Scientific studies as well as anecdotal evidence suggest that Nike's Vaporfly running shoes, equipped with super-springy foam soles encasing a carbon-fibre plate, can increase running efficiency by between 4% and 6%.
British amateur runner and triathlete Holly Grundon was delighted to smash a long-held personal speed target - to finish a half marathon in less than 1 hour and 30 minutes.
But the 36-year-old's decision to switch to Nike's Vaporfly trainers before securing her new personal best (PB) time has her wondering whether she cheated herself, or dirtied a sport she's always loved for its purity.
"When you're running you just want to run for yourself," Grundon said during a lunchtime run at her local park. "It's just you against the road and the time that you have in mind to beat.
"So, investing in a pair of trainers that are meant to make you automatically faster - well, there were ethical considerations, and whether it could be cheating, I suppose."
Scientific studies as well as anecdotal evidence suggest that Nike's Vaporfly running shoes, equipped with super-springy foam soles encasing a carbon-fibre plate, can increase running efficiency by between 4% and 6%.
That has persuaded millions of amateur athletes to ditch their old favourites and give them a go.
The shoe has also raised question at World Athletics, the sport's governing body, which looks set to review its rules for elite racing in light of technological advances while allowing recreational runners to wear what they want.
Nike's chief financial officer Andrew Campion told analysts in December the company's market share in running had reached a record high in the second quarter of 2019, led by the success of the VaporFly 4% and Next% models.
And the company's chief executive Mark Parker joked: "The time it takes me to walk across the Nike campus here at headquarters has dropped by at least 4%. So I'm feeling good about it."
Grundon's race distance was a little more ambitious. She described how she'd been chasing the 1:30 half marathon time for a few years, and "inching closer and closer to it".
When she entered the Madrid half marathon she decided it was going to be her last shot at a PB for a while before taking a training break. She opted to buy Nike's Vaporfly shoes after seeing other fast runners wearing them and reading reviews.
"They feel very strange to walk in, but as soon as you're racing and you're running, they really do feel fast," she said. "I smashed my target actually quite considerably.
"But even after the event, that made me wonder whether I should have worn them - because I think I probably could have beaten my target just in my ordinary trainer."
