By Catherine M. Koetters
Although slip-resistant footwear can play a key role in preventing accidents related to wintry conditions, most styles don’t do the job, according to researchers from the Toronto Rehabilitation Institute in Canada who have set out to change the situation.
In a study published in the May 2016 issue of Ergonomics, Geoff Fernie, PhD, and his team showed that testing slip resistance using their Maximum Achievable Angle (MAA) method resulted in a more consistent and accurate evaluation of winter shoes and boots than the industry standard, mechanical bench testing, which does not involve human participants or realistic outdoor conditions.
“Other testing methods used tend to get variable results,” said Fernie, a senior scientist and director of the research institute. “You get different results in different labs at different times. We seem to get rather consistent results.”
For the Toronto study, eight participants of similar age, height, and weight tested six styles of men’s winter footwear on wet ice, dry ice, and dry ice after walking over soft snow. Using Toronto Rehab’s state-of-the-art WinterLab facility, researchers tipped the floor one degree at a time until each participant was unable to initiate gait or both feet slipped simultaneously midslope. Testers wore harnesses to prevent slip-related injury.
MAA ranged from 4° to 10° on dry ice and up to 13° on wet ice; the bench testing values were the equivalent of MAAs from 2° to 8° on wet ice and up to 14° on dry ice. There was even less agreement between MAA and bench testing for the snow-plus-ice condition (4° to 8° for MAA vs the equivalent of 10° to 25° for bench testing).
The researchers also studied the participants’ gait as they walked across various surfaces. They found that step length and width, gait speed, foot angle at heel strike, and upper body flexion all adapted to changing conditions. According to the study’s authors, this finding underscores the importance of using human participants when testing the slip resistance of footwear.
Fernie and his team have already put their MAA method to practical use. They tested the slip resistance of 98 new winter boots and published the results in November on a website they created, ratemytreads.com.
Fernie was surprised at how badly the footwear performed overall. One pair of expensive boots advertised the wearer’s ability to “walk to the arctic.” Yet a student tester could not walk up a 2° ice slope while wearing those boots, he said.
Boots received a rating of one, two, or three snowflakes. If testers could successfully negotiate the 7° angle—similar to the slope of many street curb ramps—under wet ice and dry ice conditions, the shoe got one snowflake. Only 10 boots out of 98 were awarded a single snowflake. The rest got zero.
The researchers found the shoes that performed best used either Green Diamond or Arctic Grip sole technologies. Green Diamond soles have a rough surface, resembling a scattering of little diamonds, that scratches at the ice, Fernie said. Arctic Grip soles feature fibers that penetrate the molecular layer of water on the ice’s surface and stick to the frozen part underneath.
“People think the tread is most important, but when you’re on a hard patch of ice, or really packed snow, or a sheet of black ice, the tread doesn’t really penetrate,” Fernie noted. “It’s the material that matters. It’s the material that interacts with the surface, which increases the friction, that matters.”
Jeff Jacobs, CPed, owner of Foot Dynamics comfort shoe store in Boise, ID, is grateful for the research.
“Every bit of Information that can be brought forward to help us do our job is much appreciated,” Jacobs said. “As the shoe industry can embrace this information and create better products, we all benefit.”
Interest in the Rate My Treads website has been tremendous, Fernie said. The site received more than 250,000 hits in its first six weeks, and the researchers got phone calls from consumers frustrated that every boot with a one-snowflake rating was sold out before Christmas.
Fernie said retailers are now asking his group to test boots they’re looking to carry next winter to ensure they will sell well. A few manufacturers also want the research team’s help making better boots. The group has already tested one company’s prototype that Fernie thinks may get a two-snowflake rating next year—that would mean the wearer could successfully negotiate an 11° slope. When testing next season’s boots, the researchers will also perform wear studies to determine how long the better-rated footwear continues to perform well.
Fernie believes that in two to three years, winter footwear will achieve higher ratings as more manufacturers look to science, not fashion, when striving for truly slip-resistant winter footwear.
Hsu J, Shaw R, Novak A, et al. Slip resistance of winter footwear on snow and ice measured using maximum achievable incline. Ergonomics 2016; 59(5):717-728.