There is word that Liberty Mutual (LM) is going to publish a new version of its venerable Manual Materials Handling Tables. In the words of the insurance company, they are used “to perform ergonomic assessments of lifting, lowering, pushing, pulling, and carrying tasks with the primary goal of supporting ergonomic design interventions.” The company isn’t ready to say when the new version will be available and what users can expect. In a search for clues, the Ergonomics Report® talked to the scientist regarded as the father of the program, Stover Snook, Ph.D., CPE.
LM, the largest carrier of workers compensation insurance in the United States, hired Dr. Snook in 1962 as director of a project aimed at reducing and preventing workplace injuries. At the company’s research center in Massachusetts, now the Liberty Mutual Research Institute for Safety, he continued the research started by Willem S. Frederik, M.D.
The data yielded the first version of the tables. Authored by Dr. Snook, it was published in a 1978 issue of Ergonomics. The scientist authored and published an expanded set with Vincent Ciriello, Sc.D., CPE, in the same journal in 1991. Dr. Snook has since retired. Dr. Ciriello, who joined Liberty Mutual in 1969, is now the principal researcher on the project. Both scientists, who are also lecturers at the Harvard School of Public Health-Department of Environmental Health, have won international recognition for their decades-long contribution to ergonomics.
The development of the first two versions of the tables reveals a painstaking process, a clue that the third could still be years away.
“The tables are based on 11 different experiments that we ran at Liberty Mutual … over a period of 25 years,” Dr. Snook explained. The data were integrated after seven of the experiments. He recalled that the paper covering the first set of tables, “The Design of Manual Handling Tasks,” was the Ergonomics Society’s lecture for 1978, and that he delivered it in England at the Cranfield Institute of Technology.
Afterwards, he said, the team ran four more studies, resulting in the current psychophysical tables published in 1991. Called, “The Design of Manual Handling tasks: Revised Tables of Maximum Acceptable Weights and Forces,” the 1991 version combined all 11 studies.
“Those were major experiments. We spent a lot of time with the subjects. It just wasn’t a one-day thing. They would come in two or three days a week over a 10-week period.” He explained that the details varied, depending on the study. “We ran both males and females. We investigated age differences, and we looked at the different task variables – lifting vs.lowering, pushing vs. pulling, different heights, frequencies, distances, etc. That’s why it took 25 years to do it, because there are so many variables to these tasks.”
No one had ever done this research before, he explained. “It was more of an observational study in which we wanted to find out how lifting did differ from lowering and how males did differ from females. … It’s just a matter of actually coming up with the numbers.”
He continued the physiology direction of his predecessor, investigating lifting tasks primarily because that was a task associated with low back pain. The disorder is a particular concern of worker’s compensation companies like Liberty Mutual, he added.
The Move to Psychophysics
Dr. Snook said it soon became apparent to him that the issue was more complicated than he had supposed. “I was looking for the maximum number of lifts that a person could safely perform, but there is more to lifting than frequency.” He realized that the weight of the object is also important, he said, “but the work physiology measurements were limited in what they could tell us about weight. Sometimes the weight is only lifted once a day, and the work physiology measurements are not appropriate for an intermittent task like that.”
The scientist turned to psychophysics, in part because it had been successful in developing the decibel scale for noise. “I thought this could be a good method to investigate manual handling. … It had never been done before, so we were pleased with the outcome. It was able to quantify just what people would do, and what they wouldn’t do – what were the maximum acceptable weights and forces, and so forth.”
Thirty years after the groundbreaking version, the tables are available on the LM web site. They include both the male and female population percentages capable of performing manual material handling tasks without over exertion, rather than maximum acceptable weights and forces. And they are available with an added resource – a manual material handling analysis tool for ergonomic assessments. The analysis tool, created from a computer program called CompuTask, a joint effort between Dr. Ciriello and computer programmer Carl Harris, was based on the data from the Snook/Ciriello paper. CompuTask calculates the percent of the population that found the task acceptable.
Pressed to speculate about the potential shape of changes in the new version of the tables, Dr. Snook recalled that when he was at LM the goal was always “to improve the tables and make them more accurate by increasing the sample size, for example, and investigating different combinations of variables in manual handling tasks.”
He referred to a paper by Dr. Ciriello et al in the April 2008 issue of Ergonomics. It questioned, according to the abstract, whether the present generation of workers have the same psychophysically determined weights and forces as those reflected in the guidelines. The trials of present day industrial workers focused on whether secular changes had occurred. Dr. Snook explained that these refer to changes in height, weight, etc. from generation to generation. “For example, there were differences between recruits in the army from World War I to World War II, and these are what we call generational changes or secular changes. The soldiers in WWII were taller and heavier than they were in WWI. These are public health issues – better nutrition, better sanitation and so forth.”
There are indications that the workforce is heavier as a result of the obesity epidemic, he noted, and the question is whether this affects their ability to lift and lower and push and pull and carry. The LM researchers found differences, Dr. Snook noted, but they say they don’t have enough data yet to conclude that these are secular changes.
“They are always trying to make the tables more accurate, and more current according to the types of people who are doing these types of task. … I suppose that’s what they [Dr. Ciriello and his team] are doing – trying to find out if indeed these are significant changes that are occurring now.”
He added that the Ciriello paper identified a need for more data confirmed by other investigators at other locations with a greater subject sample.
The need Dr. Ciriello expressed reinforces the clue that the new version is still some time away, and that users can expect refinements and expanded data rather than radical change.
There is no question that the LM tables are distinctive. Only the news ages as fast as research data, yet this tool has been in use for 30 years. The present version is 17 years old.
Asked to explain why they are distinctive, Dr. Snook pointed to their value to ergonomists, insurance companies, manufacturers, therapists and others. They provide data “to evaluate or to analyze a task in terms of whether people can actually do this without increasing the risk of getting low back pain,” he explained, and they have been used in international and national guidelines and standards. “They are valuable because they give somebody the tools [with] which to evaluate a manual handling task and decide whether they should change it and provide some sort of aid.” He added that the tables have also been used to help decide when people should return to work.
Dr. Snook also doesn’t see the present version as a tool that has outlived its usefulness. “I believe Liberty Mutual is trying to … make them more accurate and current. Are there secular changes occurring? That’s one of the questions they have to answer. If there are not, then they don’t have to worry about them. If there are, then there might be changes coming. They need more data.
Dr. Snook’s observations and the concluding statement in the Ciriello et al paper explain why LM isn’t ready to talk about when the new version will be published and what users can expect: “It was concluded that consideration to change existing guidelines, reflecting this new psychophysical set point, may be appropriate if these significant performance decreases are confirmed in other locations, with greater subject numbers, and by other investigators.”
Sources: Dr. Stover Snook; Liberty Mutual; Ergonomics
This article originally appeared in The Ergonomics Report™ on 2008-06-25.