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You are now beginning Module 6.
In Module 5, you were introduced to biomechanical modeling as a robust assessment of manual material handling (MMH) tasks. In Module 6, you will be introduced to three assessment tools that are developed for use by practitioners in the workplace. The tools will not provide the same level of detail but do provide a screening to the load limit that is recommended based on the task parameters. Each tool is based on research that incorporates injury risk based on biomechanical criteria. The first two tools, known as the Snook Tables and the MITAL tables, respectively, are based on psychophysical approach to determining MMH limits. The third tool is the NIOSH lifting equation which provides a recommend weight limit for the task based on 7 variables describing the lifting task.
By the end of this module, you will be able to:
The psychophysical approach seeks to provide limits and guidelines for manual work that represents "maximum acceptable" work loads that minimize the injury potential of the work.
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Select the accordion below to view a message about using the term "maximum acceptable".
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From Dr. Wayne Albert
I want to bring your caution to the term "maximum acceptable" because when we look at our next assessment tool, the NIOSH Lifting Equation, the terminology will be "recommended weight limit". If we determine "maximum acceptable", we're talking about the top end of things; when we determine a "recommended weight limit", we're probably being a lot more conservative in our approach to the numbers; therefore, we may see some differences between the results of the two tools.
Psychophysics is a branch of psychology that deals with the relationship between a stimulus and a sensation. It seeks to find the relationship between some external stimuli, how an individual reacts to it, and what the sensation is when they react to it.
The psychological magnitude (sensation) grows as a power function of the physical magnitude (stimulus) as shown in this image.
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For our purposes, we will look at how we apply a psychology approach to ergonomic human factors.
Select the accordions below.
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Psychophysical experiments were developed to try to determine what people perceive that they could do over the course of an 8-hour day or a 12-hour day, and so on. The subjects of these experiments may be asked to lift a box of a certain load at a certain rate. They would have to be able to lift the load for a certain period without straining themselves or becoming unusually tired, weakened, overheated, or out of breath to the point where they cannot easily speak.
The participant is essentially being asked to maintain their sensation level of being comfortable while lifting. If the sensation level is being held at a constant, then the researcher could vary the stimuli by increasing or decreasing the weight and/or frequency of lifting.
The main idea behind developing these experiments is to determine the participant's perception of a task. The parameters that we glean from these experiments can be used later to ensure that workers are not overexerting themselves when they are working and whether modifications are required.
Example | Psychophysical Approach to Task Analysis
This image below shows a generalized flowchart for workplace assessment of manual material handling tasks. Take a minute to examine how the information gathered in this approach determines a solution.
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After observing employees working, a decision can be made to record relevant tasks and workplace parameters. This could include:
Using appropriate assessment tools, a determination can be made regarding the percentage of the population capable of completing the task. For example, with knowledge of the strength requirement to complete the MMH task, it can be determined what percentage of the population is capable of the task. Typically, the desire is to accommodate 90% of the population, however there may be cases for 75% to be used.
If an appropriate percentage is deemed unable to complete the task, there is a need to determine engineering or administrative redesign options for meeting this accommodation standard. For example, can the task be redesigned with so that the weight of the load being handled is reduced? Can a device be used to reduce the load weight or eliminate the need to manual handle the load? Can the repetitive component of the task be reduced? Can the task be complete by two people?
If a redesign is possible, it should be implemented, and the task be re-evaluated. If it can't be redesigned, determine if there is a need to have selective higher practices, such as a pre-employment screening. Pre-employment screening is incorporated in fire fighting given the need to be able to complete tasks that have both physical and physiological limitations, including manual material handling. The adoption of Bone Fide Occupational Requirement are, however, strictly regulated as a Human Rights consideration and require strict protocols for their adoption.
The thinking behind the psychophysical experiments is to determine a perception of a task and to make sure we know the relevant work task parameters and then determine whether or not those parameters can be modified to make sure that people are not overtaxed when they're doing a job.
