Vertical manipulations are defined precisely according to the NIOSH1 (National Institute for Occupational Safety and Health) equation. After a creation in 1981, it was revised in 1991 to include more parameters (especially the factor asymmetry). Very complete, it takes into account biomechanical, physiological and psychophysics criteria. It is used to determine the maximum permissible load supported by 99% of men and 75% of women during the bimanual handling tasks of lifting or removing loads without moving the operator. It applies to assess the Risks of MSD Back level.
Therefore, it does not use for handling2 :
- has one hand
- For more than 8hr work
- Sitting or kneeling
- With unstable objects
- At the same time as lifting, pushing, pulling or walking
- With outside help
- Very fast
- With a bad ground/foot couple (slippery floor…)
- Handling under specific conditions of temperature or humidity
The calculation method
The maximum load (CMA) is calculated according to the equation an equation that depends on 7 factors:
CMA (KG) = FP * FH * FV * FD * FF * FA * FI
We detail in the paragraphs following the details of the factors.
FP: The weight factor
The weight factor is constant and fixed at 23Kg. According to experts, this means that 23Kg is the highest load that 99% of men and 75% of women can rise in the reference posture: upright, straight back charge held at 2 hand at 75 cm from the ground and against the body.
FH: Horizontal Factor
FH = 25/H , where H is the distance in cm that separates the middle of the virtual segment that connects the ankles to the projection of the hands on the ground at the beginning of the gesture. In practice, H Usually varies from 25 to 75cm.
- FH = 1: object is against the body. Plus H Increases and more FH decreases. In other words, the more an object is removed from the body on the horizontal plane, the more difficult it becomes.
- FH < 1: carry out actions to bring the load closer to the person’s body by moving the barriers or reducing the size of the load.
FV: Vertical Factor
FV = 1 – 0.003 * (V – 75)
V is the distance in cm that separates the soil from the hands at the beginning of the charge socket. In practice, for most operators, V is between 0 (ground level load) and 175 cm.
- FV = 1 for an object located 75 cm from the ground. This distance is the optimum height of an object in the vertical plane. FV decreases as the hands move upward or downwards from this 75 cm mark value.
- FV < 1: reduce the position of the load drop. Make sure to have the load farther from the ground and eradicate lifting above the shoulders.
FD: Displacement Factor
FD = 0.82 + (4.5/VD)
VD represents the vertical movement in cm of an object between the beginning and the end of the handling, usually in lifting. For VD less than 25cm, FD = 1.
Thus, the longer a move, the more FD is small and the more the CMA decreases. Minimize the distance to be carried out.
FA: Asymmetry Factor
FA = 1 – (0.0032 * A)
A is the angle between the sagittal plane and the degree asymmetry plane. The plane of asymmetry is defined as a vertical plane that passes through the middle of the right that connects both hands and the axis of the body. A is measured at the beginning or at the end of the movement.
In the case of FA < 1, modify the origin and/or destination of the object to minimize the torsion to be carried out.
FF: Frequency factor
FF is obtained by integrating 3 information that is the frequency of uplift, the posture of the subject and the continuous duration of the handling. It reads in a specific table3.
It is defined as the average number of lifts per minute. Depending on the frequency, FF ranges from 1 for a frequency less than 0.2 per minute to 0 for 9 to 15 movements per minute. This frequency is considered unacceptable.
The posture is evaluated by V defined previously. The operator is considered standing if v>75cm and standing bent if v<75cm.
This is the time that the lifting is done. 3 situations are retained:
- handling lasts less than an hour: It consists of repetitive and continuous handling followed by a recovery period representing at least 120% of the handling phase.
- handling lasts less than 2 hours: It consists of repetitive and continuous handling, followed by a recovery period representing 30% of the handling phase.
- handling lasts up to 8 hours: It consists of repetitive and continuous handling, with no other breaks than the usual ones.
If FF is less than 1, reduce the frequency of the uplift, reduce the duration of the movement or lengthen the rest periods.
FI: Interface Factor
The main/object interface influences the CMA. Thus, the Shape of the Handled object, the presence or absence of handles will change FI. This value is identified in a specific table.
The raise index
Once the previous steps are completed, the CMA is obtained which represents the maximum acceptable weight to be lifted under the defined conditions. It is therefore possible to calculate the raise index, which will allow to give direction to the actions to be carried out.
This index is calculated using the following formula:
LI = element/CMA weight
Five cases show up to us4. If:
- LI ≤ 1: The load is therefore acceptable for the majority of people.
- 1.1 < LI < 1.5: The load must be evaluated and changes must be put in place but without urgency.
- 1.5 < LI < 2.9: modifications must be carried out to reduce the risk of MSD.
- 3 ≤ LI: The load presents a clear risk to the majority of us, and immediate action must be taken.
Multiple Job cases
We can see that the measurement and analysis of the data may only be done in the condition that we are with a repetitive and identical task. If we are in the case of repetitive but different tasks, the procedure must be adapted as follows5.
1. Calculate the STRWL for each task
Calculate the CMA by frequency (STRWL) for each task. This is exactly the same calculation as in the case of a single spot. The STRWL for a task reflects the global requests for this task, assuming that it was the only task being performed.
2. Calculate the FIRWL for each task
Calculate the Macs (FIRWL) for each task by using the respective task variables and placing the frequency multiplier at a value of 1.0. The FIRWL for each task reflects the force of compression and the force required to the muscles for a simple repetition of this task.
3. Calculate the IPPs for each task
Calculate the lifting index (IPPS) for each task by dividing the weight of the average load for this task by the respective STRWL. The average weight is used to calculate the IPPs because the average weight provides a better representation of metabolic demands, which are distributed through the tasks, rather than the dependant on different tasks. In cases where the FILI exceeds the IPPs for any task, the maximum weights may represent a significant problem and careful evaluation is necessary.
4. Calculate the FILI for each task
Calculate the Frequency Lift index (FILI) for each task by dividing the maximum load weight for this task by its FIRWL. The maximum weight is used to calculate the FILI because the maximum weight determines the maximum biomechanical loads to which the body will be exposed, irrespective of the frequency of the occurrence.
5. Calculate the CLI for work
Finally, the compound lifting index (CLI) is determined for the overall work. The CLI is calculated as follows:
- Tasks are numbered in descending order of their IPPs value. The tasks are numbered in this way so that the more difficult tasks are considered first.
- The CLI for the work is then calculated according to the following formula:
CLI Global = IPPs (largest) + FILI2* (1/FF1.2 – 1/FF1) + FILI3* (1/FF1, 2, 3 -1/FF1.2) + FILI4* (1/FF1, 2, 3, 4 -1/FF1, 2.3)…
The CLI is interpreted strictly in the same way as the LI.
1-M. Sohail, P. Downey (1995)-Maximum permissible loading load
2 – G. LaPorte, S. Sellers (2010) – Common Tools for assessing ergonomics risk factors
3-M. Sohail, P. Downey (1995)-Maximum permissible loading load
4 – T. Ellis (2011) – Ergonomics: Assessments and evaluations for job improvements
5-T. R. Waters, V Putz-Anderson, A. Garg (1994) – Application Manual for the revised Niosh facelift equation
J. Machaire (2001) – Evaluation and prevention of lumbar risks: Classification of methods