Have you ever slipped on ice and fallen on your derrière? At best, it is unpleasant; at worst, it can send you to the hospital with a broken bone (or even kill you). Icy sidewalks and parking lots are a hazard with which those of us living far from the equator must deal, and one would hope that winter boots would offer protection. Shopping for a new pair, you will see convincing names for outsole technology. You would be forgiven for thinking that science has delivered a modern winter boot that digs into the ice and never lets you down.
You would be mistaken.
Slip-resistant winter boots are severely understudied by scientists, but there is one laboratory in Toronto that specializes in putting this type of footwear to the test. You may have seen it on CBC Marketplace or when Rick Mercer paid it a visit. Its staff rates boots based on how they perform when walking on ice. Their latest results, however, show that even the best winter boots have a secret problem.
We tip till you slip
While cancer research has received a massive amount of funding over the decades, the same is not true of winter boot research. There is, as scientists like to say, a “paucity of data” in the academic literature. And yet, the problem is real: for the winter of 2021-2022, the province of Quebec registered over 2,200 hospitalizations due to falls on ice. The total for Canada? A little over 8,000. We know from research done with healthcare workers and restaurant employees that providing people with slip-resistant footwear indoors cuts down on the number of falls, but what works on a wet hospital linoleum floor may not work on snow-covered ice.
The overall problem, though, is the same: a reduction in friction. Slip-resistant shoes and boots have treads on their soles that are separated by channels (or grooves), and these channels are meant to provide a way to drain liquids away so that the treads can make a firm contact with the ground. There is a common way to test for the slip-resistance of a shoe or boot: a mechanical device on which the footwear is mounted which pushes the shoe down onto a mini-treadmill. This treadmill can be outfitted with a refrigerated ice tray to simulate walking on ice. Friction between the shoe and the treadmill can then be calculated.
However, this is not how we use our boots. We don’t apply the same pressure as we push down on the ground, and the ground itself is not uniform. We also don’t all walk the same: gait differs from person to person. This is why this type of mechanical testing lacks what scientists call “ecological validity.” Like a wild animal plucked from its home and put into a zoo, the boot does not behave the same when mounted onto a machine. So, how else can we test winter boots for their slip resistance?
Figure 1: WinterLab, seen from the outside (source: KITE Research Institute)
Enter WinterLab. From the outside, it looks like a massive ice cube, big enough to host a small party inside of it. The cube rests on electrically powered jacks that can tilt it up to 15 degrees, creating a hazardous incline. Inside, it may be the world’s smallest ice rink: a 4.5-metre-by-4.9-metre ice floor, created by running glycol tubes underneath the surface. People put on boots and try walking on the ice. If they do slip, a fall harness connected to an overhead gantry ensures they won’t end up in the hospital.
WinterLab is the brainchild of Geoff Fernie, the former director of what is now called the KITE Research Institute at University Health Network in Toronto. Given the importance of preventing falls, he wanted to test winter boots in a more ecologically valid way. WinterLab tests this footwear as part of research projects, but also as a paid service to footwear manufacturers who want to put their latest offers through their paces.
The basic idea behind the testing is best summarized on their website: “We tip till you slip.” All in all, four different people will put on and walk around a particular brand of boots inside WinterLab. After each trial, the slope is increased. They walk uphill and they walk downhill, and they do it when the ice is cold and dry, but also when it is melting and has a layer of water on top. The score that is retained is not the best of all four conditions, but the worst. It is the maximum achievable angle or MAA. Increase the slope by one more degree and the person slips. The boot itself then receives the worst MAA recorded out of the four people that tested it. To pass the test, a boot is expected to have an MAA of at least 7 degrees. This is because it meets the accessibility guidelines in Ontario for a curb ramp, and people should be able to walk up and down these ramps in the winter without slipping and falling.
Figure 2: A man testing a pair of boots inside WinterLab (source: KITE Research Institute)
The boots that have been tested by WinterLab are listed on the website Rate My Treads. You can see the MAA scores for each boot, but there is also an overall snowflake rating to make things easier. Any boot performing below a 7-degree angle gets no snowflake. An MAA score of 7 and higher results in the boot receiving one, two, or three snowflakes depending on the steepness of the angle. How good a boot is at preventing you from slipping on ice seems to be mainly determined by its outsole. There may also be minor factors such as the height of the boot, which can restrict ankle mobility, and more research is needed to better understand their contributions to slip resistance.