Remember, from an occupational and biomechanics or an ergonomics perspective, we do not want to put people in a position where they are overexerting themselves either biomechanically, cognitively, or physiologically.
There are a several criteria that inform the psychophysical approach:
Remember, the psychophysical approach seeks to provide limits and guidelines for manual work that represents "maximum acceptable" workloads that minimize the injury potential of the work.
Select each accordion below.
To have a good basis of knowledge from which to create the psychophysical experiments, we need to understand how injuries happen. We have already spoken a fair bit about this topic in this course, specifically cumulative exposure.
Play the video below to learn about epidemiological criteria.
Biomechanics is considered an epidemiological criterion. As an ergonomist, you want to strive to accommodate 90% of the population with your designs and recommendations in the workplace. This will not always be possible, of course, but this is what you are striving for.
If you designed for 90% of the male population, you would probably only be designing for the 50th or 60th percentile of women. Now, if there are only males, this makes sense and would be fine. The best strategy, however, is to design for 90% of females as this would incorporate all males as well.
The table below illustrates the differences between men and women in terms of how many newtons of force are required for maximum compression and 75% compression on L4 and L5 with a 30% margin of safety. Regardless of the frequency of lifts, you have a consistent criterion that the compressive load should not exceed the 75% compression mark: 3,900 newtons for men or 2,700 newtons for women.
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As you can see in the graph below, this sets a weight limit for women somewhere around 21 kg and for men about 27 kg. This means that if a maximum weight limit were set for a job task, the sex of the person could be considered based on our knowledge of differences in strength capabilities.
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As discussed previously, the participants of psychophysical experiments are provided with an empty box and given 2 or 3 kg weights. They are asked to add weight to the box, do a few test lifts, and to keep increasing the load until they reach a weight that they think they can handle for an 8-hour period. Once they set their limits, they would lift for 8 hours with the caveats we already discussed, such as being able to speak without losing their breath.
This experience could also be done in reverse wherein the box is filled with heavy weight and then participants are asked to remove the weight until they felt it was light enough to handle. In other words, both a top-down and bottom-up approach can be used to determine the weight that a participant can handle.
This can be done using a number of different scenarios; they can lift from the floor to knee height or from the floor to shoulder height, for instance. They can also perform twisting lifts, such as if someone were to lift a box from the floor to knee height at a 45o angle.
The graph below shows the results of when various psychophysical studies are brought together.
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Frequency (lifts per minute) is on the x-axis and load weight (kg) is on the y-axis. The load weight is the weight that people perceived they could handle over an 8-hour period of time. If you have very low frequencies, 2 lifts per minute, for example, participants felt they could handle a heavier load. The load at 1 lift per minute is close to 40 kg for men and close to 25 kg for women. The load weight drops off quite dramatically as the frequency goes up, as you would expect as the faster you have to lift a weight, the quicker you are going to fatigue.
There are a few findings to remember here.
Select each image below to learn about the level of accuracy of the judgment of participants.
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Participants overestimated how much they could handle when they were lifting more than 6 times a minute.
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Participants were accurate at estimating in ranges of frequency between 2 and 6 lifts per minute.
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Participants underestimate how much they can handle when they had to lift 1 to 2 times per minute.
It is interesting to see how individuals are not great predictors of their own strength, especially when they are predicting for long periods of time. It is possible that the participants may be able to more accurately estimate their abilities if the trials ran for shorter periods of time (15 minutes, for instance).
This brings us to the physiological criteria. Again, the load weights that was predicted or established for males and females are very similar but are different based on the kilocalorie (kcal) approach.
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You can see the load weights drop off dramatically with frequency as we saw on the psychophysical but again, if you look at the numbers very quickly, you can see the numbers are quite different for the load weight that someone could handle certainly based on a physiological approach only.
