Last week, we looked at some ways to keep warm while running during the winter. Cold weather affects not only your body, but your equipment too—watch displays slow down, water bottles can freeze up, and cold temperatures can even change the stiffness of your shoes.
It’s this last problem that we’ll be looking at this week.
Why would cold impact shoe cushioning and performance?
The insole and midsole of virtually all running shoes are made from ethylene-vinyl acetate or EVA, a synthetic foam that provides a good balance between cushioning and energy return.
Like all normal materials, EVA gets softer when it is heated and stiffer when it is cooled. But, the question remains. Is this something that you need to worry about in the winter?
The actual effect of cold weather on the stiffness of your shoe in the real world is something that needs to be measured in an experiment, since your shoes are being cooled by the ambient temperature during a winter run, but also heated up by your feet and the impact when you hit the ground.
Effect of temperature on shoe properties
This topic was explored in a 1996 paper by Hiroshi Kinoshita and Barry Bates at the University of Osaka and the University of Oregon, respectively. Kinoshita and Bates designed two experiments to gauge how different temperatures affect the mechanical properties of your running shoes.
To measure the internal temperature of a running shoe during a training run, the researchers had two different runners wear a test shoe in a series of runs in different weather over the course of a year. Each session consisted of a 40-minute run, during which the temperature of the shoe’s midsole was monitored constantly via a sensor implanted into the shoe.
In a separate experiment, the researchers used a mechanical impact tester to measure the temperature-dependence of the midsole stiffness of shoes with a firm, moderate, or soft midsole (differing only from the test shoe worn by the runners in the stiffness of the sole).
Test results
- During the course of the 40 minute runs, Kinoshita and Bates found that the shoes warmed up by about 14° F in the first 10-15min of the run, then stayed at the same level—about fourteen degrees warmer than the ambient temperature—for the rest of the run, regardless of the weather.
- Through the mechanical testing, they also found that, as predicted, the stiffness of the shoes was much higher at colder temperatures, rising to 1.5 times the room-temperature stiffness when the shoe’s temperature was around freezing (32° F).
- Though the cushioning in soft shoes did fare better at colder temperatures, the mechanical properties of all three types of shoes converged at extremely cold temperatures. When the shoes were at well below freezing, there wasn’t much difference in the amount of energy lost, the deformation of the foam, or the time it took for an impact to reach its peak.
Related study results
Kinoshita and Bates’ findings were replicated in a 2010 study by Shariatmadari, English, and Rothwell at Liverpool John Moores University in the UK. In this study, the researchers took a computational approach to studying the behavior of shoe cushioning in warm and cold temperatures. By creating a digital model of a running shoe’s midsole and insole, the researchers were able to accurately model the interaction between the heel of your foot and the cushioning in the heel of a running shoe.
The temperature dependence of EVA foam’s stiffness, the changing impact and deceleration parameters, and the behavior of firm, medium, and soft materials were all accurately modeled by the simulation. The computational approach also allowed Shariatmadari, English, and Rothwell to measure the interaction between the foot and the insole of a shoe during impact in cold conditions, something Kinoshita and Bates weren’t able to examine.
- Because of the increased stiffness of the midsole and insole at cold temperatures, the shoe deforms less on impact, meaning that a smaller area of your foot is in contact with the shoe. This necessarily leads to an increase in peak plantar pressure—the forces distributed over the area of your foot which is in contact with the shoe. This is much akin to running on a harder surface, and will result in more localized pressure on your foot.
- You will be putting more pressure on a smaller area of your foot although the overall impact forces shouldn’t change (just as they don’t change when you run on grass or asphalt). This hasn’t yet been connected with running injuries but it is still something you should keep in mind, especially if you’ve had foot problems or other injuries that can be exacerbated by running on hard surfaces or in stiff shoes.
Conclusion
Despite a good bit of information we have learned about how shoes behave in the cold, there isn’t much we can do about it.
Cold temperatures will stiffen the foam in the midsole and insole of your running shoes, changing their cushioning properties.
- Because your shoes are stiffer when running outside in the winter, they cause more localized pressure on the sole of your foot. Although a shoe with a softer midsole will be more resistant to these changes, the mechanical properties of any shoe will still be substantively different at sub-freezing temperatures.
- In many ways, cold shoes are similar to shoes with a lot of miles on them: as shoes age, they also get stiffer as their EVA foam wears out. If you run outside over the winter, you should pay special attention to how old your shoes are. And if you are especially sensitive to stiff shoes, you might even want to consider wearing a shoe with a softer midsole when it is cold out.
With new shoe materials on the horizon, EVA foam’s temperature sensitivity may not be as much of a problem in the future. Adidas, for example, claims that the cushioning in its latest shoes aren’t as temperature-dependent as traditional foam. But given the history of running shoe companies making unsubstantiated claims about the technology in their shoes, I wouldn’t get your hopes up.
For now, keep in mind the implications of having stiffer shoes when running in cold weather.