How do boots fare in the WinterLab? “With the early testing we did, we learned very quickly that most of the winter footwear we find on the market is terrible.” This is Dr. Tilak Dutta, a scientist at the KITE Research Institute and an assistant professor at the Institute for Biomedical Engineering at the University of Toronto. I spoke to him to learn more about what goes on at WinterLab. “We get scores in the range of 2 or 3 degrees for most of the footwear that’s out there. Then, we started to find a few types of outsole technologies that could achieve much better, like 7, 10, up to 15 now.” It’s still a minority, but these are boots with cutting-edge technology, where the manufacturer either puts metal spikes or cleats into the rubber outsoles (so-called spiked footwear) or they create a composite material that uses hard minerals (Green Diamond technology) or microscopic glass fibres (Icelandic Grip and Vibram Arctic Grip technologies) embedded into the sole. Sometimes, these technologies are combined, such as in the IceFX soles which have both hard minerals and microscopic glass fibres in them to improve friction.
Figure 3: Composite material on a winter boot outsole (source: KITE Research Institute)
This should be the end of the story. Irrespective of brand, buy winter boots with one of these cutting-edge outsole technologies and you will significantly reduce your risk of slipping and falling in the winter. There is, however, a giant qualification to this recommendation.
Worse for wear
When boots are tested, they are new. But boots don’t stay new forever.
“I think there is some incentive,” Dr. Dutta tells me, “for footwear manufacturers to game this footwear testing system by coming up with materials that work really well when they’re brand new but that could wear out.” Indeed, his team published the results of a pilot study last year that I found damning. They picked two brands of winter boots (one for men, one for women) that had either a Vibram Arctic Grip or a Green Diamond outsole—top performing soles in terms of slip resistance. They had volunteers walk 100,000 steps around the block in them, and they retested the boots every 25,000 steps. Shockingly, there was “a significant loss of slip resistance after relatively little use at 75,000 steps,” the paper concludes. If you walk 3,000-4,000 steps a day, you will reach this threshold in less than a month. How many of us are going to buy multiple pairs of boots for ourselves every winter? Practically no one.
What prompted this study was an earlier one they did with personal support workers, who were given winter boots to test. Some of them complained that the boots were becoming more slippery at the end of the eight-week period. This has now been confirmed by last year’s small pilot study.
Is it worth investing in a well-rated boot, then? “That is exactly the question that we’re trying to answer,” Dr. Dutta says, specifically just how much worse will a good boot perform after a full winter season. If a new boot allows you to ascend a 14-degree slope without falling, will this same boot’s performance drop down to 9 degrees in April… or 2 degrees? Will it still be superior to most boots on the market or just as bad as your average winter boot? According to Dr. Dutta’s follow-up study, which is still in the works, that 14-degree boot dropped down to just above a 7 after four months of use. This is encouraging, but will it be the same for every high-performing outsole technology? Can soles become clogged with ice particles during use, thus reducing friction? Does a person’s gait affect how quickly a sole wears down? And will this performance remain above 7 for more than a single winter season? More studies, as always, are needed. The Rate My Treads website may one day boast two scores for each boot: one for a new pair and one for a worn-down pair.
A way around the problem of worn outsoles is cleats and spikes, which you can attach to a pair of boots. They are cheap and work very well on ice, but they have major issues: they are uncomfortable to walk in and they can increase the risk of slipping when transitioning to an indoor surface. Both Dr. Dutta and I have personally experienced this. This is why Dr. Dutta’s team is now also testing winter boots and cleats on indoor surfaces, to quantify how well they perform on surfaces other than ice.
Ultimately, we need better outsole technology that resists deterioration. A former post-doctoral fellow of the WinterLab, Zahra Shaghayegh Bagheri, developed a new outsole technology which has been patented. “We’re seeing promising results,” Dr. Dutta tells me. “It has much better wear resistance than the other composite materials we are seeing on the market.” If all goes well, it could turn into a type of overshoe (like galoshes) that would improve the slip resistance of any winter boot.
Fall-related injuries are a much bigger problem than we might think. In 2018, for example, the direct cost of these injuries (which include falls on ice) in Canada was $10.3 billion dollars, which represented 4% of our public spendings on healthcare. To this, we should add the cost of people taking time off work to recover, as well as the pain and disability caused by these injuries. If we can develop better winter boots, we will save both money and suffering, and we might start to unclog our ailing healthcare system at the same time.
Take-home message:
- Most winter boots on the market in Canada are not good at preventing falls from slipping on ice
- Sole technologies like Green Diamond, Vibram Arctic Grip, Icelandic Grip, and IceFX tend to perform much better when tested
- Preliminary results however indicate that some of these advanced soles degrade rapidly with use and lose part of their slip resistance
