One Step at a Time

March 1, 2008
Pinpointing the causes of a same-level fall is like investigating a crime scene there's always more than meets the eye. And, like the crime scene investigator,

Pinpointing the causes of a same-level fall is like investigating a crime scene … there's always more than meets the eye. And, like the crime scene investigator, Liberty Mutual research scientists use a multifaceted investigative approach to study the physical, psychological and environmental factors that can impact the big picture.

Bringing together experts from Liberty Mutual's Center for Physical Ergonomics, Center for Behavioral Sciences and Center for Injury Epidemiology, we examined a range of potential contributors to slip-and-fall incidents, including flooring choices, conditions of walking surfaces, foot protection and employee perceptions of slip hazards.

Exposing Causes of Falls

A team of Liberty Mutual researchers completed a field investigation that illustrates a multifaceted approach to slips-and-falls research. The study applied tribology (the interactions between sliding surfaces with respect to friction, wear and lubrication), psychology and epidemiology to examine, among other things, the relationship between worker perceptions of slipperiness and actual friction measures.

Isolated friction measurements historically have been used to gauge floor surface safety. However, we know these measurements do not tell the whole story. Other factors such as friction variations and individual perceptions of slipperiness have important safety implications. Friction variations, which are the changes in friction that can be present on a given surface, are a critical factor.

When people unknowingly encounter a change in friction due to contamination or wear, they may not have time to adjust their gaits and avoid falls. Worker perception of floor surface slipperiness is another critical area of study. It is important to understand individual perceptions of floor surface slipperiness because perceptions often influence behaviors. When individuals perceive a floor surface to be slippery, they typically alter their gait to avoid a slip or fall. If, on the other hand, the individual does not perceive the floor to be slippery, or underestimates its slipperiness, he or she is more likely to be caught off guard and fall.

10 Participants

We recruited 10 U.S. quick-serve restaurants to participate in the study. We targeted the restaurant industry because of its historically high numbers of same-level slips and falls. In earlier epidemiological studies, Liberty Mutual researchers found that one out of every three disabling restaurant worker injuries is caused by a slip, trip or fall, with 26 percent of these being same-level falls.

“Based on these and similar findings from the scientific literature, we knew that restaurants would be a good setting for examining work-related, same-level slips and falls,” explains Theodore Courtney, M.S., CSP, director of research operations, Liberty Mutual Research Institute for Safety.

During an 8-week data collection phase, we used standardized slipmeters — devices that help determine the slip resistance of surfaces under various conditions — to collect friction measurements from six critical working areas in each restaurant. These areas included the fryer, back vat, grill, sink, front counter and walk-through. Liberty Mutual tribology experts took measurements immediately after the lunchtime rush — a time when floor contamination can be high.

At the same time, institute psychologists and epidemiologists administered surveys to gather subjective data from employees, who were asked to rate the slipperiness of the six identified major working areas from “extremely slippery” to “not slippery at all.” The survey also asked questions relative to recent slip history, including whether or not the employee had experienced a slip or fall at the restaurant in the previous 4 weeks.

Perceptions vs. Reality

As part of the initial data analyses, the institute's tribology researchers compared the friction measurements taken in all 10 restaurants. Among the early findings, we found that the most slippery of the six kitchen areas studied was the sink area, followed by the fry vat, the walkthrough, the grill and the back vat. The front counter was the least slippery of the areas.

We also used the friction measurements to calculate friction variations in the key areas studied. An in-depth analysis of the measured friction variations indicated that workers had a greater than 50 percent chance of experiencing a friction reduction when stepping from one foot to the other in the studied areas.

Our findings illustrated how frequently employees encounter friction reductions in real life. Most field safety measurement practices do not include friction variations, so this was an opportunity to consider the role of such variations in slipperiness measurement.

We used statistical methods to calculate the correlation between actual friction measurements and subjective slipperiness ratings. The results describe a modest correlation between the average measured friction and the average perception rating score for each working area across all the restaurants.

For certain kitchen areas, workers could estimate actual floor slipperiness with some accuracy. In some cases, perceptions may be a useful complement to friction measurements. Equally important, the findings suggest that perceptions of floor slipperiness may be impacted by factors other than friction. For example, human factors, such as visual cues, sensory feedback and motor control, and environmental factors, such as lighting, shoe sole condition and contaminants, also may impact perception. It is important to examine the many different factors influencing perceptions in order to understand how to best focus prevention efforts.

To begin to identify some of the factors other than friction that can impact perceptions, Liberty Mutual epidemiologists and psychologists examined the remaining subjective survey data from the restaurant study, such as age, gender, ethnic group, tenure, shoe type, etc.

Among the initial findings from this analysis, we noted that workers over the age of 45 did not perceive slippery conditions as well as their younger counterparts, and that slip history had a significant impact on worker perceptions.

Wen-Ruey Chang, Ph.D., is a senior research scientist at the Liberty Mutual Research Institute for Safety. Yueng-Hsiang (Emily) Huang, Ph.D., is a behavioral psychologist.

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