When the lift rate is about 35% of the maximum voluntary oxygen consumption, it leads to overexertion and undue fatigue. The 30% range of maximum voluntary oxygen consumption was ideal and where people were dealing with about 4 kcal or 3 kcal for males and females respectively. Age would, of course, also have a part to play in these results, but it is not necessarily incorporated in most of these tables.
We have just covered research that has gone into understanding how we set weight limits. Databases have been developed from this research to present maximum acceptable weight and forces.
These psychophysical results have both positive and negative features.
Select each image below to learn about these features.
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The ability to set frequencies and determine what the changes would be with respect to what an individual could handle.
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Psychosocial research is based on the perception of the participants on what loads they can and cannot handle at different frequencies over time. This means that it may not be as accurate as other values that we can calculate directly, such as with physiology and biomechanics. There is a variety of finding between studies.
Let's use an integrative approach to compare data from the three approaches that we just covered: biomechanical, psychophysical, and physiological. The data has been compiled separately for male and female results.
Select each accordion below.
Select the '+' symbols on each curve on the graph below to learn how each approach compares for male data.
Select the '+' symbols on each curve on the graph below to learn how each approach compares for female data.
Remember that the physiological and psychophysical perspectives take fatigue rates into consideration, so they vary on the graph with frequency of lifts whereas the biomechanical perspective does not. To be conservative, it is wise to take the lowest load weight value from the integrated approach (the recommended or maximum weight limit) as the maximum limit.
Ayoub (1992) proposed a conceptualized model of the recommended weight limit if each area was looked at independently.
Take a minute to examine the graph below and how it shows how a trade-off exists between the approaches.
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The following reading is available in Course Reserves.
First, read the description of the reading.
Then, visit the Course Reserves link provided at the bottom of the reading list and read the reading.
Select the link below to open it in a new window.
In the 90's, large databases of people lifting in a variety of scenarios were developed. The tables modify the load lifted, the lifting start and end positions, and the frequency and the angle of twist required to complete the lift.
Two main research groups published extensive look up tables to assess maximum acceptable weight tables for manual material handling tasks.
(This is the reading you just read.)
Snook, S.H., Ciriello, V.M. (1991). The design of manual handling tasks: revised tables of maximum acceptable weights and forces. Ergonomics, 34(9),1197-1213.
Mital, A. (1997). Guide to Manual Materials Handling. CRC Press; 2 edition.
We will examine both of these tables. But first, select the accordion below.
As a reminder, psychophysical experiments consist of:
Asking individuals to perceive
…how much they can lift
…for a certain frequency (twice a minute, for instance)
…for a specific amount of time (typically an 8-hour day)
…based on a set of lifting parameters (such as not straining).
A test could be to lift
…from the floor to a knee-high shelf
…from the floor to a waist-high shelf
…from the floor to a shoulder-high shelf
…from the floor to above the shoulder
…or other permutations.
These are all considered to be the lift envelope - where the lift starts and ends.
The lifts could also be done
…at different angles of rotation.
This was all done to figure out averages for prolonged lifting for a period of one work day. It was a big undertaking to develop these databases because of the variety of permutations; they were done over a series of years. The subjects were industrial workers who did manual materials handling. They received five days of lifting training in order to make sure that they understood proper lifting technique. The oxygen consumption and heart rate of the participants were monitored throughout the testing, among other physical and biomechanical measures.
The maximum weight tables from Snook and Ciriello are numbered 2-10 for a total of 9 tables; table 1 just provided information about the experimental design.
Here is a guide as to the subject matter of each table:
Select each accordion below.
The phenomenal part of this is that tables for both males and females were provided, which was rarer in the 1990s; research was still quite male oriented at the time.
They also, importantly, provided information on weight limit and list several percentages of the population (10%, 25%, 50%, 75%, or 90%) who would be accommodated based on strength value curves and understanding of anthropometrics and physiology. You could argue that this is not needed as the goal is to accommodate the 90th percentile of the population, but having this information is important as it allows us to calculate backwards.
