As it so often is, history was made in Berlin on September 16. Kenyan runner Eliud Kipchoge smashed the marathon world record with a time of 2:01:39. To put this astonishing feat into context, that’s the equivalent of running 100 meters in 17.3 seconds and continuing at the same pace for another 26.2 miles.

The fact he took 78 seconds off the previous best set by Dennis Kimetto in 2014 is staggering in itself, but Kipchoge running more than 10 miles of the race alone after his pacemakers dropped off earlier than intended will see his achievement go down as one of the most iconic moments in sporting history.

Kipchoge, 33, is now regarded as the greatest long-distance runner of all time. He has won 10 of his last 11 marathons, including an Olympic gold in Rio de Janeiro and three victories in the London marathon. His victory in Berlin last month was his third in the German capital.

To top it off, he also seems like a genuinely wise, happy, and humble kind of guy. A recent New York Times profile tells of how he never ducks chores at his training camp high in the Kenyan hills: “He is a wealthy man, but he still scrubs the toilet.”

But his record-breaking run in the German capital has thrown up a few interesting questions. They don’t relate to performance-enhancing drugs — dozens of Kenyan runners have tested positive for banned substances, but Kipchoge’s record is clean and few would suspect him of involvement — but they do concern what does or does not constitute an unfair advantage.

Kipchoge had something most other runners in the field did not: Nike’s Vaporfly 4% running shoe.

Nike says the $250 shoe, which was conceived as part of the brand’s ambitious Breaking2 project (also featuring Kipchoge) to attempt a two-hour marathon, is named 4% because it offers an average 4 percent running economy compared with other top runners. Or in other words, how much energy runners use while running in them.

If you’re not a runner, that might seem small or inconsequential, but even small improvements in running economy can equate to shaving off minutes, not just seconds, over long distances.

It’s an astonishing claim, and one that could easily be dismissed as yet more marketing hype to sell more shoes. But this time it actually appears to be true.

The New York Times recently analyzed data from around 500,000 marathon and half marathon running times — taken from public race reports and fitness app Strava — over the last four years. It found that “runners in Vaporflys ran 3 to 4 percent faster than similar runners wearing other shoes, and more than 1 percent faster than the next-fastest racing shoe.”

The findings back up the earlier Nike-funded study at the University of Colorado, published in peer-reviewed research journal Sports Medicine, that discovered the 4 percent increase in running economy and gave the shoe its name.

If you’re a runner who takes races seriously, it’s likely you’re at least considering getting the Vaporfly. The efficiency improvement could in theory be worth more than seven minutes to a three-hour marathoner, or nearly 10 minutes to a four-hour marathoner.

It could arguably turn out to be the biggest innovation in athletics since rubberized artificial running tracks replaced crushed cinders — and for that Nike deserves tremendous credit.

But what does it mean for Kipchoge’s record-breaking Berlin run using the shoes? Well, as sport scientist Ross Tucker pointed out after the race, if what Nike says and what the NYT found are true, it essentially explains the record-shattering run.

Kipchoge improved Kimetto’s record by 1.07 percent, which suggests Kipchoge and Kimetto were equal runners wearing unequal shoes. Lets Run‘s Jonathan Gault noted that, if you go by the New York Times study, you could even argue that Kimetto is the superior runner: “The Vaporflys were 1 percent faster than the next-fastest shoe in the study, but they were about 2.5 percent faster than the adidas Adizero Adios — the shoes Kimetto ran

[in] during his world record run.”

A week after Berlin, Kenyan Emmanuel Saina Kipkemboi smashed the Buenos Aires marathon course record by four minutes 25 seconds with a time of 2:05.21. It was his debut marathon and his time is now the fastest ever recorded in South America. He was, of course, wearing the Vaporfly, or as physiology and biomechanics professor Dr. Peter Weyand called them, “performance enhancing shoes.”

The effectiveness of the shoe is just another chapter in a saga that has plagued sporting officials for decades. At what point does new technology create an unfair or unsporting advantage? The US Golf Association banned balls that fly straighter than others, the NFL prohibited the use of a substance that helped players catch the ball, and swimming officials barred high-tech suits that reduced drag and increased buoyancy.

The difference between the Vaporfly and other running shoes is that it has a carbon fiber plate in the midsole. That plate stores and releases energy with every step, meaning the runner expends less energy as the shoe propels them forward.

The International Association of Athletics Federations (IAAF) rules around footwear state that “shoes must not be constructed so as to give athletes any unfair assistance or advantage” and that they “must be reasonably available to all in the spirit of the universality of athletics.”

In a statement, Nike told Highsnobiety, “The Nike Zoom Vaporfly 4% meets all IAAF product requirements and does not require any special inspection or approval. The IAAF has also offered public support of the innovation. Furthermore, the shoe is available to every athlete via and stores.”

It seems unlikely that the IAAF will ban carbon fiber plates, nor do most runners think they should. At least not yet. But the issue does throw up some interesting dynamics for the future of running shoes.

Will other brands try to catch up by using similar technology to level the playing field? How far can manufacturers push the technology? At what point does a world record become a creative technological feat rather than an outright physical one? Is the Vaporfly any different from, say, improvements in our understanding of nutrition?

As David Epstein asks in his excellent 2014 TED talk, are athletes really getting faster, better, stronger?

Nobody can really answer any of those questions right now, nor does anybody seem all that willing to try. To do so would unfortunately take some of the sheen off Kipchoge’s incredible achievement.

In terms of the sport in a wider sense, it has been suggested that Kipchoge (and the Vaporfly) has effectively ended the era of other athletes chasing world records — simply because it seems unlikely that anyone but Kipchoge himself could seriously hope to break the marathon record now.

As for Nike, Kipchoge’s time of 2:01:39 on a fully sanctioned race course such as Berlin offers serious hope that the two-hour barrier can be broken. As part of Breaking2, Kipchoge has already run a 2:00:25 marathon in perfect though not world record-eligible conditions. The Beaverton brand might yet be tempted into a Breaking2 Part 2 in 2019.

Kipchoge, meanwhile, has a more modest goal in mind. “I have run 2:00, 2:01, 2:03, 2:04, and 2:05,” he said, smiling, after the race in Berlin. “Next season, I want to run 2:02.”