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 an hour and 30 minutes.

But her decision to switch to Nike Vaporfly trainers before securing her new personal best 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,” said Grundon 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.”..