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From Dr. Wayne Albert
If you are reviewing a work task and know what the current weight limit is, you can look at the table and say that only 25% of the population would be able to accomplish that task. This could motivate employers to want to reach 90% accommodation sooner due to the optics and risk of their current low number.
Lifting and lowering are considered separate tasks and are given their own tables.
Lifting
Precision is required to lift an object to a specific location, such as a shelf, bin, or truck
Lowering
Is assisted by gravity
Less precision is needed to place the object in its final destination, such as the ground
Lifting and lowering values accommodate:
As you can appreciate, the tables cannot list every single scenario, so there are more gross categories to try and capture most activities. This means that you will have to do some interpretation and approximation based on what you are trying to assess.
Play the video below to learn about Snook and Ciriello lookup tables for lifting and lowering tasks.
Pushing
Pulling
Pushing and pulling values accommodate:
Play the video below to learn about Snook and Ciriello lookup tables for pushing and pulling tasks.
Carrying
Carrying is often part of a sequence of manual material handling such as:
Carrying values accommodate:
Here you need to know how far you're going to be carrying it. You need to know the vertical height of carry so obviously there are a little different binning because males tend to be taller so they've gone a little bit higher with the vertical heights.
Play the video below to learn about Snook and Ciriello lookup tables for carrying tasks.
The Mital tables are very similar in their style and in their presentation to the Snook tables. They are modified a bit because they take in some biomechanical, physiological, and epidemiological criteria that is not as evident in the Snook tables.
The Mital tables provide values for:
Select the accordion below.
To extend applicability, the multipliers that are provided penalize or enhance the number based on:
The Mital tables provide a bit more of a penalty for some of these other criteria:
In the video presented in the accordion below, you will review information about 3 different types of manual materials handling which all could be incorporated in many different industrial jobs.
Select the accordion below.
Play the video below to learn about these Mital lookup tables.
The following reading is available in Course Reserves.
First, read the description of the reading.
Then, visit the Course Reserves link provided at the bottom of the reading list and read the reading.
Select the link below to open it in a new window.
We will now begin to cover the National Institute of Occupational Safety and Health (NIOSH) Lifting Equation in depth.
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The information in this topic builds on what we reviewed about psychophysical tables. The NIOSH Lifting Equation is another tool that can be used to assess manual materials handling. As the name implies, it focuses only on lifting and does not consider carrying, pushing, or pulling. Please keep this in mind as if carrying, pushing, or pulling are part of the task, you must switch to the appropriate tables.
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Select the accordion below.
The original lifting equation was developed in the early 1980s. It was updated about 10 years later by NIOSH and their research team. This new NIOSH lifting equation was derived from extensive research. Waters, Putz-Anderson, and Garg were the three researchers behind the development of the equation. Waters worked for NIOSH for most of his career. NIOSH was similar to Liberty Mutual, the insurance company, in that they had a research arm that dealt with Snook Tables.
Over 4,000 research papers and studies were reviewed in the development of the NIOSH equation. The resulting equation was based on psychophysical, biomechanical, physiological, and epidemiological findings related to injury risk.
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It should be noted that these various approaches sometimes yield contradictory results. For example, metabolic data suggests that it is more efficient to lift heavier loads less frequently; whereas biomechanical data suggests that the load should be minimized to lift the lighter loads more frequently to reduce stress on muscles and vertebrae. Psychophysics suggests that workers can typically lift heavier loads than those estimated from either biomechanical or physiological studies.
The NIOSH committee chose to deal with these contradictions by using both scientific findings and expert judgment to select the most conservative values.
Select the accordion below.
Take a minute now to review the table below to see how the biomechanical, physiological, and psychophysical disciplines compare. Select the '+' symbols on the image below to learn how each approach has its own design criteria.
Select the accordion below.
An initial assumption of the research was that people have a maximum aerobic capacity. That capacity would be the expenditure of energy that would limit your repetitive lifting.
