A failed shoe became a talking point. Now we know why it happened.
When Zion Williams’ shoe exploded on him in late-February, people searched for answers. Weeks removed from the NCAA tournament, there was suddenly a very real possibility that the most-exciting player in college basketball would be absent from its brightest stage.
Nike called it an “isolated occurrence,” assuring the public it was looking into why WIlliamson’s PG 2.5s blew out. Some speculated the shoes were faulty, others blamed Zion’s propensity to play in shoes that weren’t made for big men as part of the issue.
Dr. Nate Hunt is an assistant professor at the University of Nebraska at Omaha’s department of biomechanics. He conducts experimental research on high-performance movement in humans and other animals, in an effort to address movement degradation that comes with aging or movement disorders.
This research evaluates the vertical and horizontal ground reaction forces humans generate when they balance and accelerate. Hunt’s work doesn’t explicitly measure forces on shoes, but he explained how opposing forces, like those Zion generated against North Carolina on February 20, can cause a shoe to fail.
“A shoe blowout, like Zion Williamson generated, takes huge opposing forces happening simultaneously on different parts of the shoe,” Hunt explained. “Typically, the largest forces where the foot meets the ground are vertical. But during rapid direction changes, large horizontal forces are produced. Looking back at the video, Zion plants his foot at a steep angle - roughly 60 degrees from vertical. And at that point, he’s supporting himself on just one leg. To vertically support his 284 pound body, with his leg at that extreme angle, he’d need to produce a minimum of double his body weight, or 560 pounds of force.”
The issue, greater than basic answers like Zion was wearing the wrong shoe or Nike made it incorrectly, is that there simply isn’t a great analog for Zion Williamson playing basketball today. His size and play style is in a class of its own, something Nike showed at the beginning of the NCAA Men’s Tournament when they gave Zion specially re-enforced shoes for him to play in.
The closest comparisons for a player of Williamson’s size and athleticism in the NBA are Julius Randle or LeBron James, who, much like Williamson, prefer to wear lighter shoes. However, while Randle and LeBron might cut like Zion does, neither have same frame behind it.
This means that Zion is putting roughly 70 pounds more force on shoes when he cuts than Randle or James. It’s also unclear if wearing a big man shoe, one made specifically for a center, would hold up to the kinds of stress Zion puts on his shoes, because they’re not designed to take that kind of shear force. Hunt explains why a blowout like this takes a specific set of circumstances.
“This enormous force wouldn’t be a problem if he was leaping into the air and pushing straight down through the sole of the shoe with the pressure distributed across the bottom surface of his foot. But he was leaping sideways off one foot, and was likely producing much of the horizontal force against the upper of the shoe, while the horizontal ground-reaction force was pushing back against the sole of the shoe. The misalignment of these large forces would cause enormous shear stress within the shoe’s materials, and could lead to separation of the upper from the sole.”
Nike hasn’t publicly released their findings into why Zion’s shoe failed, but did seek to fix the issue by giving him some specially-designed shoes, which presumably addressed the forces he puts on them. These new sneakers appear to be reinforced Kyrie 4s (Zion’s preferred shoe), as he explained during a press conference before the tournament.
“I couldn’t really specifically tell you if I wanted to,” Williamson said. “I just know they’re a little stronger than the regular Kyrie 4’s, so I want to thank Nike for making these, but, yeah, they felt very comfortable.”
The final part of the equation is shoe technology. That, paired with Zion’s size is part the issue too, explains Hunt.
“Beyond the mass and power of Zion, another reason the shoe blew out was because it didn’t slip. Shoe companies design soles for high traction. But that allows large powerful players to cut even harder, putting greater stress on other aspects of the design.”
The final answer might not be as sexy or intriguing as you hoped. Based on everything we know it’s less a case of Zion wearing the incorrect shoes, or Nike having a product failure — and more that there simply hasn’t been a player like Zion before.
His unprecedented mixture of speed and power is putting stress on shoes that is incomparable to anything in the NBA, and we may not know what footwear innovations are required until he reaches the NBA and gets his first signature shoe.
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