutions for reducing the physical stresses associated with manual lifting. The implementation of this tool in the digital human will help in evaluating lifting tasks, which could prevent lower back injury.
The lifting equation is a tool designed for a two handed, manual lifting operation. The equation is based on biomechanical, physiological and psychophysical data. We are primarily interested in implementing it to analyze a lifting task. The NIOSH equation cannot account for all possible factors leading to injuries, but it is likely to protect most healthy workers performing lifting tasks from lower back injuries.
The equation for calculating the recommended weight limit is based on a multiplicative model that provides a weighting for each six-task variables. The weightings are expressed as coefficients that serve to decrease the load constant, which represents the maximum recommended load weight under ideal condition.
Methods / Current Research
Briefly the NIOSH equation is
RWL = LC x HM x VM x DM x AM x FM x CM (NIOSH Guidelines,1993)
RWL- Recommended Weight Limit
LC - Load Constant
HM- Horizontal Multiplier
VM- Vertical Multiplier
DM - Distance Multiplier
AM- Asymmetric Multiplier
FM- Frequency Multiplier
CM- Coupling Multiplier
RWL-Recommended Weight Limit
The Recommended Weight Limit is defined as the weight of the load, of a particular lifting task, that nearly all healthy workers could perform without an increased risk of developing lifting related lower back pain.
LC- Load Constant
The Load Constant is fixed at 23 kg. This corresponds to the maximum recommended weight under optimal conditions. Selection of the load constant is based on biomechanical and psychophysical criteria (Waters, 1993). This corresponds to a weight that would be acceptable to 90% of the male and 75% of the female population and the disc compression forces resulting from such a lift would be less than 3-4kN.
Horizontal Multiplier - HM
The horizontal multiplier is determined primarily from the horizontal location. The horizontal location of the load is determined as the distance between the midpoint of the line joining avatar’s ankle joints and the center of gravity of the load object in the Z direction.
Vertical Multiplier - VM
Similar to the horizontal multiplier the vertical multiplier is determined from the vertical location. The vertical location of the load is determined from the midpoint of the line joining the avatar’s ankle joints and the center of gravity of the load object in the Y direction
Asymmetry Multiplier - AM
The asymmetry multiplier is determined from the angle between the line joining the center of gravity of the load and the mid –point of the line joining the ankles and the line joining the avatar’s ankles as seen in figure 1.
Figure 1
Frequency Multiplier - FM
Currently the Santos environment does not have a defined protocol to determine the frequency of a lifting action. The frequency multiplier will be determined as part of a task eventually when task planning is implemented. The frequency multiplier is set to 1.
Coupling Multiplier - CM
The coupling multiplier is dependent on the nature of the coupling handles of the object. Couplings have three classification ratings ‘good’, ’fair’ and ‘poor’. The coupling rating will be included in the properties of the object, based on a table. The table is available index the coupling multiplier section in the NIOSH report available in the appendix.
Load Index - LI
The Load Index is the ratio of the objects weight to the recommended weight limit for that current posture.
Using the NIOSH Equation
On enabling the NIOSH equation a Load Index is calculated for the current posture of the avatar based on the recommended weight limit. By translating the object or changing its properties different load index’s can be compared. The avatar’s posture can also be changed to compare different indexes. The NIOSH equation is connected to the Avatar and certain tasks associated with the avatar, such as box lifting.
Figure 2: Interface in Santos™ for utilizing the NIOSH lifting equation
Works Cited
(NIOSH), National Institute of Occupational Safety and Health. "Applications Manual for the Revised NIOSH Lifting Equation." 1994.
Contact Info
https://www.ccad.uiowa.edu/vsr/contact