The research determined a:
There can be some margin of error in the baseline, so the calculation goes to 70% of that baseline for maximum lifts that are predominantly arm work. So, if the lift is above 75 cm, automatically the calculation drops or penalizes the lifting maximum required weight limit to 70%. The reference is:
Then, the calculation goes to 50%, 40% or 33% to account for duration of lifting, as described here:
You will now complete a workplace scenario activity.
Select each accordion below.
The table below shows a method to determine the energy expenditure limits for repetitive lifting, as described in the previous section.
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Take a minute now to determine what the energy expenditure limit would be for the following scenario:
Select the Feedback button below.
The Recommended Weight Limit (RWL) is the output of the NIOSH Lifting equation. The RWL is a recommended value based on the parameters (factors) of the lift, which are used within the equation. There are 7 factors in the lifting equations. Each factor will be discussed as to how it is measured. Based on the measurement, it is assigned a multiplier if a value between 0 and 1 which will be the factor in the lifting equation.
Select the accordion below.
There are several body measurements that are needed when considering a lifting task.
Select the arrow to the right of the presentation below to learn about each measurement.
Now, let's look at the NIOSH Lifting Equation and define its factors.
RWL = LC x HM x VM x DM x AM x FM x CM
Recommended Weight Limit = load constant x horizontal multiplier x vertical multiplier x distance multiplier x asymmetry multiplier x frequency multiplier x coupling multiplier
Select the accordion below.
Select each equation factor listed below for a definition of how it is used to calculate a recommended weight limit.
The recommended weight limit (RWL) is derived from multiplying a load constant (LC), a horizontal multiplier (HM), a vertical multiplier (VM), a distance multiplier (DM), an asymmetry multiplier (AM), a frequency multiplier (FM), and a coupling multiplier (CM). The multipliers provide a weighting factor, between 0 and 1, for each of the six components important in determining the recommended weight limit (RWL).
An analyst must take some steps before the data collection stage of the NIOSH procedure begins.
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They must first analyze the job as a single or multitask manual lifting job.
Questions they might task are:
They might find that significant control is needed at the lift destination if the worker has to:
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If this is the case, both the start position/origin and the end position/destination should be assessed. It is best to err on the side of caution.
Select each accordion below.
Play the video below.
The second way to use the equation is to do the math for each factor yourself, then plug it into the equation. The look up tables were developed using a set of equations that determined the multiplier as a factor of the ideal lifting measurement for each factor.
You can also do the math yourself to find the values of the equation factors. The table below can be used to do these calculations. The metric column has been highlighted for you.
Here is the equation for reference again:
RWL = LC x HM x VM x DM x AM x FM x CM
Here is how each factor in the equation breaks down:
RWL= 23 x (25/H) (1-0.003 x |V-75|) (0.082 + 4.5/D) (1-0.0032 A) (F factor) (C factor)
Review the table below. Select the '+' symbols on the table below to learn more about each factor of the equation.
Select the accordion below.
Play the video below to learn more about how to use the lifting equation. In this video, we will use data from an example scenario.
Select the accordion below.
The last thing to review is the worksheet that we typically use when analysing a job. It keeps you systematic in your approach. As you can see on the image below, it leads you through a 3-step process.
Select the arrow to the right of the presentation below to learn about each step.
What are the strengths and weaknesses of the NIOSH Lifting Equation?
Select each image below to learn about the positives and negatives of using the equation.
/Module5.3/../images/05-module05/NIOSH_Graphics/Strength.png)
The NIOSH Lifting Equation is useful because:
/Module5.3/../images/05-module05/NIOSH_Graphics/Weakness.png)
The NIOSH Lifting Equation does not apply to lifting and lowering:
It also does not apply to:
You will now complete an activity concerning an example dish-washing machine unloading task. This will give you practice completing a NIOSH lifting assessment.
Select each accordion below and complete each step.
