We've written previously about slip, trip and fall accidents and accident prevention, noting that it's an important topic that falls within the realm of ergonomics practice (for example: Slips, Trips, Falls: An Untapped ROI Opportunity for Ergonomists, Britain Boosts Campaign to Cut Slip, Trip and Fall Accidents, Slip, Trip, Fall Research Gaining Pace). I was recently reminded of the topic while speaking with a colleague that specializes in forensic ergonomics, including slip/trip/fall (STF) incidents, and through a chance meeting with another forensic ergonomics colleague who is involved with standards development related to the devices used to test walking surfaces for STF related characteristics. I was involved with STF research and forensics in the mid-1980's, and was surprised to learn that the underlying science to understand the mechanics of STF events, as well as the equipment used to test walking surfaces for their STF characteristics, has apparently advanced very slowly since that time. The research summarized below sheds some light on why so little progress has been made: STF, as with many ergonomics topics, is a complicated multi-factorial problem, and simple explanations and agreeable solutions are therefore hard to find. The magnitude of the problem, nonetheless, is truly significant. For example, the authors cite research that states that roughly 25% of USA workplace STF accidents result in 31 lost work days per event, costing the US economy $10 billion per year.
In this study University of Alberta, Canada based researchers Osis, Worobets and Stefanyshyn set out "to examine ground kinetics early in stance while walking on a contaminated surface and assess the potential of kinetics to quantify risk of slipping." The authors suggest that previous research has focused primarily on what is referred to as the utilized coefficient of friction (UCoF) between the floor surface and the foot/shoe interface. Coefficient of friction (CoF) is a standard engineering measure that quantifies a measure of slip resistance by dividing the horizontal force (shear) by the vertical force (normal) between two surfaces (referred to as the ground reaction forces in the case of walking). The coefficient changes between static and dynamic events: the static CoF, which characterizes the slip resistance between surfaces that have not yet begun to slide relative to one another is always higher than the dynamic CoF, once two surfaces are sliding against each other. Though much research has been conducted to correlate such UCoF measures with the incidence and severity of STF accidents, the authors suggest "… it is still unclear … how to consistently identify a slip hazard based on a standardized measure of UCoF." They postulate that the time at which the ground reaction forces are measured, from initial heel-strike to the point when the foot/shoe leaves the walking surface is a key research question. They review research that measured UCoF values at various points in the foot-floor interaction, from 25 milliseconds to 200 milliseconds following heel-strike. Since the physics of what happens at 25 resistance is dictated by what occurs before that point in time, they turned their attention to this early heel-strike period, specifically for a "slippery surface" condition.
The researchers built a scaffold system that included a moving trolly with a fall restraint harness worn by the 11 healthy male experimental participants. Ground reaction forces were measured using an in-ground force measurement plate, and 3D body kinematics were collected using a motion capture system. Following some training/experience walking under the experimental conditions, the researchers began the study by having each participant walk across the force plate under randomly designed dry and contaminated surface conditions. The participants wore goggles that blocked their view of the surface condition. Interested readers are referred to the original research article, cited below, for complete details of the experimental methods.
Significantly reduced UCoF values were recorded as early as 11.34 ms following heel-strike under contaminated conditions, suggesting that kinetic changes are occurring prior to those time points examined in previous research;
Changes in shear force were seen as early as 0.42 ms after heel-strike, suggesting the same;
22 trails under contaminated conditions resulted in < 10 mm of movement/displacement ("microslip"); 22 trials with displacement between 10 mm and 100 mm ("macroslip"); and 5 trials with > 100 mm displacement ("fall," however no one in the study actually did fall);
Shear forces (horizontal force under foot) correlated better with slip displacement than did the traditional CoF measurements, suggesting it may be a better predictor of slip conditions than CoF.
What this Research Contributes to Applied Ergonomics
This research applies primarily to flooring and footwear design and analysis. In particular, those who get involved with forensic ergonomics applications, such as testifying in a court of law as to the causative factors in a STF accident, are the biggest immediate benefactors of this type of basic research. In addition, it is likely to play a role in the debates over, and subsequent standardization of testing equipment used to understand slip potential for walking surface and shoe sole characteristics.
I selected this article for review for several reasons:
- It demonstrates the need for basic research, and the challenges researchers face when attempting to fully understand the contributing factors to an accident;
- It demonstrates that advancements in measurement/experimental technologies also advance our understanding of the world (previously, accurately measuring data like this, at the millisecond level, was difficult if not impossible); and
- It demonstrates the need for ergonomists to be careful with their own understanding and application of ergonomics theory.
This is an example of why the statement that "ergonomics is common sense" is often untrue. While it may be common sense that a contaminated walking surface could be more slippery than a dry one, the actual mechanics of how that may come about, and the design improvements that will reduce risk, remains a question. There are many theories, of course, but in order to apply theories in practice, we need evidence, and scientific studies like this one are where evidence based ergonomics comes from. As professionals, we must be careful to recognize the difference.
The research will go on, but in the mean time, keep floors clean and clear of contaminants — that much is clear (e.g., see Slips, Trips, Falls: An Untapped ROI Opportunity for Ergonomists).
Sean T. Osis, Jay T. Worobets, Darren J. Stefanyshyn, Early Heelstrike Kinetics Are Indicative of Slip Potential During Walking Over a Contaminated Surface, Human Factors: The Journal of the Human Factors and Ergonomics Society 2012 54: 5 originally published online 2 December 2011, DOI: 10.1177/001872081142790
Subscribers to Human Factors: The Journal of the Human Factors and Ergonomics Society may access this article online at: http://hfs.sagepub.com/content/54/1/5
This article originally appeared in The Ergonomics Report™ on 2012-03-07.