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The Snook tables provide a value of the proportion of the population being able to perform some manual task.

Introduction

The Snook tables are due to the work of Drs. Snook and Ciriello at the Liberty Manual Research Institute. They provide a value of the proportion of the population being able to perform a manual task as part of the daily work. These values are based on a research where each subject had to adjust the load in such a way that it could ” work all day with as much effort as possible on the basis of incentives, without experiencing trauma or feeling fatigue or weakness, or being breath or too heated . “

These tables provide values adapted mainly for the lower back, the legs, the shoulders but also the physiological stresses: cardiovascular system, anthropometric data…

With regard to the ” psychologic” aspect of the method of evaluating efforts, its effectiveness could be questioned. Researchers carried out a comparative study, on the uplift movement, between the Niosh equation That references and the tables of Snook. The study of 353 Position studies consisted of identifying the percentage of cases that the 2 methods were able to assess in the right way.

Niosh

Snook Table

Significant risk

73%

40%

Average risk

21%

36%

Low risk

55%

91%

Source: W. S. Marras, L. J. Fine, S. A. Ferguson, T. R. Waters (1999) – The effectiveness of commonly used lifting assessment methods to identify industrial jobs associated with elevated risk of low back disorders.

 

The researchers observed that the Niosh method tends to overestimate risks, while the Snook method tends to be more accurate.

Historical

The ” psychological ” methods of determining efforts date back to the 19th century. The first is Dr. Ernst Heinrich Weber (1795-1878). He worked on the meaning of the touchdown and determined that the weight should evolve by about 2.5% in order to perceive a noticeable difference.

It is in this context that the Liberty Manual Research laboratory wanted to use these methods to apply it to the efforts of uplift, lowering… They conducted a 25-year study. In total, it was 11 experiments from 2 to 3 years each, which were conducted. Each subject of the experiment was measured on 41 anthropometric parameters as well as on physiological parameters (heartbeat, oxygen…), and the tests were conducted on at least 80 hours for each subject, all factors being under control ( weight, distance…).

Use

The method adapts to any type of work environment. Using the appropriate table, the values can be used to evaluate efforts of lifting, lowering, transporting or pushing/pulling.

These same tables allow to take into account the effort, but also the posture, the duration of the effort and the frequency of it.

Method

1-Select the correct table according to the situation

There are a total of 20 different tables. These depend on the type of movement (5 in total: lifting, lowering, transporting, pushing, pulling), distances of these movements and whether the operator is a man or a woman.

The first task is to measure the conditions of the movement to select the table that we are going to use.

2-Identify the value

For lifting, lowering and transporting tables, the weight of the object must be measured.

For the firing and pushing tasks, we will have to measure the value of the required force, move the object (called ” Initial Force “) and then measure the effort to keep the element in motion (called ” sustained Force “).

Other elements are also to be measured, including the frequency of movement, as well as different distances such as the distances to be travelled or the position of the hands (see table below).

Hands/Body Distance

The Distance between the body and the hands when they maintain the object. If the object is moved against the body, then that distance is half the width of the object.

Vertical Distance

Vertical movement of the hands during lifting or lowering of the object.

Transport Distance

Distance travelled during thrust, traction or transport.

Hand height

Height of the operator’s hands on the object being pushed, pulled or transported.

3 – The rules for identifying the value

Some rules apply depending on the specificity of the situation.

Description

Rule

Example

The same task can be done by a woman or a man.

Then we will take the value of the women’s table.

On a Position in 3 * 8, 2 are men, and the last person is a woman.

For the calculation of the permissible effort, the value of the effort corresponding to the women’s table will necessarily be taken.

No proper value in the tables.

We will have to take the approximate value, the most ” critical “.

The frequency of movements that the table offers us is 15 or 30 sec. Our movement is all 22 seconds. The corresponding value will then preferably be taken at 15 sec, which is necessarily more restrictive.

On the same day there may be several types of movements.

We’ll analyze each posture but we will take for each independent analysis the total frequency for all tasks.

A Position requires a lifting task every 2 minutes, a push and a transport every 5 minutes.

The operator performs 4.5 tasks every 5 minutes, or 5 tasks.

The Position will be analyzed by comparing the values given for thrust, transport and uplift at a frequency of one movement per minute.

The elements to be lifted, lowered or transported do not have a handle or gripping is complex.

Reduce the weight by 15%.

4-Analysis

Once the value is identified, the analysis is simple: changes to the Position will have to be made if the value of the percentage of person able to do so is below 75%.

Method limit

The Snook method does not apply in the following cases where the movement:

  • Requires rotation or twisting of the trunk
  • is done with one hand
  • is carried out with obstacles on the floor or a slippery floor.

Moreover, this method being global, it does not take into account individual factors such as age, medical history…

Source

S. H. Snook, V. M. Ciriello (1991) – The Design of manual Handling tasks: Revised tables of maximum acceptable weights and forces.

W. Karwowski, W. S. Marras (1999) – The Occupatinal Ergonomics Handbook

G. Salvendy (2001) – Handbook of industrial engineering

F. Rebelo, M. Soares (2014) – Advances in ergonomics in design, usability and special populations.

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