A worker manually lifts trays of clean dishes from a conveyor at the end of a dishwashing machine and loads them on a cart as shown in figure below. The trays are filled with assorted dishes and silverware. The job takes between 45 minutes and one hour to complete and the lifting frequency rate averages 5 lifts/minute. Workers usually twist to one side of their body to lift the trays (i.e., asymmetric lift) and then rotate to the other side of their body to lower the trays to the cart in one smooth continuous motion. The maximum amount of asymmetric twist varies between workers and within workers however there is usually equal twist to either side. During the lift the worker may take a step toward the cart. The trays have well designed hand hold cutouts and are made of lightweight materials.
The task variable data are measured and recorded on the job analysis worksheet. At the origin of the lift, horizontal distance H is 50.8 centimeters the vertical distance is 111.8 centimeters and the angle of symmetry is 30°. At the destination of the lift H is 50.8 centimeters V is 17.8 centimeters and A is 30°. The trays normally weigh from 2.3 kilograms to 9.1 kilograms but for this example assume that all the trays weigh 9.1 kilograms.
Select the link below to open the document in a new window. You can save it and edit it from there.
Select the Feedback button below.
To wrap up this module, take a minute now to reflect on any insights you gained.
Select the accordion below.
/Module3/../images/00-shared/avatar-instructor.jpg){kind=avatar}
From Dr. Wayne Albert
The NIOSH Lifting Equation and its factors are:
RWL = LC x HM x VM x DM x AM x FM x CM
Recommended Weight Limit = load constant x horizontal multiplier x vertical multiplier x distance multiplier x asymmetry multiplier x frequency multiplier x coupling multiplier
You can calculate the recommended weight limit (RWL) in either of two ways: using NIOSH lookup tables or using the calculations that originally developed these tables.
You will complete the Manual Material Handling Assignment next.
[Flowchart] Dempsey, P. G. (2006). Psychophysical approach to task analysis. The Occupational Ergonomics Handbook: Fundamental and assessment tools for occupational ergonomics. Boca Raton: CRC Taylor and Francis Group.
[Graph showing cumulative injury rate and job severity index]. Mital, A. (1997). Guide to Manual Materials Handling. CRC Press; 2 edition.
[Graph showing load weight and frequency]. Ayoub, M. M. (1992). Problems and solutions in manual materials handling: The state of the art. Ergonomics, 35(7-8), 713-728.
[Graphs showing load weight and frequency, and recommended weight lifting population limit and frequency]. Ayoub, M. M. (1992). Problems and solutions in manual materials handling: The state of the art. Ergonomics, 35(7-8), 713-728.
[Illustrations of a person lifting a box.] Waters, T.R., Putz-Anderson, V., & Garg, A., (1994). Applications Manual for the Revised NIOSH Lifting Equation. DHHS (NIOSH) Publication No. 94-110, accessed July 30, 2020, https://www.cdc.gov/niosh/docs/94-110/pdfs/94-110.pdf
[Illustrations of a person unloading a dishwasher.] Waters, T.R., Putz-Anderson, V., & Garg, A., (1994). Applications Manual for the Revised NIOSH Lifting Equation. DHHS (NIOSH) Publication No. 94-110, accessed July 30, 2020, https://www.cdc.gov/niosh/docs/94-110/pdfs/94-110.pdf
[Image of table] Waters, T.R., Putz-Anderson, V., & Garg, A., (1994). Applications Manual for the Revised NIOSH Lifting Equation. DHHS (NIOSH) Publication No. 94-110, accessed July 30, 2020, https://www.cdc.gov/niosh/docs/94-110/pdfs/94-110.pdf
[Images of various tables] Waters, T.R., Putz-Anderson, V., & Garg, A., (1994). Applications Manual for the Revised NIOSH Lifting Equation. DHHS (NIOSH) Publication No. 94-110, accessed July 30, 2020, https://www.cdc.gov/niosh/docs/94-110/pdfs/94-110.pdf
[Table and graph showing load weight and frequency]. Ayoub, M. M. (1992). Problems and solutions in manual materials handling: The state of the art. Ergonomics, 35(7-8), 713-728.
