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ATLANTA — The story of the U.S. Olympic Marathon Trials, for so long about who finishes first, second and third to qualify for the Games, has an added primary question here on Saturday.

What shoes will those runners be wearing?

An arms race within a foot race, as two-time Olympian Des Linden put it, was, unknowingly at the time, sparked at trials four years ago. There in Los Angeles, some Nike-sponsored runners raced in unreleased prototypes of what later became known as the Vaporfly.

Reported studies claim the latest version — the unusually tall Alphafly with extra foam and a carbon fiber plate — can boost a runner’s efficiency by several percentage points.

Other shoe companies have been playing catch-up to the technology, releasing their own prototypes and new versions ahead of Saturday’s trials (12 p.m. ET, NBC, NBCSports.com/live and the NBC Sports app).

In versions of the Vaporfly: Kenyan Eliud Kipchoge broke the marathon world record by 78 seconds in 2018. Kipchoge became the first person to break two hours for a marathon in 2019 in a non-record-eligible event (in an Alphafly). The next day, Kenyan Brigid Kosgei lowered the 16-year-old women’s marathon world record by 81 seconds.

“[Marathon] times don’t make sense anymore, necessarily,” Linden, who is sponsored by Brooks Running, said Thursday as she bids to become the first woman to make three U.S. Olympic marathon teams. “It’s hard to figure out if it’s the athletes, if it’s the shoes or what combination it is that you’re watching.”

The shoes caused World Athletics, the sport’s international governing body, to say that there was “sufficient evidence to raise concerns that the integrity of the sport might be threatened.”

On Jan. 31, World Athletics ruled that, for this spring and beyond, any shoe must have been available to buy for at least four months prior to competition use. It also limited the height of shoes, though, according to Nike, its Vaporflys and Alphaflys, including the version Kipchoge wore for his sub-two marathon, meet those limits.

“I feel like every conversation I have is: What shoe is that person wearing. Do you think that helped them run faster? I feel like the conversations are taking away from the athletes,” said Emily Sisson, a New Balance runner among Saturday’s favorites. “People don’t really still know even how much work these shoes are doing. Innovation is great, and can be great for the sport, but at the same time, I don’t like seeing shoes getting bigger and bigger and with more plates and things like that. … I’m hoping eventually the conversations will start drifting back to the athletes, not what shoes are they wearing.”

A tweet this week suggested that Nike is offering every one of the men and women racing on Saturday a free pair of Alphaflys. A record 771 runners qualified.

Most of the Olympic team contenders are not sponsored by Nike. Many intend to race in recent versions of their own sponsors’ shoes, believed to have similar technology to Nike’s.

“Three or four years ago, the shoe industry was turned on its side with the shoe that was released that was 15 years ahead of its time,” said Saucony-sponsored Rio Olympian Jared Ward, one of the favorites in the men’s race, along with Nike-sponsored Galen Rupp. “For decades, I feel like the emphasis was on making shoes lighter and lighter and lighter, and that was all we were focused on. Then, all of a sudden, there was this idea that maybe adding weight the right away is going to actually help performance.”

Ward, a BYU adjunct statistics professor, did his own research on the Nike effects, though he said he has never worn them.

He plans to race Saturday in a version of a Saucony shoe that he first competed in at the New York City Marathon on Nov. 3. He was the top American male runner in sixth place in his fastest time in three New York City starts by 99 seconds.

“I feel like the Saucony is very much competitive,” Ward said. “The results that I’m seeing in terms of energy-cost benefit are enough to make me smile.”

Linden said she will wear a Brooks shoe version that will soon be available for purchase. She has had them for about a month. Before this high-tech shoe era, Linden never had such a short amount of time with her race shoes before a major marathon. Linden, the 2018 Boston Marathon winner, starts her 20th marathon on Saturday.

“That’s the thing with the shoes right now. It’s not only does the athlete’s speed matter, the company’s speed matters,” said Linden, noting at a recent Boston Marathon that she had her shoes six days before the race. “How quick are they turning around the new innovations and the newest, greatest thing? It’s a bit of an arms race within a foot race.”

Two of Nike’s top male U.S. marathoners, Bernard Lagat and Leonard Korir, said as of Thursday night they had not decided whether they will race in the new Alphafly or a previous Vaporfly version.

“The most advantage that I get is that when I run 20 miles, or even 25, if I wear that, I can recover faster,” Lagat said of the Alphafly.

There’s no defined answer as to how much a specific shoe can boost a specific athlete — same as it’s always been. Saturday will crown six U.S. Olympic team members and provide another set of data to analyze.

“I’m sure studies are going to surface everywhere comparing everybody’s versions of the shoe,” Ward said. “So we should have answers to that before long.”

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