Can there be too much stickiness in a shoe?

Gretchen Reynolds / New York Times News Service /


Sometimes innovative science requires innovative machinery, like a four-legged robotic sled that can wear shoes, a contraption recently developed and deployed by researchers at the University of Calgary to test whether grippy athletic shoes affect injury risk.

It’s well-known, of course, that shoe traction influences athletic performance, especially in sports that involve sprinting or cutting, meaning abrupt rapid shifts in direction. In broad terms, more traction leads to better results.

In a 2009 study of soccer players and their footwear, for instance, researchers tested forward sprinting and sideways cutting speed while the players wore their normal soccer shoes, and again after the shoes’ cleats had been shaved down in length by 50 percent and then by 100 percent, meaning they were flat against the outsole. While wearing the shortened cleats, the players had less traction on the field and were significantly slower moving forward or sideways.

But these and similar studies did not establish whether more shoe traction is always desirable or if there is such a thing as too much stickiness in a shoe.

Athletic shoes have two primary types of traction. One keeps you sticking to the ground as you move forward. The other, called rotational traction, kicks in when you move sideways or shift direction. The amounts of each type of traction depend on a shoe’s outsole material and on whether it has cleats and, if so, how many, their size and shape, and how they are positioned.

For some time, most researchers have believed that forward traction does not have much effect on injury risk, while rotational traction does.

But that idea had been difficult to test in real-world situations. So researchers at the Human Performance Lab at the University of Calgary created their shoe-wearing robotic tester. Mounted on rails, it can move forward or sideways on a field at whatever speed the researchers choose, while its “feet” stay in contact with the ground and various sensors determine forward and rotational traction.

With this robot tester at the ready, the researchers recruited hundreds of local high school football players and borrowed their shoes. They fitted each shoe onto the robot tester and determined its unique forward and sideways traction. The various shoes varied widely in terms of traction, said John Wannop, the University of Calgary kinesiologist who led the study.

The scientists then returned the shoes to the players and asked each team’s trainer to track all noncontact leg injuries throughout the season. This experiment was repeated for two more years, during which time the playing fields were switched from grass surfaces to artificial turf. At the end of the three seasons, the scientists compared traction levels and injury reports.

Many of the players had experienced ankle, knee and ligament injuries that didn’t involve contact. The incidence was highest and the severity of the injuries greatest among those players whose shoes provided the most rotational traction. At the same time, the players whose shoes had provided the most forward traction developed fewest injuries.

This finding was unexpected, Wannop said, because it had been thought that any shoe with high forward-motion traction would also automatically have high rotational traction, and so would increase injury risk. But this was not the case; some shoes gripped as players ran forward but didn’t stick when they cut sideways. Those shoes were the safest.

Playing surface, meanwhile, had almost no effect on injury risk. Injury rates were similar on grass and artificial turf, whatever shoes the players wore.

What these findings mean in practical terms, Wannop says, is that for the ideal mix of athletic performance and reduced injury risk, a shoe should have “high translational traction values and relatively low rotational traction values.”

Good luck, however, finding precisely that shoe. Companies don’t advertise shoes’ traction values and probably can’t, Wannop said, since they will vary, depending on your body size and movement patterns and on such ephemera as the conditions of a field or trail on any given day.

Still, there are some broad guidelines to consider when purchasing athletic shoes, especially for team sports like football, soccer or basketball, Wannop said. Avoid models with multiple large, toothy cleats or rubbery nodules along the outside of the sole, he advises, since they can create too much rotational traction. Look instead for groupings of shorter cleats in the forefoot, which can provide reliable forward-oriented traction.

Most important, try the shoes before buying, if at all possible. Ask the salesperson if you can go outside while wearing them. Find some grass and sprint, halt, pivot and cut. If your foot slips when you move forward or noticeably sticks when you pivot, Wannop said, try another pair. You might want to stick to your exercise regimen, but you don’t want to be stuck to the ground.